The mechanical response of the Ni−Ni₃Nb eutectic composite: Part II. Cyclic behavior

Abstract

In an effort to more fully understand fatigue crack propagation in composite materials, tension-tension fatigue studies of the Ni−Ni₃Nb eutectic composite were conducted. This composite displayed excellent fatigue resistance in a notched configuration by exhibiting an endurance limit of 55 pct of the 108.7 ksi smooth bar tensile strength. Under high stress-low cycle fatigue conditions, the fatigue resistance of the composite was controlled by the {112} twinning and subsequent twin boundary fracture of the Ni₃Nb phase. However, under low stress-high cycle fatigue conditions, Stage I crack propagation was observed in the nickel matrix. It was concluded that the nickel matrix controls the high cycle fatigue resistance of the composite. The fatigue behavior of the Ni−Ni₃Nb eutectic composite was compared with that of the Al−Al₃Ni composite. The similarity in the operative crack growth mechanisms in these two composites suggests a trend which may apply to all composites.