A neutron diffraction and modeling study of uniaxial deformation in polycrystalline beryllium (original) (raw)
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Role of twinning and slip during compressive deformation of beryllium as a function of strain rate
International Journal of Plasticity, 2012
An experimental and theoretical investigation was carried out to study the strain rate dependent plastic response of beryllium over a wide range of applied compression strain rates, 10 À4-10 4 /s. At each rate, the evolution of flow stress and the final texture with deformation was obtained from a non-textured hot-pressed (HP) sample and a textured rolled sheet. The rolled sheet material was compressed in both the in-plane (IP) and throughthickness (TT) direction for comparison. The twin volume fraction was determined from the change in texture. The activity of twinning was strongly dependent on strain rate in the IP and HP samples. We applied a multi-scale constitutive model for hexagonal close packed polycrystals that accounts for crystallographic slip and twinning on individual systems in each crystal, as well as twin reorientation. Rate effects enter the calculations only through thermally activated dislocation glide on the active slip modes. The importance of this study is that it points to the necessity of using a crystallographic model based on microstructure evolution to understand the role played by plastic anisotropy, slip-slip competition, and slip-twin competition, in the mechanical response of HCP aggregates. The model reproduces the observed flow curves and texture evolution for all tests with a unique single crystal set of parameters.
Neutron diffraction provides a unique, non destructive method for studying deformation in polycrystalline materials. At pulsed sources like LANSCE the strain information typically comprises all possible lattice reflections. This is attractive because of its comprehensive description of the strain state. Indeed the gross aspects of the results, almost invariably, describe the long range or mean phase strain behavior reasonably well. However it is more of a challenge to interpret the data associated with the microscopic or intergranular strains which typically exceed our ability to model them. Research performed during this LDRD project addressed three areas that relate to the distinction of micro and macrostrains in engineering materials; 1) validation and development of self consistent models, 2) provision of a routine and reliable analysis method for "engineering" interpretation of data from a pulsed neutron source, & 3) assessment of problems in which strain, texture and phase fraction all evolve simultaneously.
International Journal of Plasticity, 2013
A polycrystalline material, deformed to large plastic strains and subsequently reloaded along a distinct strain path, exhibits a change in flow stress and hardening behavior. Such changes upon reloading depend on the level of mechanical anisotropy induced by texture and sub-grain microstructure developed during prior loading. In order to comprehend such material behavior, we extend a previously developed rate-and temperature-sensitive hardening law for hexagonal single crystals that accounts explicitly for the evolution of dislocation densities by including the effects of reverse dislocation motion and de-twinning on strain hardening and texture evolution. The law is implemented within a visco-plastic self-consistent polycrystalline model and applied to simulate macroscopic behavior of polycrystalline beryllium during strain-path changes. We show that the model successfully captures the mechanical response and evolution of texture and twin volume fraction during pre-loading in compression and subsequent cross-reloading in compression along two orthogonal directions at two different strain rates. These predictions allow us to elucidate the role played by various slip and twin mechanisms, de-twinning, and reverse dislocation motion on strain hardening and texture evolution of beryllium during strain-path changes. The model is general and can be applied to any metal deforming by slip and twinning.
Metallurgical and Materials Transactions A, 2005
Weakly textured hot-pressed (HP) beryllium and strongly textured hot-rolled beryllium were compressed using a split-Hopkinson pressure bar (SHPB) (strain rate ϳ4500 s Ϫ1 ) to a maximum of 20 pct plastic strain as a function of temperature. The evolution of the crystallographic texture was monitored with neutron diffraction and compared to polycrystal plasticity models for the purpose of interpretation. The macroscopic response of the material and the active deformation mechanisms were found to be highly dependent on the orientation of the load with respect to the initial texture. Specifically, twinning is inactive when loaded parallel to the strong basal fiber but accounts for 27 pct of the plastic strain when loaded transverse to the basal fiber. In randomly textured samples, 15 pct of the plastic strain is accomplished by twinning. The role of deformation mechanisms with components out of the basal plane (i.e., twinning and pyramidal slip) is discussed.
Journal of Applied Crystallography
The software DISEMM is designed to analyse diffraction data from in situ loading experiments on polycrystalline samples for the determination of single-crystal elastic constants (SECs) and elasto-plastic self-consistent (EPSC) modelling of lattice strains. The SECs can be obtained from powder-diffraction elastic constants using a variety of grain-to-grain interaction models, namely Voigt, Reuss, Hill, Kröner, de Wit and Matthies approaches. The texture of the polycrystalline sample can be taken into account using the orientation distribution function of the grains. For the analysis of two-phase materials, an approach was implemented to calculate the stress transfer between the phases and its impact on the apparent elastic properties. The calculated SECs can then be used as input into the EPSC model, which allows the user to predict the elasto-plastic behaviour for comparison with experimental lattice strain data and to investigate the activation of individual slip systems. For this ...
Advanced neutron diffraction techniques for strain measurements in polycrystalline materials
Le Journal de Physique IV, 1993
Three unique high resolution experimental arrangements for nondestructive strain measurements which are based on neutron Bragg diffraction optics with cylindrically bent perfect crystals are reviewed. Using focusing in momentum and real space thgfe techgiques yield Ad/d (d-lattice spacing) resolution of 10 -10-and considerably higher luminosity in comparison with the current dedicated instruments. They permit measurements not only macrostrain components resulting in angular shifts of diffraction peaks but also of microstrains by means of profilebroadening analysis.
Acta Materialia, 2009
The intergranular thermal residual stresses in texture-free solid polycrystalline beryllium were determined by comparison of crystallographic lattice parameters in solid and powder samples measured by neutron diffraction during cooling from 800°C. The internal stresses are not significantly different from zero >575°C and increase nearly linearly <525°C. At room temperature, the c axis of an average grain is under $200 MPa of compressive internal stress, and the a axis is under 100 MPa of tensile stress. For comparison, the stresses have also been calculated using an Eshelby-type polycrystalline model. The measurements and calculations agree very well when temperature dependence of elastic constants is accounted for, and no plastic relaxation is allowed in the model.
2008
The room-temperature plastic behavior of several FCC alloys was examined with in-situ neutron-diffraction and with polychromatic X-ray microbeam diffraction (PXM). The measurements characterize the local dislocation density distribution as a function of loading and combined with modeling, provide insights into damage and failure in polycrystalline materials. Both, monotonic-tension and low-cyclefatigue experiments were conducted as a function of stress. The plastic behavior during deformation is discussed in light of the relationship between the stress and dislocationdensity evolution. The observed dislocation density evolution finds that the monotonic tensile and low-cycle-fatigue samples have similar dislocation densities at small strain, but that latter have much lower dislocation densities than the former at high strain.
Metallurgical and Materials Transactions A, 2014
A single crystal superalloy with initial sample axis 10 deg deviated from [001] was creep deformed at 1273 K (1000°C) 235 MPa and its triaxial strain/stress state and subgrain defects were studied by neutron diffraction. Normal internal stresses with their directions close to the loading axis and their scales smaller than those perpendicular to the axis were observed and attributed to a lattice rotation toward [001] pole. The internal stress at a level approaching to the loading stress and mostly in the state of interphase stress was induced during the first stage of creep prior to rafting and associated to lattice rotation, microstrain relaxation and line-up of misoriented c¢-precipitates. The internal stress was diminished and released at final stage of creep associated with a reduction in unit-cell volume and a transition of strain/stress state between the two phases. The observation was explained by development of dislocations and raft structure during creep.