Influence of Heat Treatment on Fatigue Crack Growth Rate of As-cast Nickel-Aluminum Bronze

The as-cast nickel-aluminum bronze (NAB) was normalized at 920 ℃ and annealed at 675 ℃ respectively, and then the microstructures and tensile properties of the NAB in different states were studied. The fatigue crack growth rates were measured by direct current potential drop (DCPD) method and the fatigue crack growth path and fracture morphology were observed. The results show that the microstructures of the annealed and as-cast samples both consisted of α phase, residual β phase and three kinds of Ni-Fe-Al intermetallic compounds (κⅡ, κⅢ, κⅣ), while that of the normalized sample consisted of residual β phase and evenly distributed κⅣ phase, which had a much higher strength and a decreased plasticity. The fatigue crack growth rate of the as-cast sample was the fastest while that of the normalized sample was the slowest. The fatigue crack in as-cast and annealed samples propagated from the interface of κ phases and showed a cleavage fracture mode; in normalized sample the fatigue crack propagated through α phase with fatigue striation on the fracture surface and the fatigue crack growth path was the most tortuose.