Generation, development and degradation of the intrinsic two-way shape memory effect in different alloy systems

Abstract

A uniaxial two-way shape memory effect is induced in wire specimens by a thermomechancial treatment. In order to enable a systematic variation of the microstructure with respect to dislocation density, second phase particles and grain size, the investigations are carried out on three different alloy systems. 4000 thermal cycles are performed on the trained shape memory elements, continuously observing the changes in the deformation behavior. The influence of work hardening, grain size and high internal stress-fields on the development and the stability of the intrinsic two-way shape memory effect is discussed, supported by microstructural investigations. The degradation due to cyclic application is related to instabilities in the high- and low-temperature modification. The decrease of the effect size is found to consist of two characteristic stages, where the decay is distinguished by different origin and rate. Definite conclusions about the degradation mechanisms are drawn by analyzing the influence of the preliminary heat treatment on the fatigue behavior. It is shown that the degradation can be minimized and hence the stability of the two-way shape memory effect can be optimized by establishing an appropriate microstructure.