Proliferation of twinning in hexagonal close-packed metals: Application to magnesium

Abstract

Plastic deformation of metallic alloys usually takes place through slip, but occasionally involves twinning. In particular, twinning is important in hexagonal close packed materials where the easy slip systems are insufficient to accommodate arbitrary deformations. While deformation by slip mechanisms is reasonably well understood, less remains known about deformation by twinning. Indeed, the identification of relevant twinning modes remains an art. In this paper, we develop an universal framework combining fundamental kinematic definition of twins with large scale atomistic calculations to predict twinning modes of crystalline materials. We apply this framework to magnesium where there are two accepted twin modes -tension and compression, but a number of anomalous observations. Surprisingly, our framework shows that there are a very large number of twinning modes that are important in the deformation process of magnesium consistent with the anomalous observations. Thus, in contrast to the traditional view where plastic deformation is kinematically partitioned between a few modes, our result argues that the physics of deformation in HCP materials is governed by an energetic and kinetic competition between numerous possibilities. Consequently, our findings suggest that the commonly used models of deformation physics need to be revisited in order to take into account a broader and richer variety of twin modes, and potentially points to new avenues of improving the mechanical properties. *

Following the phase transitions of iron in 3D with X-ray tomography and diffraction under extreme conditions

“…[19], where the transition was induced under hydrostatic pressure. The peak labelled T is not expected by Burgers mechanism but can be obtained by 0112 0111 twinning of one Burgers variant of ǫ-Fe [38], evidencing one irreversible twinning deformation mechanism in the sample subsequent to the transformation.…”

Anharmonic effect on the thermally activated migration of {101̄2} twin interfaces in magnesium

“…Magnesium was modeled with the modified embedded atom method (MEAM) potential of Wu et al [12], which has been used extensively to model dislocation cores [13][14][15], fracture [16] and twinning [6,17].…”

Stochastic modeling of discontinuous dynamic recrystallization at finite strains in hcp metals

“…The significantly lower critical resolved shear stresses in the basal plane as compared to slip on nonbasal planes results in the activation of alternative strain-accommodating processes, which in the case of hcp metals is provided by deformation twinning (Christian and Mahajan, 1995). Although recent studies based on density functional theory (DFT) suggest the presence of eleven twin systems (Sun et al, 2017), we here constrain ourselves to the two most easily activated systems, viz. compression and extension twins (with the respective names referring to the loading along the c-axis).…”