IN-SITU REFRACTORY INTERMETALLIC-BASED COMPOSITES

With the ultimate objective of exploiting refractory intermetallics fo r high-temperature structural materials, several binary and ternary tw o-phase intermetallics/refractory-metal solid solutions were explored. The ductile solid solution is used to toughen the composite microstru cture via in-situ phase separation. While the viability of ductile pha se separation in solid state was briefly considered for systems such a s Nb3Al/Nb, much of the work focused on processing eutectic systems su ch as Cr2Nb/Nb and (Nb,Mo)(5)Si-3/Nb,Mo). This paper describes results obtained via containerless directional solidification of these high-m elting eutectic alloys using an optical neat-zone furnace. The observa tions are explained on the basis of solidification theory and paramete rs unique to the optical neat-zone furnace. It is demonstrated that, b y this technique, casting-defect- and macrosegregation-free material, with well-aligned microstructure, can be readily produced. Moreover, t he potential to approach sub-micron laminate spacing at high growth ra te in alloys with very high melting eutectics has also been establishe d. Room-temperature bend test evaluation of directionally solidified m aterial is discussed in light of prevailing theories of ductile phase toughening. The results of a preliminary exploration of the Nb-Mo-Cr-S i-Al multicomponent system are presented, showing the prevalence of eu tectic phase separation and the potential for improving oxidation resi stance.