THE 2 WAY MEMORY EFFECT IN COPPER-BASED SHAPE MEMORY ALLOYS - THERMODYNAMICS AND MECHANISMS

The mechanisms, which are according to literature underlying the two way memory effect (abbr. to TWME), made it widely accepted that the TWME is an unstable effect, i.e. it cannot do work during the forward martensitic transformation, and that it is, moreover, a small effect compared to the one way memory effect. Those and other hypothetical characteristics of the TWME are disproved in the present study. Important opposing stresses during the forward transformation can be resisted by well-trained specimens, i.e. the TWME can do work during cooling. Moreover, the transformation temperatures are shifted to lower temperatures when external stresses opposing the TWME are applied. Those new characteristics require a TWME mechanism in which the formation of the trained martensitic variants is thermodynamically favoured with regard to other crystallographically equivalent variants. This thermodynamic favouring is ascribed to the variant dependent energy of the dislocation arrays which are generated by transformation-retransformation cycling. Quantitative data on the relative thermodynamic stability of the parent phase, the preferred variants and the non-preferred variants are deduced from experimental observations. Experimental observations on the TWME available in literature are reinterpreted and explained in terms of the present study.