Improving the strength and ductility of multicomponent nickel alloys by optimal iron alloying

Experimental specimens of the precipitation-hardenable nickel alloy base+10Fe–0.3Hf–0.3Zr–0.25Ta were produced by selfpropagating high-temperature synthesis, crushing of porous sintered compacts, air classification of the target fraction, hot isostatic pressing of the precursor powder, and vacuum heat treatment. The alloy exhibited a favorable combination of strength and ductility-related deformation characteristics at 20 and 800 °C (compressive strength = 1592 MPa, strain = 6.5 % at 20 °C; compressive strength = 623 MPa, strain = 32 % at 800 °C) owing to strengthening of the matrix phase by coherent, highly dispersed precipitates of the α-(Fe, Cr) phase and nanoparticles of the intermetallic topologically close-packed σ phase. Using in situ transmission electron microscopy to study solid-solution transformations during heating of a lamella directly in the microscope column, the optimum vacuum heat-treatment temperature was established as 900 °C. At this temperature, primary α-(Fe, Cr) particles give rise to highly dispersed secondary (Fe, Cr) precipitates measuring 10–80 nm and intermetallic σ-phase particles measuring 100–250 nm. Alloying with zirconium and iron preserves a high level of resistance to high-temperature oxidation at 1000 °C owing to the formation of a dense protective Al₂O₃ layer containing inclusions of the complex oxide (Hf, Zr)O₂. Oxidation follows a logarithmic law and almost completely ceases after 25 h of thermal cycling. Oxygen diffusion from the specimen surface into the metal interior proceeds along grain boundaries through conglomerates of (Hf, Zr)O₂ oxides. Despite the relatively high iron content, the oxidation resistance of the alloys remains high, amounting to 12.24 g/m² for base+5Fe–0.3Hf–0.3Zr–0.25Ta and 14.23 g/m² for base+10Fe–0.3Hf–0.3Zr–0.25Ta.

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