Fatigue fracture of directionally solidified superalloy Mar-M247 at room temperature and dependence on the solidification direction
Tsutsumi Mitsuyoshi; Okada Mitsutoshi; Ohtani Ryuichi; Kitamura Takayuki
Abstract:Low cycle fatigue tests using smooth specimens of Mar-M247 are conducted at room temperature to simulate the fracture under out-of-phase thermal fatigue and the behavior of initiation and growth of small cracks is identified. Three kinds of specimen are cut from a cast plate such that their axes possess angles of 0 degree , 45 degree and 90 degree with respect to the less than 001 greater than orientation that is parallel to the solidification direction; these specimens being denoted the specimen 0 degree , the specimen 45 degree and the specimen 90 degree , respectively. The results are discussed in connection with the anisotropic and composite microstructures. They are summarized as follows. (1) Transgranular cracks are initiated in all specimens. The first crack in Specimen 0 degree nucleates at 8000 cycles and those in other two specimens at about 1000 cycles. Cracks of Specimen 0 degree are initiated from casting defects at surface of the specimen, whereas cracks of Specimens 45 degree and 90 degree originate mostly in slip bands and the number of cracks per unit area is 200 times as large as that of Specimen 0 degree . (2) Cracks in Specimen 0 degree grow each other independently. On the other hand, cracks in Specimens 45 degree or 90 degree coalesce frequently. Grain boundaries, dendrite arms and gamma prime -phase precipitates do not work as barriers to the crack growth unlike the creep fatigue crack. However, the direction of crack growth has a strong dependence on the crystallographic orientation of the gamma -matrix. (3) The crack growth rate tends to be the lowest in Specimen 0 degree for the same half crack length. This is caused by the smallest Young's modulus, the largest resistance for slip band formation, and the lowest crack density and hence the crack growth without coalescence. Key Words:directionally solidified superalloy, Mar-M247, fatigue, small crack, crack initiation, crack growth, anisotropy