Acoustic emission study of martensitic transition kinetics in Cu-based shape-memory alloys F. J. Pérez-Reche, E. Vives, LI. Manosa and A. Planes Departament d'Estructura i Constituents de la Matèria, Facultat de Fisica, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain Abstract : In this paper we study the acoustic emission generated during the martensitic transition of Cu-based shape-memory alloys. We analyze the evolution of the acoustic emission activity on thermal cycling through the transition. A correlation function between the acoustic activity of consecutive cycles is proposed, which gives a quantitative estimation of the reproducibility of the transformation. The results enable the evolution of the kinetic (athermal/isothermal) character of the martensitic transition to be assessed in this alloy. 1. INTRODUCTION Martensitic transitions in shape-memory alloys have been traditionally regarded as athermal. This means that the transition is not dominated by thermal fluctuations, and, consequently, it can only occur under conditions of temperature change or external stress. In these systems the transition path is strongly influenced by the existence of disorder (dislocations, grain boundaries, vacancies, internai stresses, local composition fluctuations, etc...) which determine an energy barrier distribution that separate metastable states. The kinetics is characterized by jumps from one metastable state to another. This gives rise to sudden changes in the local strain field which are at the origin of the acoustic emission (AE) generated during the transition. This purely athermal picture has been recently questioned by a number of experimental observa- tions which show that the martensitic transition undergone by some shape-memory alloys displays a certain isothermal character [1, 21. In particular, it has been reported that after an incubation time at a given temperature above the nominal ilS8 temperature, nucleation takes place spontaneously. On the other hand, the possible evolution of disorder from cycle to cycle could result in changes of the transformation features. The present paper is aimed at establishing to what extent the martensitic transition in Cu-based alloys must be classified as purely athermal. In addition, for Cu-Zn-Al, for which the martensitic transition shows no appreciable isothennal component, the reproducibility of the transformation from cycle to cycle is also studied. 2. EXPERIMENTAL We have detected the AE generated during the martensitic transition in a (Su68ho Zn] 6. 0 sin- gle crystal which transforms from the B phase (ordered-bee) towards an 18R martensite and in a CU64. sAI27, 8Ni3, à which transforms to an hep martensite.