Shape memory effect and phase transitions in TiNi alloys electron microscopy studies of the phase transitions after thermomechanical treatment

Abstract

Diffuse scattering is observed in Ti–53 to 56 wt% Ni alloys after high-temperature treatment (within temperature range 650 to 800 °C) which can be explained as the result of the short-range order arrangement of the structural vacancies. During aging within the temperature range 400 to 650 °C this short-range order state transforms to a long-range order state which can be interpreted as a long-period superstructure with lattice constant a = 1.58 nm (x-phase). The orientation relationships between β-phase and x-phase are: {111} 〈321〉β ‖ {111} 〈110〉x. The quantity of the x-phase formed during aging at intermediate temperatures is limited (≈︁ 15%). The x-phase particles arrange in two to three directions and look like thin elongated lentiform plates coherent with the parent β-phase matrix. These particles are inherited partly by the martensite formed near room temperature.