Role of twinning and slip during compressive deformation of beryllium as a function of strain rate (original) (raw)
Related papers
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.
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.
Materials Science and Engineering: A, 2015
In this work, we develop a polycrystal mean-field constitutive model based on an elastic-plastic self-consistent (EPSC) framework. In this model, we incorporate recently developed subgrain models for dislocation density evolution with thermally activated slip, twin activation via statistical stress fluctuations, reoriented twin domains within the grain and associated stress relaxation, twin boundary hardening, and de-twinning. The model is applied to a systematic set of strain path change tests on pure beryllium (Be). Under the applied deformation conditions, Be deforms by multiple slip modes and deformation twinning and thereby provides a challenging test for model validation. With a single set of material parameters, determined using the flowstress vs. strain responses during monotonic testing, the model predicts well the evolution of texture, lattice strains, and twinning. With further analysis, we demonstrate the significant influence of internal residual stresses on: 1) the flow stress drop when reloading from one path to another, 2) deformation twin activation, 3) de-twinning during a reversal strain path change, and 4) the formation of additional twin variants during a cross-loading sequence. The model presented here can, in principle, be applied to other metals, deforming by multiple slip and twinning modes under a wide range of temperature, strain rate, and strain path conditions.
STATISTICAL ANALYSIS OF HCP POLYCRYSTALS USING CRYSTAL PLASTICITY MODELING AND SIMULATION
STATISTICAL ANALYSIS OF HCP POLYCRYSTALS USING CRYSTAL PLASTICITY MODELING AND SIMULATION, 2019
Three-dimensional crystal plasticity finite element modeling and simulation of polycrystalline magnesium (Mg) alloys is performed. The focus of this work is on extracting the relation between the initial texture and the mechanical behavior of a polycrystalline Mg microstructure under uniaxial loading, particularly along principal directions of a rolled material - rolling (L), in-plane transverse to rolling (T), and out-of-plane transverse (S) to L and T. Starting with an initial rolled texture of an Mg alloy as a basis, the effect of deviations in the initial texture on the macroscopic and microscopic characteristics is analyzed through quantitative analysis of the deviations in the overall stress-strain behaviors, activities of the slip and twinning mechanisms, and textural evolution.
Comparisons of crystal hardening laws in multiple slip
International Journal of Plasticity, 1985
This paper brings together and concisely reviews results from recent analytical investigations on single crystals (variously clone alone or with students) in which predictions from different theoretical hardening laws are contrasted and compared with experimental studies. Finitely deforming f.c.c, crystals in both constrained and unconstrained multiple-slip configurations are considered. Four crystal hardening laws are given prominence. Two of these belong to a class of theories in which the physical hardening moduli relating rates-of-change of critical strengths (in the 24 crystallographically equivalent slip systems) to slip-rates are taken as symmetric. These are G. I. Taylor's classic isotropic hardening rule (proposed in 1923), which is almost universally adopted in the metallurgical literature for various approximate analyses of single and poly-crystal deformation, and a 2-parameter modification of Taylor's rule that has an empirical basis in the qualitative features of experimentally determined latent hardening in single slip. The other two hardening laws featured here belong to a class of theories that were introduced in 1977 by this author. This class requires the above modu[i to be nonsymmetric and explicitly dependent upon the current stress state in such a manner that the following consequences are assured. (1) The deformation-dependent hardening of latent slip systems necessarily develops anisotropically if there is relative rotation of gross material and underlying crystal lattice. (2) The theories admit self-adjoint boundary value problems for crystalline aggregates, hence a variational formulation. (The fact that symmetric physical hardening moduli do not permit variational formulations of polycrystalline problems was shown at the 1972 Warsaw Symposium.) The two members of this class considered here are the original (and simplest possible) theory of rotation-dependent anisotropy, which was proposed by this author in 1977 and commonly has been referred to as the "simple theory," and a modification of this theory introduced in 1982 by Peirce, Asaro and Needleman that lies between Taylor's rule and the simple theory in its predictions for finitely deforming f.c.c, crystals. (In a series of five papers during 1977-79, the simple theory was shown to universally account for the experimental phenomenon of "overshooting" in single slip in both f.c.c, and b.c.c, crystals.) Theoretical results from the various hardening rules are contrasted and compared with finite strain experiments in the metallurgical literature. Both tensile-loaded crystals in 4, 6 and 8-fold symmetry orientations and compressively loaded crystals under conditions of channel die constraint are treated. A postulate of minimum plastic work introduced in 1981 plays a prominent role in the theoretical analyses, in many cases providing a unique solution to the slip system inequalities and deformation constraints (where applicable). The rather remarkable ability of the simple theory to reconcile diverse qualitative features of both constrained and unconstrained finite deformation of f.c.c, crystals is demonstrated. Finally, conditions for total loading (all systems active) in 6-fold symmetry are investigated, and certain concepts regarding the selection of active systems under prescribed straining are critically assessed. i i4
A neutron diffraction and modeling study of uniaxial deformation in polycrystalline beryllium
Metallurgical and Materials Transactions A, 2003
The deformation of polycrystalline beryllium to strains of 60.8 pct in uniaxial tension and compression was studied by neutron diffraction and modeled using an elasto-plastic self-consistent (EPSC) model. The beryllium response is asymmetric with respect to tension and compression in both the macroscopic behavior, as displayed in the stress/strain curve, and the microscopic lattice response. The EPSC model qualitatively reproduces the lattice strain curves in tension and compression with the assumption of pyramidal slip being active, in addition to prism and basal slip and with the inclusion of thermal residual stresses developed during processing. Although it underpredicts the magnitude of the observed strains, it demonstrates that accounting for residual stresses of thermal origin is crucial for understanding the evolution of lattice strains during uniaxial loading.
2011
When a strongly textured hexagonal close packed (HCP) metal is loaded under an orientation causing profuse twinning or detwinning, the stress-strain curve is sigmoidal in shape and inflects at some threshold. Authors have largely attributed the dramatic stress increase in the lower-bound vicinity of the inflection point to a combined effect of a Hall-Petch mechanism correlated to grain refinement by twinning, and twinning-induced reorientation requiring activation of hard slip modes. We experimentally and numerically demonstrate that these two mechanisms alone are unable to reproduce the stress-strain behaviors obtained under intermediate loading orientations correlated to in-between profuse twinning and nominal twinning. We argue based on adopting various mechanistic approaches in hardening model correlations from the literature. We used both a physics dislocation based model and a phenomenological Voce hardening model. The HCP material is exemplified by an extruded AM30 magnesium alloy with a 1010 -fiber parallel to the extrusion direction.