Numerical Simulation of Solidification, Homogenization, and Precipitation in an Industrial Ni-Based Superalloy
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L-BASED superalloys are employed to manufacture turbine blades for aeronautical applications owing to their good mechanical properties at high temperature.[1] The microstructural features of these alloys, i.e., the volume fraction and the size of the c¢ precipitates embedded in the c phase, largely determine the properties at high temperatures. For this reason, heat treatments are carried out to control the microstructure. The c¢ phase forms first during solidification as a coarse interdendritic eutectic microstructure with poor mechanical properties. A solution heat treatment followed by a quench is then performed to dissolve the c¢ eutectic and attenuate the composition gradients in the primary c dendrites. The alloy is then LUC ROUGIER, Ph.D. Student, is with Ecole Polytechnique Fe´de´rale de Lausanne, 1015 Lausanne, Switzerland, and also with Snecma-SAFRAN Group, 92702 Colombes, France. ALAIN JACOT, formerly Senior Scientist with Ecole Polytechnique Fe´de´rale de Lausanne, is now Group Leader with the ESI Group, Calcom ESI SA, Route Cantonale 105, 1025 St-Sulpice, Switzerland. Contact e-mail: [email protected]. CHARLES-ANDRE´ GANDIN, CNRS Researcher, is with MINES ParisTech, CEMEF UMR CNRS 7635, 06904 Sophia Antipolis, France. DAMIEN PONSEN, Research Engineer, and VIRGINIE JAQUET, Research Manager, are with the Snecma-SAFRAN Group, Boulevard de Valmy - BP 31, 92702 Colombes Cedex, France. Manuscript submitted November 16, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
aged at temperatures below the c solvus to form c¢ precipitates with the desired average size and volume fraction. The final microstructure and properties of the alloy after heat treatment are determined by the precipitation response during aging, which depends on the chemical composition of the matrix. To obtain a perfectly homogenized c matrix of nominal composition, the material needs to be held at the solution heat treatment temperature during an extremely long time, which is not achievable in practice. Residual concentration gradients and small amounts of c¢ eutectic are thus often present at the initial stage of aging. The precipitation response of the alloy during aging is therefore determined by the history of the material, starting from the solidification stage. Numerical simulation can be a very valuable approach to investigate the microstructure evolution in the material during its transformation, and potentially optimize the processing conditions in terms of properties and production cost. Numerical simulation has been used for several decades with the aim of predicting microsegregation and precipitation in metallic alloys. Microsegregation during solidification can be quantified with analytical models such as Gulliver–Scheil and back-diffusion approaches.[2] An important limitation of analytical models is that they cannot simulate the dissolution of interdendritic eutectics and the evolution of concentration profiles in the primary phase during the solution
heat treatment. In multicomponent systems, another aspect is cross-diffusion
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