Microstructural investigation of the growth of large grains in prealloyed powder extrusions of a nickel base superalloy

The metallurgical conditions controlling the growth of large grains in consolidated Rene 95 superalloy powder were investigated. The starting material was — 35 mesh powder produced by the rotating electrode process from low carbon, (0.08 wt pct) vacuum remelt stock. The powder was batch consolidated at temperatures between 1228 K (1750°F) and 1561 K (2350°F) and extruded to 10∶1 ratio at temperatures between 1339 K (1950°F) and 1450 K (2150°F). High temperature conventional and gradient anneals to 1505 K (2250°F) employing both slow and fast heating rates were used to investigate the large grain growth phenomena. Results indicated that growth of large grains, 2 to 3 mm in length, occurs when extrusion temperatures were above the γ′ solvus temperature and the tendency for large grain growth increased with decreasing heating rate. The grain boundary γ′ is believed to be responsible for the prevention of large grain growth but these precipitates are less effective when a slow heating rate anneal is used. The results are found to conform to the theoretical formulation of Hillert.