Microsegregation and Secondary Phase Formation During Directional Solidification of the Single-Crystal Ni-Based Superall
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NICKEL-BASED superalloys, solidified as single crystals (SX), show superior mechanical properties compared to their polycrystalline counterparts. This is due to the absence of grain boundaries and the presence of coherent and uniformly dispersed c’ (L12) precipitates within a c (A1-face-centered cubic) matrix.[1,2] LEK94 is a second-generation SX Ni-based superalloy developed by MTU Aero Engines (Munich, Germany) and is typically used for low-pressure and intermediate-pressure turbines.[3] It owes its excellent high-temperature strength to the large amount of a finely dispersed and ordered phase c’, which has a very high solvus temperature.[1–3] At the end of solidification, the interdendritic areas show significant amounts of bulky c’ particles, commonly referred to as the c/c’ eutectic, although it has been suggested that the mechanism is a peritectic one.[4,5] During further cooling, finely dispersed c’ also forms inside the dendrites. The formation of the microstructure during casting of the SX Ni-based superalloy can be regarded as one of the most influential factors for further processing and the final quality of the superalloy because it governs INMACULADA LOPEZ-GALILEA, Scientist, is with the Lehrstuhl Werkstofftechnik, Ruhr-Universita¨t Bochum, 44780 Bochum, Germany, and also with the Research Department IS3/HTM, RuhrUniversita¨t Bochum. Contact e-mail: [email protected] STEPHAN HUTH, Junior Professor, and WERNER THEISEN, Professor and Dean, are with the Lehrstuhl Werkstofftechnik, Ruhr-Universita¨t Bochum. SUZANA G. FRIES, Project Leader, is with the STKS, Scale Bridging Thermodynamic and Kinetic Simulation, Ruhr-Universita¨t Bochum. NILS WARNKEN, Researcher, is with the Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K. INGO STEINBACH, Professor, is with the STKS, Scale Bridging Thermodynamic and Kinetic Simulation, Ruhr-Universita¨t Bochum. Manuscript submitted September 20, 2011. Article published online August 2, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
many material properties, especially the mechanical properties due to its influence on the distribution of porosities and residual microsegregation after heat treatment.[6] Therefore, simulation of microstructure formation and evolution during casting can provide useful information, not only for subsequently required heat treatments but also for the casting process itself. One characteristic of directional solidification to be considered here is that the primary spacing of the dendrites is not free to adjust to the casting conditions. Depending on the process history, a band width of a factor of roughly two between minimum and maximum primary spacing has been predicted theoretically and observed experimentally.[7,8] This variation corresponds to a factor of 16 in the cooling rate according to the following generally acknowledged model of primary spacing selection. (see References 9 and 10) k G0:5 v0:25 G0:25 T_ 0:25
½1
if the temperature gradient is kept constant. k is the primary spacing, G is the tempe
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