长期时效条件下Ta/(Ta+Ti)变化对镍基高温合金铸态组织与性能的影响

Effect of Ta/(Ta+Ti) Variation on the As-cast Microstructure and Properties of Nickel-based Alloy During Long-term Aging

摘要:

在DD98M镍基高温合金的基础上，保持合金中γ"相形成元素(Ta+Ti)总量不变，利用真空感应熔炼制备A1- Ta/(Ta+Ti)=0、A2- Ta/(Ta+Ti)=0.34、A3- Ta/(Ta+Ti)=0.66、A4- Ta/(Ta+Ti)=1四种合金，并对铸态合金进行固溶时效和1273 k下的长期时效。采用X射线衍射(XRD)、扫描电镜(SEM)、电子探针(EPMA)等测试方法对时效后的组织进行研究，分析高温长期时效和Ta/(Ta+Ti)变化对合金铸态组织和性能的影响规律。结果表明，长期时效会引起γ"相的部分分解，促进元素扩散，加剧元素偏析。随长期时效时间的延长，γ"相粗化长大，硬度降低，错配度绝对值减小，γ"相立方度降低。随Ta在(Ta+Ti)中占比增加，错配度绝对值减小，γ"相立方度降低，硬度增加，长期时效条件下Cr、Mo、W、Ta偏析加剧，Ti偏析得到缓解，Ta的加入会将W排挤至γ相。A2和A3合金长期时效后在晶界处有σ相和MC型碳化物析出，A1和A4合金长期时效晶界无析出，即Ta和Ti的协同作用会促使σ相和MC型碳化物的析出。A2合金有着相对高的γ"相体积分数、γ"相立方度、硬度和强度，最小的γ"相尺寸和最高的延伸率，因此四种合金中A2合金有着最好的综合力学性能。

Abstract:

Based on DD98M nickel-based superalloy, four kinds of alloys of A1-Ta /(Ta+Ti)=0, A2-Ta /(Ta+Ti)=0.34, A3-Ta /(Ta+Ti)=0.66 and A4-Ta /(Ta+Ti)=1 were prepared by vacuum induction melting, keeping the total amount of γ" phase forming elements of (Ta+Ti) in the alloys unchanged. The as-cast alloys were subjected to solution aging and long-term aging at 1273k. X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe (EPMA) were used to study the microstructure of the four alloys after aging. The effects of long-term aging at a high temperature and the change of Ta/(Ta+Ti) on the as-cast microstructure and properties of the alloys were analyzed. The results show that long-term aging causes partial decomposition of γ" phase, promotes element diffusion and intensifies element segregation. With the extension of long-term aging time, the γ" phase coarsens, the hardness decreases, the absolute value of misfit decreases, and the cubic degree of γ" phase decreases. With the increasing proportion of Ta in (Ta+Ti), the absolute value of misfit decreases, the cubic degree of γ" phase decreases, and the hardness increases. Under the long-term aging condition, the segregation of Cr, Mo, W and Ta is intensified, and the segregation of Ti is alleviated. The addition of Ta will squeeze W into the γ phase. After long-term aging, σ phase and MC carbide precipitated at the grain boundaries of A2 and A3 alloys, while no precipitation was observed at the grain boundaries of A1 and A4 alloys, indicating that the synergistic effect of Ta and Ti promotes the precipitation of σ phase and MC carbide. A2 alloys have relatively high γ" phase volume fraction, γ" phase cubed degree, hardness and strength, the smallest γ" phase size and the highest elongation, so A2 alloys have the best comprehensive mechanical properties of the four alloys.