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

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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