Synthesis of a new tungsten-free γ–γ′ cobalt-based superalloy by tuning alloying additions

The paper presents the synthesis of a new class of γ–γ′ cobalt-based superalloy that is free of tungsten as an alloying addition. It has much lower density and higher specific strength than the existing cobalt-based superalloys. The current superalloys have a base composition of Co–10Al and are further tuned by the addition of a binary combination of molybdenum and niobium, with the optimum composition of Co–10Al–5Mo–2Nb. The solvus temperature of the alloy (866 °C) can be further enhanced above 950 °C by the addition of Ni to give the form Co–xNi–10Al–5Mo–2Nb, where x can be from 0 to 30 at.%. After heat treatment, these alloys exhibit a duplex microstructure with coherent cuboidal L1₂-ordered precipitates (γ′) throughout the face-centred cubic matrix (γ), yielding a microstructure that is very similar to nickel-based superalloys as well as recently developed Co–Al–W-based alloys. We show that the stability of the γ′ phase improves significantly with the nickel addition, which can be attributed to the increase in solvus temperature. A very high specific 0.2% proof stress of 94.3 MPa g⁻¹ cm⁻³ at room temperature and 63.8 MPa g⁻¹ cm⁻³ at 870 °C were obtained for alloy Co–30Ni–10Al–5Mo–2Nb. The remarkably high specific strength of these alloys makes this class of alloy a promising material for use at high temperature, including gas turbine applications