Nonelastic behavior of Ni₅₀Ti_50-xZr_x alloys during B₂ ⇆ B19′ martensite transformation

Abstract

The results of an investigation of martensite transformations (MT), the shape memory effect (SME), and their interaction in Ni₅₀Ti_50-xZr_x alloys (1≤x≤50 at. %) are presented. The temperature dependencies of electroresistance and the magnitudes of deformation accumulation and recovery in a torsion mode under conditions of a continuously applied external load were investigated. Structural testing of the alloys was carried out by means of x-ray diffractometry methods. It was revealed that during “cooling—heating” cycling in alloys containing not more than 15 at. % of Zr, B2 ⇆ B19′ MT occurs. In alloys with 30 and 50 (binary NiZr) at. % Ar, under these conditions phase transformations are not observed. With increase in the concentration of the alloying element an increase in the critical temperatures of the MT was observed, the principal shape of the_ρ(T) curve changed, and the sequence of the B2 ⇆ B19′ transition was preserved. The SME in Ni₅₀Ti_50-xZr_x alloys was conditioned by the B2 ⇆ B19′ MT, had a single-stage character, and appeared when the Zr content in the alloy did not exceed 15 at. %. The maximum level of accumulation of inelastic deformation γ_max did not depend on the quantity of the alloying element and was 10–12%.