γ/γ\u27 Co-base superalloys – new high temperature materials beyond Ni-base Superalloys?

In 2006 a new L12 phase, Co3(Al,W), was discovered in the Co-Al-W system which has led to the development of novel Co-base superalloys with g/g¢ microstructures similar to those of the well-established Ni-base superalloys. First investigations on simple ternary alloys could show that these Co-Al-W based alloys exhibit higher solidus temperatures and show less segregations after casting compared to typical Ni-base superalloys. This leads to the question whether this g/g¢ Co-base superalloys can be regarded as new class of high temperature materials that can compete with or even supersede established Ni-base superalloys. In the first part of the talk it will be shown how alloy properties change, when the base element Ni is gradually substituted by Co in a series of Ni-Co-Al-W-Cr alloys with otherwise constant element contents of Al, W and Cr. All alloys form g/g¢ microstructure after a standard aging treatment with a similar g¢ volume content. Liquidus and solidus temperatures are hardly influenced by the Ni/Co content, but the g¢ solvus temperature is strongly decreasing with increasing Co content. This indicates that the potential application temperature of g/g¢ Co-base superalloys will not be beyond the maximum application temperature of advanced single crystal Ni-base superalloys. However, this also shows that g/g¢ Co-base superalloys have a great potential as wrought alloys since the solvus temperature of the intermetallic compound is comparatively low, which gives a large processing window, and because a high volume fraction of the L12 phase at temperatures up to 900°C can be achieved. Please click Additional Files below to see the full abstract