Study Of A Modified New Low Expansion Thermo-Span Superalloy

In this dissertation, the Thermo-Span alloy was modified with Al, Cr, P and B to improve its combined properties. The goal of this work lies in finding a way to develop a new low thermal expansion superalloy with lower thermal expansion coefficient and better mechanical and oxidation resistant properties. The Cr content was decreased to lower the thermal expansion coefficient, the Al content was increased to compensate the loss of oxidation abilities. In addition, P and B were added into the alloy modified with AI and Cr and heat treatment was adjusted to improve the grain boundary strength and get rid of notch sensitivity. The behaviors of Al, Cr, P and B during processing and their effects on the microstructure and properties of Thermo-Span alloy have been investigated systematically.The thermal expansion coefficient is decreased noticeably when the Cr content is decreased from 5.5wt.％to 3.0wt.％and the Al content is increased from 0.45wt.％to 1.5wt.％. In addition, the increment of Al content increases the tensile strength at room temperature and 650℃because it increases theγ' precipitation. The modification of Al and Cr increased the smooth stress rupture life at 650℃and 600MPa from 345h to 597h, while the modified alloy has a serious notch-sensitivity. Compared to the standard Thermo-Span alloy, the oxidation resistant property 650℃of the alloy modified with Al and Cr is slightly lower, but both the alloy belongs to the complete oxidation resistant class according to the standard oxidation evaluation regulation. Al is concentrated at the boundary of oxidation scale andγmatrix and hence it is helpful to increase the oxidation resistance.The effect of P and B on as-cast microstructure was studied firstly. P and B increases the elemental segregation and the precipitation of Laves phase during solidification. The Ti-P phase is precipitated in the alloy with 0.05wt％P, and the (Nb-B phas +γ) eutectic forms in the alloy with 0.5wt.％B. P hardly influences the melting point of Thermo-Span alloy. B also has little effect on the melting point when its content is lower than 0.1wt.％, but largely reduces the melting point of the alloy when its content is as high as 0.5wt.％due to the (Nb-B phase+γ) eutectic reaction.P and B do not affect the tensile properties of the modified alloy at room temperature and 650℃. The stress rupture life at 650/600MPa is decreased with the increasing P content due to the precipitation of P-bearing phase at grain boundaries and interior. The addition of B at the level of 0.1wt.％lowers the stress rupture life of the modified alloy significantly because it reduces the grain boundary precipitation, but that at the level of 0.5wt.％extends the life largely because it increases the grain boundary precipitation.The oxidation rate at 650℃for 100h is lowest when the P content is 0.023wt.％. All of the alloys follow the parabolic rule when oxidized at 650℃for 100 hours. The oxidation film includes two layers, the outer-layer rich in Fe, Co and O and transition layer rich in Nb, Ti, Al. The surfacial oxidation scale is made up of needle-like Fe and Co oxide and small particles of Co oxide. The needle-like Fe and Co oxide is loosely arranged and the small particles of Co oxide is densely arranged. When the P content is 0.023wt.％, the Fe and Co oxide fraction is less and hence the alloy has a higher oxidation resistance.By keeping the aging treatment unchanged, the effect of solution temperature and cooling rate on the stress rupture properties has been studied. The stress rupture life of the modified alloy at 650℃and 600MPa is largely reduced by increasing the solution temperature from 1093℃to 1160℃. P increases the rupture life under this condition because the P-beating phase was mostly dissolved, but the longest one is still shorter than 100h due to the dissolution of the grain boundary precipitates. The notch sensitivity is unchanged.By increasing the cooling rate from air cooling to water quenching, the amount ofγ' phase is not changed noticeably, but the size of theγ' phase is reduced. Under this condition, the alloy with 0.5 wt％B had higher stress rupture life without notch sensitivity.By modification with Cr, Al and B, and the increasing of the cooling rate following solution, a new alloy with better combined properties has been developed, which has a lower expansion coefficient, good oxidation resistance, and high mechanical properties.