Zr对高Al、Ti含量镍基变形高温合金凝固偏析的作用机制研究

Nickel-base superalloys with high Al and Ti contents have a high tendency to form the eutectic (γ+γ′), which is unfavorable for the hot-working and homogenization process. Zr is an important strengthening element in superalloys, but the Zr addition could significantly promote the formation of coarse eutectic (γ+γ′) at interdendritic area and this harmful effect must be taken into account. However, the mechanisms by which Zr affects eutectic (γ+γ′) formation are not well understood. Recently, we found that γ matrix solidification rate is an important factor that influences the eutectic (γ+γ′) precipitation, but the previous studies about the role of Zr on solidification segregation did not consider this factor. Besides, since Zr could dissolve in MC carbides, the addition of C can cause interference to the effect of Zr. This research will produce ingots added with Zr, C and C+Zr respectively by vacuum induction melting (VIM) to study how Zr affects the solidification segregation eliminating the interference of C. The quantitative relation between the volume fraction of eutectic (γ+γ′) and Zr contents will be established. The distribution and existing form of Zr will be determined. The influence mechanism of Zr on the γ matrix solidification will be revealed by combining the differential thermal analysis (DTA), high temperature water quenching experiment and in situ observation of the solidification process of the experimental alloys. Based on the above results and analysis, the mechanism by which Zr influencing the precipitation of eutectic (γ+γ′) will be clarified. This work will provide a theoretical basis for controlling the solidification segregation of nickel-base superalloys with high Al and Ti contents and Zr microalloying process.

高Al、Ti含量镍基变形高温合金在凝固过程中容易析出(γ+γ′)共晶，对其热加工和均匀化处理极为不利。Zr是高温合金中的一种关键强化元素，但其添加会促进有害(γ+γ′)共晶析出。目前Zr对凝固偏析的作用机制尚不清楚。申请人发现γ基体凝固速度是影响(γ+γ′)共晶析出的重要因素，但以往相关研究并未考虑该因素。另外，由于Zr可溶解在MC碳化物中，合金中C的添加会对Zr的作用造成干扰。本项目拟采用真空感应炉熔炼单加Zr、单加C和同时加C+Zr的高Al、Ti镍基变形高温合金铸锭，排除C的干扰，研究Zr对其凝固偏析的作用机制。建立(γ+γ′)共晶体积分数与Zr含量的定量关系；明确Zr的分布规律和存在形式；结合差热分析、高温水淬实验和凝固过程原位观察，揭示Zr对γ基体凝固速度的影响规律及机理，进而阐明Zr对(γ+γ′)共晶析出的作用机制，为先进高Al、Ti含量镍基变形高温合金的凝固偏析控制和Zr的微合金化工艺提供理论指导。

高Al、Ti含量镍基变形高温合金在凝固过程中容易析出(γ+γ′)共晶，对其热加工和均匀化处理极为不利。Zr是高温合金中的一种关键强化元素，但其添加会促进有害(γ+γ′)共晶析出。目前Zr对凝固偏析的作用机制尚不清楚。申请人发现γ基体凝固速度是影响(γ+γ′)共晶析出的重要因素，但以往相关研究并未考虑该因素。另外，由于Zr可溶解在MC碳化物中，合金中C的添加会对Zr的作用造成干扰。本项目采用真空感应炉熔炼了不同Zr含量和不同C含量的高Al、Ti镍基变形高温合金铸锭，研究Zr对其凝固偏析的作用机制并阐明C的干扰作用。建立了(γ+γ′)共晶与Zr含量的定量关系，发现Zr含量的增加显著促进了(γ+γ′)共晶的形成。Zr能少量溶解在共晶γ′中，成为(γ+γ′)共晶的形成元素。结合差热分析(DTA)和高温水淬实验得出，添加Zr显著增加了残余液相的面积分数，明显扩大了γ基体的形成温度区间，阻碍了合金的凝固。合金中的C元素主要以MC碳化物的形式存在，C含量的增加明显增大了MC碳化物的尺寸和体积分数，但显著降低了(γ+γ′)共晶的尺寸和体积分数。DTA分析表明在凝固过程中MC碳化物比(γ+γ′)共晶更先形成，更多碳化物的形成消耗了更多的Ti原子，进而减少了(γ+γ′)共晶的析出。另外，电子探针(EPMA)分析表明Zr会较明显地溶解在MC碳化物中，C元素的添加明显弱化了Zr对(γ+γ′)共晶析出的作用。总之，Zr促进(γ+γ′)共晶的析出主要是由于添加Zr延缓了合金的凝固并增加了(γ+γ′)共晶的形成元素。项目研究结果为先进高Al、Ti含量镍基变形高温合金的凝固偏析控制和Zr的微合金化工艺提供了理论指导，对我国航空发动机性能的提升起到了推动作用。