STRESS-INDUCED MARTENSITIC-TRANSFORMATION OF A NITI ALLOY IN ISOTHERMAL SHEAR, TENSION AND COMPRESSION

The thermomechanical behaviour of stress-induced martensitic transform ation in an equiatomic NiTi alloy was investigated with respect to dif ferent deformation modes including uniaxial tension, compression and s hear of plate specimens at different temperatures above the M-s temper ature. Results showed that loading conditions have significant influen ces on the deformation behaviour of the alloy. In particular, deformat ion behaviour was observed to be asymmetric in tension and in compress ion. The physical origins of such an asymmetry are explored. Compariso n among the results obtained from the tension, compression and shear t ests disproves the use of classical Von Mises equivalent in the modell ing of the three-dimensional behaviour of martensitic transformation i n this alloy. Based on this analysis, another criterion for yielding, which involves the third stress invariant, is therefore suggested. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All ri ghts reserved.