In situ transmission electron microscopy of stress-induced martensite with focus on martensite twinning

At the proper composition and after receiving the appropriate thermomechanical treatment, NiTi shape memory alloys show shape memory and superelastic behaviour. These effects are due to the martensitic transformation, and a proper understanding of the microstructural aspects of this transformation is essential to develop accurate models describing the macroscopical behaviour. In this work, transmission electron microscopy combined with in situ straining is used to investigate microstructural features of the stress-induced transformation. Two kinds of samples are investigated: a superelastic polycrystalline NiTi sheet and a single crystal of NiTi, both overstoichiometric in Ni. Regarding the superelastic material, the martensite plates induced by straining are reversible upon relaxing and do not seem to consist of more than one variant. In the single-crystal sample, the induced martensite plates are not reversible and contrasts possibly due to twinning are observed.