FRACTURE-BEHAVIOR OF AN ALUMINIDE COATING ON A SINGLE-CRYSTAL NICKEL-BASE SUPERALLOY

Measurement of the coating fracture strain of an aluminide coating on a single crystal nickel base superalloy has been performed both in three-point bending and using variable wall thickness testpieces. As-aged specimens, 28 to 33 mum in thickness, were tested at room temperature, 600, 700 and 750-degrees-C; specimens pre-exposed for 140 h at 850 and 1100-degrees-C in air and vacuum were tested at room temperature. Fracture strains varied from 0.52 to 0.70% for as-aged specimens tested at temperatures up to 700-degrees-C and specimens exposed at 850-degrees-C and tested at room temperature. The crack path for these conditions was intergranular or transgranular in the main coating, along carbide-matrix interfaces in the coating transition zone, and at an angle of 30-45-degrees to the original crack path in the substrate. The as-aged coating tested at 750-degrees-C was ductile; a ductile-brittle transition occurs between 700 and 750-degrees-C for the strain rate used (1 x 10(-5) s-1). Following 1100-degrees-C pre-exposure, specimens were ductile at room temperature with fracture strains of several percent. In this condition the crack morphology changed to one of subsurface nucleation in beta grains and at beta-gamma' interfaces.