Characterization of phase evolution under load by means of phase contrast imaging using synchrotron radiation

In the last few years many studies have been done on the characterization of the phase evolution, during martensitic transformation of shape memory alloys (SMAs). From the experimental point of view this topic has many difficulties. Detailed information is needed on the very small modifications locally induced by the transforming interface and, at the same time, on the phase evolution in the sample as a whole. Microscopic techniques give detailed information on local interfaces, twinning and self- accommodation structures, but in order to develop a complete description of the link between the microscopic and macroscopic levels, proper techniques must be identified and verified which can describe the intermediate level. In this paper we report on the application of Phase Contrast Imaging (PCI) in the study of the phase evolution during pseudoelastic transformation in NiTiCu shape memory alloys. PCI is a quite novel technique, which gives image information linked to very small differences in the density of the material under analysis. The method has some clear advantages when compared with common microscopic tech- niques: (a) no special preparation of the sample is needed, (b) the investigated area is very large, (c) it allows the setting up of complex experimental apparatus. In order to perform stress-strain tests on NiTiCu samples, an "ad-hoc" experimental set-up was prepared. With this, control of different parameters and data acquisition during tests were ensured by dedicated soft- ware, developed in the LabVIEW language, while images of the sample were acquired.