Martensitic transformations in TiNi alloys with Ti3Ni4 precipitates

Abstract

The thermoelastic martensitic transformations in Ti21Ni31 and Ti21Ni32 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) after initial aging stages depresses B2→B19′ transition and initiates B2→R ones. The Ti3N4 particles provide the nucleation sites for the R-martensite. When size of Ti3N4 particles run from 100 nm to 1-2 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.