Influence of the gamma ' Volume Fraction on the High-Temperature Strength of Single Crystalline Co-Al-W-Ta Superalloys

Understanding the influence of gamma' and secondary-phase fractions on the mechanical properties of superalloys is very important to optimize these high-strength materials. So far, this has not been systematically investigated for the novel class of Co-based superalloys. In this study, a Co-Al-W-Ta model alloy series was designed with compositions of gamma/gamma' on the tie-line and an increasing gamma' volume fraction of up to 70% after heat treatment at 900 degrees C, while a few alloys are unexpectedly out of gamma/gamma' two-phase region with an additional secondary phase fraction of up to 15%. The high-temperature strength and creep properties were evaluated by compression tests up to 1050 degrees C and compressive creep experiments at 950 degrees C, respectively. At temperatures of up to 1050 degrees C, an increasing gamma' volume fraction consistently increased the yield strength, which was not dramatically changed by the presence of secondary phases. Significant work hardening was found in alloys with gamma' volume fractions of 65-70% during compression testing, but not in alloys with either a lower gamma' volume fraction (<50%) or a high fraction of secondary phases (similar to 15%). Similar to the yield strength, the creep strength also increased continuously with the gamma' volume fraction, but was greatly reduced with an increasing fraction of secondary phases. The best creep performance at 950 degrees C and 200 MPa was found in the alloy with the highest gamma' volume fraction and no secondary phases. At higher creep stresses, rafting contributed significantly to the hardening and, again, the alloy with a high gamma' volume fraction and a small amount of secondary phases exhibited the highest strength.