Solidification Characteristics and Segregation Behavior of Nickel-Base Superalloys in Dependence on Different Rhenium an
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TECHNICAL applications such as turbines in power plants are governed by nickel-base superalloys, which can sustain severe thermal and mechanical stresses under extreme conditions and long-term exposure. Systematic alloy development to increase the turbine gas inlet temperature is fundamental for a continuous efficiency improvement, which simultaneously leads to a cost decrease in energy production as well as lower CO2 emissions. Solely as a result of enhanced alloy chemistry in single-crystal (SX) turbine-blade superalloys, the material capability has been increased by more than 100 C in the past 30 years.[1] This has been achieved to a large degree by continuously rising levels of refractory elements such as W, Mo, and Re. In particular, Re is known as a very efficient solid-solution strengthener that promotes higher tensile strength as well as better creep properties.[1–4] However, it also features a strong segregation tendency to the dendrite core during solidification and therefore causes a high degree of inhomogeneity.[1,3,5–8] This, in turn, causes several disadvantages, as follows: (1) Inhomogeneous distribution during casting enhances grain defect formation, which deteriorates the microstructural orientation and SX castability;[9] (2) due to a higher segregation together with its low diffusion coefficient,[7] the necessary
A. HECKL and R. RETTIG, Research Assistants, and R.F. SINGER, Head of the Chair of Science and Technology in Metals, are with the Institute of Science and Technology of Metals, Department of Materials Science and Engineering, University of Erlangen, D-91058 Erlangen, Germany. Contact e-mail: [email protected] Manuscript submitted July 7, 2009. Article published online October 30, 2009 202—VOLUME 41A, JANUARY 2010
heat-treatment time increases and, thus, the process gets more expensive; and (3) enhancement of Re in the dendrite core facilitates the formation of brittle tpc phases, which can drastically degrade the mechanical properties of the material.[10–12] In particular, the latter led to the addition of Ru in the latest generation of nickel-base superalloys to overcome the problem. The Ru appears to be advantageous through stabilization of the microstructure with respect to tcp phase formation and increased high-temperature creep strength.[8,13–17] However, there are only limited studies of the solidification characteristics and as-cast microstructures of Ru- and Re-containing superalloys. Hence, a detailed knowledge of the alloy composition influence on segregation behavior could lead to an improvement in the development and processing of new alloys, because the solution heat treatment of highly alloyed nickel-base superalloys appears to be controlled by the need to reduce segregation instead of c¢ eutectic dissolution.[18,19] The present article investigates a series of 12 experimental nickel-base superalloys containing different contents of Re and Ru that were designed based on the commercial alloy CMSX-4. Additions were made in well-defined atomic percent contents to gu
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