Role of carbides on hot deformation behavior and dynamic recrystallization of hard-deformed superalloy U720Li

Cracks are easy to occur during cogging of U720Li ingots due to the high deformation resistance and poor hot ductility. In this paper, the low-carbon alloy (0.014 wt% C) and high-carbon alloy (0.071 wt% C) ingots were fully homogenized. The effect of carbides on hot deformation behavior and dynamic recrystallization (DRX) of as-homogenized U720Li alloy was investigated with aim of providing some guidance for improving hot working performance of such hard-deformed superalloys. It has been found that the blocky MC carbide is the major form of carbon in both alloys. Compared with low-carbon alloy, the size, number density and area fraction of MC carbides in high-carbon alloy are much greater, but those of gamma ' are similar. The existence of more MC carbides leads to finer initial grains in the high-carbon alloy. During compression deformation of both alloys at 1060 degrees C, intergranular cracking occurred preferentially along initial grain boundaries (GBs), but cracking of the highcarbon alloy occurred at larger strains than that of low-carbon alloy. Before the occurrence of cracking, the yield and flow stress of high-carbon alloy were obviously lower than those of low-carbon alloy. An interesting phenomenon was discovered that the DRX always occurred preferentially around the MC carbides. Based on this phenomenon and electron backscatter diffraction analysis, the MC carbides were assumed to act as nucleation sites during initiation of DRX by a particle stimulated dynamic recrystallization mechanism. In addition, initial GBs are also preferential sites for DRX initiation. The greater size and number density of MC carbides and finer initial grains significantly increased the stored energy and nucleation sites, which distinctly promoted DRX in the high-carbon alloy. The faster DRX is the major contributor in reducing deformation resistance and cracking of the high-carbon alloy.

Automated summary

The presence of more MC carbides in the high-carbon alloy led to finer initial grains and promoted dynamic recrystallization, which reduced deformation resistance and cracking.