Strength differential effect in pseudoelastic NiTi shape memory alloys

Pseudoelastic shape memory alloys of NiTi-type can be reversibly deformed up to maximum strains of 8%. In strain-controlled tension/compression testing of pseudoelastic NiTi shape memory wires, compression recovery forces were found to be markedly higher than tension forces. An explanation for this ''strength differential effect'' is proposed: The analysis of the generation process of stress-induced martensite (SIM) variants in a single crystal reveals that for a given load axis, the largest possible transformation strains in tension loading can be more than twice as high as respective compression strains. Thus, the formation of martensite in tension is facilitated, and high force levels are generated in compression loading. Despite this asymmetric behaviour of transformational SIM strains being most pronounced near [111](B2) load directions, even for untextured polycrystalline samples the averaged SIM strains in tension exceed compression strains by 46%. Consequently, the strength differential effect must not only be considered in single crystals, but also in commercial shape memory materials and applications. (C) 1997 Acta Metallurgica Inc.