Dual phase steels Research Papers (original) (raw)

Dual Phase steels (DP) are part of the Advanced High Strength Steels (AHSS) family and consist in a ferritic matrix with a fraction of dispersed martensite between 5 and 50%, which gives the material a good combination of strength and ductility, with a significant capacity to absorb energy. Steel wires called in Argentina ATR500N, are used to manufacture steel welded framework, wires mesh and lattice girders for reinforcement of concrete structures and their mechanical requirements have defined in this country by the IRAM-IAS U500 526 standard. The current manufacturing process uses a wire rod of low carbon steel, hardened by cold working, producing a low ductility and low yield strength to tensile strength ratio product, although meet the requirements of the standard. The objective of the present work is to develop DP steels for ATR500N product, starting from the raw material used today and compare their mechanical properties to the commercial product. Several grades of DP steels were obtained and characterized microstructural and mechanically. Expressions of technological interest were developed, relating properties with fraction of martensite. Certain DP steels were slightly hardened by cold working and compared with the commercial ATR500N product. The DP steels developed fully satisfy the requirements of the standard and, in addition, a significantly higher elongation, hardening exponent and yield strength to tensile strength ratio. These characteristics are interesting for earthquake resistant applications. A new manufacturing route could be developed for ATR500N product.

- by and +1
- •
- Mechanical Engineering, Materials Science, Thermodynamics, Mechanical properties

The microstructure of dual phase steels can be compared with a composite composed of a matrix of ferrite reinforced by small islands of martensite. This assumption has been used in several attempts to model the mechanical properties of dual phase steels. However, recent measurements show that the properties of the ferrite phase change with distance from the martensite grains. These measurements showed that the grains of the ferrite phase are harder in the vicinity of martensite grains. As a consequence of this local hardening eﬀect,the ferrite phase has to be considered as an inhomogeneous matrix in modeling dual phase steels. This experiment inspired the idea thatlocal hardening is caused by geometrically necessary dislocations. The idea is investigated experimentally and numerically in the presentanalysis, which for the ﬁrst time leads to good agreement with experimental observations of the mechanical stress–strain behavior

- by Dierk Raabe
- •
- Microstructure, Finite Element Methods, Micromechanics, Mechanical properties

This paper presents a three dimensional Computational Fluid Dynamics (CFD) model to investigate the flow dynamics of solid-gas phases during fine grinding in an air jet mill. Alpine 100AFG fluidized bed air jet mill is considered for the study and the jet milling model is simulated using FLUENT 6.3.2 using a standard k-ε model. The model is developed in GAMBIT 2.3.16 and meshed by tet/hybrid (T-Grid) and Triangular (Pave) meshes. The effects of operating parameters such as solid feed rate, grinding air pressure and internal classifier speed on the performance of the jet mill are analyzed. The CFD simulation results are presented in the forms of dual phase vector plot, volume fraction of phases and particle trajectories during fine grinding process. The mass of ground feed entering and leaving the cyclone (underflow) is also computed by simulation. The proposed model gives realistic predictions of the flow dynamics within the jet mill. Experiments are conducted on the Alpine 100AFG jet mill to study the particle size, morphology and mass of the ground product. The numerical results are found in good agreement with the experimental results.

Ferritic–martensitic dual phase (DP) steels deform spatially in a highly heterogeneous manner, i.e. with strong strain and stress partitioning at the micro-scale. Such heterogeneity in local strain evolution leads in turn to a spatially heterogeneous damage distribution, and thus, plays an important role in the process of damage inheritance and fracture. To understand and improve DP steels, it is important to identify connections between the observed strain and damage heterogeneity and the underlying microstructural parameters, e.g. ferrite grain size, martensite distribution, martensite fraction, etc. In this work we pursue this aim by conducting in-situ deformation experiments on two different DP steel grades, employing two different microscopic-digital image correlation (lDIC) techniques to achieve microstructural strain maps of representative statistics and high-resolution. The resulting local strain maps are analyzed in connection to the observed damage incidents (identified by image post-processing) and to local stress maps (obtained from crystal plasticity (CP) simulations of the same microstructural area). The results reveal that plasticity is typically initiated within ‘‘hot zones’’ with larger ferritic grains and lower local martensite fraction. With increasing global deformation, damage incidents are most often observed in the boundary of such highly plastified zones. High-resolution lDIC and the corresponding CP simulations reveal the importance of martensite dispersion: zones with bulky martensite are more susceptible to macroscopic localization before the full strain hardening capacity of the material is consumed. Overall, the presented joint analysis establishes an integrated computational materials engineering (ICME) approach for designing advanced DP steels.

In this study wear behaviour of 0.1% C containing dual phase
steels with three different microstructures have been examined. Intercritical annealing, step quenching and intermediate quenching heat treatments have been applied to the alloy in order to obtain different morphologies of ferrite and martensite. It has been observed that, intercritical annealing lead to highest strength but lowest wear resistance in this alloy. Step quenching suggested to have no beneficial effect on tensile properties but increased the wear resistance. Intermediate quenching was found to be the best heat-treatment condition. The fibrous microstructure lead to the
optimum tensile strength, ductility and wear resistance in this alloy.

A series of dual-phase (DP) steels containing finely dispersed martensite with different volume fractions of martensite (V m) were produced by intermediate quenching of a boron- and vanadium-containing microalloyed steel. The volume fraction of martensite was varied from 0.3 to 0.8 by changing the intercritical annealing temperature. The tensile and impact properties of these steels were studied and compared to those of step-quenched steels, which showed banded microstructures. The experimental results show that DP steels with finely dispersed microstructures have excellent mechanical properties, including high impact toughness values, with an optimum in properties obtained at ∼0.55 V m. A further increase in V m was found to decrease the yield and tensile strengths as well as the impact properties. It was shown that models developed on the basis of a rule of mixtures are inadequate in capturing the tensile properties of DP steels with V m>0.55. Jaoul-Crussard analyses of the work-hardening behavior of the high-martensite volume fraction DP steels show three distinct stages of plastic deformation.

The phase transformation kinetics under continuous cooling conditions for intercritical austenite in a cold rolled low carbon steel were investigated over a wide range of cooling rates (0.1–200 °C/s). The start and finish temperatures of the intercritical austenite transformation were determined by quenching dilatometry and a continuous cooling transformation (CCT) diagram was constructed. The resulting experimental CCT diagram was compared with that calculated via JMatPro software, and verified using electron microscopy and hardness tests. In general, the results reveal that the experimental CCT diagram can be helpful in the design of thermal cycles for the production of different grades of dual-phase–advanced high-strengh steels (DP-AHSS) in continuous processing lines. The results suggest that C enrichment of intercritical austenite as a result of heating in the two phases (ferrite–austenite) region and C partitioning during the formation of pro-eutectoid ferrite on cooling significantly alters the character of subsequent austenite phase transformations.

Dual-phase (DP) steel is the flagship of advanced high-strength steels, which were the first among various candidate alloy systems to find application in weight-reduced automotive components. On the one hand, this is a metallurgical success story: Lean alloying and simple thermomechanical treatment
enable use of less material to accomplish more performance while complying with demanding environmental and economic constraints. On the other hand, the enormous literature onDP steels demonstrates the immense complexity ofmicrostructure physics inmultiphase alloys: Roughly 50 years after the first reports on ferrite-martensite steels, there are still various open scientific questions. Fortunately, the last decades witnessed enormous advances in the development of enabling experimental and simulation techniques, significantly improving the understanding of DP steels. This review provides
a detailed account of these improvements, focusing specifically on (a) microstructure evolution during processing, (b) experimental characterization of micromechanical behavior, and (c) the simulation of mechanical behavior, to highlight the critical unresolved issues and to guide future research efforts.

Two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 μm by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM).

- by Dierk Raabe
- •
- Electron Backscatter Diffraction (EBSD), EBSD, Grain size, Grain size distribution

The microstructure and texture of rolled and annealed dual-phase steels with 0.147 wt. % C, 1.868 wt. % Mn, and 0.403 wt. % Si were analyzed using SEM, EDX, and EBSD. Hot rolled sheets showed a ferritic-pearlitic microstructure with a pearlite volume fraction of about 40 % and ferrite grain size of about 6 µm. Ferrite and pearlite were heterogeneously distributed at the surface and distributed in bands at the center of the sheets. The hot rolled sheets revealed a throughthickness texture inhomogeneity with a plane-strain texture with strong α-fiber and γ-fiber at the center and a shear texture at the surface. After cold rolling, the ferrite grains showed elongated morphology and larger orientation gradients, the period of the ferrite-pearlite band structure at the center of the sheets was decreased, and the plane-strain texture components were strengthened in the entire sheet. Recrystallization, phase transformation, and the competition of both processes were of particular interest with respect to the annealing experiments. For this purpose, various annealing techniques were applied, i.e., annealing in salt bath, conductive annealing, and industrial hot-dip coating. The sheets were annealed in the ferritic, intercritical, and austenitic temperature regimes in a wide annealing time range including variation of heating and cooling rates.

- by Dierk Raabe
- •
- Phase Transformations, Cellular Automata, Recrystallization, Cellular Automaton Model

Although Nernst observed ionic conduction of zirconia-yttria solutions in 1899, the field of oxygen separation research remained dormant. In the last 30 years, research efforts by the scientific community intensified significantly, stemming from the pioneering work of Takahashi and co-workers, with the initial development of mixed ionic-electronic conducting (MIEC) oxides. A large number of MIEC compounds have been synthesized and characterized since then, mainly based on perovskites (ABO 3−ı and A 2 BO 4±ı ) and fluorites (A ı B 1−ı O 2−ı and A 2ı B 2−2ı O 3 ), or dual-phases by the introduction of metal or ceramic elements. These compounds form dense ceramic membranes, which exhibit significant oxygen ionic and electronic conductivity at elevated temperatures. In turn, this process allows for the ionic transport of oxygen from air due to the differential partial pressure of oxygen across the membrane, providing the driving force for oxygen ion transport. As a result, defect-free synthesized membranes deliver 100% pure oxygen. Electrons involved in the electrochemical oxidation and reduction of oxygen ions and oxygen molecules respectively are transported in the opposite direction, thus ensuring overall electrical neutrality. Notably, the fundamental application of the defect theory was deduced to a plethora of MIEC materials over the last 30 years, providing the understanding of electronic and ionic transport, in particular when dopants are introduced to the compound of interest. As a consequence, there are many special cases of ionic oxygen transport limitation accompanied by phase changes, depending upon the temperature and oxygen partial pressure operating conditions. This paper aims at reviewing all the significant and relevant contribution of the research community in this area in the last three decades in conjunction with theoretical principles.

- by Wilhelm Meulenberg
- •
- Engineering, Membrane Science, Scientific Communication, Phase Change

We present a fully embedded implementation of a full-field crystal plasticity model in an implicit finite element (FE) framework, a combination which realizes a multiscale approach for the simulation of large strain plastic deformation. At each integration point of the macroscopic FE model a spectral solver, based on Fast Fourier Transforms (FFTs), feeds-in the homogenized response from an underlying full-field polycrystalline representative volume element (RVE) model which is solved by using a crystal plasticity constitutive formulation. Both, a phenome-nological hardening law and a dislocation density based hardening model, implemented in the open source software DAMASK, have been employed to provide the constitutive response at the mesoscale. The accuracy of the FE-FFT model has been benchmarked by one-element tests of several loading scenarios for an FCC polycrystal including simple tension, simple compression, and simple shear. The multiscale model is applied to simulate four application cases, i.e., plane strain deformation of an FCC plate, compression of an FCC cylinder, four-point bending of HCP bars, and beam bending of a dual-phase steel. The excellent capabilities of the model to predict the microstructure evolution at the mesoscale and the mechanical responses at both macroscale and mesoscale are demonstrated.

In dual-phase (DP) steels, inherited microstructures and elemental distributions affect the kinetics and morphology of phase transformation phenomena and the mechanical properties of the final material. In order to study the inheritance process, we selected two model materials with the same average DP steel composition but with different initial microstructures, created by coiling at different temperatures
after hot rolling. These samples were submitted to a DP-steel heat treatment consisting of a short isothermal annealing in the pure austenite region and a quenching process. The evolution of microstructure, chemical composition and mechanical properties (hardness) during this treatment was investigated.
The initial samples had a bainitic–martensitic (B + M) microstructure for the material coiled at lower temperature and a ferritic–pearlitic (P + F) microstructure for that coiled at higher temperature. The P + F microstructure had a much more inhomogeneous distribution of substitutional elements (in particular of Mn) and of carbon. After complete heat treatment, both materials showed a typical DP microstructure (martensite islands embedded in ferrite) but the P + F material showed lower hardness compared to the B + M material. It was found that the inhomogeneous elemental distribution prevailed in the P + F material.
The inheritance process was studied by combining measurements of the elemental distribution by Wavelength-Dispersive X-ray spectroscopy (WDX), simulations of the evolution of the elemental composition via the DICTRA (diffusion-controlled reactions) computer programme, dilatometry to observe the kinetics of phase transformation, and observation and quantification of the microstructures by
Electron Backscatter Diffraction (EBSD) measurements. For the P + F material it was found that the a–c transformation during annealing is slowed down in regions of lower Mn content and is therefore not completed. During the subsequent cooling the incompletely autenitized material does not require ferrite
nucleation and the c–a transformation starts at relative high temperatures. For B + M, in contrast, nucleation of ferrite is needed and the transformation starts at lower temperatures. As a result the B + M material develops a higher martensite content as well as a higher density of geometrically necessary
dislocations (GNDs). It is speculated that for the B + M material the c–a transformation occurs through a bainitic (i.e. partly displacive) process while the transformation at higher temperatures in the P + F material proceeds exclusively in a diffusive way.

This work aims to evaluate the effects of hydrogen in three high- strength steel grades. The phenomena of hydrogen (H) entry, transport and trapping inside the metals, together with the different types of damages due to the presence of hydrogen are presented. The study materials are a range of AHSS steel grades: Dual Phase Steel (DP 1000 and DP 1200) and Martensitic Steel (M 190). The hydrogen entry was performed by cathodic charging, which is suitable for industrial applications. In order to evaluate the influence of H on the steel mechanical properties, the following tests were done: H charging, to measure total H content (saturation point) and diffusible H content (embrittlement susceptibility); uniaxial tensile test of uncharged samples to determine notched tensile strength values and the strength levels at the end of elastic region and constant load tensile testing carried out in hydrogen environment, to determine the threshold values where hydrogen has an effect on the material. DP 1200 and M 190 were strongly affected by H pre-charging, as shown by the significant drop in stress required to break them. On the other hand, DP 1000 showed a lower embrittlement susceptibility, which is attributed to its lower mechanical strength. The current densities effects (0.2 up to 1.0 mA/cm²) were evaluated during H charging to measure diffusible H content. All steels showed a drop in the tensile strength i.e. experienced hydrogen embrittlement. Steels with higher tensile strength, as DP 1200 and M 190, showed a much bigger drop that is related to the favorable characteristics of martensitic microstructure regarding to the hydrogen permeability and diffusivity.

An ultrafine grained (UFG) ferrite/cementite steel was subjected to intercritical annealing in order to obtain an UFG ferrite/martensite dual-phase (DP) steel. The intercritical annealing parameters, namely, holding temperature and time, heating rate, and cooling rate were varied independently by applying dilatometer experiments. Microstructure characterization was performed using scanning electron microscopy (SEM) and high-resolution electron backscatter diffraction (EBSD). An EBSD data post-processing routine is proposed that allows precise distinction between the ferrite and the martensite phase. The sensitivity of the microstructure to the different annealing conditions is identified. As in conventional DP steels, the martensite fraction and the ferrite grain size increase with intercritical annealing time and temperature. Furthermore, the variations of the microstructure are explained in terms of the changes in phase transformation kinetics due to grain refinement and the manganese enrichment in cementite during warm deformation.

The risk of hydrogen embrittlement (HE) is currently one important factor impeding the use of medium Mn steels. However, knowledge about HE in these materials is sparse. Their multiphase microstructure with highly variable phase conditions (e.g. fraction, percolation and dislocation density) and the feature of deformation-driven phase transformation render systematic studies of HE mechanisms challenging. Here we investigate two austenite-ferrite medium Mn steel samples with very different phase characteristics. The first one has a ferritic matrix (~74 vol.% ferrite) with embedded austenite and a high dislo-cation density (~10 14 m −2) in ferrite. The second one has a well recrystallized microstructure consisting of an austenitic matrix (~59 vol.% austenite) and embedded ferrite. We observe that the two types of microstructures show very different response to HE, due to fundamental differences between the HE mi-cromechanisms acting in them. The influence of H in the first type of microstructure is explained by the H-enhanced local plastic flow in ferrite and the resulting increased strain incompatibility between fer-rite and the adjacent phase mixture of austenite and strain-induced α'-martensite. In the second type of microstructure, the dominant role of H lies in its decohesion effect on phase and grain boundaries, due to the initially trapped H at the interfaces and subsequent H migration driven by deformation-induced austenite-to-martensite transformation. The fundamental change in the prevalent HE mechanisms between these two microstructures is related to the spatial distribution of H within them. This observation provides significant insights for future microstructural design towards higher HE resistance of high-strength steels.

- by Dierk Raabe
- •
- Phase Transformations, Microstructure, Hydrogen, Grain Boundaries

Hydrogen embrittlement affects high-strength ferrite/martensite dual-phase (DP) steels. The associated micromechanisms which lead to failure have not been fully clarified yet. Here we present a quantitative micromechanical analysis of the microstructural damage phenomena in a model DP steel in the presence of hydrogen. A high-resolution scanning electron microscopy-based damage quantification technique has been employed to identify strain regimes where damage nucleation and damage growth take place, both with and
without hydrogen precharging. The mechanisms corresponding to these regimes have been investigated by employing post-mortem
electron channeling contrast imaging and electron backscatter diffraction analyses, as well as additional in situ deformation experiments. The results reveal that damage nucleation mechanism (i.e. martensite decohesion) and the damage growth mechanisms (e.g. interface decohesion) are both promoted by hydrogen, while the crack-arresting capability of the ferrite is significantly reduced. The observations are discussed on the basis of the hydrogen-enhanced decohesion and hydrogen-enhanced localized plasticity mechanisms. We discuss corresponding microstructure design strategies for better hydrogen-related damage tolerance of DP steels.

- by Dierk Raabe
- •
- Hydrogen, Damage detection, Fracture Mechanics, Ferrite

The performance of cold rolled dual-phase (DP) steels depends on their microstructure, which results from the thermomechanical processing conditions, involving hot rolling, cold rolling, and continuous annealing. The knowledge on the influence of each annealing stage on the microstructure formation is essential for manufacturing high-quality DP steels. In the present work, the effects of some intercritical annealing parameters (heating rate, soaking temperature, soaking time, and quench temperature) on the microstructure and mechanical properties of a cold rolled DP steel (0.08% C-1.91% Mn) were studied. The microstructure of specimens quenched after each annealing stage, simulated on a Gleeble, was analyzed using optical, scanning, and transmission electron microscopy. The tensile properties, determined for specimens submitted to complete annealing cycles, are influenced by the volume fractions of martensite, bainite, martensite/austenite (MA) constituent, and carbides, which depend on annealing processing parameters. The results obtained showed that the yield strength (YS) increase and the ultimate tensile strength (UTS) decrease with the increasing intercritical temperature. This can be explained by the increased formation of granular bainite associated with the increased volume fraction of austenite formed at the higher temperatures. The experimental data also showed that, for the annealing cycles carried out, UTS values in excess of 600 MPa could be obtained with the steel investigated.

Dual-Phase steel is an advanced high strength steel that
are used recently in auto-body for energy absorption.
Fracture is an noticeable factor in crashworthiness of
vehicle impact loading. Failure prediction of DP800 was
investigated in this studying. There are different fracture
criteria for damage modeling, but the material behavior
cannot be simulated by using them, so a new approach
have been used in this studying. The Gurson model in
conjunction with the Johnson-Cook damage model is
applied to simulate correctly the material behavior. By
using a new method in this research, in shear fracture
and ductile fracture the active mechanisms are JohnsonCook damage model and Gurson model respectively.
Finally, the crashworthiness of a B-Pillar in different
conditions is simulated and the fracture is predicted.

- by solat noruzi
- •
- Damage modelling, Dual phase steels, Gurson

The mechanical response of multiphase alloys is governed by the microscopic strain and stress partitioning behavior among microstructural constituents. However, due to limitations in the characterization of the partitioning that takes place at the submicron scale, microstructure optimization of such alloys is typically based on evaluating the averaged response, referring to, for example, macroscopic stress-strain curves. Here, a novel experimental-numerical methodology is introduced to strengthen the integrated understanding of the microstructure and mechanical properties of these alloys, enabling joint analyses of deformation-induced evolution of the microstructure, and the strain and stress distribution therein, down to submicron resolution. From the experiments, deformation-induced evolution of (i) the microstructure, and (ii) the local strain distribution are concurrently captured, employing in situ secondary electron imaging and electron backscatter diffraction (EBSD) (for the former), and microscopic-digital image correlation (for the latter). From the simulations, local strain as well as stress distributions are revealed, through 2-D full-field crystal plasticity (CP) simulations conducted with an advanced spectral solver suitable for heterogeneous materials. The simulated model is designed directly from the initial EBSD measurements, and the phase properties are obtained by additional inverse CP simulations of nanoindentation experiments carried out on the original microstructure. The experiments and simulations demonstrate good correlation in the proof-of-principle study conducted here on a martensite-ferrite dual-phase steel, and deviations are discussed in terms of limitations of the techniques involved. Overall, the presented integrated computational materials engineering approach provides a vast amount of well-correlated structural and mechanical data that enhance our understanding as well as the design capabilities of multiphase alloys.

The industrial interest on light weight components has contributed in the last years to a significant research effort on new materials able to guarantee high mechanical properties, good formability and reasonable costs together with reduced weights when compared to traditional mild steels. Among such materials advanced high strength steels (AHSS) such as transformations induced plasticity (TRIP) and dual phase (DP), and light weight alloys proved their usefulness in stamping of automotive components. As AHSS are concerned, one of the main drawbacks is related to springback occurrence. Many aspects have to be taken into account when springback reduction is investigated: material behavior issues, process conditions, numerical simulations parameters calibration, geometrical aspects and so on. Moreover, springback minimization problems are typically multi-objective ones because springback reduction may conflict with other goals in stamping design such as thinning reduction. In this paper, such problem was investigated through integration between numerical simulations, Response Surface Methodology and Pareto optimal solutions search techniques. The design of a U-channel stamping operation was investigated utilizing two different dual phase steel grades: DP 1000 and DP 600. An explicit/forming-implicit/springback approach was utilized for the numerical simulations. Friction conditions and blank holder force were optimized as design variables in order to accomplish two different objectives: reduce excessive thinning and avoid excessive geometrical distortions due to springback occurrence.

The purpose of the present work is the implementation and validation of a model able to predict the microstructure changes and the mechanical properties in the modern high-strength dualphase steels after the continuous annealing process line (CAPL) and galvanizing (Galv) process. Experimental continuous cooling transformation (CCT) diagrams for 13 differently alloying dual-phase steels were measured by dilatometry from the intercritical range and were used to tune the parameters of the microstructural prediction module of the model. Mechanical properties and microstructural features were measured for more than 400 dual-phase steels simulating the CAPL and Galv industrial process, and the results were used to construct the mechanical model that predicts mechanical properties from microstructural features, chemistry, and process parameters. The model was validated and proved its efficiency in reproducing the transformation kinetic and mechanical properties of dual-phase steels produced by typical industrial process. Although it is limited to the dual-phase grades and chemical compositions explored, this model will constitute a useful tool for the steel industry.

The present paper deals with the investigation of the propagation of harmonic plane waves with assigned frequency by employing the thermoelasticity theory with dual-phase-lags (Tzou [7], Chandrasekharaiah [10]). The exact dispersion relation solutions for the plane wave are obtained analytically and asymptotic expressions of several characterizations of the wave fields, such as phase velocity, specific loss, penetration depth and amplitude ratios are obtained for both the high frequency as well as low frequency values. In order to illustrate the analytical results, the computational tool Mathematica is used to find the numerical values of different wave fields at intermediate values of frequency and results are depicted in different figures. A detailed analysis of the effects of phase-lags on plane wave is presented on the basis of analytical and numerical results and significant points are highlighted.

- by Dr Rajesh Prasad
- •
- Civil Engineering, Applied Mathematics, Modeling, Low Frequency

The effects of pre-strain on the crushing properties of a crash-box structure are reported for two Advanced High Strength Steels produced by ArcelorMittal (Dual-Phase DP600 and Transformation Induced by Plasticity TRIP780 steels). In addition, the effect of a bake-hardening treatment is studied on the TRIP780 steel. The material's behaviors have been determined in tensile loading in a wide range of strain-rates (8 × 10 −3 s −1 ≤ε ≤ 10 3 s −1 ) including strain-rate jump tests from quasi-static to dynamic loading. These interrupted tests allow us to characterize the effect of the (quasi-static) forming process on the subsequent dynamic behavior associated to the crash event. A phenomenological visco-plastic model, based on the evolution of an internal variable, has been implemented in PAMCRASH using a user-defined subroutine in order to simulate the crushing of the crash-boxes. A simple approach is proposed to account for the bake-hardening treatment in numerical simulations. A good correlation between experimental and predicted mean crush forces is obtained.

- by L. Durrenberger and +1
- •
- Materials Engineering, Mechanical Engineering, Strain Rate, Numerical Simulation

Microstructures of multi-phase alloys undergo morphological and crystallographic changes upon deformation, corresponding to the associated microstructural strain fields. The multiple length and time scales involved therein create immense complexity, especially when microstructural damage mechanisms are also activated. An understanding of the relationship between microstructure and damage initiation can often not be achieved by post-mortem microstructural characterization alone. Here, we present a novel multi-probe analysis approach. It couples various scanning electron microscopy methods to microscopic-digital image correlation (l-DIC), to overcome various challenges associated with concurrent mapping of the deforming microstructure along with the associated microstrain fields. For this purpose a contrast- and resolution-optimized l-DIC patterning method and a selective pattern/microstructure imaging strategy were developed. They jointly enable imaging of (i) microstructure-independent pattern maps and (ii) pattern-independent microstructure maps. We apply this approach here to the study of damage nucleation in ferrite/martensite dual-phase (DP) steel. The analyses provide four specific design guidelines for developing damage-resistant DP steels.

The microstructure and texture evolution of dual-phase steel sheets with a cold reduction of about 50%, annealed at ferritic and intercritical temperatures, were analyzed by scanning electron microscopy and electron backscatter diffraction. The competition between recrystallization and phase transformation was of particular interest. The sheets were annealed in salt bath or were annealed in a MULTIPAS annealing simulator under variation of annealing temperature, annealing time, and heating rate.

- by Dierk Raabe
- •
- DP Steel, Dual phase steels

Numerical simulations of straight tube hydroforming of a dual phase (DP600) advanced high strength steel were performed using a variant of the Gurson–Tvergaard–Needleman (GTN) constitutive model to account for the influence of void shape and shear on coalescence. The effect of axial-feed (end-feed) on damage development and formability is investigated for end-feed loads of zero and 133 kN. A parametric study was conducted to determine an appropriate void nucleation stress and strain and the numerical values compared with the experimental data. The calibrated GTN damage model gives good agreement with the experimentally determined burst pressure, formability and failure location with the best performance occurring for the high end-feed load.

- by Cliff Butcher
- •
- Materials Engineering, Mechanical Engineering, Civil Engineering, Modeling

In situ tensile tests have been carried out on a high-strength, dual-phase steel. These experiments show that it is possible to follow and quantify the evolution of damage nondestructively and in three dimensions in this type of material. The measurements were analyzed in terms of density, size, aspect ratio of the cavities but also of the local deformation and the stress triaxiality in the sample. It was found that damage initiation is progressive with the applied tensile strain and that the initiation of new small cavities reduces the average diameter while the growth of the previously created ones increases the average diameter. This information was used to develop a new model for void growth based on the classical Rice and Tracey approach. This simple approach was modified to account for progressive damage initiation. The results of the proposed model are in good agreement with the measurements.

- by E. Maire
- •
- Materials Engineering, Mechanical Engineering, X-ray Tomography, X Rays

The main goal of this study is to evaluate the influence of work-hardening modeling in springback prediction in the first phase of the Numisheet’05 “Benchmark 3”: the U-shape “Channel Draw”. Several work-hardening constitutive models are used in order to allow the different materials’ mechanical behavior to be better described: the Swift law (a power law) or a Voce type saturation law to describe the classical isotropic work-hardening; a Lemaître and Chaboche type law to model the non-linear kinematic hardening, which can be combined with the previous two; and Teodosiu’s microstructural work-hardening model. This analysis was carried out using two steels currently used in the automotive industry: mild (DC06) and dual phase (DP600). Haddadi et al. [Haddadi, H., Bouvier, S., Banu, M., Maier, C., Teodosiu, C., 2006. Towards an accurate description of the anisotropic behaviour of sheet metals under large plastic deformations: Modelling, numerical analysis and identification. Int. J. Plasticity 22 (12), 2226-2271] performed the mechanical characterization of these steels, as well as the identification of the constitutive parameters of each work-hardening model, based on an appropriate set of experimental data such as uniaxial tensile tests, monotonic and Bauschinger simple shear tests and orthogonal strain-path change tests, all at various orientations with respect to the rolling direction of the sheet. All the simulations were carried out with the in-house FE code DD3IMP. The selected sheet metal formed component induces high levels of equivalent plastic strain. However, for the several work-hardening models tested, the differences in springback prediction are not significantly higher than those previously reported for components with lower equivalent plastic strain levels. It is shown that these differences can be related to the predicted through-thickness stress gradients. The comparative significance of both equivalent plastic strain levels and strain-path changes in the through-thickness stress gradients is discussed.

- by Luis Menezes
- •
- Materials Engineering, Mechanical Engineering, Civil Engineering, Plasticity

This study describes the correlation between microstructure, mechanical and tribological properties of TiC x coatings (with x being in the range of 0-1.4), deposited by reactive magnetron sputtering from a Ti target in Ar/C 2 H 2 mixtures at~200°C. The mechanical and tribological properties were found to strongly depend on the chemical composition and the microstructure present. Very dense structures and high hardness, combined with low wear rates and friction coefficients, were observed for coatings with chemical composition close to TiC. X-ray diffraction and X-ray photoelectron spectroscopy analysis, used to evaluate coating microstructure, composition and relative phase fraction, showed that low carbon contents in the coatings lead to sub-stoichiometric nanocrystalline TiC x coatings being deposited, whilst higher carbon contents gave rise to dual phase nanocomposite coatings consisting of stoichiometric TiC nanocrystallites and free amorphous carbon. Optimum performance was observed for nanocomposite TiC 1.1 coatings, comprised of nanocrystalline nc-TiC (with an average grain size of~15 nm) separated by 2-3 monolayers of an amorphous a-DLC matrix phase.

In this article, we investigated the effect of martensite morphology on the mechanical properties and formability of dual phase steels. At first, three heat treatment cycles were subjected to a low-carbon steel to produce ferrite–martensite microstructure with martensite morphology of blocky-shaped, continuous, and fibrous. Tensile tests were then carried out so as to study mechanical properties, particularly the strength and strain hardening behavior of dual phase steels. In order to study the formability of dual phase samples, Forming Limit Diagram was obtained experimentally and numerically. Experimental forming limit diagram was obtained using Nakazima forming test, while Finite Element Method was utilized to numerically predict the forming limit diagram. The results indicated that the dual phase samples with fibrous martensite morphology had the highest tensile properties and strain rate hardening out of the three different microstructures. Blocky-shaped martensite morphology, on the other hand, had the worst mechanical properties. The study of the strain hardening behavior of dual phase sample by Kocks–Mecking-type plots, evinced two stages of strain hardening for all specimens with different microstructures: stages III and IV. The forming limit diagram of dual phase steels also proved that samples with fibrous martensite morphology had the best formability compared to other two microstructures. The simulated forming limit diagram manifested that there is a good agreement between experimental results and those obtained by FEM.

A detailed qualitative and quantitative examination of the microstructure and mechanical properties of three different classes of DP600 and DP450 dual-phase (DP) steels was carried out. The tested DP steels are characterized by different alloying elements: aluminum, boron, and phosphorus. Among them, aluminum DP steels showed the lowest percentages of hard phases, while phosphorus DP steels exhibited the highest resistance values. The Hollomon, Pickering, Crussard-Jaoul (CJ), and Bergstrom models were used to reproduce the strain hardening behavior of DP steels. Relationships that correlate the fitting parameters with the chemical composition and the thermal cycle parameters were found, and the predictive abilities of different models were evaluated. The Pickering equation, among the tested models, is the best one in the reproduction of the experimental stress-strain data.

The effects of some intercritical annealing parameters including heating rate, soaking temperature, soaking time, and quench media on the microstructure and mechanical properties of cold-rolled dual phase steel were studied. The microstructure of specimens quenched after each annealing stage was analyzed using optical microscope. The tensile properties, determined for specimens submitted to complete annealing cycles, were influenced by the volume fractions of multiphases (originated from martensite, bainite, and retained austenite), which depend on the annealing process parameters. The results obtained showed that the yield strength and the ultimate tensile strength increase with increasing the intercritical temperature and cooling rate. This can be explained by higher martensite volume ratio with the increased volume fraction of austenite formed at the higher temperatures and cooling rates.

- by Havva Zeytin
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- Materials Engineering, Heat Treatment, Cooling Rate, Mechanical Testing

Introduction Massive haemobilia carries a mortality of 25% in most reports. Although previously it was mainly due to road accidents or homicidal attempts it is now more often due to iatrogenic trauma like percutaneous liver biopsy and biliary drainage. However the management protocol is not established and there have been few reports of this serious condition from India. Aim To review the causes of massive haemobilia and outline its management in an Indian hospital. Patients and methods We retrospectively analysed the records of 20 consecutive patients with massive haemobilia (blood requirement more than 1400 ml/day) admitted to our department over six years from a prospectively maintained database. There were 10 males and 10 females who had a mean age of 43 (range 15–65) years. Results Haemobilia accounted for 9 percent of patients admitted with upper gastrointestinal bleeding who were seen over this period. The commonest cause was iatrogenic (11) including laparoscopic cholecystectomy (6), Whipple’s operation, endoscopic retrograde cholangiography (ERC), percutaneous transhepatic cholangiography (PTC), hepatic stone extraction and removal of biliary stent (1 each). The others had accidental trauma (4), visceral aneurysms (2), biliary stones (2) and chronic pancreatitis (1). The commonest clinical presentation was massive gastrointestinal bleeding. The dual phase computed tomography (CT) scan correctly identified the site of bleeding and other associated conditions in all the 11 patients in whom it was done. Conventional angiography was done in 8 patients with transarterial embolisation (TAE) being attempted in 6 and successful in 2 patients. Operations were performed in 18 patients for the following indications — failure of angiographic embolisation (6), failure of endoscopic sclerotherapy (EST) (1), duodenal erosion (2), portal biliopathy (1), haemoperitoneum (1), bile leak (1), pseudocyst (1), liver necrosis (1) and other hepatobiliary conditions (4). The surgical procedures to control bleeding were ligation of aneurysms (8), repair of the hepatic artery (4), right hepatectomy (3), lienorenal shunt, cholecystectomy and under-running of the duodenal papilla (1 each). The overall mortality was 4 patients (20 percent). There was no mortality in patients with bleeding aneurysms; the mortality being significantly higher in patients with non-aneurysmal bleeding (p=0.0049: Fishers’ exact test). Conclusions In our experience haemobilia was usually due to an iatrogenic cause with a pseudoaneurysm following a diagnostic or therapeutic intervention(most often laparoscopic cholecystectomy) being the commonest aetiology. A dual phase CT scan accurately identified the site of bleeding. Angiographic embolisation often failed to stop bleeding and mortality was significantly higher in patients with non-aneurysmal bleeding. We should perhaps consider early surgery for haemobilia once the bleeding site has been localised by CT scan.

- by Mudit Singh
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- Computed Tomography, IT Management, Ct Scan, Chronic Pancreatitis

Room temperature uniaxial tensile and biaxial Viscous Pressure Bulge (VPB) tests were conducted for five Advanced High Strength Steels (AHSS) sheet materials, and the resulting flow stress curves were compared. Strain ratios (R-values) were also determined in the tensile test and used to correct the biaxial flow stress curves for anisotropy. The pressure vs. dome height raw data in the VPB test was extrapolated to the burst pressure to obtain the flow stress curve until fracture. Results of this work show that the flow stress data can be obtained to higher strain values under biaxial state of stress. Moreover, it was observed that some materials behave differently if subjected to different state of stress. These two conclusions, and the fact that the state of stress in actual stamping processes is almost always biaxial, suggest that the bulge test is a more suitable test for obtaining the flow stress of AHSS sheet materials for use as an input to Finite Element (FE) simulation models.

- by Ahmed Nasser and +1
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- Materials Engineering, Mechanical Engineering, Finite Element, Advanced high strength steels

A combination of micropillar compression tests and microstructure-based numerical simulations were used to determine the flow strength and strain rate partitioning in uniaxial tension in two commercial low-carbon dual-phase sheet steels, DP980 (0.09% C–2.15% Mn–0.60% Si (wt.%)) and DF140T (0.15% C–1.45% Mn–0.30% Si (wt.%)). The two steels have different microstructures, with the martensite volume fraction in DP980 being ∼60%, compared to ∼40% in DF140T. Nevertheless, they exhibit similar uniaxial stress–strain behavior. To determine the microstructural origin of this behavior, micropillar compression specimens from ferrite and martensitic phases in both steels were deformed in uniaxial compression to obtain their individual response. A microstructure-based crystal plasticity model that accounts for non-Schmid behavior in the ferrite phase and contains a detailed description of the hierarchical microstructure of martensite was developed and material parameters were determined by fitting model predictions to the micropillar compression data. The crystal plasticity model was then used to predict the flow stress and strain rate partitioning during uniaxial tensile deformation of the two steels. The ferrite phase in the two steels was found to have similar flow strength. In contrast, the flow stress of martensite in DF140T was found to be approximately twice that in DP980. This strength difference is offset by the difference in martensite volume fraction in the two steels, resulting in nearly identical uniaxial tensile behavior. The strain rate partitioning and interfacial stress distributions in the two steels differ significantly, however, and have important implications on their tensile ductility.

- by Peng Chen
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- Dual phase steels, Mechanical Behavior of Dual-Phase Steels

Measurements of oxygen permeation through dense Sr 1Àx Fe,Al ð ÞO 3Àδ À SrAl 2 O 4 composite membranes showed a considerable influence of processing conditions on the surface exchange kinetics, while the bulk ambipolar conductivity is almost unaffected by microstructural factors. Compared to the materials prepared via the glycine-nitrate process (GNP), the surface limitations to oxygen transport are significantly higher for dual-phase SrFe ð Þ 0:7 SrAl 2 ð Þ 0:3 O 3:3Àδ made of a commercial powder synthesized by spray pyrolysis. This difference in behavior may be related to compositional inhomogeneities in the grains of A-site deficient perovskite phase and an enhanced surface concentration of grain boundaries in the case of GNP-synthesized composite, which has also smaller grain size, slightly higher thermal expansion and lower total conductivity. No essential effects on Vickers hardness, varying in the range 6.3-6.5 GPa, were found. The deposition of porous catalyst layers onto the composite surface exposed to reducing environment leads to mem-brane decomposition. For the fabrication of tubular membranes, the cold isostatic pressing technique was, hence, combined with mechanical treatment to increase the specific surface area without incorporation of catalytically active components.

- by Frans Snijkers and +1
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- Kinetics, Solid State Electrochemistry, Grain size, Specific surface area
- by Alireza Asgari
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- Automotive Industry, Finite Element, Advanced high strength steels, Design process

Computational crystal plasticity (ABAQUS FE) simulations are presented for dual phase alloy Ti-6Al-4V subjected to cyclic loading in the high cycle fatigue (HCF) regime. Relations between remote loading conditions and local plasticity are discussed as a function of stress amplitude and microstructure. Based on computational micromechanics, effects of microstructure heterogeneity and R-ratio are examined in terms of their influence on cyclic microplastic strain within the microstructure of unnotched specimens. It is shown that bulk-dominated fatigue damage at high R-ratios (>0.7) is associated with the onset of percolation of ratcheting of shear strain in the HCP a phase through connected channels within the microstructure. A high cumulative plastic strain gradient across the a-b phase boundaries is the likely driving force for decohesion at phase boundaries as the manifestation of bulk damage in the HCF regime. Effects of texture are also examined using random periodic microstructure representations. Application of the same crystal plasticity model for Ti-6Al-4V in fretting fatigue contact at positive R-ratios for the bulk fatigue stress also reveals a dominance of ratcheting strain in shear bands emanating from the contact surface, ostensibly in the HCF regime.

- by Chung Hyun Goh
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- Materials Engineering, Mechanical Engineering, Civil Engineering, Microstructure

Cold-rolled steels based on 0.15C–1.5Mn–1.5Si–0.5Cu containing Cr or/and Ni were prepared, and intercritical annealing and isothermal treatment, were carried out. The addition of Cu or Cu+Ni resulted in a large increase of the retained austenite volume fraction as well as an improvement of elongation and the strength-ductility balance. However, the addition of Cr or Cu+Ni showed a dual-phase deformation behavior having higher tensile strength and lower elongation.

- by Sudhir Jangra
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- Materials Engineering, Mechanical Engineering, Dual phase steels, Tensile Strength

Dual-phase continuity and phase inversion of polystyrene (PS)/poly(methyl methacrylate) (PMMA) blends processed in a twin-screw extruder was investigated using a selective extraction technique and scanning electron microscopy. Emphasis was placed on investigating the effects of viscosity ratio, blend composition, processing variables (mixing time and annealing) and diblock copolymer addition on the formation of bi-continuous phase structure (BPS) in PS/PMMA blends. The experimental results were compared with the volume fraction of phase inversion calculated with various semi-empirical models. The results showed that the formation of a BPS strongly depends on the blend composition and the viscosity ratio of the constituent components. Furthermore, BPS was found in a wide volume fraction interval. Increasing the mixing time and the addition of diblock copolymer, both led to a narrowing range of volume fraction in which BPS exists. Quiescent annealing coarsened the structure but indicated no qualitative changes. Some model predictions for phase inversion could predict qualitative aspects of the observed windows of co-continuity but none of the models could account quantitatively for the observed data. q

- by Kristoffer Almdal
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- Engineering, Scanning Electron Microscopy, Polymer, CHEMICAL SCIENCES

The fields of applicability of headspace sorptive extraction (HSSE) and stir bar sorptive extraction (SBSE) using polydimethylsiloxane (PDMS) as sorbent have been intensively discussed and widely described. One of the limits of sorptive extraction is that PDMS (i.e. an apolar phase) is the only polymer currently in use making it difficult to recover polar analytes from complex or multi-ingredient matrices and those with very volatile components (C1-C4 analytes). Dual-phase twisters are here introduced as new tools for HSSE and SBSE to overcome the above limits. Dual-phase twisters combine the concentration capabilities of two or more sampling materials operating in different ways (in this case sorption and adsorption). The new twisters consist of a short PDMS tube the ends of which are closed with two magnetic stoppers, thus creating an inner cavity that can be packed with different types of adsorbents like activated carbons. The concentration capability of dual-phase twisters was evaluated by using them for the HSSE and SBSE sampling of a number of matrices in the vegetable, food and environmental fields. The contributions made by different carbons to recovery, repeatability and intermediate precision were also investigated.

- by Pat Sandra and +2
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- Engineering, Technology, Activated Carbon, Gas Chromatography
- by Tatiana Gurova
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- Materials Engineering, Mechanical Engineering, Materials Science, Heat Treatment

We have developed a novel method to construct porous hydroxyapatite (HA) by dual-phase mixing-in other words, generating a porous ceramic body and pore-forming template simultaneously. The technique is based on mixing two immiscible phases: HA slurry and polymethylmethacrylate (PMMA) resin. Naphthalene particles are necessary when greater porosity (>50%) is wanted. After shaping in a mold, the mixture is subjected to polymerization, drying, pyrolysis, and sintering. The porous HA has been thoroughly characterized with Fourier transformation infrared spectrometry, X-ray diffractometry, environmental scanning electron microscopy coupled with energy-dispersive X-ray analysis, and image analysis. The demanding specifications for bone ingrowth are met: (i) the size of pores and their fenestrations are adjustable, ϳ80% within 300 -800 m; (ii) uniform and isotropic porous structure is observed in three directions; (iii) pores are fully interconnected throughout; (iv) the porosity is adjustable up to 60%; and (v) sufficient mechanical strength is present for cell culture and implantation handling. The porous HA can be applied as either implant material or scaffold for bone-tissue engineering.

- by Pierre Layrolle
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- Materials Engineering, Mechanical Engineering, Microstructure, Manufacturing

A Nb-microalloyed structural steel with ferrite-pearlite microstructure was subjected to cold rolling and intercritical annealing to produce ultra-fine grained dual phase microstructure. Optical and transmission electron microscopy techniques were employed to characterise the microstructure. Initial results showed that the intercritical annealing (at 790°C for 90s) of samples rolled to a true strain of 2.4 resulted in a significant grain refinement from the average initial grain size of 20 μm to 1–2 microns. The microstructure primarily consisted of UFG ferrite matrix with homogeneously distributed islands of plate martensite with volume fraction of 27%.

- by papa rao Mondi
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- Transmission Electron Microscopy, Grain size, Microstructures, Dual phase steels
- by Elena Prieto and +1
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- Kinetics, Positron Emission Tomography, Brain Tumor, Glioblastoma

Fuel efficiency of vehicles and increase the safety of passengers lead the automotive industry to incorporate of new materials and technologies into today's vehicles. Advanced high strength steels offer excellent strength and formability properties. One the main important grade of advanced high strength steels is Dual Phase (DP) steel. Dynamic behavior of the material must be considered in crashworthiness studying of the vehicles. In this paper the dynamic tensile characteristics of Dual Phase (DP) steel sheets at low and moderate strain rates ranging from 0.0009 s-1 to 0.1 s-1 are investigated. All the tests were performed by the tensile test machine with the cross-head speed of 4 mm/min and 500 mm/min. Digital Image Correlation method was employed to study the strain rate sensitivity, stress-strain curves and the strain fracture of DP800. In the numerical analysis, an isotropic hardening model with rate dependence was used to predict the experimental behavior. The fracture behavior of the material was simulated by using of ductile criterion model. The local strains in fracture zone obtained by the experiments and finite element analysis are compared and good agreement is obtained.

Large strain warm deformation at different temperatures and subsequent intercritical annealing has been applied to obtain fine grained (2.4 m) and ultrafine grained (1.2 m) ferrite/martensite dual-phase (DP) steels. Their mechanical properties were tested under tensile and impact conditions and compared to a hot deformed coarse grained (12.4 m) reference material. Both yield strength and tensile strength follow a Hall-Petch type linear relationship, whereas uniform elongation and total elongation are hardly affected by grain refinement. The initial strain hardening rate as well as the post-uniform elongation increase with decreasing grain size. Ductile fracture mechanisms are considerably promoted due to grain refinement. Grain refinement further lowers the ductile-to-brittle transition temperature and leads to higher absorbed impact energies. Besides the common correlations with the ferrite grain size, these phenomena are explained in terms of the martensite particle size, shape and distribution and the more homogeneous dislocation distribution in ultrafine ferrite grains.

- by Dierk Raabe and +1
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- Microstructure, Electron Backscatter Diffraction (EBSD), Advanced high strength steels, Mechanical Properties of Materials