THERMOMECHANICAL ASPECTS OF NITI

The uniaxial behavior of a nearly equiatomic NiTi alloy is studied experimentally. Experiments are conducted in a temperature and deformation regime in which the alloy exhibits the shape memory effect and pseudoelasticity. These characteristics are due to the displacive nature of transformation between the two major phases of the material, austenite and martensite, and to the fact that in this alloy martensite accommodates deformation by twinning. A series of uniaxial experiments is conducted on NiTi wire at temperatures in the range of approximately -20 to 100 degrees C where the fundamental material response changes drastically. In addition, the loading rate and the choice of ambient medium were found to have a significant influence on the recorded stress-strain responses due to a complex interaction between the inherent mechanical properties of the material and the prevailing heat transfer conditions of the experiment. It is demonstrated that local measurements of strain and temperature can help clarify events that take place at different stages of a typical loading-unloading history. These local measurements are used to track the movement of the interfaces between phases during stress-induced phase transformations at different loading rates.