Twinning and antitwinning in body-centered cubic metals

R Gröger and J Holzer and T Kruml, COMPUTATIONAL MATERIALS SCIENCE, 216, 111874 (2023).

DOI: 10.1016/j.commatsci.2022.111874

Deformation twinning in body-centered cubic (BCC) metals occurs by shearing the crystal along 112 planes parallel to (111) directions. One of these directions (twinning shear) produces a twin, but it is often argued that the opposite sense of shearing (antitwinning shear) does not lead to twin formation. However, recent slip trace and orientational mapping analyses made on many BCC metals after low- temperature plastic deformation show clear evidence of fine misoriented lamellae along the traces of 112 planes sheared in the antitwinning sense. To resolve this controversy, we have utilized molecular statics simulations to determine the energy barriers for uniformly shearing all transition and alkali BCC metals in the twinning and antitwinning sense. The results of these simulations show that twins in transition BCC metals of the 5th and 6th groups can be produced on both types of 112 planes, irrespective of whether the shear is applied in the twinning or the antitwinning sense. However, this is not the case for alpha-Fe and the BCC structures of the alkali metals Li, Na, and K, where twins are likely to occur only on 112 planes sheared in the twinning sense. Furthermore, we have used TEM diffraction pattern analyses to investigate the characters of misoriented deformation lamellae in Nb and Cr compressed at 77 K, which were found along 112 planes sheared in the antitwinning sense. We demonstrate that these regions constitute regular twins. Similar studies on alpha-Fe compressed at 77 K prove that twinning in this material takes place exclusively on 112 planes sheared in the twinning sense.

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