Evolution of precipitates during creep deformation for alloy 718

Abstract

This study investigated the microstructural evolution of the gamma &apos;&apos;, gamma &apos;, and delta phases during creep for alloy 718. Creep tests on two-step aged samples were conducted under different stresses and temperatures ranging from 600 to 700 degrees C. Analysis of precipitates was performed on two-step aged and crept samples using scanning electron microscopy and transmission electron microscopy. The creep rupture lives were decreased with increasing stresses and temperatures and showed a linear correlation between experimental data and Larson-Miller parameter curve predictions. The gamma &apos;&apos; phase was formed as a disc shape in the grain interior and had an orientation relationship of (001)gamma &apos;&apos;//{001}gamma and [100]gamma &apos;&apos;//<100>gamma with gamma matrix. The length of the gamma &apos;&apos; phases increased with increasing temperatures and creep exposure time. The activation energy for gamma &apos;&apos; lengthening was 323 kJ/mol, similar to that for lattice diffusion of Nb in Ni. It was found that the formation mechanisms of delta phase were different from temperatures. The delta phase was formed in grain interiors and grain boundaries and had a blocky shape during a sub-delta solvus annealing process. On the other hand, the delta phase was formed as a plate shape and had an orientation relationship of (010)delta//(111)gamma and [102]delta//[011]gamma with gamma matrix at creep temperatures. The growth rates of plate delta phase were faster than blocky delta phase at all creep temperatures.