THE MICROSTRUCTURAL RESPONSE OF AS-HIP P/M U-720 TO THERMOMECHANICAL PROCESSING

Udimet 720 is a high strength nickel-base superalloy which in this case was produced via powder metallurgy processing: the billet was in the as-HIP condition. The microstructural response was studied as a function of thermal exposure in the range of 1080°C to 1162OC. Compression specimens were deformed in a matrix of five different temperatures (105O'C - 114O'C) and five strain rates (0.006 min-1 - 3.0 min-l). Resultant grain size, y' precipitate volume fraction and propensity for cracking were evaluated. The grain size coarsened, as expected, with increased temperature for both the thermal exposure and deformed samples. Grain size also coarsened as the deformation strain rate was reduced. Grain boundary cracking (location and density) varied with deformation temperature and strain rate. The resultant grain size correlated well with the volume percent primary Y' present in the microstructure. A relationship between the percent Y' in the structure, and the thermal exposure and deformation conditions is presented along with a discussion of grain boundary pinning. It is suggested that the primary y' and the prior particle boundaries (ppbs) have significant influences on the resultant grain sizes.