M. Arul Kumar

Grain neighbour effects on twin transmission in hexagonal close-packed materials.

Materials with a hexagonal close-packed (hcp) crystal structure such as Mg, Ti and Zr are being used in the transportation, aerospace and nuclear industry, respectively. Material strength and formability are critical qualities for shaping these materials into parts and a pervasive deformation mechanism that significantly affects their formability is deformation twinning. The interaction between grain boundaries and twins has an important influence on the deformation behaviour and fracture of hcp metals. Here, statistical analysis of large data sets reveals that whether twins transmit across grain boundaries depends not only on crystallography but also strongly on the anisotropy in crystallographic slip. We show that increases in crystal plastic anisotropy enhance the probability of twin transmission by comparing the relative ease of twin transmission in hcp materials such as Mg, Zr and Ti.

Grain size constraints on twin expansion in hexagonal close packed crystals

Deformation twins are stress-induced transformed domains of lamellar shape that form when polycrystalline hexagonal close packed metals, like Mg, are strained. Several studies have reported that the propensity of deformation twinning reduces as grain size decreases. Here, we use a 3D crystal plasticity based micromechanics model to calculate the effect of grain size on the driving forces responsible for expanding twin lamellae. The calculations reveal that constraints from the neighboring grain where the grain boundary and twin lamella meet induce a stress reversal in the twin lamella. A pronounced grain size effect arises as reductions in grain size cause these stress-reversal fields from twin/grain boundary junctions to affect twin growth. We further show that the severity of this neighboring grain constraint depends on the crystallographic orientation and plastic response of the neighboring grain. We show that these stress-reversal fields from twin/grain boundary junctions will affect twin growth, below a critical parent grain size. These results reveal an unconventional yet influential role that grain size and grain neighbors can play on deformation twinning.

First-principles study of crystallographic slip modes in ω -Zr

We use first-principles density functional theory to study the preferred modes of slip in the high-pressure ω phase of Zr. The generalized stacking fault energy surfaces associated with shearing on nine distinct crystallographic slip modes in the hexagonal ω-Zr crystal are calculated, from which characteristics such as ideal shear stress, the dislocation Burgers vector, and possible accompanying atomic shuffles, are extracted. Comparison of energy barriers and ideal shear stresses suggests that the favorable modes are prismatic 〈c〉, prismatic-II

Multi-scale Investigation on Yield “Symmetry” and Reduced Strength Differential in an Mg–Y Alloy

\langle 10\bar{1}0\rangle

Effect of local stress fields on twin characteristics in HCP metals

〈101¯0〉 and pyramidal-II 〈c + a〉, which are distinct from the ground state hexagonal close packed α phase of Zr. Operation of these three modes can accommodate any deformation state. The relative preferences among the identified slip modes are examined using a mean-field crystal plasticity model and comparing the calculated deformation texture with the measurement. Knowledge of the basic crystallographic modes of slip is critical to understanding and analyzing the plastic deformation behavior of ω-Zr or mixed α-ω phase-Zr.

Yield symmetry and reduced strength differential in Mg-2.5Y alloy

HCP magnesium metals are widely used in different industries due to their low density and high specific strength. Their applicability is restricted due to poor formability and high anisotropy in deformation behavior. The formability of magnesium can be improved by alloying additions and this modification can affect macroscopic plastic anisotropy. Alloying additions can also significantly control the twinning process. In this work we use a crystal plasticity based Fast Fourier Transform model to characterize deformation twinning in different alloy types. In the model, the influence of alloying additions is represented through their effect on the critical resolved shear stress (CRSS) values for all slip and twinning modes. From this study, we build an understanding of the influence of alloying elements on twin growth and twin transmission behavior. A new plastic anisotropy measure is proposed to quantify the effects of alloying elements on some important twinning characteristics in magnesium alloys.

A measure of plastic anisotropy for hexagonal close packed metals: Application to alloying effects on the formability of Mg

Mg and its alloys are promising candidates for light-weighted structural applications, e.g., aircraft, automobile, electronic, etc. However, the inherent hexagonal close packed crystal structure makes the deformation of Mg anisotropic, namely deformation only occur by dislocation slip in the close-packed (0001) plane (i.e., basal plane), or by deformation twinning in \( \{ 10\bar{1}2\} \) planes. Consequently, polycrystalline Mg alloys undergone thermos-mechanical processing usually contain strong texture, i.e., preferred crystallographic orientation in grains. The texture in turn leads to anisotropic deformation in wrought Mg alloys. For example, in extruded Mg alloys, the compressive yield strength is usually much lower than the tensile yield strength (so-called yield asymmetry and strength differential). It is the anisotropy that hinders the broader application of Mg alloys. Recent modeling study on Mg predicts that certain alloying elements, particularly rare-earth elements (e.g., Y, Ce, Nd, Gd, etc.), could alter the active deformation modes, and promote more homogeneous deformation and overall mechanical properties in Mg. Therefore, this work aims to investigate experimentally the effects of alloying element Y in reducing the intrinsic and extrinsic anisotropy, modifying texture, and enhancing the overall strength and ductility for Mg. Fine-grained Mg 2.5 at.% Y alloy (FG Mg–2.5Y) was prepared by powder metallurgy method, including gas atomization for producing Mg–2.5Y powder, degassing and hot isostatic pressing (HIP), and hot extrusion. Both the as-HIPed and the as-extruded materials were characterized by electron back-scattered diffraction (EBSD), transmission electron microscopy (TEM), and/or atom probe tomography (APT). Tension and compression tests were carried out along the extrusion direction (ED) for FG Mg–2.5Y. Unlike common Mg alloys exhibiting yield asymmetry, the FG Mg–2.5Y exhibits virtual yield “symmetry” and significantly reduced strength differential. Namely the deformation is more isotropic. In addition to post-mortem TEM characterization for deformed FG Mg–2.5Y, in situ TEM was also performed at the National Center for Electron Microscopy (NCEM), in an effort to understand the fundamental deformation mechanisms in FG Mg–Y that lead to reduced anisotropy. In situ TEM for single-crystal Mg–Y nano-pillars reveals that deformation twinning is replaced by dislocation slip in non-basal planes (i.e., prismatic planes), which diametrically differs from any other Mg alloys.