In this dissertation, the effects of Rhenium (Re) and Cobalt (Co) on the microstructures and properties and the interactions between them in single crystal superalloys are investigated.
With the increasing of rhenium content, the size of as-cast reduces and the incipient melting points of alloys enhance. With the rising of rhenium content, the dissolution of eutectics and the alleviation of chemical segregation becomes more difficult. When the content of Re is rising, the primary ageing temperature heightens. With the increasing of rhenium content, the diffusion coefficient reduces and coarsening is inhibited. Re is beneficial to improve the creep rupture lives.
With the increasing of cobalt content, the volume fraction of as-cast increases and the size of decreases. When the content of Co is rising, the incipient melting points of alloys lower. With the the increasing of Co content, the dissolution of eutectics and the alleviation of chemical segregation becomes easier. With the the rising of Co content, the diffusion coefficient elevates, which accelerates coarsening and suppresses the formation of TCP phase. With the increasing of cobalt content, the creep rupture lives of alloys free of Re improve and that of alloys containing Re do not degrade. In alloys containing high levels of Re, the deleterious phases can be eliminated by adding some contents of cobalt.
The influences of cobalt on deformation mechanisms in Re-containing alloys are quite different from that in the alloys free of Re. For the alloys containing Re, the particles are cut by stacking faults (SF) in the alloy having 12wt.% Co when creep ruptured at 850℃/586MPa. However, for the alloys free of Re, when they creep ruptured at 1010℃/248MPa, the particles are cut by dislocation pairs coupled with APB in the alloy containing 12wt.%Co, while the particles are cut by stacking faults in the alloy free of Co. The cutting of particles by stacking faults or by dislocation pairs coupled with APB depends on the interactions of Re and Co on the energies of SF and APB.
After long-term ageing at 950℃ and 1050℃, when was cubic, global or irregular, its coarsening is more controlled by diffusion; when formed rafts, its coarsening is more controlled by interface reaction. A model of describing the process of forming rafts is established.
In this dissertation, a low cost third generation single crystal superalloy containing 4wt.% Re and 12wt.%Co is developed. The creep rupture lives of this alloy approach or achieve the levels of alien third generation single crystal superalloys having 6wt.% Re. This alloy has also high tensile strength, good oxidation and hot corrosion resistance properties, excellent microstructural stabilities and high strength after long time exposure.
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