In situ transmission electron microscopy-investigations on the strain-induced B19′-phase in NiTi shape memory alloys structured by focused ion beam

In the present study, we investigate the deformation of a pseudo elastic NiTi shape memory alloy, micro structured with the focused ion beam technique, by in situ transmission electron microscopy (TEM). A polycrystalline Ni-rich NiTi shape memory alloy with a nominal composition of 50.7 at.% Ni was solution heat treated at 850 °C for 15 min. Subsequent water quenching was carried out to stabilize the B2-phase at ambient temperatures. Dog bone shaped TEM tensile specimens were prepared by spark-erosion and ground to a thickness of 120 μm followed by electropolishing in order to obtain thin regions which are transparent for the electron beam. Then, a rectangular hole was sputtered into the specimen using the focused ion beam technique. In situ TEM straining was carried out at ambient temperatures and the stress induced formation of martensite was observed near the rectangular hole. On straining, the sample shows a martensitic transformation from B2 (austenite) to B19′ (martensite). Details of these in situ observations are reported.