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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.

The dilatometric studies of Ti-50.5 at. % Ni alloy have shown that the thermal expansion coefficient (TEC) of B19'-martensite varies considerably for the solution-treated and tensile-strained samples. In the undeformed state the TEC has the positive value c=9.7.10-6K-. After tensile straining the TEC decreases sharply and takes on a negative value (down to 10.8.10-6 K-). The reasons for the variation of the TEC of the B19'-phase of the solution-treated and deformed samples are discussed. The tensile straining was found to create in the samples an extremely sharp texture of the phase transformation, with the texture axis parallel to the strain direction. The TEC of monoclinic B 19'-lattice is different in various crystallographic directions and assumes a negative value in the [0 01]B9, direction.B 19-martensite; Thermal expansion coefficient; Thermal expansion

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