THERMODYNAMIC ANALYSIS OF THE MARTENSITIC-TRANSFORMATION IN NITI .1. EFFECT OF HEAT-TREATMENT ON TRANSFORMATION BEHAVIOR

The effect of heat treatment on the martensitic transformation behavio ur of NiTi has been investigated. Transformation temperature measureme nts are interpreted in terms of the chemical, elastic and irreversible components of the overall transformation free energy change. It is sh own that the occurrence of a two-stage parent-R-martensite transformat ion on cooling and a single-stage reverse transformation in specimens annealed at intermediate temperatures is a consequence of the irrevers ible energy loss associated with the martensitic transformation. The i ncrease in the R transformation temperature in specimens heat treated at low temperature is shown to result from changes to the reversible, elastic or chemical, components of the transformation free energy chan ge.