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

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 M_s. 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 M_s, 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.