镍基粉末高温合金热加工变形过程中显微组织不稳定性对热塑性的影响

Effect of Microstructure Instability on Hot Plasticity During Thermomechanical Processing in PM Nickel-Based Superalloy

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g′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g+g′显微双相晶粒组织。晶粒尺寸细小,达到1.2~6.8μm,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g+g′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 perfor- mance and damage tolerance properties, and good creep resistance at 750 =C, 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 y＋y＇ microduplex structure during the thermo- mechanical processing always results in a decrease in flow stress and a promotion of hot plasticity. How- ever, the stability of the y-y microduplex structure has not been evaluated. The high temperature flow be- havior 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 ＂（2 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 7＋7＇ microduplex structures are generated when steady state flow or highest strains achieved at tempera- tures below the y solvus. The formation of the 7＋7＇ 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 7＋7＇ microduplex structure were analyzed. The possibility of the microstructure controlling during hot working was discussed.