Comparative Study of Prior Particle Boundaries and Their Influence on Grain Growth during Solution Treatment in a Novel Nickel-Based Powder Metallurgy Superalloy with/without Hot Extrusion

The prior particle boundaries (PPBs), as one of the typical defects in the nickel-based powder metallurgy superalloy, largely affect the microstructure and thus properties/performance of alloys. However, the effect of PPBs on the microstructure evolution in nickel-based powder metallurgy superalloy during heat treatment is still unclear. In this paper, a comparative study of PPBs and their influence on grain growth during solution treatment in a novel nickel-based powder metallurgy superalloy FGH4113A (i.e., WZ-A3 from Shenzhen Wedge, China) with/without hot extrusion (HEX) was conducted. Firstly, through a combination of scanning electron microscope (SEM), electron probe microanalyzer (EPMA) and transmission electron microscope (TEM) techniques, PPBs in FGH4113A alloys were characterized to be Al2O3, carbides (TiC, M6C, M23C6) and large-size gamma ' particles. After HEX, the oxides broke, carbides deformed, and gamma ' phase redistributed. After solution treatment at 950 degrees C, the TiC decomposed to M6C and M23C6, while no such decomposition occurred in FGH4113A alloys after solution treated at 1050 degrees C and 1150 degrees C. Secondly, the evolution of grain size in FGH4113A alloys was analyzed using the electron backscattered diffraction (EBSD) technique. At 950 degrees C, the decomposition of carbide TiC resulted in the increase of PPBs and the enhancement of their pinning effect on grain boundaries, thus inhibiting grain growth. At 1050 degrees C, the nucleation rate due to recrystallization is comparable to the grain growth rate, leading to the stable distribution of grain size. While at 1150 degrees C, the higher temperature can induce a higher content of PPBs. However, the driving force for grain growth surpassed the pinning force of PPBs, making the grains quickly coarsen. Finally, it was concluded that the HEX process is an effective method to modify the microstructure of powder metallurgy superalloy after HIP that can heavily refine the grains in the powder metallurgy superalloys. Furthermore, based on the present experiment and analysis, an appropriate solution treatment mechanism (i.e., 1050 degrees C for 2 h) was proposed for FGH4113A alloys.