Creep crack growth in the absence of grain boundary precipitates in UDIMET520

The effects of grain size and environment on creep crack growth (CCG) in Ni -base superalloy, UDIMET 520, were studied through experiments at 540 degre esC. Specially designed solution and aging treatments were used to produce gamma ' strengthened microstructures with different grain sizes but without any M23C6 grain boundary precipitates. Five grain sizes, which fall into t hree groups (i.e., small, medium, and large), were employed. The creep crac k growth rates (CCGRs) in specimens with small grain sizes were approximate ly 2.5 times lower than those with medium and large grain sizes, as a resul t of crack branching and the presence of some undissolved primary MC carbid es at the grain boundaries. Otherwise, the CCGRs were insensitive to the gr ain size. Fractographic observations on the fracture surfaces and metallogr aphic examinations on the cross sections of the interrupted CCG specimen re vealed intergranular microcracks and a faceted intergranular mode of fractu re in both air and argon environments. The test results suggest that the fo rmation and propagation of intergranular cracks by grain boundary sliding ( GBS) is the main micromechanism responsible for CCG in both air and argon e nvironments at the relatively low test temperature employed. Grain boundary oxidation attack in the air environment simply accelerates the crack growt h process. The present results are in agreement with the theoretical predic tions of the GBS-controlled CCG model previously developed by the authors.