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Atomistic Analysis of Stress-induced Local Amorphization in NiTi Alloy

In situ tensile straining experiments have been carried out on thin films of the ordered intermetallic compound NiTi (B2 structure) in a high-voltage electron microscope. Real-time observations of stress-induced local amorphization of moving crack tips were followed by high-resolution transmission electron microscopy examination of nanocrystalline regions formed near the crack tip and in the region between the amorphous and crystalline regions. We have also independently confirmed the occurrence of local amorphization by ultra-high speed deformation at an atomistic level in bulk samples of the same NiTi sample. A fast Fourier transformation (FFT) image analysis technique was used to determine the root mean square displacement (RMSD) parameter within the nanocrystalline regions. The RMSD parameter increases to a critical fraction of the nearest neighbor distance of 0.12±0.01 as the amorphous region is approached, and this finding agrees with the generalized Lindemann melting criterion for amorphization proposed by Okamoto and Lam [1,2]. The present study shows that FFT-real-space analysis technique can provide local measurements of the RMSD parameter in an inhomogeneous system.