Influence of precipitate morphology on the high temperature fatigue properties of SRR99

Fatigue crack growth (FCG) tests have been conducted in air at 650°C and 850°C on 〈001〉 oriented single crystals of SRR99 having the γ′ particles in the form of: (A) 0.3 μm cuboids; (B) 0.2 μm ogdoadical cuboids; and (C) a coarse, rafted γ′ structure. In general, reducing the frequency and increasing the temperature enhances crack-tip shielding at low ΔKs due to increasing oxide induced crack closure. In material A at 650°C the crack path changes from one of γ′ precipitate cutting on {001} to propagation within the matrix as ΔK increases. Enhanced crack branching at 850°C improves the Paris regime behaviour compared with that seen at 650°C. In material B at 650°C, greater cross slip at lower frequency reduces slip reversibility, thus enhancing the fatigue crack growth rate (FCGR). At 850°C crack tip blunting and meandering, associated with γ′ cutting, improves the high ΔK FCG response and on a strength/modulus normalized basis is comparable with that seen for material A. Material C shows a similar FCG resistance to A at 650°C, but there is an acceleration in FCGR at 850°C, which can be accounted for in terms of the lower proof stress and modulus of this microstructure.