Homogenizing a Nickel-Based Superalloy: Thermodynamic and Kinetic Simulation and Experimental Results
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MANY Ni-based superalloy castings and ingots are given a homogenization heat treatment prior to further processing or hot working in order to evenly distribute the alloying elements throughout the microstructure.[13] Solute inhomogenieties can adversely affect the corrosion/oxidation resistance, strength (use of alloy additions), service temperature (resulting from artificially lowering the melting temperature in interdendritic regions), hot workability (from grain boundary liquation or cracking), and induce formation of undesired topologically close-packed phases. Alloys that are used in the cast form are not subjected to hot working, which can very effectively redistribute solute atoms during wrought processing.[4] Thus, the homogenization heat treatment of cast Ni-base superalloys is especially critical and can be viewed as the last ‘‘processing’’ step the component receives before being put into service. In the case of solid-solution-strengthened alloys, the homogenization heat treatment is the last chance the metallurgist has to optimize the microstructure of the alloy for the required service conditions it will experience. Solid solution strengthening in Ni-based superalloys is typically accomplished by alloying with refractory elements such as W and Mo. Due to their large atomic size, high melting points, and hence sluggish diffusion kinetics, the refractory elements will typically be the most difficult elements to homogenize. For second-phase-strengthened alloys that require aging heat treatments to induce precipitation reactions (for example, gamma-prime-strengthened alloys), the PAUL D. JABLONSKI, Metallurgist, is with the National Energy Technology Laboratory, United States Department of Energy, Albany, OR 97321. CHRISTOPHER J. COWEN, Metallurgist, is with the National Energy Technology Laboratory, United States Department of Energy, and Parsons Corporation, South Park, PA 15129. Contact e-mail: [email protected] Manuscript submitted November 6, 2008. Article published online March 4, 2009. 182—VOLUME 40B, APRIL 2009
homogenization heat treatment is also critical, because the success of the homogenization heat treatment directly dictates the distribution of the fine dispersion of second-phase strengthening precipitates that are developed in the microstructure during aging.[5,6] Parameters for homogenization heat treatments for newly developed alloys are defined by trial and error, past practice on alloys of similar composition (parent alloys), or extensive experimental lab work.[2,3,6] If a sample of the cast microstructure can be obtained, the segregation across a dendrite arm can be profiled and the homogenization modeled with thermodynamic modeling software such as diffusion-controlled transformations (DICTRA).[7] Such a homogenization treatment can be important, especially for Ni-based superalloys that are designed to be slow diffusing for high-temperature mechanical and microstructural stability. While a traditional brute force approach to diffusion calculations may work to design a homoge
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