鎳基粉末高溫合金熱加工變形過程中顯微組織不穩(wěn)定性對熱塑性的影響

鎳基粉末高溫合金熱加工變形過程中顯微組織不穩(wěn)定性對熱塑性的影響Effect of Microstructure Instability on Hot Plasticity During Thermomechanical Processing in PM Nickel-based Superalloy

采用單軸熱壓縮試驗，研究了熱等靜壓態(tài)鎳基粉末高溫合金FGH98的熱加工變形行為。觀察了形變過程中合金組織演變，分析了顯微組織不穩(wěn)定性對熱塑性的影響。熱壓縮試驗在等溫、恒應變速率下進行，真應變分別為0.2、0.4和0.6，溫度分別為1060、1105、1138和1165 ℃，應變速率分別為0.01、0.1、1和10 s-1。結果表明，隨著真應變的增加，合金的真應力-真應變曲線上出現(xiàn)硬化-軟化-穩(wěn)態(tài)流變階段。在低于γ′相完全溶解溫度合金處在穩(wěn)態(tài)流變或高應變條件下時，應變誘發(fā)動態(tài)再結晶發(fā)生并形成特殊形態(tài)的γ + γ′顯微雙相晶粒組織。晶粒尺寸細小，達到1.2~6.8 μm，合金顯示良好的熱塑性。分析了變形過程中晶粒尺寸和流變應力的變化和γ + γ′顯微雙相晶粒組織形成機理，對熱加工過程中顯微組織調控的可能性進行討論。

High alloying Ni-based powder metallurgy (PM) superalloys show excellent fatigue performance 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 γ + γ′ microduplex 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 nickel-base superalloy FGH98 prepared by hot isostatic pressing has been examined by means of uniaxial compression testing. The microstructural evolution and instabilities during plastic flow have been studied. The tests were done isothermally at 1060, 1105, 1138 and 1165 ℃ and at constant true strain rates between 0.01 and 10 s-1. 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 sizes 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 are analyzed. The possibility of the microstructure controlling during hot working is discussed.

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