Acoustic Emission as a Probe of the Kinetics of the Martensitic Transformation in a Shape Memory Alloy

The kinetics of the martensitic transformation in a CuAlMn shape memory alloy (SMA) has been studied using the acoustic emission (AE) technique. It is demonstrated that the volume fraction of martensite as a function of time and temperature can be derived from the measured AE power. The fraction data obtained can be described by the Koistinen and Marburger (Acta Metall. 7 (1959) 59) equation with high accuracy, which indicates that the nucleation of martensite takes place heterogeneously and that the average volume of martensite crystals is constant over the extent of the transformation. The martensite-start temperature determined from the measured AE data is in good agreement with the value found by differential scanning calorimetry (DSC). Furthermore, the results of AE experiments on the SMA are compared with optical Confocal Laser Scanning Microscopy (CLSM) observations of the surface of the SMA. The observations show that both small and large martensite plates are formed both at the beginning and at the end of the transformation, which is in agreement with the assumption of a constant average volume of martensite crystals used in the Koistinen and Marburger model.