Microstructural evolution and dynamic recrystallization mechanism of a heavily-alloyed Ni-based superalloy during hot extrusion

Direct hot forging of heavily-alloyed Ni-based superalloy castings usually ends up with severe cracking due to serious macro-segregation. To address this issue, we propose to use hot extrusion as the replacement technology to process this kind of superalloys. An advanced cast and wrought Ni-based superalloy, GH4151, was thus subjected to hot extrusion. It was found that the cast GH4151 ingot can be successfully processed by hot extrusion without cracking. Extensive twinning happened and played a crucial role in accommodating large and sudden macroscopic plastic deformation during hot extrusion. Dynamic recrystallization happened through twinning and grain boundary bulging, leading to significant grain refinement. Along the radial direction, the central region shows a relatively larger grain size than the outer region, which is believed to be associated with the less extent of recrystallization in the central region. Despite a certain microstructural scatter, the hot-extruded GH4151 demonstrates superior tensile strengths and stress rupture properties as compared with a number of the state-of-the-art disc superalloys. With proper heat treatment, the microstructure was further refined and the strengths were further enhanced. The present study indicates that hot extrusion is capable of processing heavily-alloyed Ni-based superalloys and producing desired microstructure that allows for acquisition of excellent mechanical properties.