A review of very-high-temperature Nb-silicide-based composites
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11/9/03
3:25 PM
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A Review of Very-High-Temperature Nb-Silicide–Based Composites B.P. BEWLAY, M.R. JACKSON, J.-C. ZHAO, and P.R. SUBRAMANIAN The temperatures of airfoil surfaces in advanced turbine engines are approaching the limits of nickelbased superalloys. Innovations in refractory metal–intermetallic composites (RMICs) are being pursued, with particular emphasis on systems based on Nb-Si and Mo-Si-B alloys. These systems have the potential for service at surface temperatures 1350 °C. The present article will review the most recent progress in the development of Nb-silicide–based in-situ composites for very-high-temperature applications. Nb-silicide–based composites contain high-strength silicides that are toughened by a ductile Nb-based solid solution. Simple composites are based on binary Nb-Si alloys; more complex systems are alloyed with Ti, Hf, Cr, and Al. In higher-order silicide-based systems, alloying elements have been added to stabilize intermetallics, such as Laves phases, for additional oxidation resistance. Alloying schemes have been developed to achieve an excellent balance of room-temperature toughness, high-temperature creep performance, and oxidation resistance. Recent progress in the development of composite processing-structure-property relationships in Nb-silicide–based in-situ composites will be described, with emphasis on rupture resistance and oxidation performance. The Nb-silicide composite properties will be compared with those of advanced Ni-based superalloys.
I. INTRODUCTION
HOT-SECTION materials developments have made substantial contributions to performance improvements of both aircraft engines and land-based gas turbines. In addition, advances in casting technologies for Ni-based superalloy airfoils have provided the evolution from equiaxed microstructures to directionally solidified multigrain and single-crystal components. As a result of the developments in both alloy composition and casting technology, high-pressure turbine blade temperatures have increased by 125 °C in the past 30 years. Metal surface temperatures at the hottest locations on advanced turbine engine airfoils now approach 1150 °C, which is essentially the limit for nickel-based superalloys. Thermal barrier coatings (TBCs) can also provide airfoils with insulation from the hot gas, so that the TBC surface temperatures can be appreciably higher than 1150 °C. Further increases in the temperature capability of nickelbased superalloys beyond that of the fourth-generation single-crystal alloys will be very difficult to achieve, because most advanced superalloys melt at 1350 °C. Depending on the solidification processing method and subsequent heat treatment, chemical segregation in the superalloy can lead to incipient melting at 1270 °C. In addition, the interaction zone between the bond coat and the single-crystal airfoil can melt at temperatures less than 1250 °C. Thus, there are clear B.P. BEWLAY, M.R. JACKSON, and P.R. SUBRAMANIAN, Staff Metallurgists, and J.-C. ZHAO, Materials Scientist, are wit
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