Stress-induced phase transformations studied by in-situ transmission electron microscopy

Abstract

In this work, we carry out a detailed study, by in-situ Transmission Electron Microscopy (in-situ TEM), focused on two single-crystals of Cu-Al-Ni shape memory alloys with different transformation temperatures. The first single crystal is in beta phase at Room Temperature (RT) and has been cycled under stress, by super-elastic effect, inside the TEM. Two different mechanisms for the nucleation of β'3 and γ'3 martensite phases were observed: a) Martensite can nucleate on dislocations during super-elastic tests and when withdrawing the stress, the reverse transformation takes place by the disappearance of the martensite plate on the dislocation. b) During mechanical cycling martensite plates nucleate in other plates. The second single crystal is in martensite phase at RT, and when the stress is applied different mechanisms are observed: a) Reorientation and interface motion of the plates under the external applied stress, b) nucleation of mobile dislocations inside the martensite. A quantitative analysis of the experimental results, having into account the images and the diffraction patterns, has been realized and different mechanisms have been proposed to explain the experimental results.