Niobium Silicide High Temperature In Situ Composites

Abstract

The requirements for greater aircraft-engine performance, greater thrust-to-weight ratios, and greater fuel efficiency have resulted in significant increases in turbine gas-path temperatures. Present-day aircraft engines have combustion-gas temperatures well in excess of the melting temperatures of the airfoil alloys, and therefore rely on sophisticated cooling methods to keep the alloys solid. Both materials and airfoil blade designs have evolved to sustain these increasing demands (Bewlay et al., 1999a; Subramanian et al. 1996). Advances in high-temperature materials have had a major impact on the efficiency of gas-turbine engines, so that currently superalloys provide a maximum surface temperature capability of*1150 8C. The evolution in HPTB (high pressure turbine blade) cooling technology is illustrated schematically in Figure 1. In the 1960s, equiaxed-grain Ni-based superalloys were cooled with radial cooling passages and with film cooling holes at the leading and trailing edges to reduce the interaction with the combustion gases. This advance over uncooled hardware improved blade durability, and allowed an increase in turbine inlet temperatures to greater than 1100 8C (2000 8F). Once cooling of hardware became routine, and the ability was developed to cast airfoils with more complex cooling schemes, these gains in cooling effectiveness were coupled with improved investment casting technology, and with the introduction of directional solidification for the production of HPTBs with either columnar or single-crystal microstructures. In the past decade, the potential of new alloys strengthened with intermetallic compounds with low densities, high elastic moduli, and high melting ranges (Dimiduk et al., 1993; Subramanian et al., 1997) has been explored. Intermetallic-based compound materials, such as Nb or Mo silicides, have been combined with metallic second phases in order to generate composites with a combination of attractive hightemperature properties and acceptable low-temperature properties. Nb-silicide based in-situ composites with Nb3Si and/or Nb5Si3 silicides have been shown to have great potential because of their attractive balance of highand low-temperature mechanical properties (Mendiratta et al., 1993; Bewlay et al., 1996, 1997). These materials have the potential to surpass the performance of Ni-based superalloys. This chapter will describe directional solidification and single-crystal technologies, with particular consideration to their role in present and future aircraftengine applications. Areas that will be covered include Ni-based superalloys strengthened by Ni3Al, DS eutectics of Ni-based superalloys, and DS Nb-silicide-based in-situ composites. This chapter will compare the role that Ni-based superalloys and Nbsilicide composites play in improving the performance of gas-turbine engines. Microstructures, phase compositions, and mechanical behavior will be reviewed.