热等静压温度对镍基高温合金初生γ′相演变的影响以及粉末再结晶机理

Effect of HIPing temperature on evolution of primary γ′ phases and mechanism of powder recrystallization in a Ni-based superalloy

摘要:

本文研究了柱状初生γ′相转变为块状二次γ′相的过程以及粉末再结晶的机理。分别在γ′相亚、超固溶温度采用热等静压法制备了高γ′相含量的Ni-Co-Cr高温合金。结果表明,未溶解的柱状初生γ′相阻碍了晶界的迁移,只有少数粉末颗粒在亚固溶温度热等静压过程中发生了再结晶。位错在亚晶界附近堆积,低角度晶界转变为高角度晶界,这说明在亚固溶温度热等静压过程中,连续动态再结晶粉末再结晶的主要机制。在超固溶温度热等静压过程中,柱状初生γ′相在扩散控制下溶解于γ基体,再析出块状二次γ′相。没有初生γ′相的钉扎作用,粉末颗粒间局部应变驱动的晶界移动加速。动态再结晶核是通过晶界迁移产生的,然后再结晶的循环形核是由于结晶膨胀而发生的,这说明对于在超固溶温度热等静压的试样,非连续动态再结晶是主要机制。同时,随着热等静压温度的升高,晶界迁移到了原始颗粒边界(PPBs)之外,这是PPBs减少的原因。结果表明,在超固溶温度热等静压时PPBs的减少和充分再结晶,导致在室温和750℃的测试温度,拉伸试样从混合断裂模式转变为韧性穿晶断裂模式,具有较高的伸长率和较低的屈服强度。

Abstract:

The evolution of columnar primary γ′ to blocky secondary γ′ and the mechanism of powder recrystallization were investigated. Ni-Co-Cr superalloys with high γ′ fraction were prepared by hot-isostatic-pressing (HIPing) at γ′ sub- and super-solvus temperatures. The results show that undissolved columnar primary γ′ phases impede grain boundaries (GBs) migration and only a few of powder particles have experienced recrystallization during sub-solvus HIPing process. Dislocations pile up around sub-grain boundaries and low-angle grain boundaries (LAGBs) turn into high-angle grain boundaries (HAGBs), which indicate that continuous dynamic recrystallization (CDRX) is the main mechanism of powder recrystallization at sub-solvus HIPing temperature. HIPing at super-solvus temperature, columnar primary γ′ phases dissolve into γ-matrix under diffusion-controlled process and then re-precipitate blocky secondary γ′ phases. The migration of GBs driven by localized strain between powder particles accelerates without pinning of primary γ′ phases. Dynamic recrystallization (DRX) nuclei has been generated through GBs′ migration, and then cyclic nucleation of recrystallization happens due to GBs′ bulging, which means that the discontinuous dynamic recrystallization (DDRX) is the dominant mechanism of super-solvus HIPed samples. Meanwhile, GBs migrate beyond prior particle boundaries (PPBs), which account for a reduction of PPBs with elevating HIPing temperature. As a result, a decline in PPBs and sufficient recrystallization at super-solvus HIPing temperature change from mixed fracture mode to ductile transgranular fracture mode, leading to a higher elongation and lower 0.2% yield strength at room temperature as well as 750°C testing temperature.