Diffusion in the nickel-rhenium system

Abstract

Most Ni-based metal matrix composites (MMC) studied to date were reinforced with ductile refractory metal fibers, mostly W or W-alloys. However, Ni embrittles and weakens cold-drawn W fibers by (1) forming intermetallic phases at the fiber-matrix interface and (2) inducing recrystallization of W at temperatures as low as 950 C by rapid grain-boundary diffusion. To prevent these problems, ceramic coatings with low solubility, reactivity, and diffusion coefficients for both W and Ni can be applied at the interface. However, ceramic coatings are brittle, and typically exhibit a large mismatch of coefficient of thermal expansion with Ni; therefore, these coatings tend to crack when the composite is subjected to thermal cycling. To prevent cracking of the diffusion barrier, ductile, metallic coatings can be used. Rhenium is an attractive candidate for W fibers in a Ni matrix, since it exhibits high-temperature strength significantly higher than W, while retaining excellent ductility at all temperatures, even after recrystallization. Also, its coefficient of thermal expansion is between those of W and Ni. Diffusion in the Re-W system at the potential use temperatures of Ni-based MMC (1,200--1,500 K) is negligible due to the very high melting points of both W and Re (3,695 K and 3,459more » K, respectively). Furthermore, Re additions have been found to improve the mechanical properties of both W and Ni. Finally, the Ni-Re phase diagram shows a simple peritectic system with no intermetallic present. However, one potential drawback is the relatively high solubility of each metal in the other. The authors present an experimental investigation of diffusion phenomena in the Ni-Re system. The parameters controlling diffusion (activation energy and frequency factor) of each metal into the other are determined in order to predict long-term diffusion behavior at elevated temperatures and to assess the suitability of Re diffusion coatings for W fibers in Ni-based matrices.« less