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Design of new Co-Al-W-based alloys

Final Report Summary - COALWSUPER (Design of new Co-Al-W-based alloys)

New Co-Al-W-base superalloys will become an essential technology for high efficiency energy generation and transportation. As more pressure is placed on currently available single-crystal Ni-based superalloys to perform at even higher temperatures, Co-Al-W-base superalloys are likely to become their replacement in the future. Recent studies have shown that this new class of alloys possesses interesting high-temperature properties, including pronounced anomalous yield strength, good high temperature creep properties, and low segregation upon solidification. The small gamma/gamma-prime lattice misfit means that a high fraction of gamma-prime precipitates can grow semi-coherently within the gamma matrix maintaining a cuboidal morphology, the optimal microstructure for components in the hottest parts of gas turbines and jet engines. Unfortunately, the stability of the gamma-prime strengthening phase is still questionable, and adopting experimentation to study the full alloy design space is impractical.

The overall aim of this proposal is to develop and apply state-of-the-art modelling methods to address the stability and performance of these alloys, reducing the alloy design space considerably. A few attempts to use ab initio simulations to investigate the stability of the gamma-prime phase have been made, but these have failed to either prove or disprove the presence of the gamma-prime phase in the Co-Al-W ternary system as no entropy contributions were evaluated. Understanding whether the gamma-prime phase is stable, and determining how alloying can be used to stabilise it, will significantly impact the design of these alloys as well as provide insights on several aspects of physical metallurgy. The results of this work are expected to guide future experimentation and lead to new high-performance alloy candidates for use in the gas turbines and jet engines, building considerable positive industrial, financial and environmental impact.

Since the beginning of the project, work has focussed on investigating the stability of the gamma-prime phase relative to other conflicting phases, such as the D019-Co3W (delta) phase. This is of particular importance, since the stability of the gamma-prime phase relative to the delta phase has dramatic effects on the high-temperature properties of these alloys. One of the major weaknesses of Co-Al-W-base superalloys is that the gamma-prime phase transforms to the delta phase during service. In addition, the ease of transformation between the gamma-prime phase and the delta phase means that the gamma-prime precipitates are easily sheared by superlattice partial dislocations separated by wide superlattice instrinsic stacking faults.

Results indicate that the stability of the gamma-prime phase is strongly dependent on Al/W ratio. Although the gamma-prime phase in these alloys has been traditionally assumed to have equivalent concentrations of Al and W, it appears that a higher concentration of W promotes the stability of the gamma-prime phase. This is a problem, since the amount of W required would render these alloys too dense and unsuitable for use in jet engines. The solution may be using other solute atoms to replace W in these alloys. Our results indicate that elements such as Ti, V, Zr, Nb and Ta are beneficial to the stability of the gamma-prime phase relative to other detrimental phases. On the other hand, elements like W, Mo, Cr and Re promote the formation of unwanted phases. These findings have since been validated by experiments, and the community has now focussed on using elements such as Ti and Ta to produce stable gamma/gamma-prime microstructures.