MARTENSITIC TRANSFORMATIONS IN TINI ALLOYS WITH TI3NI4 PRECIPITATES

The thermoelastic martensitic transformations in Ti49Ni51 and Ti48Ni52 alloys after isothermal aging have been studied by means of transmiss ion electron microscopy and dilatometry. The presence of Ti3Ni4 precip itates and tile difference in precipitates size affect the martensite nucleation and martensitic transformation temperatures and sequences i n TiNi alloys. Highly coherent and densely dispersed Ti3Ni4 particles (less than or equal to 20-50 nm length) after initial a ln stages depr esses B2-->B19' transition and initiates B2-->R one. The Ti3Ni4 partic les provide the nucleation sites for the R-martensite. When size of Ti 3Ni4 particles run from 100 nm to 1-3 mu m, decreasing of number of Ti 3Ni4 variants and thus R-phase ones is observed. Transformation sequen ce is B2-->R-->B19'. When sizes of Ti3Ni4 particles I un as large as s everal microns. the! provide nucleation sites of the B19'-martensite. Transformation sequence is B2-->B19' as in solution treated specimen.