Microstructural Evolution and Deformation Mechanisms in Nickel-Base SuperAlloys

Nickel-base superalloys are used at high temperature applications in aerospace and power generation. The objective of present work is to gain a better understanding of microstructural evolution and deformation mechanisms in Nickel-base superalloys. The microstructure of the Nickel base superalloy is basically composed of gamma matrix with eta, delta, gamma prime or gamma double prime/or both, carbides and nitrides. Cold working of IN 718 and Waspaloy to 50% reduction led to an increase in hardness. This hardening was related to the continuous increase in dislocation density in both alloys. Cold working of IN 718 to levels of 10% and higher also led to shearing of the gamma double prime precipitates present initially, leading to their dissolution and the redistribution of the alloying elements into the matrix. Shot peening of both alloys introduces near surface compressive residual stresses and a significant increase in the surface and near-surface hardness to a distance of ~200-400 m in both alloys. Shot peening of both alloys followed by aging at 900°C quickly led to a large drop in hardness to near that of the bulk material. Aging shot peened the IN 718 at 700C led to an increase in the hardness throughout the sample. Microstructural characterization revealed that this hardening is related to the formation of new precipitates of gamma prime or gamma double prime or both within the gamma matrix. Aging shot peened Waspaloy at 700C led to an increase or decrease at near surface region at short time, depending on the Almen intensity. Microstructural characterization shows that these changes are related to partial reduction in % cold work by recrystallization and/or new gamma prime precipitation, depending on the Almen intensity. The hardening, microstructural evolution and stress rupture behavior of IN 740 were studied. Aging of the IN 740 alloy led to significant hardening due to the gamma prime precipitation. The gamma prime coarsening in aged and tensile tested and stress rupture tested IN740 alloy was observed to follow the LSW theory. Gamma prime coarsening was promoted by the applied stress and accelerated between 750 and 800°C. Applied stress was also found to significantly enhance the eta phase formation. Various models for strengthening in Ni-base superalloys were considered and their contributions to strength following aging were calculated utilizing the experimentally measured gamma prime sizes and volume fractions. Good agreement between the calculated and experimentally measured yield strengths was obtained. Based on these calculations and characterization of the deformation microstructure in the aged and tensile-tested samples, it was concluded that apart from solid solution strengthening, the main strengthening contributions from the gamma prime precipitates is associated with precipitate shearing involving either weak pair dislocation coupling or strong pair dislocation coupling/ or both at 700C for 100h, strong pair dislocation coupling following aging at 700C for 1000, 3000h and 750C for 100, 1000h, strong pair coupling or Orowan process/or both following aging at 750C for 3000h and 800C for 100h, and mainly Orowan process following aging at 800C for 1000 and 3000h.