Improving the weldability and service performance of nickel-and iron-based superalloys by grain boundary engineering
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NICKEL- and iron-based superalloys are normally specified for high-temperature applications such as jet engines and gas turbine components, including blades (e.g., alloy 738), disks, rotors (e.g., alloy V-57), and combustor/burner cans (e.g., alloy 625),[1,2,3] as well as compressor and turbine cases. Superalloys are differentiated from more conventional ‘‘stainless’’ alloys on the basis of their ability to retain tensile strength at temperatures exceeding 80 pct of the absolute melting temperature. Sustained temperatures of between 800 7C and 1000 7C (in the presence of sulfur, which diffuses along grain boundaries forming Ni3S2, CrS, or Cr2S3, commonly referred to as ‘‘spiking’’) render these alloys susceptible to intergranular degradation by ‘‘hot corrosion,’’ fatigue, and creep.[3] Hot corrosion and sulfide spiking at intergranular sites ultimately result in a loss of tensile, fatigue, and impact strength.[3] In contrast to cobalt-based superalloys, precipitationstrengthened Ni- and Fe-based alloys are generally less weldable, limiting use of alloys such as V-57, 738, and alloy 100 in applications where complex geometries are constructed by joining individual components. This has been the main limitation for using higher temperature precipitation-strengthened alloy formulations for combustorcan components.[3] Weldability correlates directly with the Al and Ti content in the alloy, as illustrated in Figure 1.[4] Gamma prime (g') phases formed by these constituents (i.e., Ni3Al,Ti), which are responsible for high-temperature strength, precipitate along grain boundaries in the weld heat-affected zones (HAZs) resulting in hot cracking (weld cracking) and postweld heat treatment (PWHT) cracking.[4] Increasing resistance to intergranular degradation has been the primary focus of superalloy development with turE.M. LEHOCKEY, Senior Scientist, G. PALUMBO, Principal Scientist, and P. LIN, Scientist, are with Ontario Hydro Technologies, Toronto, ON, Canada M8Z 5S4. Manuscript submitted February 27, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
bine entry temperatures continually increasing in modern industrial turbines in an effort to improve fuel and combustion efficiency.[1,3] At the same time, service life of industrial turbines (105 hours) being an order of magnitude longer than those expected of jet engine/aerospace components (for which these alloys were originally developed) has placed additional emphasis on long-term reliability and endurance of these materials.[3] Performance improvements associated with successive generations of superalloys have traditionally been derived from alloying additions to control the content, distribution, and growth (ripening) of intermetallic g' (NiAl3) and carbide (MC, M23C6 M6C) phases.[5,6] Unfortunately, thermal conductivity and phase stability considerations place practical limits on alloying as a means of further improving corrosion, creep, fatigue, and strength performance. Single-crystal, directionally solidified, ceramic, and diffusion barrier overlay components such as
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