The transformation strain dependence on applied stress in equiatomic nickel-titanium alloys of powder metallurgical origin

Abstract

The strain-temperature behavior of Ni-Ti samples upon martensitic phase transformation was investigated as a function of applied uniaxial tensile and compressive stresses. Axial and transverse strains were measured. The samples were fabricated from stock bars of powder metallurgical origin. They were machined into tubes or solid bars with square cross sections. The tubular samples were cycled in temperature by circulating pre­ heated or precooled silicone oil through them. At the same time, they were held in grips, which permitted a deadweight axial stress to be applied. Rosette-type strain gages were used to measure axial, circum­ ferential, and 45°-inclined strains continuously as a function of temper­ ature. The strain-temperature hysteresis loops were analyzed for the dependence of their shape (including magnitude and direction of trans­ formation strain) on the type (tensile or compressive) and magnitude of the applied stress. Furthermore, the difference in behavior for Ni-Ti in the as-fabricated and annealed conditions was studied. In the absence of an applied stress, the strains upon transformation were nearly isotropic, and a net expansion took place upon transformation on cooling. By con­ trast, as the uniaxial tensile stress was increased, the axial and cir­ cumferential strains became increasingly different, i.e., the axial strain corresponded to an expansion upon transformation on cooling and the circumferential strain to a contraction. The magnitudes of these strains increased progressively with increasing stress, with the axial