FATIGUE CRACK-PROPAGATION BEHAVIOR OF UNIDIRECTIONALLY SOLIDIFIED GAMMA-GAMMA'-DELTA EUTECTIC ALLOYS

Fatigue crack propagation (FCP) studies were conducted on a series of γ/γ′-δ (Ni-Nb-Al) alloys by subjecting them to cyclic four-point bending loads at room temperature. The aluminium contents of the alloys investigated ranged from 1.5% to 2.5% by weight, with the niobium contents adjusted to maintain controlled eutectic microstructures. In addition to studies of as-grown alloys, heat treatments were performed on several of the alloys to determine the effect of resultant changes in microstructure on FCP behaviour. Crack growth rates from approximately 2×10-6 to 10-3 mm/cycle were recorded as a function of the crack tip stress intensity factor range. The growth rates for the heat-treated alloys differed little from the as-grown FCP behaviour. Comparison with other published results indicated that the addition of aluminium was beneficial to FCP resistance, although the level of aluminium addition (within the investigated range of 1.5 to 2.5% by weight) did not influence the crack growth rates. Based on a comparison with previously reported results, chromium additions were seen to have a detrimental effect on FCP behaviour. Fractographic studies revealed the superior fatigue behaviour of the γ/γ′-δ eutectic composite to be a result of repeated grain boundary delamination as the crack progressed through the microstructure. © 1979 Chapman and Hall Ltd.