Microstructure and Phase Composition of Rhenium and Ruthenium-Alloyed Ni–Al–Co System after Thermal Treatment

Abstract

The paper presents the transmission electron microscopy investigations of the microstructure and phase composition of the nickel-based superalloy after high temperature annealing. All states of the superalloy are characterized by the single-crystal structure with [001] crystallographic orientation. The Ni–Al–Co alloy system also contains such elements as Mo, Cr, W, Ta, Re and Ru. The Ni–Al–Co system is studied in four states: initial (after directional crystallization) and after 1000°С annealing during 118, 372 and 1274 hours. The major phases forming the alloy system are γ- and γ′-phases. After annealing for 118 hours, the formation of Al6(Re, Ru) phase is observed. After longer high-temperature annealing new phases occur, such as σ-, δ- and Laves phase. The obtained alloy microstructure is classified into four types: 1) γ′-phase quasi-cuboids with γ-phase layers, 2) ribbon anisotropic microstructure of γ′+ γ-phase, 3) ribbon anisotropic microstructure with σ-phase particles in γ-phase layers, 4) γ-phase with δ-phase and Laves phase inclusions. The introduction of a large amount of various alloying elements and the annealing process modify the crystallographic texture of the superalloy.