STRESS-INDUCED MARTENSITIC TRANSFORMATION OF A NiTi ALLOY IN ISOTHERMAL SHEAR, TENSION AND COMPRESSION L. ORGE  AS{ and D. FAVIER Laboratoire Sols, Solides, Structures, UMR CNRS 5521, Universite Joseph Fourier Grenoble I, Institut National Polytechnique de Grenoble, BP 53X, 38041 Grenoble Cedex 09, France (Received 22 September 1997; accepted 20 April 1998) AbstractÐThe thermomechanical behaviour of stress-induced martensitic transformation in an equiatomic NiTi alloy was investigated with respect to dierent deformation modes including uniaxial tension, com- pression and shear of plate specimens at dierent temperatures above the M s temperature. Results showed that loading conditions have signi®cant in¯uences on the deformation behaviour of the alloy. In particular, deformation behaviour was observed to be asymmetric in tension and in compression. The physical origins of such an asymmetry are explored. Comparison among the results obtained from the tension, compression and shear tests disproves the use of classical Von Mises equivalent in the modelling of the three-dimen- sional behaviour of martensitic transformation in this alloy. Based on this analysis, another criterion for yielding, which involves the third stress invariant, is therefore suggested. # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Re sume ÐLe comportement thermomeÂcanique de la transformation martensitique induite sous contrainte dans un alliage NiTi eÂquiatomique a eÂte eÂtudie aÁ dieÂrentes tempeÂratures supeÂrieures aÁ M s et sous dieÂr- ents chemins de sollicitation, aÁ savoir la traction, la compression et le cisaillement. Les reÂsultats ont montre que les conditions de chargement ont une in¯uence signi®cative sur la deÂformation du mateÂriau. En parti- culier, les comportements en traction et compression ne sont pas symeÂtriques: les raisons physiques d'une telle dissymeÂtrie sont discuteÂes. La comparaison des reÂsultats de traction, de compression et de cisaillement montre l'incapacite des eÂquivalents classiques de type Von Mises aÁ modeÂliser le comportement tri-dimen- sionnel de la transformation martensitique du preÂsent mateÂriau. A partir des reÂsultats preÂceÂdents, un autre criteÁre macroscopique faisant intervenir le troisieÁme invariant du tenseur des contraintes est alors proposeÂ. # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. 1. INTRODUCTION Shape memory alloys (SMAs) have attracted exten- sive interest in scienti®c research in the past several decades, owing to their novel properties. Among these properties, superelasticity, shape memory eect and two-way memory eect are known to be closely related to thermoelastic martensitic trans- formation, a ®rst-order displacive thermoelastic solid±solid phase transformation [1]. Owing to a vast variety of experimental evidence, signi®cant progress has been made in the characterization and the understanding of these behaviours. Nevertheless, there still remains a lack of knowl- edge. As a matter of fact, most experimental studies of the thermomechanical behaviour of SMAs to date have been conducted in uniaxial tension, with few being carried out in other homogeneous stress state conditions. It has been demonstrated in the litera- ture with these studies that the thermomechanical behaviour of a specimen is sensitive to the defor- mation mode. Signi®cant dierences have been observed between tension and compression for both NiTi [2, 3] and CuZnAl [4] alloys. Furthermore, Manach and Favier [5] have demonstrated that the Von Mises assumptions, that are usually adopted in establishing tensorial constitutive equations, are not always valid in simple shear tests for NiTi alloys. This implies that conclusions derived from tensile deformation tests cannot be directly translated to other deformation modes and that independent ex- perimental investigations of deformation modes other than tension are needed for a full characteriz- ation of the thermomechanical behaviour of NiTi. To date, however, no directly comparable studies have been carried out to investigate the eects of deformation modes, such as tension, compression and shear, using the same SMA to describe the behaviour of stress-induced martensitic transform- ations. Such experimental evidence is essential both for the understanding of the phenomena of shape memory eects and for the establishment of three- dimensional constitutive equations [6±10]. For this reason, this study was carried out aiming at investi- gating the eects of deformation modes, including Acta mater. Vol. 46, No. 15, pp. 5579±5591, 1998 # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 1359-6454/98 $19.00 + 0.00 PII: S1359-6454(98)00167-0 {To whom all correspondence should be addressed. 5579