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The major advancements in mechanical and thermal properties of the most recently developed single crystal (SX) superalloys can be attributed to the addition of specific refractory elements into base alloy compositions. The present study investigates the effect of modifying refractory addition levels on the solidification behaviour of SX superalloys systems. Specifically, a series of six Ni-base alloy compositions are set in a controlled manner, such that the chemical microsegregation effects of Re, W, and Ru can be independently assessed. Fabrication of grain-free SX bars from each alloy composition is achieved by utilizing a modified Bridgman casting process, with subsequent compositional analysis of the solidification structures via electron microprobe analysis (EPMA) methods. Further validation of these EPMA microsegregation results are supported by means of eutectic phase fraction analysis and differential scanning calorimetry (DSC) methods. Qualitative partitioning results indicate typical SX alloy segregation behavior with elements such as Cr, Co, Re, Mo, and W all segregating towards the dendrite core regions, while the forming elements of Al, Ti, and Ta partition to the interdendritic-eutectic regions. Both Ni and Ru exhibit ideal segregation behaviour with no favorable partitioning to either liquid or solid phase. Quantitative EPMA results indicate that as the nominal Re level increases, the severity of microsegregation to the dendrite core regions rises dramatically for Mo, Cr, and Re. Evidence is presented that demonstrates the role that Ru plays in counteracting the microsegregation effects of both increased Re and higher overall total refractory levels. In addition to experimental trials, chemical partitioning predictions are also presented for the alloy system, utilizing a solid-liquid phase equilibria model generated using a customized chemical thermodynamic database. Using this CALPHAD approach, a comparison of the computational predictions and the actual experimental segregation results is also provided for discussion.

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The fully lamellar microstructure of powder metallurgy Ti-48Al-2W after cooling from the ␣ region to 1280°C, followed by air cooling and aging at 950°C for up to 96 hours, is presented. Aging times as short as 5 hours result in acicular-shaped precipitates of W-rich ␤ 0 along lamellar interfaces, with the ␤ 0 size increasing with aging time. The ␤ 0 precipitates nucleate and grow in the ␣ 2 lamellae. Concurrently, with the formation of ␤ 0 , the ␣ 2 decomposes into discontinuous lamellae. Aging to precipitate ␤ 0 along lamellar interfaces increases the 760°C tensile strength (with a slight reduction of ductility) and reduces the instantaneous creep strain, since ␤ 0 precipitates at lamellar interfaces hinder interface dislocation mobility. The deformed microstructures from interrupted creep tests at 140 to 276 MPa at 760°C indicate that the precipitation of ␤ 0 along interfaces substantially reduces the primary creep strain, primarily due to the influence of ␤ 0 on interface dislocation emission and motion. These results are discussed in terms of the influence of lamellar morphology on the instantaneous creep strain and primary creep transient, and the possible creep mechanisms are highlighted.

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