ASYMMETRY OF STRESS-STRAIN CURVES UNDER TENSION AND COMPRESSION FOR NITI SHAPE-MEMORY ALLOYS

The stress-strain curves of polycrystalline martensitic NiTi shape mem ory alloys are often different for loading under tension and compressi on. Under tension, a flat stress-plateau occurs, while under compressi on, the material is quickly strain hardened and no flat stress-plateau is observed. Cyclic deformation under tension-compression also shows that it is more difficult to deform the material during compression th an during tension, where an asymmetric stress-strain loop is obtained. TEM observations show that, under tension to 4% strain, martensite va riants are partially reoriented via migration of variant interfaces wi th formation of dislocation networks mainly along the junction plane a reas, and no significantly plastic deformation has been observed insid e the martensite twin bands. While under compression to 4% strain, a h igh density of dislocations has been generated in both the martensite twin bands and the variant accommodation area, and no significant mart ensite reorientation via variant interfacial migration has been observ ed. This shows that the deformation mechanism of martensitic polycryst alline NiTi SMAs under tension is different from that under compressio n. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.