Date of Award
Doctor of Philosophy
The strain-temperature behavior of Ni-Ti samples upon martensitic phase transformation was investigated as a function of applied uniaxial tensile and compressive stresses. The samples were fabricated from stock bars of powder metallurgical origin;The samples were cycled in temperature when they were held in grips, which permitted a deadweight axial stress to be applied. The resulting strain-temperature hysteresis loops were analyzed for the dependence of their shape on the type and magnitude of the applied stress;It was found in the absence of an applied stress that the strains upon transformation were nearly isotropic. By contrast, as the uniaxial tensile stress was increased, the axial and circumferential 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. Similar results were obtained for uniaxial compressive stresses, except that the axial strain corresponded to a contraction and the circumferential strain to an expansion upon transformation on cooling;The observation that the transformation strain magnitudes increase with increasing applied stress magnitudes is explained on the basis of a preferred activation model (PAM). The PAM incorporates the concept of applied stress induced preferentially activated martensite variants. The phenomenological theory of Wechsler-Lieberman and Read (WLR) is applied to the case of martensitic phase transformation in Ni-Ti, and the semi-axes of the total distortion ellipsoid are calculated. In conjuction with the PAM, the calculated total distortions predict the saturation limit of phase transformation strains with increasing applied uniaxial tensile stress.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Ramon, Peter, "The transformation strain dependence on applied stress in equiatomic nickel-titanium alloys of powder metallurgical origin " (1983). Retrospective Theses and Dissertations. 7687.