Improving the High-Temperature Strength of L1₂-Co₃(Al,W) by Ni and Ta Additions

Microstructures of the ternary Co-Al-W system consist of two phases, an ordered Co₃(Al,W) phase ( L1₂ crystal structure, usually designated as the γ’ phase) and a face centered (fcc) Co-rich solid solution. There has been a significant research activity in the development of Co-based superalloys where Co₃(Al,W) is the strengthening phase with an initial expectation that high-temperature mechanical properties of Co-based superalloys would outperform those of Ni-based alloy. This was related to the fact that precipitation of the γ’ phase in the γ matrix occurs coherently and forms cuboidal precipitates like in Ni-based superalloys with the advantage that both the melting temperature and the elastic stiffness are higher for Co than for Ni. However, in spite of the expectation, the high-temperature creep strength of Co-based superalloys which were so far developed are modest, only comparable to those of Ni-based superalloys of the first generation. A lower phase stability of γ’ phase, which causes a low high-temperature strength was suggested to be responsible for this disappointing find. In fact, the addition of elements that increase the γ’ phase stability (L1₂-stabilizer) is reported to improve the creep properties of the Co-based alloys. The γ’ high-temperature strength is also expected to be improved in these alloys but nothing is known at present about how a higher high-temperature strength of the γ’ phase is achieved by L1₂-stabilizer additions.<br/>In the present study, we investigate the compression behavior of L1₂-compounds Co₃(Al,W) with Ni and Ni+Ta additions, which are known as L1₂-stabilizers, in a temperature range from room temperature to 1000^oC, in order to elucidate the effects of stability of the L1₂ phase on the mechanical properties. Emphasis is placed on the deformation microstructures of these L1₂ compounds through detailed transmission electron microscopy (TEM) investigations, paying special attention to how the anomalous increase in yield stress (often referred to as ‘yield stress anomaly’ in L1₂ compound research) occurs in these L1₂ compounds.<br/>The result shows that the addition of Ni and Ta increases the high-temperature strength of the γ’ phase. 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. The reduction of the onset temperature of yield stress anomaly (YSA-onset) due to the increased complex stacking fault (CSF) energy and the increased anti-phase boundary (APB) energy is considered to account for the strength increase at high 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.