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' martensit e also challenged the well-known basal-plane shear/shuffle theory of marten sitic transformation in beta phase alloys. Recently we proposed that the B2 -B19' transformation stems from a strong coupling between a non-basal-plane shear (c(44) shear) with the basal-plane shear (c' shear), being manifeste d by a low-lying and decreasing anisotropy factor (c(44)/c') towards M-s. T his model has gained experimental support from elastic constants measuremen t on Ti50Ni30Cu20 alloy which exhibits B2-B19 transformation (i.e. absence of the monoclinic shear). In the present study, we attempt to further verif y this model by measuring the elastic constants of Ti50Ni40Cu10 alloy which undergoes a two-stage transformation B2-B19-B19'. The results clearly demo nstrated once again that whenever the parent phase B2 does not transform di rectly into B19', the anisotropy factor exhibits an increase towards M-s, i ndicating that the c(44) shear is not included into the transformation. The refore, the present study gives additional support to the coupling model of TiNi-based alloys. Furthermore, we tried to understand the multi-stage tra nsformation in terms of coupling strength between the c' shear and c(44) sh ear. (C) 1999 Elsevier Science S.A. All rights reserved.