NiTi形状记忆合金相变过程的正电子湮没研究

采用正电子湮没技术研究了NiTi合金在降温和升温过程中缺陷和电子密度随温度的变化.在降温过程中,当温度从295K降至225K时,合金的电子密度nb随温度的降低而下降,在225K时降至最小;随后,nb随温度的降低而升高.当温度从295K降至221K时,合金缺陷的开空间随温度的下降而升高,在221K时达到最大值;随后,缺陷的开空间随温度的下降而下降.在升温过程中,当温度从25K升至253K时,合金的电子密度nb随温度的升高而降低,并在253K时达到最小;当温度从253K升至295K时,合金电子密度nb随温度的升高而升高.在相变临界温度点Ms=222K,Mf=197.2K,As=237.5K,Af=255.5K附近,合金的电子密度nb及合金缺陷的开空间均有异常的变化.

PHASE TRANSFORMATION IN NiTi SHAPE MEMORY ALLOYS STUDIED BY POSITRON ANNIHILATION

The variation of defects and electron density in NiTi shape memory alloy in the processes of temperatures going up and down has been studied by positron annihilation techniques. As the temperatures decreasing from 295K to 225K in the temperatures going down process, the electron density n_b of the alloy decreases and reaches a minimum value at 225K, and then increases with the decrease of temperatures. As the temperatures decreasing from 295K to 221K, the open volume of defect in the alloy increases with the decrease of temperatures and reaches a maximum value at 221K, and then decreases with the decrease of temperature. As the temperatures rising up from 25K to 253K in the temperatures going up process, n_b decreases and reaches a minimum value at 253K, and then increases with the increase of temperatures. The open volume of defect and the electron density n_b of the alloy exhibit enormous changes at the critical points of phase transformation temperatures: M_s=222K, M_f=197.2K, A_s=237.5K, A_f=255.5K.