Martensitic Transformations in TiNi Alloys with Ti₃Ni₄ Precipitates

Abstract
The thermoelastic martensitic transformations in Ti₄₉Ni₅₁ and Ti₄₈Ni₅₂ alloys after isothermal aging have been studied by means of transmission electron microscopy and dilatometry. The presence of Ti₃Ni₄ precipitates and the difference in precipitates size affect the martensite nucleation and martensitic transformation temperatures and sequences in TiNi alloys. Highly coherent and densely dispersed Ti₃Ni₄ particles (≤20-50 nm length) afrer initial aging stages depresses B2 → B19' transition and initiates B2 → R one. The Ti₃Ni₄ particles provide the nucleation sites for the R-martensite. When size of Ti₃Ni₄ particles run from 100 nm to I-2 µm, decreasing of number of Ti₃Ni₄ variants and thus R-phase ones is observed. Transformation sequence is B2 → R → B19'. When sizes of Ti₃Ni₄ particles run as large as several microns, they provide nucleation sites of the B19'-martensite. Transformation sequence is B2 → B19' as in solution treated specimen.