Anisotropy Of Plastic Flow Research Articles

Texture Evolution and Anisotropy of Plastic Flow in Hot Compression of Extruded ZK60-T5 Magnesium Alloy Plate

The texture evolution during hot compression of extruded ZK60A-T5 magnesium alloy plate loaded along the extrusion direction (ED) and the normal direction (ND) has been examined with the help of pole figures obtained on specimens deformed in the ranges of 200 °C to 500 °C and 0.0003 s−1 to 10 s−1. The results are interpreted in terms of the operating slip systems and mechanisms identified based on processing maps developed for the above two initial specimen orientations. The processing map for the initial ED orientation exhibited three domains. In Domains 1 and 3, first-order pyramidal slip {10 1 ¯ l} <11 2 ¯ 0> occurs, while in Domain 2, second-order pyramidal slip {11 2 ¯ 2} <11 2 ¯ 3> occurs. The pole figures obtained on specimens deformed in Domains 1 and 3 are strikingly similar, indicating that the operating slip system controls the texture evolution. Compression in Domains 1 and 3 nearly randomizes the intense basal texture in the as-received specimens, while a new texture is generated in Domain 2 with basal poles at 45° to ND or transverse direction (TD). This new texture will promote basal slip when loaded in a transverse direction. When loaded in the normal direction (ND), the processing map exhibited four domains. In Domains 1 and 4, {10 1 ¯ l} <11 2 ¯ 3> slip occurs, while {11 2 ¯ 2} <11 2 ¯ 3> slip occurs in Domains 2 and 3. The pole figures obtained from specimens deformed in Domains 1 and 4 have similar features, while those deformed in Domains 2 and 3 exhibited similar features to one another, confirming that the operating slip systems control the texture development since they are the same in each pair. The compression along ND produces strong basal textures with the basal planes normal to the ED. The texture gets intensified with increased temperature of deformation and causes strong anisotropy in mechanical properties.

Complexity and Anisotropy of Plastic Flow of α-Ti Probed by Acoustic Emission and Local Extensometry.

Current progress in the prediction of mechanical behavior of solids requires understanding of spatiotemporal complexity of plastic flow caused by self-organization of crystal defects. It may be particularly important in hexagonal materials because of their strong anisotropy and combination of different mechanisms of plasticity, such as dislocation glide and twinning. These materials often display complex behavior even on the macroscopic scale of deformation curves, e.g., a peculiar three-stage elastoplastic transition, the origin of which is a matter of debates. The present work is devoted to a multiscale study of plastic flow in α-Ti, based on simultaneous recording of deformation curves, 1D local strain field, and acoustic emission (AE). It is found that the average AE activity also reveals three-stage behavior, but in a qualitatively different way depending on the crystallographic orientation of the sample axis. On the finer scale, the statistical analysis of AE events and local strain rates testifies to an avalanche-like character of dislocation processes, reflected in power-law probability distribution functions. The results are discussed from the viewpoint of collective dislocation dynamics and are confronted to predictions of a recent micromechanical model of Ti strain hardening.

Complexity and anisotropy of plastic flow of commercially pure Ti on multiple scales

Complexity and anisotropy of plastic flow of commercially pure Ti on multiple scales

On non-associative anisotropic finite plasticity fully coupled with isotropic ductile damage for metal forming

In this work, non-associative finite strain anisotropic elastoplasticity fully coupled with ductile damage is considered using a thermodynamically consistent framework. First, the kinematics of large strain based on multiplicative decomposition of the total transformation gradient using the rotating frame formulation, is recalled and different objective derivatives defined. By using different anisotropic equivalent stresses (quadratic and non-quadratic) in yield function and in plastic potential, the evolution equations for all the dissipative phenomena are deduced from the generalized normality rule applied to the plastic potential while the consistency condition is still applied to the yield function. The effect of the objective derivatives and the equivalent stresses (quadratic or non-quadratic) on the plastic flow anisotropy and the hardening evolution with damage is considered. Numerical aspects mainly related to the time integration of the fully coupled constitutive equations are discussed. Applications are made to the AISI 304 sheet metal by considering different loading paths as tensile, shear, plane tensile and bulge tests. For each loading path the effect of the rotating frame, the equivalent stress (quadratic or non-quadratic) and the normality rule (with respect to yield function or to the plastic potential) are discussed on the light of some available experimental results.

Plastic flow anisotropy of pure zirconium after severe plastic deformation at room temperature

The present work investigates the plastic flow anisotropy of zirconium following one pass of equal-channel angular extrusion (ECAE) at room temperature in a 90° die. Samples were oriented with their strong basal pole texture either aligned with or perpendicular to the extrusion direction prior to processing, which leads to two significantly different starting textures. After ECAE, the samples were compressed to 30% along one of three sample directions. A visco-plastic self-consistent polycrystal model was used to determine the mechanisms responsible for the observed anisotropy. Using the same single-crystal material parameters, the model reasonably predicts texture evolution and the post-compression stress–strain response. We found that basal slip is crucial in reproducing the experimental results, and twinning makes an important contribution. These findings are noteworthy because basal slip is not active in zirconium at room temperature, and grain size reductions associated with ECAE are expected to suppress deformation twinning.

Crystal structures and textures in the hot-forged Ni-Mn-Ga shape memory alloys

Three ferromagnetic shape-memory alloys with the chemical compositions of Ni53Mn25Ga22, Ni48Mn30Ga22, and Ni48Mn25Ga22Co5 were prepared by the induction-melting and hot-forging process. The crystal structures were investigated by the neutron powder diffraction technique, showing that Ni53Mn25Ga22 and Ni48Mn25Ga22Co5 have a tetragonal, 14/mmm martensitic structure at room temperature, while Ni48Mn30Ga22 has a cubic, L21 austenitic structure at room temperature. The development of textures in the hot-forged samples shows the in-plane plastic flow anisotropy from the measured pole figures by means of the neutron diffraction technique. Significant texture changes were observed for the Ni48Mn25Ga22Co5 alloy after room temperature deformation, which is due to the deformation-induced rearrangements of martensitic variants. An excellent shape-memory effect (SME) with a recovery ratio of 74 pct was reported in this Ni48Mn25Ga22Co5 polycrystalline alloy after annealing above the martensitic transformation temperature, and the “shape-memory” influence also occurs in the distributions of grain orientations.

Textures and compressive properties of ferromagnetic shape-memory alloy Ni48Mn25Ga22Co5 prepared by isothermal forging process

A ferromagnetic shape-memory alloy Ni48Mn25Ga22Co5 was prepared by the induction melting and isothermal forging process. Dynamic recrystallization occurs during the isothermal forging. The deformation texture was studied by the neutron diffraction technique. The main texture components consist of (110)[112] and (001)[100], which suggested that in-plane plastic flow anisotropy should be expected in the as-forged condition. The uniaxial compression fracture strain in the forged alloy reaches over 9.5%. The final room-temperature fracture of the polycrystalline Ni48Mn25Ga22Co5 is controlled mainly by intergranular mode.

Anisotropy of plastic flow and microstructure evolution during hot working of laser-deposited Ti–6Al–4V

Hot compression tests were conducted at subtransus temperatures both parallel and perpendicular to the columnar-grain direction of laser-deposited Ti–6Al–4V. Negligible anisotropy in stress–strain behavior and the kinetics of dynamic globularization was noted, thus indicating the importance of alpha platelet thickness, not grain shape, on plastic flow and structure evolution.

Effect of texture and slip mode on the anisotropy of plastic flow and flow softening during hot working of Ti-6Al-4V

The effect of crystallographic texture and slip mode on the plastic flow of Ti-6Al-4V with either a colony- or globular-alpha microstructure was determined by conducting isothermal, constant-strainrate, hot-compression tests on specimens cut at various orientations (rolling direction (RD), transverse direction (TD), 45 deg, and normal) from hot-rolled plate. Testing was performed using a fixed strain rate (0.1 s−1) and various temperatures below the beta transus. The flow curves from all of the experiments exhibited a peak flow stress followed by a large and a small amount of flow softening for the colony and globular microstructures, respectively. Although the flow softening response did not depend noticeably on test direction for a given microstructure and test temperature, the peak flow stress and development of sample ovality did. This orientation dependence was interpreted using both lower-bound (isostress-type) and upper-bound (isostrain, Taylor/Bishop-Hill) models to deduce the operative slip systems in the alpha phase. These analyses suggested that prism 〈a〉 and basal 〈a〉 slip are considerably easier than pyramidal 〈c+a〉 or 〈a〉 slip at hot-working temperatures. A comparison of the flow curves for the colony and globular alpha microstructures suggested that slip transfer across alpha/beta interfaces and loss of Hall-Petch boundary strengthening can account for a substantial portion of the flow softening observed during hot working.

Plastic potentials for anisotropic porous solids

Abstract The aim of this paper is to incorporate plastic anisotropy into constitutive equations of porous ductile metals. It is shown that plastic anisotropy of the matrix surrounding the voids in a ductile material could have an influence on both effective stress–strain relation and damage evolution. Two theoretical frameworks are envisageable to study the influence of plastic flow anisotropy: continuum thermodynamics and micromechanics. By going through the Rousselier thermodynamical formulation, one can account for the overall plastic anisotropy, in a very simple manner. However, since this model is based on a weak coupling between plasticity and damage dissipative processes, it does not predict any influence of plastic anisotropy on cavity growth, unless a more suitable choice of the thermodynamical force associated with the damage parameter is made. Micromechanically-based models are then proposed. They consist of extending the famous Gurson model for spherical and cylindrical voids to the case of an orthotropic material. We derive an upper bound of the yield surface of a hollow sphere, or a hollow cylinder, made of a perfectly plastic matrix obeying the Hill criterion. The main findings are related to the so-called ‘scalar effect’ and ‘directional effect’. First, the effect of plastic flow anisotropy on the spherical term of the plastic potential is quantified. This allows a classification of sheet materials with regard to the anisotropy factor h ; this is the scalar effect. A second feature of the model is the plasticity-induced damage anisotropy. This results in directionality of fracture properties (‘directional effect’). The latter is mainly due to the principal Hill coefficients whilst the scalar effect is enhanced by ‘shear’ Hill coefficients. Results are compared to some micromechanical calculations using the finite element method.

Anisotropy of plastic flow in strongly textured γ-TiAl-based alloys

Abstract Uniaxial compression applied parallel to the lamellae of γ-TiAl based PST-crystals (polysynthetically twinned) results in a plain strain deformation along the lamellae. Similarly, uniaxial tension along the rolling direction of a strongly textured γ-TiAl sheet causes contraction in the normal direction of the sheet which is twice the contraction in the transverse direction. For both cases the anisotropies of plastic flow are discussed in terms of texture, microstructure and contributing slip systems.

Deformation modes in γ-TiAl as derived from the single crystal yield surface

Based on a detailed analysis of the topography of the single crystal yield surface, a deformation mode map for slip and twinning of L1 0 ordered γ-TiAl is derived. Various different deformation systems have been considered, namely, glide of ordinary and superdislocations as well as shear contributions by ordered twins. In the framework of the Taylor model for all combinations of critical resolved shear stress, parameters for polycrystal deformation are determined such as Taylor factors, slip distribution among the various deformation modes and internal stresses set up between grains. The comparison of theoretical with experimental textures, and of predicted with observed anisotropy of plastic flow, is utilized to determine the slip geometry and the relative strength levels of the different deformation systems. The results are used for a general discussion of deformation processes observed in TiAl and how these are affected by work hardening characteristics. It is concluded that superdislocations of one kind or another are necessary deformation modes, and that all modes have similar CRSS values at large strains.

Lamellar Interfaces and their Contribution to Plastic Flow Anisotropy in Tial-Based Alloys.

Lamellar Interfaces and their Contribution to Plastic Flow Anisotropy in Tial-Based Alloys.

Shock response of pentaerythritol tetranitrate single crystals

The shock response of single crystals of the explosive pentaerythritol tetranitrate was studied from 3.4 to 19 GPa. Four orientations, 〈110〉, 〈001〉, 〈101〉, and 〈100〉 were studied. The shock Hugoniot and shock initiation sensitivities were measured in wedge tests. Intermediate velocity transitions were noted. A sensitivity anomaly near 4 GPa was discovered for 〈110〉 orientations. Chemiluminescent phenomena associated with this anomaly were studied using framing photography and uv/visible spectroscopy. Both plane-wave explosive and gas-gun impact experiments were done. Analysis of the steric hindrance occurring during edge dislocation motion in the shear flow associated with shock-induced uniaxial strain was done. The results corroborate the hypothesis that the orientation dependence of shock initiation sensitivity is due to the anisotropy of plastic flow, that is, the relative ease of slip in different orientations.

Plastic anisotropy and texture development in calcite polycrystals

In the past, most information on the plasticity of rocks has been derived from axisymmetric compression experiments. These data are the basis for Arrhenius‐type flow laws which relate stress and strain, treating them essentially as scalars and not accounting for the deformation history. This assumption is valid for a very limited range of mechanisms, for example, superplastic flow, dislocation climb, and some mechanisms of recrystallization. Wherever dislocation glide is involved in the deformation process, preferred orientation develops which depends on the strain mode. The development of texture has a profound influence on the stress‐strain curve: (1) a geometric factor due to crystal orientation and expressed by the Taylor factor, (2) strain hardening due to interaction of dislocations in active slip systems, and (3) latent hardening because of microstructural difficulties in activating inactive slip systems at expected critical shear stresses. We have analyzed the influence of geometric factors on the plasticity of calcite polycrystals using the full constraint Taylor theory and observed that effects of strain mode are even stronger than in metals where the anisotropy of plastic flow was first documented. For example, at low temperature in axial deformation the specimen hardens rabidly, whereas in plane strain it softens during straining. The differences can be attributed to activation of different slip systems (number and relative importance) in each case, particularly the role of mechanical twinning. Wherever dislocation glide is significant, anisotropic flow laws should be considered, also in geological materials, to explain deformation. Particularly emphasized is the importance of deformation experiments in geometries different from axisymmetric compression.

Dislocations in Ice

The dislocation structures of dendritic ice crystals have been directly observed by x-ray diffraction topography. Growth occurs without the intervention of dislocations and very low dislocation dencities may initially prevail. Straining expands dislocations having 11[unknown]20 direction Burgers vectors from sources at grown-in inclusions. Slip of these dislocations in the basal plane (0001) and on pyramidal planes, probably (1 101), with frequent occurrence of primatic punching, is observed. Dislocation reactions of the type (a(0)/ 3) [11 20] + (a(0)/ 3) [1 210] = (a(0)/ 3) [2 110] and a strong preference for pure screw orientation are characteristic. These dislocations generally account for the known anisotropy of plastic flow in ice.