In-situ refractory intermetallic-based composites

With the ultimate objective of exploiting refractory intermetallics for high-temperature structural materials, several binary and ternary two-phase intermetallics/refractory-metal solid solutions were explored. The ductile solid solution is used to toughen the composite microstructure via in-situ phase separation. While the viability of ductile phase separation in solid state was briefly considered for systems such as Nb3Al/Nb, much of the work focused on processing eutectic systems such as Cr2Nb/Nb and (Nb,Mo)5Si3/Nb,Mo). This paper describes results obtained via containerless directional solidification of these high-melting eutectic alloys using an optical float-zone furnace. The observations are explained on the basis of solidification theory and parameters unique to the optical float-zone furnace. It is demonstrated that, by this technique, casting-defect- and macrosegregation-free material, with well-aligned microstructure, can be readily produced. Moreover, the potential to approach sub-micron laminate spacing at high growth rate in alloys with very high melting eutectics has also been established. Room-temperature bend test evaluation of directionally solidified material is discussed in light of prevailing theories of ductile phase toughening. The results of a preliminary exploration of the NbMoCrSiAl multicomponent system are presented, showing the prevalence of eutectic phase separation and the potential for improving oxidation resistance.