Carbides and their influence on notched low cycle fatigue behavior of fine-grained IN718 gas turbine disk material

The notched low cycle fatigue properties of forged Inconel 718 superalloy that was heat-treated with conventional solution and aging cycles were evaluated. The microstructure of the material was fine-grained with average grain size of 8 mu m (ASTM 11) and with occasional grains as large as 30-40 mu m (ASTM 6-7). Tests were conducted at room, intermediate (420 K and 560 K) and high (810 K and 880 K) temperatures at several levels of notch root 'concentrated' stress (1400-1650 MPa). The cycles to failure were analyzed in terms of stress, temperature and fatigue initiation sites. In all tests fatigue originated from either grains or carbides, and the relative occurrence of each depended on the temperature and stress of the test At low temperature and high stress, carbide initiation was dominant, but with increased test temperature grain initiation was observed. The fatigue life reduction due to carbide initiation (relative to the grain initiation) was significant at low temperature/high stress conditions. Under these conditions, grain refinement did not improve fatigue life. These results are discussed in terms of cracking of carbides that was promoted by carbide brittleness at lower temperatures, a high 'concentrated' notch-root stress and larger carbide size.