HIGH TEMPERATURE CREEP DAMAGE MECHANISMS IN A DIRECTIONALLY SOLIDIFIED ALLOY: IMPACT OF CRYSTALLOGRAPHY AND ENVIRONMENT

The damage mechanisms in creep have been investigated in directionally solidified Mar-M200+Hf (DS200 + HO alloy at high temperatures. Creep tests were performed along the solidification and transverse directions. These properties have been compared to experiments using single crystalline (SX) specimens have the same chemical composition. Creep tests were performed at 750 degrees C, 900 degrees C and 1100 degrees C, with a special focus on the intermediate temperature. Experiments both in air or under high vacuum were performed at this temperature, as well as experiments in air with a variation in sample thicknesses. Compared to longitudinal testing and SX experiments, shorter creep lives are systematically observed during transverse creep testing, whatever the testing temperature. This mainly results from a lower creep ductility along the transverse direction, due to an intergranular fracture mode which is assisted by oxidation at 900 degrees C and 1100 degrees C. The variation in creep life anisotropy observed between 750 degrees C and 1100 degrees C has been analyzed by considering the contribution of oxidation and local (at the grain scale) elastic and plastic behavior anisotropy. In situ monitoring of strain heterogeneities during transverse creep tests performed at 750 degrees C/800 MPa and at 900 degrees C/400 MPa shows that cracks develop between grains prone to large creep strains and at high angle boundaries. Moreover, fast oxidation and gamma '-rafting seems to be the main factors reducing the creep life anisotropy.