Mechanism of training effect in the ferrous shape memory alloys associated with gamma->epsilon martensitic transformation.

研究了Fe-Mn-Si合金中的γ-> γ-正相变和γ-> γ-逆相变以及循环相变。主要调查结果如下。（1）在Ms高于室温的Fe-24%Mn-6%Si合金中，在室温至573 K（高于Af）的热循环过程中，观察到相变温度和显微组织的变化。与Fe-24%Mn的情况不同，Fe-Mn-Si合金的Ms几乎不受热循环的影响。此外，Fe-Mn-Si合金还具有良好的γ马氏体片的再现性，微结构存储器此外，在热循环后的奥氏体中几乎没有观察到位错等晶格缺陷。这些结果有力地表明，在Fe-Si-Mn合金的γ-> γ-相变过程中，相变位错的运动是可逆的。（2）对室温拉伸变形的Fe合金中的应力诱发马氏体进行了显微组织观察。 ...更多信息 -33%Mn-6%Si合金的Ms值略低于室温。透射电镜观察证实了位错板是由一种部分位错作用形成的。在未转变的奥氏体中观察到大量的层错。这些层错可以作为形核点，在随后的正向转变后，退火Af以上。应力诱发马氏体的逆转开始在较低的温度和完成在较高的温度相比，热诱发马氏体。（3）Fe-33%Mn-6%Si合金在77 ~ 873 K热循环条件下，形状回复率发生变化。热循环不能改善形状回复，表明γ-> γ-循环转变本身与训练处理改善形状回复无关，由此推断，训练效应的作用是在奥氏体中引入晶格缺陷，这些缺陷在随后的转变中充当形核位置。少

gamma->epsilon forward and epsilon->gamma reverse transformations and cyclic transformation in Fe-Mn-Si alloys have been investigated. The main findings are as follows.(1)The change in transformation temperature and microstructure by thermal cycling between room temperature and 573K(above Af)was observed in an Fe-24%Mn-6%Si alloy of which Ms was higher than room temperature. Unlike the case of Fe-24%Mn, Ms hardly changed by the thermal cycling in the Fe-Mn-Si alloy. Moreover, the Fe-Mn-Si alloy showed the good reproducebility of gamma martensite plates, i e., microstructure memory. Furthermore, lattice defects such as dislocations were scarcely observed in the austenite after the thermal cycling. These results strongly suggest that the movement of transformation dislocations is reversible during gamma->epsilon transformations in the Fe-Si-Mn alloy.(2)Microstructural observation of stress-induced epsilon martensite formed by tensile deformation at room temperature was performed in an Fe … More -33%Mn-6%Si alloy of which Ms was just below room temperature. It was confirmed by TEM observation that an epsilon plate formed by the operation of one kind of partial dislocations. A large amount of stacking faults were observed in the untransformed austenite. These stacking faults may act as nucleation sites in the following forward transformation after the annealing above Af. The reversion of stress-induced martensite started at a lower temperature and finished at a higher temperature compared with that of thermally-induced martensite.(3)The change in shape recovery by the addition of thermal cycling between 77K and 873K to the Fe-33%Mn-6%Si alloy was observed. The shape recovery was not improved by the thermal cycling, indicating that the gamma->epsilon cyclic transformation per se was not associated with the improvement of shape recovery by the training treatment.It was concluded from the above results that the role of the training effect is to introduce lattice defects into austenite which act as the nucleation sites in the following transformation. Less