微观尺度下晶粒尺寸和冷却速率对多晶NiTi合金相变温度的影响

Microscopic investigation to influence of grain size and cooling rate on phase transformation temperature for polycrystalline NiTi alloy

采用分子动力学方法研究了微观尺度下晶粒尺寸和冷却速率对NiTi合金相变温度的影响和相变微观机理。结果表明:当冷却速率为-5 K/ps,晶粒尺寸从17.5 nm减小到8.1 nm时,马氏体相变起始温度从230 K下降到80 K,马氏体相变形核点的数目逐渐减少且主要出现在晶粒内部,降温过程中马氏体相成核后向晶界处扩散生长,当晶粒尺寸减小至4.1 nm时,马氏体相变效应则受到抑制;而升温过程中,奥氏体相变形核点主要出现在晶界处,且随着温度的升高,形核点主要向晶粒内部聚集生长。当冷却速率从-5 K/ps增加到-15 K/ps,晶粒尺寸为17.5 nm的模型中马氏体相变结束温度从190 K减小到20 K。随着冷却速率的增加,马氏体晶粒细化程度相应增加,相变滞后宽度(Mf-Af)随着冷却速率的增加相应增大,但是冷却速率对马氏体相变生长机制影响相对较小。

The molecular dynamic simulation was conducted to analyze the influence of grain size and cooling rate to the phase transformation temperature, and the phase transformation mechanism for the alloy was also studied. The result shows that when the cooling rate is -5 K/ps, as the grain size decreases from 17.5 nm to 8.5 nm, the martensite formation temperature decrease from 230 K to 80 K, the number of nucleation point also decreases gradually and only nucleate inside the grain, the martensite grows towards the grain boundary in the process of cooling. When the grain size decreases to 4.1 nm, the transformation is suppressed. In the process of heating, the nucleation point of austenits nucleates near the grain boundary and grows towards inside of the grain. When the cooling rate increases from -5 K/ps to -15 K/ps, the finish transformation temperature of martensite decreases from 190 K to 20 K, the grain refinement increases and the transformation hysteresis width (Af-Mf) decreases with the increase in cooling rate in the model of 17.5 nm. While the effect of cooling rate on martensitic phase growth mechanism is relatively small.