STRUCTURE CONTROLL AND IMPROVEMENT OF HIGH TEMPERATURE SHAPE MEMORY PROPERYIES OF TiPdNi ALLOY WITH NEW PRECIPITATE PHASE

由于Ti-Pd-Ni合金的马氏体相变温度可以通过Pd和Ni的替代比从室温控制到800 K左右，因此该合金在各个领域的应用前景广阔。为改善材料性能，对富Ti、富Pd、富Ni合金进行了第二相析出控制的尝试，发现Ti_<45>Pd_<45>Ni_<10>合金在淬火过程中出现了特殊的相变。在DSC测量中加热时，在373 K附近首先发生吸热反应，这对应于从B19到B2结构的逆马氏体转变。随后在550 ~ 740 K观察到一对放热峰和吸热峰，这是由于具有长周期堆积结构的新有序相的出现和消失。新相的电子衍射图的特征之一是在111_ * 反射行的1/8位置上出现超晶格斑点<B2>。最后，在850 K附近发现了一个对应于Ti_2（PdNi）_3相形成的放热反应。发现了更有趣的现象，即500 K以下低温时效时母相稳定化。在稳定的B2相的电子衍射图中观察到强度相当强的ω弥散条纹。证实了弥散条纹是有序化的一种前兆现象。本文还讨论了新有序相的形态学和晶体学方面的问题。利用这种新的有序相可以产生可逆的形状记忆效应。但其形状变化量小于含有Ti_3Mi_4相的富Ni Ti-Ni合金。

Since martensitic transformation temperature of Ti-Pd-Ni alloys can be controlled from ambient temperature to around 800 K by substitutional ratio of Pd and Ni, the application of me alloys have been expected in various fields. To improve the materials properties the microstructure control with precipitation of secondary phase has been attempted in Ti-rich and Pd and Ni-rich alloys.The authors have found out that quenched Ti_<45>Pd_<45>Ni_<10> alloy shows peculiar phase transformations. Upon heating in DSC measurement, an endothermic reaction occurs firstly around 373 K, which corresponds to the reverse martensitic transformation from B19 to B2 structure. Subsequently, a pair of exothermic and endothermic peaks is observed from 550 to 740 K, which is due to the appearance and disappearance of new order phase with long period stacking structure. One of characteristic feature in electron diffraction pattern of the new phase is superlattice spots at 1/8 position in the row of reflections of 111_<B2>*. Finally, an exothermic reaction which corresponds to the formation of Ti_2(PdNi)_3 phase is detected around 850 K. More interesting phenomenon, which is the parent phase stabilization with low temperature aging below 500 K, is discovered. The ω diffuse streak with rather strong intensity is observed in electron diffraction patterns taken from the stabilized B2 phase. It is confirmed that the diffuse streak is a kind of precursor phenomena of the ordering. Morphological and crystallographic aspects of the new order phase are also discussed. The reversible shape memory effect can be generated by using the new order phase. However, the amount of shape change is smaller than that of Ni-rich Ti-Ni alloy with Ti_3Mi_4 precipitates.