Rate Of Texture Evolution Research Articles

Parallelized hybrid Monte Carlo simulation of stress-induced texture evolution

A parallelized hybrid Monte Carlo (HMC) methodology is devised to quantify the microstructural evolution of polycrystalline material under elastic loading. The approach combines a time explicit material point method (MPM) for the mechanical stresses with a calibrated Monte Carlo (cMC) model for grain boundary kinetics. The computed elastic stress generates an additional driving force for grain boundary migration. The paradigm is developed, tested, and subsequently used to quantify the effect of elastic stress on the evolution of texture in nickel polycrystals. As expected, elastic loading favors grains which appear softer with respect to the loading direction. The rate of texture evolution is also quantified, and an internal variable rate equation is constructed which predicts the time evolution of the distribution of orientations.

Role of strain-rate sensitivity in the crystal plasticity of hexagonal structures

In the first part of this paper the stress and strain-rate response of hexagonal crystal structures are examined when slip is viscoplastic according to a power law. The stress and strain-rate equi-potential surfaces are constructed and discussed as a function of the strain-rate sensitivity index m. The second part of this paper deals with the case of linear viscous slip; i.e., for the case when m is equal to one. A simple analytic solution is presented to obtain the deviatoric stress state for a given strain-rate. It is shown that the plastic spin is not zero for m = 1 in hexagonal crystal structures, contrary to the cubic case where the plastic spin vanishes. In addition, the rate of texture evolution in simple shear of a magnesium polycrystal is examined as a function of m.

Investigating the limits of polycrystal plasticity modeling

A material model which describes the rate-dependent crystallographic slip of FCC metals has been implemented into a quasistatic, large deformation, nonlinear finite element code developed at Sandia National Laboratories. The resultant microstructure based elastic–plastic deformation model has successfully performed simulations of realistic looking 3-D polycrystalline microstructures generated using a Potts-model approach. These simulations have been as large as 50,000 elements composed of 200 randomly oriented grains. This type of model tracks grain orientation and predicts the evolution of sub-grains on an element by element basis during deformation of a polycrystal. Simulations using this model generate a large body of informative results, but they have shortcomings. This paper attempts to examine detailed results provided by large scale highly resolved polycrystal plasticity modeling through a series of analyses. The analyses are designed to isolate issues such as rate of texture evolution, the effect of mesh refinement and comparison with experimental data. Specific model limitations can be identified with lack of a characteristic length scale and oversimplified grain boundaries within the modeling framework.

The role of crystallinity in the crystallographic texture evolution of polyethylenes during tensile deformation

The crystallographic texture evolution of rapidly crystallized high-density polyethylene (HDPE) and ethylene-1-octene copolymer prepared with a Ziegler–Natta (LLDPE) catalyst, was studied during tensile deformation using wide-angle X-ray diffraction WAXD. The popLA software suite, a methodology based on spherical harmonics for texture analysis, was utilized to produce recalculated pole figures and orientation distribution function plots from the raw data. An important aspect of this work has been the in situ measurement of texture during tensile deformation and the subsequent measurement of texture in the relaxed samples. The difference in molecular structure of the polyethylenes had a strong effect in the initial texture as well as in the rate of texture evolution during deformation. Texture evolves slowly in the HDPE while a fast drastic change in texture is observed in LLDPE after yield. At higher strains the texture of LLDPE was basically unchanged revealing not only a strong (100)[001] ‘ c-axis’ texture component, but also other weaker texture components such as (010)[001], (1×0)[010], (011)[100] and (201)[1̄02]. Furthermore, while relaxation after unloading mitigated or eliminated two of the preferred texture components in HDPE strengthening the (001) component aligned along the extension axis, it was observed that the texture of LLDPE did not undergo any significant changes after relaxation. At large strains ( ε>1.0), microfibers formed in both HDPE and LLDPE. A lamellar substructure that differs between the HDPE and LLDPE is observed by AFM images of drawn materials. The role of possible slip mechanisms and stress relaxation in the evolving crystallographic texture of these polyethylenes is discussed.

Crystallographic texture evolution in high-density polyethylene during uniaxial tension

This paper presents experimental measurements of crystallographic texture evolution in high-density polyethylene subjected to very large strains in uniaxial tension (up to a true strain of 2.1). The measurements presented here differ from prior studies in three important aspects: (1) The initial texture in the sample is quite strong with a large fraction of the crystallites oriented in an unstable orientation with the crystal c-axis perpendicular to the tensile axis of the sample. (2) Rigorous methods of texture analyses, based on spherical harmonics, have been applied to produce “complete, recalculated” pole figures based on diffraction data from five incomplete pole figures. (3) The measurements were performed while the samples were kept in the deformed state. The results presented here provide several new insights into texture development in tensile straining of high-density polyethylene to large strains. There are at least three distinct preferred orientations: (i) a component with (001) aligned along the extension axis, (ii) a component with (011) aligned close to the extension axis, and (iii) a component with (010) aligned along the extension axis. Note that only the first component has been reported to be stable at high strains in previous studies. The rate of texture evolution in the present study is significantly lower than that reported in previous studies. It was also observed that the natural relaxation of strain following the tensile loading had a significant impact on the texture in the sample. It was observed that the relaxation process mitigated or eliminated the second and third preferred texture components described above, while strengthening the first.

The effects of shear constraints on the lattice rotation of FCC crystals in (011) channel-die compression

T he evolution of texture during channel-die compression is examined analytically for a range of crystal orientations with a (011) compression axis. The analysis considers the effects of longitudinal transverse shear on the rotation of the crystal lattice along the α-fiber, with particular attention toward the evolution of Brass texture. Closed-form expressions are obtained for the lattice rotation rate in terms of the current orientation and the applied deformation. A relation between the lattice orientation and compressive strain is found for simple deformation histories. The results show that the rate of texture evolution can be enhanced or impeded by applied shear and that the lattice rotation is faster if the shear is restrained. Thus. for crystals with orientations restricted to the α-fiber, a stable Brass texture will develop in a full constraint Taylor model ; but lattice rotation along this fiber is not predicted in a model where longitudinal transverse shear is permitted. The results also suggest that the stress state required to deform crystals of this orientation in a Taylor model may be unrealistic.