Hysteresis and deformation mechanisms of transforming FeNiCoTi

We present an extensive set of experimental results on deformation behavior under external stress and thermal hysteresis behavior of FeNiCoTi shape memory alloys. The experiments spanned from temperatures where the martensite variant motion is responsible for shape changes to cases where stress-induced transformation from austenite to martensite occurred. A temperature hysteresis of the order of 130 degrees C and recoverable strains approaching 3.5% were observed in these materials. These recoverable strain magnitudes far exceed previously reported values on FeNiCoTi alloys. We identify the role of increased slip resistance via aging as partially responsible for increased reversibility and increase in transformation strains. The relaxation of the stored energy due to presence of dislocations at austenite-martensite interfaces was identified with transmission electron microscopy studies, and this is the primary mechanism for large thermal hysteresis as well as the limiting factor in reversibility of the transformation. The changes in transformation temperatures and the hysteresis behavior in the presence of coherent precipitates were rationalized based on a thermodynamics based mechanics model. (c) 2005 Elsevier Ltd. All rights reserved.