Rhenium additions to a Ni-base superalloy: Effects on microstructure

Abstract

Refractory metal alloying to superalloy single crystals has resulted in alloys possessing higher temperature capabilities than have previously been known. In this study, additions of rhenium to a modified MAR-M200* alloy were made at the expense of tungsten. A study was carried out on the γ′ precipitate growth characteristics, morphology, and lattice misfit as a function of rhenium content and aging time and temperature. Rhenium additions substantially lowered the γ′ coarsening kinetics and resulted in negative γ-γ′ misfit alloys. A qualitative model is developed which explains these results. Particle growth was shown to bevia the widely accepted diffusion controlled coarsening mechanism. Precipitates were also shown to grow by particle coalescence into irregular blocky shapes at high aging temperatures while remaining cuboidal at lower temperatures, and the necessity of using large precipitate volume fraction modifications to theoretical coarsening models is illustrated.