Restoration by rapid overheating of thermally stabilised martensite of NiTi shape memory alloys

The effects of thermal cycling on the transformation behaviour of NiTi shap e memory alloys have been well studied. These effects have been attributed to changes in defect structure caused by the transformation cycling. Wherea s this hypothesis appears successful in explaining some experimental observ ations, it fails with others, such as the reduction in transformation heats caused by cycling. This study aims at investigating the mechanisms of the effects of thermal cycling by thermal analysis. It was found that thermal c ycling through transformation temperature range resulted in the decrease of both transformation temperatures and transformation heats. Rapid overheati ng after the cycling was found effective in partially restoring the origina l values of both parameters. This partial restoration of transformation tem peratures and transformation heats is ascribed to the reversal of stabilise d martensite caused by thermal cycling. The remaining changes to the proper ties are attributed to the effect of structural defects introduced during t hermal cycling. (C) 2000 Elsevier Science S.A. All rights reserved.