采用单轴热压缩实验,研究了热等静压态镍基粉末高温合金FGH98的热加工变形行为。观察了形变过程中的合金组织演变,分析了显微组织不稳定性对热塑性的影响。热压缩实验在等温、恒应变速率下进行,真应变分别为0.2、0.4和0.6,温度分别为1060、1105、1138和1165 ℃,应变速率分别为0.01、0.1、1和10 s^-1。结果表明,随着真应变的增加,合金的真应力-真应变曲线上出现硬化-软化-稳态流变阶段。在低于γ′相完全溶解温度、合金处在稳态流变或高应变条件下时,发生应变诱发动态再结晶并形成特殊形态的γ+γ′显微双相晶粒组织。晶粒尺寸细小,达到1.2~6.8 μm,合金显示良好的热塑性。分析了变形过程中晶粒尺寸和流变应力的变化和γ+γ′显微双相晶粒组织形成机理,并对热加工过程中显微组织调控的可能性进行讨论。

High alloying Ni-based powder metallurgy (PM) superalloys show excellent fatigue performance and damage tolerance properties, and good creep resistance at 750 ℃, and are used for advanced gas turbine disks and other hot components. The hot-working window of high alloying PM superalloy is narrow because of its poor workability. The formation of the γ+γ′ microduplex structure during the thermomechanical processing always results in a decrease in flow stress and a promotion of hot plasticity. However, the stability of the γ+γ′ microduplex structure has not been evaluated. The high temperature flow behavior of a Ni-based superalloy FGH98 prepared by hot isostatic pressing has been examined by means of uniaxial compression testing isothermally at 1060, 1105, 1138 and 1165 ℃ and at constant true strain rates between 0.01 and 10 s^-1. The microstructural evolution and instabilities during plastic flow have been studied. Under all testing conditions, the as-hipped material exhibits flow hardening, flow softening and steady-state flow sequentially with the true strain increased. The dynamic recrystallization occurs and the γ+γ′ microduplex structures are generated when steady state flow or highest strains achieved at temperatures below the γ′ solvus. The formation of the γ+γ′ microduplex structures results in a remarkable decrease in grain size and a promotion of hot plasticity. The relationships between steady-state grain sizes and steady-state stresses during deformation and the formation mechanism of the γ+γ′ microduplex structure were analyzed. The possibility of the microstructure controlling during hot working was discussed.