Creep mechanisms of U720Li disc superalloy at intermediate temperature

The microstructures of U720Li disc superalloy have been investigated by transmission electron microscopy (TEM) before and after creep test at 725 degrees C/630 MPa. The evolution of the crept microstructures was marked as three different stages (I, II and III) corresponding to gradually increased strain 0.1%, 5% and 27%, respectively. At stage I, dislocations bypassed secondary gamma' via Orowan loops. At stage II, partial dislocations started to shear secondary gamma', leaving stacking fault (SF) behind and microtwins formed in part of grains. At stage III, grain boundary sliding occurred due to very large strain and increased effective stress. The results indicated that the creep mechanisms of U720Li at 725 degrees C/630 MPa evolved with gradually increased strain. Orowan looping process combining dislocation slip and climb and partial dislocations shearing precipitates were the main creep mechanisms. It is suggested that decreasing the interparticle spacing of secondary gamma', strengthening secondary gamma' and decreasing stacking fault energy (SFE) of gamma matrix may be effective methods to improve the creep property at relatively higher temperatures. (C) 2011 Elsevier B.V. All rights reserved.