Microstructure of the Nickel-Base Superalloy CMSX-4 Fabricated by Selective Electron Beam Melting
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SINGLE-CRYSTAL nickel-base superalloys are well known for their extraordinary high-temperature properties. Thus, these alloys are widely used in the aerospace industry for turbine blades and vanes in the hot section of today’s gas turbine engines. CMSX-4 is a second generation single-crystal superalloy containing 3 wt pct Re. This alloy is derived from CMSX-2, employing the beneficial strengthening effects of Re. It was established in the early 1990s and is used for single-crystalline turbine blades. Single-crystalline CMSX-4 is typically fabricated via investment casting and has been extensively developed to enhance the high-temperature properties. In addition, the microstructure is optimized by special heat treatments. After the conventional process route, it is not possible to dissolve the residual microsegregation in an economic way.[1] The alloy provides good high-temperature properties due to the two-phase c/c¢-microstructure, a high volume fraction of c¢ (up to 70 pct) and the solid solution strengthening effects of Cr, W, Ta, and Re. Additionally, Re decreases significantly the c¢ coarsening kinetics.[2] CMSX-4 also enhances oxidation resistance due to the reduced level of Cr and an increased level of Al (Al2O3 former, more stable than Cr2O3 at high temperatures). MARKUS RAMSPERGER, Research Associate, ROBERT F. SINGER, Professor, and CAROLIN KO¨RNER, Professor and Head of Institute, are with the Department of Materials Science, Chair of Metals Science and Technology, University of Erlangen-Nuremberg, Martensstraße 5, 91058 Erlangen, Germany. Contact e-mail: markus. [email protected] Manuscript submitted January 19, 2015. Article published online January 6, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
In the as-cast condition, the microstructure of CMSX-4 shows a dendritic pattern with a c/c¢-eutectic in between (Figure 1). Segregation is caused by constitutional undercooling during solidification. Typically, c stabilizers like the refractory metals W, Co, and Re segregate within dendrite core areas, whereas c¢ stabilizers like Al, Ti, and Ta accumulate within interdendritic areas.[3,4] Thus, the cubic c¢ precipitates are smaller within the dendritic core regions and become coarser near the interdendritic areas (eutectic regions) (Figure 1(c)) due to the increased amount of c¢ stabilizers.[5] Due to the inhomogeneous c¢-distribution and remaining c/c¢-eutectic, great efforts with respect to time-consuming heat treatments have been undertaken to homogenize the microstructure in order to improve the high-temperature properties. Usually, specific heat treatments for c¢-precipitation and aging are performed after homogenization to reach the optimal c¢ size and cubical morphology.[1,6] An important criterion for heat treatment is the primary dendrite arm spacing (DAS) k1 which represents the length scale of element segregation. With increasing DAS, homogenization heat treatment gets more and more difficult and time-consuming. The refractory metals, especially Re, play an important role due to their ve
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