Creep performance and damage mechanism for Allvac 718Plus superalloy

The creep behavior, creep damage and creep life of 718Plus alloy were investigated at temperatures ranging from 650 to 750 degrees C in air. The fracture surface, microstructure evolution and deformation mechanisms were studied by optical, scanning and transmission electron microscope (SEM and TEM). The results show that creep curves of 718Plus alloy consist of primary stage and tertiary stage without steady-state region. The Monkman-Grant relationship of 718Plus alloy can be described by t(r) = 9.32E-2/(epsilon(over dot)(min))(0.75) and apparent creep activation energy of 718Plus alloy is about 572.7 kJ/mol. Further investigation shows creep damage of alloy at 650 degrees C can be mainly ascribed to the effect of grain boundary sliding (GBS) and microtwinning. The alloy shows transgranular fracture at high stress, while it presents ductile fracture at low stress due to the nucleation, growth and linkage of creep voids. At 750 degrees C, the alloy failed in the form of ductile fracture mainly due to the microstructure instability and dislocation glide. At higher stress of 700 degrees C, the effect of GBS is mainly responsible for the creep damage of 718Plus alloy, while GBS, dislocation glide and microstructure instability work together and lead to the creep damage of alloys at lower stress of 700 degrees C. The fracture mechanism map and fracture models for 718Plus alloy were constructed. Remaining life assessment of 718Plus alloys was also evaluated by Larson-Miller parameter method.