Influence of Processing Parameters on the Solidification Behavior of Single-Crystal CMSX-4 Superalloy
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-BASED single-crystal superalloys, having high temperature creep and fatigue properties, have found increasing applications in the hot sections of aero-engines and industrial gas turbines (IGTs): such as turbine blades and vanes.[1–6] To obtain higher turbine entry temperatures and therefore enhanced efficiency of the engines and turbines, increased additions of refractory elements, such as W, Ta, Ru, and Re, are employed in the modern generation nickel-based single-crystal superalloys.[7–14] These elements improve the high-temperature strength via solid solution strengthening within the c matrix,[15] and refine c¢ precipitates by acting as a retarder.[16] However, the sluggish diffusivity of these elements in the solid-state leads to significant segregation and the formation of highly cored non-equilibrium eutectic microstructures during solidification.[17] Accordingly, a subsequent heat treatment process is required to diffuse the interdendritic eutectic and to achieve compositional homogeneity across the dendrite cross-section. In addition to this, the formation of hot tears and porosity, which are commonly encountered defects in the nickel-based single-crystal superalloy castings, are strongly affected by the solid volume FU WANG, Scientific Staff, DEXIN MA, Senior Scientist, SAMUEL BOGNER, Ph.D. Student, and ANDREAS BU¨HRIGPOLACZEK, Professor, are with the Foundry Institute, RWTH Aachen University, Intzestraße 5, Aachen 52072, Germany. Contact e-mail: [email protected]. Manuscript submitted December 16, 2015. Article published online May 4, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
fraction’s (fs) evolution during the crystallization.[18] Consequently, to reasonably control the solidification and heat treatment process and minimize the defects and cost, it is imperative to obtain knowledge of the microstructural development during solidification. During the last few years, a number of investigations have conducted studies on the microstructural development of the superalloys. Some of these studies are founded on calculations using the Thermo-Calc software.[19] The calculation is normally based on simplified solidification models which cannot precisely demonstrate the actual microstructural evolution. Several researchers[20] elicit the microstructural development by using resolidification and isothermal quenching experiments. However, owing to variations in the solidification conditions, it is difficult to characterize the real microstructural development. Some investigations[21] depict the microstructural evolution by using an enthalpy method. However, this method is also based on a specific solidification model, and it cannot therefore represent the precise solidification characteristics. In addition to this, several researchers[22,23] characterize the microstructural development of final stage solidification in the nickel-based superalloy by using three-dimensional (3D) reconstructions from a series of two-dimensional (2D) planar transverse sections of the directionally solidified samples. Notwithstanding this, 3D imag
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