Role of defects in fatigue damage mechanisms of cast polycrystalline superalloy MAR-M 247

Abstract

High-cycle fatigue life of nickel-based superalloy MAR-M 247 was experimentally determined for as-cast material and material processed by hot isostatic pressing (HIP). Fatigue testing was conducted at temperatures 650, 800, 900 ∘ C in laboratory air. HIP was done at the following conditions: 1200 ∘ C/100 MPa/240 min. It has been found that HIP significantly improves the fatigue life. Obtained results indicate that main factors which determine the fatigue strength of material in both conditions are grain size, grain orientation and size and distribution of casting defects. The distribution and size of the casting defects were evaluated by light microscopy on metallographic sections. The data were processed by the extreme value statistics, which enables to estimate the maximum size of a defect likely to occur in a defined volume. Light and scanning electron microscopy were used for fractographic investigation of fracture surfaces and fatigue crack initiation sites. Focused ion beam technique and transmission electron microscopy were applied with the aim to reveal the microstructure in the nearest vicinity of the early cracks. The mechanism of crack initiation, early crack propagation and the role of casting defects were described and discussed.