Phase stability and yield stress of Ni-base superalloys containing high Co and Ti
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ADVANCED supersonic commercial and military aircrafts will require turbine engines capable of sustaining supersonic cruise for extended time periods. This will impose severe challenges for the high pressure compressor and turbine disks, which will operate with rim temperatures in excess of 700 °C during the entire supersonic flight.[1,2] To achieve this goal, advanced Ni-base disk superalloys, such as ME3 (General Electric, Cincinnati, OH),[1,2] NR3 (SNECMA, Futuroscope Chasseneual),[3] RR1000 (RollsRoyce, Derby),[4] and LSHR (NASA, Cleveland, OH),[5] have been developed. These alloys are all strengthened by high refractory element contents. To avoid the risk of elemental segregations and unacceptable contents of carbonitride stringers,[4] these heavily alloyed alloys are usually fabricated using a powder metallurgy processing route, which is more complex than a cast and wrought (CandW) processing route. Recently, we proposed a new idea to design Ni-base disk superalloy by mixing a UDIMET* Ni-base superalloy
II.
ALLOY DESIGN
Figure 1 shows a schematic pseudo-binary phase diagram of a (Ni, X)-(Al, Y) system. It is known that a Nibase superalloy mainly consists of g and g9 phases in an equilibrium state at a given temperature, and the compositions of g and g9 phases can be indicated with B and C. Thus, any alloy with different g9 volume fraction on the tieline BC can be fabricated by mixing the two alloys with compositions of B and C. If the compositions of g and g9 phases are given as Xi and Xi9, respectively, the composition (Ci) of a Ni-base superalloy with a given g9 fraction (f) can be represented by Ci 5 ð1 f ÞX i 1 f X 0 i ði: Ni, Al, Co, Cr, Mo . . .Þ
[1]
The compositions of g and g9 phases in the new alloys on the g/g9 tie-line are kept the same; thus, the lattice misfit between g and g9 phases does not change. Using the tieline alloys, Harada et al.[9] first found that the creep rupture life of the Ni-base superalloy INCONEL* 713C is the
*UDIMET is a trademark of Special Metals, Huntington, WV.
U720LI with a g/g9 two-phase Co16.9 wt pct Ti (henceforth referred to as the ‘‘Co-Ti’’) in various contents for application at temperatures above 700 °C.[6,7] The results showed that the additions of Co-Ti to U720LI can greatly suppress the formation of topological closed packed phases.[6] In addition, new alloys exhibit higher yield strength than that of the baseline alloy, U720LI, at temperatures below 850 °C. However, they show similar strength at the processing temperature about 1100 °C, which means that these new alloys could be fabricated by the CandW route. However, in some of these alloys, Ni3Ti (h) phase with a hexagonal close-packed structure is formed in the interdendritic region. The h phase is reported to have a deleterious effect on an alloy’s ductility.[8] Up to now, the effects of Co-Ti contents on the formation of h phase and mechanical properties are not clear. C.Y. CUI and A. SATO, Research Fellows, Y.F. GU and D.H. PING, Senior Researchers, and H. HARADA, Director, are with the High Temperat
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