挤压态FGH4096镍基粉末高温合金缓冷过程中γ′相析出行为与机理研究

Study on the Precipitation Behavior and Mechanism of γ′ Phase in As‑Extruded FGH4096 Nickel‑Based Powder Metallurgy Superalloy during Slow Cooling

摘要: 为探究缓冷条件下晶界γ′相析出行为与机理，从而为通过组织调控改善合金热塑性提供依据，研究通过中断冷却实验，将挤压态FGH4096镍基粉末高温合金在1110℃亚固溶热处理后，分别以75℃/h与7.5℃/h的速率缓慢冷却至1050℃，并在1100℃~1050℃温度区间内选取6个温度节点进行水淬以保留高温组织，利用扫描电子显微镜(SEM)背散射电子成像及定量图像分析技术对γ′相的形貌、尺寸及面积分数进行了系统表征。结果表明：1110℃亚固溶处理后，合金中含有面积分数为2.05%细小γ′相，主要分布在晶界处，抑制缓冷过程中晶粒长大。在两种冷速下，γ′相均在晶界优先形核并析出。在75℃/h冷速下，冷却至1080℃时晶界γ′相面积分数为7.39%，颗粒密度降至0.013 μm⁻²，相邻颗粒沿晶界合并；随后温度降至1070℃时，晶界再次爆发形核，颗粒密度回升至0.056 μm⁻²，至1050℃时面积分数达14.82%，呈现粗大长条状与细小近球状混合分布的特征。在7.5℃/h冷速下，γ′相始终以块状形貌在晶界粗化长大，无二次爆发形核现象，至1050℃时面积分数达21.08%，晶内γ′相含量明显低于75℃/h冷速。EBSD分析表明，晶界大尺寸块状γ′相与基体呈非共格界面。分析认为：晶界异质形核能垒低，亚固溶后晶界γ′相形成元素偏聚以及晶界快速扩散等综合因素共同促进了γ′相在晶界优先形核。在细晶组织中，缓冷高温阶段溶质原子扩散速率快，扩散区域大，晶界优先形核的γ′相快速长大并消耗周围基体中γ′相溶质原子，降低晶内溶质原子的浓度，抑制晶内γ′相爆发形核。随晶界γ′相尺寸增大，其与基体的共格关系逐渐破坏，转变为非共格界面，此后生长主要受界面能控制，在晶界缓慢迁移条件下呈离散块状分布，形成γ + γ′双相组织，有利于提高后续热加工性能。研究阐明了缓冷过程中大尺寸块状γ′相形成机理，为热加工过程中析出相调控改善合金热塑性提供数据和理论支撑。

Abstract: To investigate the precipitation behavior and mechanism of the grain boundary γ′ phase during slow cooling, and thereby provide a basis for improving the hot plasticity of the alloy through microstructural control, this study employed interrupted cooling experiments. The as-extruded FGH4096 nickel-based powder metallurgy superalloy was subjected to sub-solvus heat treatment at 1110˚C, followed by slow cooling to 1050˚C at two rates of 75˚C/h and 7.5˚C/h. Six temperature points within the range of 1100˚C to 1050˚C were selected for water quenching to preserve the high-temperature microstructure. The morphology, size, and area fraction of the γ′ phase were systematically characterized using scanning electron microscopy (SEM) with backscattered electron imaging and quantitative image analysis. The results show that after sub-solvus treatment at 1110˚C, 2.05% (area fraction) of fine γ′ phase remained in the alloy, primarily distributed at grain boundaries, which inhibited grain growth during subsequent slow cooling. At both cooling rates, the γ′ phase preferentially nucleated and precipitated at grain boundaries. At the cooling rate of 75˚C/h, when the temperature reached 1080˚C, the area fraction of grain boundary γ′ phase was 7.39%, and the particle density decreased to 0.013 μm⁻², with adjacent particles merging along grain boundaries into an irregular elongated morphology. As the temperature further decreased to 1070˚C, burst nucleation occurred again at grain boundaries, and the particle density rebounded to 0.056 μm⁻². At 1050˚C, the area fraction reached 14.82%, exhibiting a mixed distribution of coarse elongated and fine near-spherical particles. At the cooling rate of 7.5˚C/h, the γ′ phase coarsened continuously at grain boundaries in a blocky morphology without secondary burst nucleation. At 1050˚C, the area fraction reached 21.08%, and the intragranular γ′ phase content was significantly lower than that at 75˚C/h. EBSD analysis revealed that the coarse blocky γ′ phase at grain boundaries exhibited an incoherent interface with the matrix. The analysis indicates that the preferential nucleation of γ′ phase at grain boundaries is promoted by a combination of factors, including a low heterogeneous nucleation energy barrier at grain boundaries, enrichment of γ′-forming elements at grain boundaries after sub-solvus treatment, and rapid grain boundary diffusion. In the fine-grained microstructure, solute atoms diffuse rapidly with a large diffusion zone during the high-temperature stage of slow cooling. The γ′ phase that nucleates preferentially at grain boundaries grows rapidly and consumes the γ′-forming solute atoms in the surrounding matrix, thereby reducing the solute concentration within grains and suppressing the burst nucleation of intragranular γ′ phase. As the grain boundary γ′ phase grows, its coherent relationship with the matrix is gradually lost and transforms into an incoherent interface. Thereafter, the growth is mainly controlled by interfacial energy, and the γ′ phase distributes as discrete blocky particles at grain boundaries under slow grain boundary migration, forming a γ + γ′ duplex microstructure that is beneficial for improving subsequent hot workability. This study elucidates the formation mechanism of coarse blocky γ′ phase during slow cooling, providing data and theoretical support for improving the hot plasticity of the alloy through precipitate control during hot working.