Abstract
The microstructural and thermodynamic parameters fundamental to exploitation for heat engine purposes of the shape memory phenomenon exhibited by elastic martensites have been examined. Microstructurally, perfect shape memory behaviour demands the total absence of dislocations and this means that 1. (a) there should be no creation of interfacial dislocations between parent and martensite, and 2. (b) dislocation mechanisms should not contribute to the shape change. It is convenient to state requirement (a) in terms of a geometrical analysis in shape change space. The Carnot efficiency is a function of the dependence of transformation temperature on applied stress and, in principal, a Clapeyron-type relationship may be used to evaluate this dependence. However, for sound experimental reasons, there exists no reproducible data for the entropy changes associated with elastic martensitic transformations. Consequently, it is necessary to resort to direct measurement of the slope of the transformation temperature vs applied stress relationship in order to calculate the Carnot efficiency. Efficiencies of ~ 20% are predicted and measurements using model engines demonstrate that more than half of this can be obtained as useful work.
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