| Summary: | 碩士 === 國立成功大學 === 機械工程學系碩博士班 === 101 === We employed molecular dynamics simulation to investigate the shape memory properties of Ni-Al alloy bulks. The effects of Ni composition ratio and the simulated crystal orientation on phase transformation were studied. At low temperature, different loading conditions were applied to the alloy bulk till plastic deformation is observed. Then, the deformed bulk went through the temperature cycle to examine whether it would restore to the original shape.
From the simulation, we found that Ni-Al is body-centered cubic structure at high temperature and transforms to martensite phase at certain temperature, except for the one with 50% Ni composition ratio. The phase transformation temperature was affected by the Ni composition ratio. We proposed certain analysis method to efficiently determine the phase transformation temperature. It was observed that different simulated crystal orientated models, which possess the same Ni composition ratio and atomic arrangement in space, would have the same phase transformation temperature. And the deformations of different crystal orientated models at various temperatures could be described using coordinate transformation relation.
For the same model under different loading conditions, it was found that not every plastic deformed model would restore to its original shape after the thermal cycle. With the assist of slip vector, only those were loaded to slip along 〈110〉 direction would exhibit shape memory behavior. For those loading which would restore original shape, the corresponding maximum shear stress was noticeably smaller than those did not restore.
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