纳米尺寸NiTi多晶微带中马氏体相变的原位透射电子显微镜研究

In situ TEM studies of the stress-induced martensitic transformation in nanometric NiTi polycrystal strips

纳米尺寸NiTi合金中的马氏体相变行为决定了其在微纳尺度的应用。利用透射电镜(TEM)对拉伸变形NiTi微带中的马氏体相变行为进行了原位研究。发现当应变达到0.9%时,马氏体首先在取向择优的晶粒内形核、长大和扩展。继续拉伸样品至断裂(应变为5.2%),相邻取向不择优的晶粒内没有观察到马氏体形核。由于NiTi微带包含不容易发生马氏体相变的晶粒,且发生相变的晶粒内应力集中,导致其断裂应变远小于块体样品的断裂应变。实验结果说明当将NiTi合金应用于微纳尺度时需充分考虑纳米尺寸材料中的马氏体相变行为及其力学性能的影响。

Martensitic transformation in nanosized NiTi alloys determines its application in micro- and nanoscale. The martensitic transformation behaviors in nanometric NiTi alloys were in situ studied in TEM. The results show that the martensitic plates first nucleate,grow and propagate in grains with preferred orientation when the strain reaches 0. 9%. However,no martensitic plate nucleates in grain with none-preferred orientation. Due to the none-transforming grain as a component of the strip and the stress localization in the transforming grains,the fracture strain of NiTi strip is much smaller than that of the bulk materials. The results indicate that when the NiTi alloys is applicated in micro- and nano-scale,the martensitic transformation behaviors in nanosized NiTi and its influence on the mechanical properties should be thoroughly considered.