形变诱导GH3625合金热挤压管材d相的析出行为

Precipitation Behavior of δ Phase of Deformation Induced GH3625 Superalloy Hot-Extruded Tube

采用XRD、SEM、EDS和Image-Pro Plus金相分析等方法测定了GH3625合金热挤压管材在不同冷变形量e下经过800℃时效后d相的析出含量,研究了冷变形对d相的析出规律及析出动力学的影响。结果表明,d相首先在形变孪晶界、晶界以及变形带上形核并析出,随后在晶内形核长大,并且随冷变形量的增加,d相在变形带上析出量增加;随冷变形量的增加,d相的形貌从针状向棒状或颗粒状转变;随着时效时间的延长,d相的平均尺寸不断增大,长大规律符合LSW理论。当时效温度为800℃时,d相的析出含量与时效时间的关系满足Avrami方程,且随冷变形量的增加,d相的含量增加,时间指数n减小,d相析出速率a增加,冷变形促进d相的析出。Nb的溶质拖曳与d相的钉扎共同作用抑制晶粒长大。e=35%时,合金的硬度随保温时间的延长而增加,e≥50%时硬度未发生明显变化。

GH3625 alloy is a wrought nickel-based superalloy mainly used in aeronautical, aerospace, chemical, nuclear, petrochemical, and marine applications industry due to its good mechanical properties, processability, weldability and resistance to high-temperature corrosion on prolonged exposure to aggressive environments. However, in medium and high temperature environment during long-term service, the γ＇＇ is a metastable phase, easily transformed into stable δ phase, or δ phase directly formed in the γ matrix so that alloy performance was deteriorated, leading to the result of alloy failure. At the present work, mass fraction of δ phase in GH3625 superalloy hot-extruded tube cold deformed to different reductions and then aged at 800 ℃ for different times, were measured by XRD. The effect of cold deformation on the law and kinetics of δ phase precipitation was investigated by SEM, EDS and Image-Pro Plus metallographic analysis. The results show that δ phase first precipitates at the deformation twin and grain boundaries as well as deformation bands, and then precipitates in the grains. The amount of δ phase at the deformation bands increases with the increase of cold deformation. The morphologies of δ phase change gradually from needles to spheroids or rodlike with increasing cold deformation. With the extend of ageing time, the average size of δ phase increases which grows according to LSW theory. At 800 ℃, the relationship between the precipitation content of δ phase and ageing time follows Avrami equation. As cold deformation increases, the content of δ phase increases, the time index n decreases, whereas the δ phase precipitation rate increases. Cold deformation promotes the precipitation of δ phase. The solute drags of Nb in soild solution and pinning of δ phase inhibits the grain growth during ageing process of cold deformed GH3625 superalloy hot-extruded tube. The hardness of the alloy increases with the extension of the holding time at ε =35% but no obvious change at ε ≥50%.