Higher order twin modes in martensitic NiTi-The (201̄) case

NiTi alloys in the martensitic phase deform by detwinning of the martensite variants succeeded by deformation twinning of the single crystal of martensite. One of the deformation twinning modes of the martensite is denoted as (20. 1-)[. 1-0. 2-]. In this work, we establish how twinning on the (20. 1-) planes develops via the combination of homogeneous shear and shuffle and establish its energy barrier via atomistic simulations. We calculate the slip barrier in addition to the twin barrier ruling out the potential for plastic flow via slip in the (20. 1-) plane. The (20. 1-)[. 1-0. 2-] mode succeeds the (001) and (100) compound twinning modes which have lower energy barriers. It plays a significant role in allowing deformation to higher strains in the martensitic phase. Therefore, the insight into the (20. 1-)[. 1-0. 2-] twinning mode is important in extending the shape memory strains in NiTi alloys and towards better understanding of shape memory alloys in general.