Effects of the alumina scale on the room-temperature tensile behavior of preoxidized MA 956

This article deals with the effects of the alpha-Al2O3 scale (similar to 5 mu m) developed during preoxidation (1100 degrees C/100 hours) of MA 956 on its room-temperature tensile behavior. The tensile tests were made in the strain-rate range of 10(-5) to 10(-1) s(-1). It is shown that the scale, fine and tightly adherent to the substrate, affects the tensile behavior in two relevant ways. First, the yield strength and the tensile strength are lowered with respect to those of the scale-free material. This is explained in terms of the residual stresses generated in the scale during preoxidation. From the analysis of the differences in the yield strength of preoxidized MA 956 with respect to the scale-free material, residual compression stresses in the scale of about 5500 MPa were obtained. These high stresses account for the surprisingly high tensile strain achieved (1.4 pct) before scale spallation occurs. Second, a ductile to brittle transition (DBT), which is not observed in the scale-free samples, occurs at intermediate strain rates (10(-3) s(-1)). The brittle fracture is related to the increase of the triaxiality state in the substrate near the scale/metal interface.