Solidification process control of intermetallic-based multi-phase heat resistant alloys applicable at ultra-high temperature

耐热材料的设计涉及到以金属间化合物为主要成分的两相或多相合金的微观结构控制。需要较高的工作温度时，这类化合物具有复杂有序的晶体结构，因此熔点高。由于这种金属间化合物本质上是脆性的，因此微观结构控制的策略是引入一定量的延性相，并通过例如恒定反应来设计混合物的形态。然而，在开发性能优于工业镍基高温合金的新型耐热材料的过程中，凝固过程中经常出现宏观空洞和微裂纹等凝固缺陷。它应该是由脆性金属间相（通常以初级固体形式出现）与通过不变反应形成的韧性相之间的热膨胀系数差异引起的。为了获得良好的铸锭，有必要通过控制凝固过程来避免这些缺陷，从而准确地评价铸锭的力学性能，从而设计适当的组织控制。本文研究了Mo、Ta、Nb等难熔金属元素以MoSi_2等硅化物为主要成分，每种元素与硅均有一定量的延展性初生固溶体的凝固过程参数。并与电弧熔炼法和光学浮区法进行了比较。此外，还尝试设计一种以Fe_3AlC为主要成分的新型铁基合金。结果表明，通过选择合适的凝固速率，可以制备出具有良好的高温强度和较低塑性的定向两相混合物

Design of heat resisting materials has involved the microstructure control on two- or multi-phase alloys in which an intermetallic compound is the major constituents. Higher the operating temperature is desired, such compounds are favored as having a complex ordered crystal structure and therefore a high melting point. Since such a intermetallic compound is brittle in nature, strategy for the microstructure control has been to introduce some amount of ductile phase and to design the morphology of the mixture via for example an invariant reaction. However in the previous work on developing a new class of heat resisting materials which exhibit a superior performance than the commercial nickel-base superalloys, solidification defects such as macro-voids and micro-cracks have been frequently resulted during solidification. It should be caused by the difference in thermal expansion coefficient between a brittle intermetallic metallic phase, which is most often appears as a primary solid pha … More se from a liquid, and a ductile phase formed through an invariant reaction. It is necessary to avoid those defects by controlling the solidification process in order to obtain a sound ingot so that the evaluation of mechanical properties should be accurate to design a proper microstructure control.In the present work, the solidification process parameters have been investigated in alloys based on such refractory metal elements as Mo, Ta, and Nb in which their silicides such as MoSi_2 are the major constituent with some amount of ductile primary solid solution of each element with silicon. Comparison has been made with the processes of arc melting and optical floating zone (OFZ) methods. Also the design of a new class of iron-base alloys in which Fe_3AlC is a major constituent has been attempted. It has been shown that by proper choice of solidification rate upon OFZ method, a sound ingot can be fabricated with aligned two-phase mixture to exhibit an excellent combination of high temperature strength and some ductility Less