INFLUENCE OF ORIENTATION ON THE LOW-CYCLE FATIGUE OF MAR-M200 SINGLE-CRYSTALS AT 650-DEGREES-C .1. FATIGUE LIFE BEHAVIOR

Low cycle fatigue tests on MAR-M 200 single crystals were conducted at 650-degrees-C under total axial strain control. Specimens having crystallographic orientations near [001], [111], [213] and [101] were tested under fully reversed strain. Fatigue life-total strain range curves were strongly orientation dependent but this behaviour was mainly due to variations in Young's modulus with orientation. Crack initiation occurs at MC particles at high plastic strain ranges where slip bands formed in the overall specimen gauge length. At longer lives, cracks initiate at subsurface micropores. The most part of fatigue life is spent in stage II crack propagation as shown by plastic replicas and metallographic observations of specimens. Fatigue life was thus predicted using a local approach to fracture and crack growth data from conventional compact tension fracture mechanics specimens.