Orientation and temperature dependence of some mechanical properties of the single-crystal nickel-base superalloy René N4: Part II. Low cycle fatigue behavior

Abstract

The low cycle fatigue (LCF) properties of a single-crystal nickel-base superalloy, René* N4, have been examined at 760 and 980 °C in air. Specimens having crystallographic orientations near [001], [01l], [111], [023], [236], and [145] were tested in fully reversed, total-strain-controlled LCF tests at a frequency of 0.1 Hz. At 760 °C, this alloy exhibited orientation dependent tension-compression anisotropies of yielding which continued to failure. Also at 760 °C, orientations exhibiting predominately single slip exhibited serrated yielding for many cycles. At 980 °C, orientation dependencies of yielding behavior were smaller. In spite of the tension-compression anisotropies, cyclic stress range-strain range behavior was not strongly orientation dependent for either test temperature. Fatigue life on a total strain range basis was highly orientation dependent at both 760 and 980 °C and was related chiefly to elastic modulus, low modulus orientations having longer lives. Stage I crack growth on (111) planes was dominant at 760 °C, while Stage II crack growth occurred at 980 °C. Crack initiation generally occurred at near-surface micropores, but occasionally at oxidation spikes in the 980 °C tests.