Direct observation of the NiTi martensitic phase transformation in nanoscale volumes

We report on quantitative in situ transmission electron microscopy nanocompression tests used to study the deformation behavior of NiTi pillars on the nanometer scale. By recording the diffraction patterns in real time we have obtained direct evidence that the stress-induced B2 to B19′ (austenite to martensite) transformation exists in NiTi even when the sample size is below 200 nm. Correlation of the appearance of the B19′ phase in the diffraction pattern with our quantitative data showed that the transformation starts at approximately 1 GPa. We found that the transformation occurred through a multi-step process, and that the reverse transformation did not occur due to extensive deformation of the B19′ phase. Our results have direct implications for the application of the shape memory effect in nanoscale NiTi devices.