Microstructure and Strength of NbCr<SUB>2</SUB>/Cr <I>in-situ</I> Composites

Abstract

To assess the potential of intermetallic-matrix composites as high temperature structural materials, the microstructures of the cast-annealed, the hand milled-HIPed and the ball milled-HIPed NbCr2/Cr in-situ composites were observed by optical, scanning and transmission electron microscopy. The as-cast Cr-24.5 mol%Nb is composed of primary NbCr2 Laves plates with C15 structure and lamellar eutectics consisting of NbCr2 and a Cr solid solution (Crss). The NbCr2 phase has a lot of planar faults and Crss contains a high density of dislocations. Fine NbCr2 precipitates form in Crss of the cast and annealed Cr-24.5 mol%Nb, although planar faults remain unchanged in the NbCr2 phase. Microstructural observation indicates that the eutectic composition exists between Cr-17.2 mol%Nb and Cr-18.3 mol%Nb. In the as-cast and the cast and annealed Cr-5.5 mol%Nb and Cr-12.2 mol%Nb, primary Cr solid solution particles are delineated by networks based on the lamellar eutectic consisting of Crss and NbCr2.Primary Laves plates in the cast and annealed Cr-24.5 mol%Nb were fragmented by hand-milling and pulverized into nearly round shapes by ball-milling. By hot isostatically pressing (HIPing) the hand-milled or ball-milled powder, porosity-free compacts were fabricated.At a high volume fraction of the NbCr2 phase the NbCr2/Cr in-situ composites possess extremly high yield stresses at elevated temperatures, although their ductile-brittle transition temperature is considerably high. Ductility is increased by refining the microstructure via hand- or ball-milling followed by HIPing and by increasing the volume fraction of Crss.