Quantification of the effect of transformation-induced geometrically necessary dislocations on the flow-curve modelling of dual-phase steels (original) (raw)
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Free dislocations and boundary dislocations in tempered martensite ferritic steels
Materials Science and Engineering: A, 2004
The present paper shows how total dislocation densities evolve during heat treatment, long term aging and creep of a 12 wt.% Cr tempered martensite ferritic steel. Special emphasis is placed on the evolution of the density of free dislocations (dislocation density: ρ f) and dislocations in micro-grain boundaries. Only micro-grain boundaries with misorientation angles between adjacent grains between 1 and 5 • are considered. These are grouped into (i) sub grain boundaries and (ii) low angle grain boundaries with misorientation angles of 1 and 2-5 • , respectively. It is shown that the overall density of free dislocations which form in order to facilitate the formation of martensite decreases during tempering, long term aging and creep. The density of free dislocations and of dislocations in low angle boundaries decreases. The density of dislocations in sub grain boundaries stays constant due to a balance between an increase of the relative frequency of sub grain boundaries and an increase of micro-grain size (which corresponds to a decrease of micro-grain boundary area per volume).
Acta Materialia 59 (2011) 4387
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 that local hardening is caused by geometrically necessary dislocations. The idea is investigated experimentally and numerically in the present analysis, which for the first time leads to good agreement with experimental observations of the mechanical stress–strain behavior.
Sādhanā, 2020
Deformation band localization modes, uniform tensile strength, and uniform elongation of Ferrite-Martensite Dual-Phase (DP) steels are analyzed by finite element (FE) study. Treating the microstructure inhomogeneity as the sole cause of imperfection, failure initiation is predicted as the natural fallout of plastic instability caused by load drop because of localized plastic strain in the Representative Volume Element (RVE) during straining. Strain partitioning between two phases (ferrite matrix and martensite island) are investigated on RVEs, and it reveals that the increase of martensite yield stress decreases the plastic deformation and increases the stress state in martensite. Whereas, a decrease in martensite island volume fraction (V m) results in the reduction of plastic deformation and stress state in the island. Studies are then carried out to investigate the effects of the ferrite-martensite flow properties and martensite volume fraction on the macroscopic tensile deformation behavior and band localization of DP steels. Micromechanical based FE simulation results emphasize that an increase in initial yield strength and volume fraction of martensite increases the ultimate tensile stress (UTS) with the decrease in uniform elongation. Similarly, as the hardening rate of ferrite increases, it increases the ultimate tensile stress (UTS) and uniform elongation. Additionally, deformation band localization modes alter from inclined to perpendicular to the loading axis with an increase in martensite volume fraction and initial yield strength of martensite. The knowledge of this work can be used to design DP steels with desired mechanical properties.
Deformation response of ferrite and martensite in a dual-phase steel
Deformation response of ferrite and martensite in a commercially produced dual-phase sheet steel with a nominal composition of 0.15% C-1.45% Mn-0.30% Si (wt.%) was characterized by nanoindentation and uniaxial compression of focused ion beam-milled cylindrical micropillars (1-2 lm diameter). These experiments were conducted on as-received and pre-strained specimens. The average nanoindentation hardness of ferrite was found to increase from 2GPaintheas−receivedconditionto2 GPa in the as-received condition to 2GPaintheas−receivedconditionto3.5 GPa in the specimen that had been pre-strained to 7% plastic tensile strain. Hardness of ferrite in the as-received condition was inhomogeneous: ferrite adjacent to ferrite/martensite interface was $20% harder than that in the interior, a feature also captured by micropillar compression experiments. Hardness variation in ferrite was reversed in samples pre-strained to 7% strain. Martensite in the as-received condition and after 5% prestrain exhibited large scatter in nanoindentation hardness; however, micropillar compression results on the as-received and previously deformed steel specimens demonstrated that the martensite phase in this steel was amenable to plastic deformation and rapid work hardening in the early stages of deformation. The observed microscopic deformation characteristics of the constituent phases are used to explain the macroscopic tensile deformation response of the dual-phase steel.
Journal Mechanics Based Design of Structures and Machines, 2019
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.
Strain Hardening Dependence on the Structure in Dual‐Phase Steels
Steel Research International, 2020
Advanced high strength steels (AHSS) is a family of steels finding increased applications in automotive industry because they help to meet the tightened regulations on lower emissions while improving crash worthiness in a cost-effective manner. This steel group mainly includes dual-phase (DP) steels, transformation induced plasticity (TRIP), quenching-partitioning (Q&P), complex-phase (CP), and twinning induced plasticity (TWIP) steels. DP steels, whose microstructure consists of a ferrite matrix and a second hard phase of martensite or bainite, are widely utilized in automotive industry, especially in passenger cars due to their superior combination of strength and ductility, better weldability, and relatively simple processing route. [1,2] Their application is constantly growing in automotive industry; e.g., it increased from 12% in the 2003 GM models to reach 36% in recent models, making this the most commonly used steels in the current products. No DP steels were used in GM models of the 1990s. [2] Indeed, the start of developing and characterization of this steel grade dates back to the end of 1970s and beginning of 1980s by privileged efforts of researchers like Davies et al., [3,4] Gladman et al., [5,6] and Sarosiek et al. [7,8] with studies presenting the basis for developing and comprehending the characteristics of the recent grades of this retrieved industrially valuable steel. DP steels can be processed to different mechanical characteristics by varying the volume fractions of the constituent phases, their distribution, morphologies, and grain sizes. [9-15] Karimi and Kheirandish concluded that the DP steels having ferrite and bainite phase microstructure possess more ductility and higher work hardening exponent, whereas those with ferrite-martensite microstructure show higher ultimate tensile and yield strength. [12] The dependence of the DP steel strength on the martensite volume fraction (V m) is a matter of debate in the literature. Many authors showed a linear increase in yield and tensile strength by increasing V m [2,3,13,16] others posited about 0.5 as the optimal V m at which the strength is at its peak. [15,17] On the other hand, Sun et al. [18] showed that the yield-to-tensile strength ratio is reduced when the content of ferrite increases. Bag et al. illustrated that the DP steel having dispersed martensite distribution shows better impact toughness and ductility than the one with a banded microstructure. [15] Sun and Pugh showed that the mechanical properties of DP steels not only depend on the martensite fraction and its distribution but also on its morphology; they illustrated that DP steels with elongated martensite "fiber" show increased strengths but lowers their ductility. [19] The martensite cracks more easily in fibrous martensite than in a blocky one. [20] Moreover, Kim and Nakagawa concluded that the fine fibrous DP structure produced by intermediate quenching has a much higher ductile-to-brittle transition temperature (DBTT) than the fine globular structure produced by intercritical annealing, regardless of the volume fraction of martensite. [21] The martensite morphology affects also the strain hardening capacity; the DP steels with fibrous martensite exhibit less strain hardening than that ones with blocky martensite. [20] Being a polycrystalline material, DP steels show a considerable enhancement of strength with refining its grain size.
Micromechanisms of deformation in dual phase steels at high strain rates
A B S T R A C T The effect of strain rate (0.001–800/s) on two commercial ferrite-martensite dual phase steels (DP600 and DP800), having different martensite contents (10.2% and 33.2%, respectively) were investigated. Microstructures of the deformed samples have been studied to understand the influence of strain rate on the micro-mechanisms of deformation in dual phase steels. The observations reveal that the volume fraction and size of the martensite play an important role in the deformation at different strain rates. Dislocation cells formed as the steels were deformed at different strain rates but the size of these cells and the extent of cell formation varied significantly with strain rate and martensite fraction. While deforming at various strain rates, martensite in DP600 was found to remain undeformed where the deformation was mainly prevalent within the ferrite grains. Although at high strain rates, extreme dislocation generation within the matrix of DP800 caused the fragmentation of martensite crystals. Secondary slip system of BCC (〈111〉{112}) was also found to coexist with the primary slip system at higher strain rates which aided the plastic deformation at high strain rates within the ferrite grains. The difference in the martensite volume fraction and distribution in these two steels also affected the void initiation and fracture morphologies when deformed at various strain rates.
Metallurgical and Materials Transactions A, 2002
A dislocation-disclination model is proposed, describing the heterogeneous nucleation of an embryo of hcp martensite at a tilt grain-boundary segment containing some extrinsic dislocations. The total energy gain due to hcp embryo nucleation is analyzed in detail, and the existence of both the equilibrium and critical embryo sizes under varying external conditions (temperature and shear stress) is shown. Depending on the external conditions, these characteristic embryo sizes may vary in wide ranges. So, the equilibrium size increases while the critical size decreases as the external shear stress increases and the temperature decreases. It is also demonstrated that a critical external stress exists which induces athermal embryo nucleation when the nucleation-energy barrier disappears and the terms of equilibrium and critical embryo sizes lose their significance. The critical external stress has been studied, depending on the temperature and characteristic parameters of the grain boundary where the fcc-to-hcp martensite transformation takes place. We have shown, in particular, that the critical external stress increases in direct proportion to both the grain-boundary misorientation angle and temperature. M.Yu. GUTKIN, Leading Researcher, and K.N. MIKAELYAN, include such important terms as the energies of elastic inter-Researcher, are with the Institute of Problems of Mechanical Engineering, actions of the transformation dislocations with remnant
Effect of ferrite-martensite interface morphology on bake hardening response of DP590 steel
ELSEVIER, 2016
The effect of martensite spatial distribution and its interface morphology on the bake hardening characteristics of a dual phase steel was investigated. In one case, typical industrial continuous annealing line parameters were employed to anneal a 67% cold rolled steel to obtain a dual phase microstructure. In the other case, a modified annealing process with changed initial heating rates and peak annealing temperature was employed. The processed specimens were further tensile pre-strained within 1–5% strain range followed by a bake hardening treatment at 170 °C for 20 min. It was observed that industrial continuous annealing line processed specimen showed a peak of about 70 MPa in bake-hardening index at 2% pre-strain level. At higher pre-strain values a gradual drop in bake-hardening index was observed. On the contrary, modified annealing process showed near uniform bake-hardening response at all pre-strain levels and a decrease could be noted only above 4% pre-strain. The evolving microstructure at each stage of annealing process and after bake-hardening treatment was studied using field emission scanning electron microscope. The microstructure analysis distinctly revealed differences in martensite spatial distribution and interface morphologies between each annealing processes employed. The modified process showed predominant formation of martensite within the ferrite grains with serrated lath martensite interfaces. This nature of the martensite was considered responsible for the observed improvement in the bake-hardening response. Furthermore, along with improved bake-hardening response negligible loss in tensile ductility was also noted. This behaviour was correlated with delayed micro-crack initiation at martensite interface due to serrated nature.
Low-energy dislocations and ductility of ferritic steels
Materials Science and Engineering: A, 1993
Extended dislocation half-loops, of a/2(111) Burgers vector, appear in ferrite of dual-phase type HSLA steels and welds as controlling to a large extent the progress of (micro)plastic deformation of the ferrite. The criss-cross configuration of such half-loops was not very sensitive to heat inputs, and on intersections of the half-loops a reaction was observed resulting in mobile a(001) edge dislocations. A recently proposed mechanism (1990) explains how these a(001) edge dislocations participate in nucleation of brittle cleavage fracture. During straining around and below the brittle/ductile transition temperature, generation of such mobile a(001) edge dislocations is substantial, and embrittled HSLA steels usually contain numerous immobilized a(001) edge dislocations which, interacting with the generated ones, nucleate the cleavage cracks.