Mechanism of grain refinement of an advanced PM superalloy during multiple isothermal forging

Nickel-base powder metallurgy (PM) superalloys are widely used as high temperature components in gas turbine engines owing to their outstanding mechanical properties and workability under intense heat. In order to meet the performance requirements of a new generation aircraft engine with a higher thrust-weight ratio, the fourth generation PM superalloy has been studied at home and abroad. Its operating temperature has been raised to 815-850℃. The alloy in this study was a newly-designed fourth generation PM superalloy, which exhibited excellent high temperature stress rupture and creep properties compared with the previous three generations' PM superalloys, FGH4095, FGH4096, and FGH4098. Based on the performance characteristics of PM superalloys of different grain sizes, dual microstructure heat treatment (DMHT) has been used to produce a turbine disk which has a fine-grained bore and a coarse-grained rim. Therefore, it was first necessary to obtain a uniform fine-grained disk. It has been demonstrated that the fine-grained disk can be gained through hot isostatic pressing (HIP) and multi-steps of high temperature working. In order to study the influence of multiple isothermal forging (ITF) on the grain refinement of the advanced PM superalloy, three steps of ITF were employed; each deformation was about 40%. The effective strain distribution of the alloy during ITF was simulated by using the commercial finite element software DEFORM 2D. Microstructures of those forgings were investigated by means of the electron back scattered diffraction (EBSD) technique. The experimental results show that during ITF, the axial section of the forging is divided into three regions. Region Ⅰ, located in the upper and lower end faces, has the smallest deformation. Region Ⅱ is located at both sides of the section, and its deformation is larger than that of region Ⅰ. And region Ⅲ, located in the center of the section, obtains the maximal deformation. After three steps of ITF, Regions Ⅱ and Ⅲ of the forging are fully recrystallized, and equiaxed fine-grained microstructures with an average grain size of 2-3 μm are generated. Nevertheless, necklace structures form near Region Ⅰ of the forging. A great amount of fine recrystallized grains distribute around the non-equiaxed deformed grains. The deformed grains contain plenty of low-angle grain boundaries (LAGBs), which mean that the dislocation density is very high. Through proper heat treatment, the necklace structure in Region Ⅰ is refined. Meanwhile, grain growth occurs in Region Ⅱ and Ⅲ. These findings suggest that fine-grained disks with uniform microstructures can be achieved, and the average grain size is 6-8 μm.