THERMOMECHANICAL ASPECTS OF NITI

The uniaxial behavior of a nearly equiatomic NiTi alloy is studied exp erimentally. Experiments are conducted in a temperature and deformatio n regime in which the alloy exhibits the shape memory effect and pseud oelasticity. These characteristics are due to the displacive nature of transformation between the two major phases of the material, austenit e and martensite, and to the fact that in this alloy martensite accomm odates deformation by twinning. A series of uniaxial experiments is co nducted on NiTi wire at temperatures in the range of approximately -20 to 100 degrees C where the fundamental material response changes dras tically. In addition, the loading rate and the choice of ambient mediu m were found to have a significant influence on the recorded stress-st rain responses due to a complex interaction between the inherent mecha nical properties of the material and the prevailing heat transfer cond itions of the experiment. It is demonstrated that local measurements o f strain and temperature can help clarify events that take place at di fferent stages of a typical loading-unloading history. These local mea surements are used to track the movement of the interfaces between pha ses during stress-induced phase transformations at different loading r ates.