Mechanical property and superplasticity mechanism of a P/M superalloy with an optimized γ/γ′ dual-phase microstructure

This study focuses on the highly alloyed, difficult-to-deform Ni-based powder metallurgy superalloy A2. By employing a microstructural preprocessing method that combines sub-solvus heat treatment with controlled slow-cooling, we successfully designed a γ/γ′ dual-phase microstructure characterized by coarsened incoherent primary γ′ particles. Microstructural characterization indicates that this slow-cooling process effectively promotes significant coarsening of the incoherent γ′ phase, with average size increasing from 0.56 to 1.49 μm, while suppressing the precipitation of the coherent γ′ phase. The critical temperature for compression cracking is found to increase with rising strain rate: 1000-1020 °C for 0.1 s -1 , below 940 °C for 0.001 s -1 . Meanwhile, tensile testing shows that the microstructure-optimized alloy exhibits a tensile elongation of up to 505% at 1080 °C/0.001 s -1 . Furthermore, the tensile elongation can be maintained at 254% even at 1000 °C under the same strain rate; it reduces the temperature requirements for forging dies. The positive influence of this γ/γ′ dual-phase microstructure on the processing plasticity is evidenced by these experimental mechanical properties. Investigations into the deformation mechanisms through EBSD and TEM methods reveal that the coarsened incoherent γ′ phase enables the superplasticity through three mechanisms: (a) the heterogeneous nucleation promoting DRX process; (b) transformation into γ, accommodating the dislocation accumulation; (c) grain boundary pinning, stabilizing the microstructure. In addition, twins formed during the deformation process also improve the alloy's plasticity. This study systematically elucidates the mechanisms of superplastic deformation in γ/γ′ dual-phase Ni-based superalloy, providing theoretical and experimental support for the superplastic forming of difficult-to-deform high-temperature alloys.