In order to clarify the crack propagation property of Nickel-based directionally solidified superalloy, longitudinal loading (L-type) specimens and transverse loading (T-type) specimens were subjected to the high temperature fatigue crack propagation tests. Fatigue crack propagation rate was correlated fairly well with effective stress intensity factor range Δ
Keff, regardless of specimen type (L or T), stress range, and
R ratio. In detail, crack propagation rate in T specimen showed eminent fluctuation and it was partially accelerated to 3 to 5 times faster than the average. Detailed observation of specimens revealed that the fluctuation was strongly dependent on the micro-structure along the crack propagation path. Both of transgranular and intergranular crack types were observed on the crack propagation path of T specimen, while that in L specimen showed only the transgranular type. Moreover, transgranular crack showed two different failure planes, they were along {111} or {001} planes, respectively. Since most part of the whole tests were governed by transgranular cracking, they showed mostly the same propagation property. Intergranular crack in T specimen showed much faster propagation than the transgranular crack, and caused eminent fluctuation of propagation rate.
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