On the stress rupture behavior and deformation mechanism of an advanced hot-extruded nickel-based superalloy

Abstract

The demand for increased aircraft engine efficiency requires continual development of high performance heavily-alloyed nickel-based superalloys. However, nickel-based superalloy castings with heavy alloying usually suffer from severe macro-segregation that results in poor hot workability or cracking during hot forging. To address this issue, hot extrusion is introduced as a transitional process between casting and forging to process a newly developed and highly alloyed nickel-based superalloy, GH4151. The results demonstrate that with hot extrusion, the GH4151 ingots can be successfully processed in the subsequent forging without cracking, thanks to the unique three-dimensional compressive stresses imposed on the billets and the development of chemical and microstructural homogeneity during hot extrusion. The refined microstructure endowed by further forging and heat treatment gives rise to superior stress rupture properties as compared with a number of the state-of-the-art disc superalloys. Moreover, it is shown that the stress rupture lifetime of the material decreases continuously with increased temperature and stress level while the elongation increases with increased temperature. The deformation at 650 °C mainly occurred by dislocation slipping in γ channels and formation of stacking faults in γ′. At 750 °C and 800 °C, microtwins were formed and cut through large regions. Paired dislocations associated with anti-phase boundaries were even found in the samples that were tested at 800 °C. In general, with increased temperature, the deformation becomes increasingly planar and easier, which accounts for the improved elongation. Large grain boundary MC carbides and primary γ′ were found to act as crack initiation sites and thus are harmful for stress rupture properties.