Fatigue fracture of directionally solidified superalloy mar-M247 at room temperature and dependence on the solidification direction

摘要

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°, 45° and 90° with respect to the <001> orientation that is parallel to the solidification direction; these specimens being denoted the specimen 0°, the specimen 45° and the specimen 90°, 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° nucleates at 8000 cycles and those in other two specimens at about 1000 cycles. Cracks of Specimen 0° are initiated from casting defects at surface of the specimen, whereas cracks of Specimens 45° and 90° originate mostly in slip bands and the number of cracks per unit area is 200 times as large as that of Specimen 0°. (2) Cracks in Specimen 0° grow each other independently. On the other hand, cracks in Specimens 45° or 90° coalesce frequently. Grain boundaries, dendrite arms and γ'-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 γ-matrix. (3) The crack growth rate tends to be the lowest in Specimen 0° for the same half crack length. This is caused by the smallest Young1s modulus, the largest resistance for slip band formation, and the lowest crack density and hence the crack growth without coalescence.