Understanding the martensitic transformations in TiNi-based alloys by elastic constants measurement

Abstract

Abstract The origin of the unique monoclinic martensite B19′ in TiNi and TiNi-based alloys has remained obscure for many years. The formation of B19′ martensite also challenged the well-known basal-plane shear/shuffle theory of martensitic transformation in β phase alloys. Recently we proposed that the B2–B19′ transformation stems from a strong coupling between a non-basal-plane shear (c44 shear) with the basal-plane shear (c′ shear), being manifested by a low-lying and decreasing anisotropy factor (c44/c′) towards Ms. This model has gained experimental support from elastic constants measurement on Ti50Ni30Cu20 alloy which exhibits B2–B19 transformation (i.e. absence of the monoclinic shear). In the present study, we attempt to further verify this model by measuring the elastic constants of Ti50Ni40Cu10 alloy which undergoes a two-stage transformation B2–B19–B19′. The results clearly demonstrated once again that whenever the parent phase B2 does not transform directly into B19′, the anisotropy factor exhibits an increase towards Ms, indicating that the c44 shear is not included into the transformation. Therefore, the present study gives additional support to the coupling model of TiNi-based alloys. Furthermore, we tried to understand the multi-stage transformation in terms of coupling strength between the c′ shear and c44 shear.