Improving the intermediate- and high-temperature strength of L12-Co3(Al,W) by Ni and Ta additions

The effects of Ni and Ta additions on the mechanical properties in the L12 compound Co3(Al,W), the strengthening phase of Co-based superalloys, have been investigated by compression tests between room temperature and 1000 °C, in order to elucidate the effects of stability of the L12 phase on the mechanical properties. The additions of Ni and Ta, both of which are L12-stabilizers that increase the L12 solvus temperature, increase the yield strength at intermediate and high temperatures. The strength increase is shown to be more significant as the amount of additions of these elements and thereby the stability of the L12 phase increases. Two factors account for the strength increase at intermediate temperatures: The reduction of the onset temperature of yield stress anomaly (YSA-onset) due to the increased complex stacking fault (CSF) energy and the increase in both the base strength and the intensity of the yield stress anomaly associated with an increased anti-phase boundary (APB) energy on (111) planes. The strength increase at high temperatures, on the other hand, arises from the increase in the peak temperature due to the increased L12 solvus temperatures. The increased strength of the L12 phase due to a higher phase stability thus partly accounts for the improved creep strength of Co-based superalloys upon alloying with Ni and Ta.