Computationally Optimized Homogenization Heat Treatment of Metal Alloys

Computationally Optimized Homogenization Heat Treatment of Metal Alloys

A computational approach has been developed to improve the homogenization heat treatment of alloys such that segregation in a casting can be greatly reduced or eliminated. The method utilizes computational thermodynamics and kinetics to determine a stepped approach to achieve optimal homogenization of the metal alloy through diffusion of the most segregation prone elements within the casting, resulting in improved material physical and mechanical properties and reduced processing costs. In addition, the approach is flexible and allows the homogenization process to be ‘tuned’ for the available heat treating furnace temperature envelop and for the intended use of the alloy within the commercial sector. This technology is available for further collaborative research with the U.S. Department of Energy’s National Energy Technology Laboratory.
Abstract: 
Alloys consist of combinations of two or more elements. During the solidification of any alloy, the constituent component elements typically segregate unevenly throughout the alloy as the solidification process proceeds with the first molten liquid chemistry different from the last molten liquid to solidify. This chemical segregation is unavoidable and as such these non-uniform regions within the solidified ingot negatively impact an alloy’s mechanical and physical properties. For example, corrosion and/or oxidation resistance, overall creep capability, upper limit service temperature, and hot workability can be improved by reducing segregation, and therefore, retarding the formation of undesirable secondary phases. Redistribution of the alloying elements within the solidified ingot is, quite simply, necessary to achieve optimal alloy performance for its desired application. This requires understanding of the thermal processes involved not only in the homogenization step but also in melting the alloy initially as well as any post homogenization thermo-mechanical processing and heat treatment. The computational approach to homogenization uses basic diffusion kinetics to determine the homogenization cycle necessary to redistribute the alloy’s segregation prone elements more uniformly throughout the alloy, based on each of these elements diffusion coefficient. In many cases, the extent of homogenization (i.e., partial or complete) directly determines the performance of the alloy, especially those used for heat resistant applications. Conventional homogenization processes are trial and error in their approach, requiring much experimentation and analysis. This usually entails numerous heat treated samples, substantial furnace use time, and associated analysis energy (sectioning specimens, evaluating them microstructurally and chemically, and then modifying the heat treatment as necessary). If the selected times and temperatures do not achieve the desired degree of homogenization, additional experimentation will be necessary to achieve an acceptable degree for that alloy’s application. The homogenization cycle so determined is certainly not optimal since many possible process paths cannot be explored. Furthermore, this "Edisonian” approach is costly and time-consuming as well as inefficient. Understanding the necessity of mating material performance with starting alloy microstructure, and realizing that the only way to get the appropriate starting alloy microstructure is through complete homogenization of the alloy, NETL researchers developed computationally-based algorithms to design alloy unique homogenization cycles through the use of commercially available thermodynamics and kinetic data.
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