Abstract: The strengthening mechanism of conventional Co-based superalloy usually include solid-solution strengthening and carbide strengthening, the reinforcement effect of which are weaker than the ordered phase γ′-strengthened of Ni-base superalloys, and this making its application limited. Stable Co3(Al,W) phase with L12 structure of Co-Al-W alloy was firstly discovered in 2006, and has many characteristics: (1) Ta-containing Co-Al-W alloys possess higher melting point than Waspaloy; (2) hardness and yield strength of Co-Al-W alloys are no less than those of Ni-base superalloys; (3) lattice mismatch between γ and γ′ is similar to that of Ni-base superalloys in numerical value and opposite in symbol, which is benefit to creep-resistance properties. Therefore the discovery of Co3(Al,W) phase supports potential for the further development of Co-base superalloys. The microstructure and properties of Co-Al-W-based superalloys have been widely investigated since 2006. In addition to γ and γ′ two phases, there are also some second phases in Co-Al-W-base alloys including B2-CoAl phase enriching in Al and Ti, topologically close-packed phase μCo7W6 enriching in refractory elements, and χ-Co3W phase likely precipitated during aging. These second phases tend to precipitate at grain boundary, which is easy to become the source of cracks. At the same time, they weaken the effect of solid solution strengthening, and deteriorate the high temperature properties of alloys. The cuboidal coherent γ′ phases are obtained in Co-Al-W-based alloys, however, they need to be alloying because of unstability of γ′ phase. As a result, this kind of γ′-strengthening Co-based superalloys are developed from simple Co-Al-W ternary alloys to complicated multi-component alloys. The alloying elements being added to Co-Al-W-based alloys include Ta, Ti, Nb, V, Mo, Ni and Cr. Among these elements, Ta, Ti, Nb, V and Mo are γ′-forming elements, which have partitioning coefficient greater than 1 and attribute to improve solvus temperature and volume fraction of γ′ phase; Cr, Fe and Re are γ-forming elements with partitioning coefficient less than 1 and reducing γ′ solvus temperature, especially, Cr can improve volume friction of γ′ phase. Besides, the excessive addition of Cr, Ni and Mo will reduce lattice misfit between γ′ and γ, resulting in transformation of shape of γ′ phase and destruction of microstructure of γ/γ′. It is noted that properties are closely related to microstructure, therein, the cuboidal and stable γ′ phase precipitated in γ phase usually make alloys display excellent properties. The temperature dependence of the flow stress of Co-Al-W-based alloys can be divided into three stages: firstly, flow stress decreases with the increase of temperature; secondly, there is an anomalous positive temperature variation where the flow stress substantially increase with temperature; finally, it backs to a negative temperature variation. So there are peak temperature and peak stress, alloys applied in peak temperature will have high strength. Besides, because of positive lattice misfit of Co-Al-W-based alloys. Moreover, during creep process, rafting parallel to tensile stress is beneficial to performance under high temperature owing to positive lattice misfit of Co-Al-W-based alloys. As for as elements, except Ta and Ti, a small amount of B element will improve mechanical properties by grain boundary strengthening. Cr-containing Co-Al-W-based alloys will form three oxide layers during oxidation under high temperature: outermost layer includes Al2CoO4, the middle layer includes Cr2O and Cr2O3 and innermost layer consisting of Al2O3. It is reported that compact and productive Cr2O3 and Al2O3 can improve oxidation resistance. This paper presents discovery and development of Co-Al-W-based superalloys. Then, the research status of Co-Al-W-based superalloys is summarized. Finally, the future progress of Co-based superalloy are proposed.
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