Martensitic phase transformations of NiTi nanocrystals

Nanocrystalline martensitic materials can show the shape memory effect and superelasticity in combination with exceptional strength. The in-depth knowledge of the mechanisms of martensitic phase transformations of these materials is a prerequisite for controlling their functional properties. Direct structural observations on the nanoscale are expected to give substantial insight in these complex physical mechanisms.

This talk reports on in-situ experiments carried out in the transmission electron microscope to study the stress induced phase transformation of individual NiTi nanocrystals. The in-situ straining was applied to thin ribbons of NiTi that contain isolated nanocrystals embedded in an amorphous matrix. During loading, in the nanocrystals (~ about 50 nm in diameter) martensitic lamellae nucleate that are only a few nm in thickness. With increasing stress, the lamellae grow in number and thickness while they shrink and finally disappear in reverse order upon unloading. Besides showing a hysteresis, the transformation is perfectly reversible since during several loading-unloading cycles, the lamellae always nucleate in the same order and at the very same positions within the nanocrystals.