1. 1. By coarsening the grain size from 22 to 91 μm in Alloy 718, it was found that: i. near threshold crack growth rates were reduced and ii. ΔKthvalues increased. 2. 2. Roughness-induced crack closure may be used to explain the influence of grain size. The coarse grained material resulted in much rougher fracture surfaces at near threshold growth rates. 3. 3. At room temperature, a fracture morphology transition occurred which coincided with the concave upward ergion in the crack growth curves above ΔKth. Below the transition, the maximum plastic zone size was less than the grain diameter and the crack propagated by crystallographic cracking. Above the transition in which the maximum plastic zone size was greater than the grain diameter, crack extension occurred through the formation the formation of fatigue striations. This transition was shifted to higher ΔK values by coarsening the grain size. © 1985.
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