FATIGUE CRACK-PROPAGATION IN NI-BASE SUPERALLOY SINGLE-CRYSTALS UNDER MULTIAXIAL CYCLIC LOADS

The effects of crystallographic orientation and stress state on the multiaxial fatigue behavior of MAR-M200 single crystals were examined. Using notched tubular specimens subjected to combined tension/torsion cyclic loads, crack growth rates were determined at ambient temperature as functions of stress intensity range, the shear stress range-to-normal stress range ratio, and crystallographic orientation. Comparison of crack growth data at the same effective DELTA K reveals a weak dependence of the crack growth rate on both the tube axis and the notch orientation. For a given set of tube axis and notch orientation, the crack growth rate might or might not vary with the applied stress state, depending on whether roughness-induced crack closure is present. In most cases, subcritical cracking occurs either along a single left brace 111 right brace slip plane or on ridges formed with two left brace 111 right brace slip planes. Neither fracture mode is altered by a change in the applied stress state. This complex crack growth behavior will be discussed in terms of the crack-tip stress field, slip morphology, and crack closure.