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Abstract

NiTi shape memory alloys (SMAs) show fascinating functional properties. They can recover large deformations during heating (thermal shape memory) or during unloading after pre-straining (pseudoelasticity). Today, NiTi alloys are often used for actuators, medical implants and surgical instruments. The shape memory effect relies on a martensitic phase transformation. In the present work, we characterize the effects of ultra-fine austenite grain structures on the transformation behavior in Ni-rich NiTi SMAs. We show how nano-crystalline grain structures can be obtained through optimized thermomechanical processing involving wire drawing and different heat treatments. The resulting materials were characterized through transmission electron microscopy and thermal analysis. It was observed that a high density of austenite grain boundaries impedes twinning accommodation because they impose geometrical constraints. Therefore, higher driving forces are required for the nucleation of martensite. Nevertheless, nano-crystalline NiTi SMAs exhibit a very stable shape memory behaviour. Irreversible effects, which rely on dislocation plasticity, are strongly reduced through very fine grain structures. Therefore, nano-crystalline NiTi alloys show a better functional performance when a large number of cycles is required.