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Abstract. The thermoelastic martensitic transformations in Ti 4 .>Ni5, and Ti 48 Nij2 alloys after isothermal aging have been studied by means of transmission electron microscopy and dilatometry. The presence of TUNi 4 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 TUNi 4 particles (<20-50 nm length) after initial aging stages depresses B2-»B19' transition and initiates B2-»R one. The Ti-,Ni 4 particles provide the nucleation sites for the R-martensite. When size of ThNi 4 particles run from 100 nm to 1-2 urn. decreasing of number of Ti-.Ni 4 variants and thus R-phase ones is observed. Transformation sequence is B2-^R->B19'. When sizes of Ti-,Ni 4 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.

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