Abstract:

Despite that Mg has a lot of possible applications, the easiest deformation mechanism in Mg alloys, basal slip, can only provide two independent slip systems, leading to large plastic anisotropy and poor mechanical properties of Mg alloys at room temperature, restricting the applications of Mg alloys. Ascertaining the contribution of twins to plastic deformation for Mg alloys, predicting twin nucleation and twin variant selection behavior in polycrystalline Mg alloys, as well as understanding the coordinative and competitive relationships between twins and slips, are the key cornerstones to designing Mg alloys with outstanding and compressive mechanical properties and to promoting the applications of Mg alloys.
A dual-textured wrought Mg-6.5Zn (wt.%) alloy with limited yield asymmetry (compressive yield strength / tensile yield strength: 0.90) is reported. Approximately 80% of the grains present the standard texture of wrought Mg alloys while the grain orientation is rotated by ~90º perpendicularly to the c axis in the remaining 20%. The deformation mechanisms responsible for this behavior are analyzed on samples deformed in tension and compression up to different strains. Compressive deformation of the grains with the standard texture is accommodated by basal slip and extension twinning while tensile deformation promotes basal and non-basal prismatic and pyramidal slip, leading to the typical yield asymmetry. However, the rotated grains present the opposite (and much stronger) yield asymmetry because tensile deformation is absorbed by basal slip and extension twinning while compression deformation requires basal slip and compression twinning. Thus, the contribution of the 20% rotated grains to the overall mechanical behavior leads to (almost) similar values of the yield strength in tension and compression. This work provides a physical understanding of the deformation behavior of dual-textured Mg alloys, that can be used to develop novel dual-textured Mg alloys with limited yield asymmetry.
Besides, the transformation of compression twins (CT) to double twins
(DT) is studied in the dual-textured Mg-6.5Zn alloy during deformation along the extrusion axis. After 7.3% compression, 85% of CT are transformed to DT. In contrast, this ratio drops ii to 22% and 36% during tension although the applied stresses and strains in tension were much higher. The Schmid factor of the actual DT variants was very low (and often negative) in tension and compression and could not explain the differences in DT activity. It is shown that the suppressed CT → DT transformation during tension is accompanied by the activation of non-basal slip -rather than basal slip- which presumably hinder the transformation because the dissociation of basal dislocations is necessary to nucleate and grow extension twins in primary CT. These findings extend our understanding of the DT mechanism and point out an effective strategy to suppress DT activities by activating non-basal slip, which may be useful to design Mg alloys with high ductility.
Machine learning provides a new approach to simultaneously examine dozens of factors, including the grain size, Schmid factor and boundary conditions in neighboring grains, on twin nucleation and propagation. In the following work, other advanced in-situ electron backscattered diffraction, high-resolution digital image correlation and diffraction contrast tomography measurements in combination of machine learning are considered to employ to better understand the deformation behavior of Mg alloys.