Detection of an Intermediate Layer Containing a Rhenium-Rich Particle at Grain Boundaries Formed Within Single Crystal N
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he superior mechanical properties of single crystal Ni-based superalloys contribute greatly to the improvement of fuel efficiency and performance, and the reduction of CO2 emissions from jet engines and power-plant gas turbines.[1–3] The absence of grain boundaries eliminates potential sites of crack initiation under thermomechanical fatigue conditions, and removes the potential creep mechanism of grain boundary sliding.[4] The excellent high-temperature properties of superalloys are derived from the ordered c¢ precipitates, with optimum size and morphology, in a c matrix.[5] Therefore, the previous studies on the development of single crystal superalloys have focused on the microstructural stability and the increase of the solvus temperature due to the addition of refractory elements.[5,6] CMSX-10 is a third-generation Ni-based superalloy, containing Re, for application in single crystal turbine
KEEHYUN KIM, Research Fellow, and W.D. GRIFFITHS, Senior Lecturer, are with the School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K. Contact e-mail: [email protected]; [email protected] PAUL WITHEY, Visiting Professor, is with the School of Metallurgy and Materials, University of Birmingham, and also Rolls-Royce Engineering Associate Fellow with the Rolls-Royce plc, PO Box 31, Derby DE24 8BJ, U.K. Manuscript submitted September 23, 2014. Article published online December 20, 2014 1024—VOLUME 46A, MARCH 2015
blades. This alloy is particularly beneficial for turbine blade applications where creep is of the major lifing concern. Rhenium is beneficial in reducing the c¢ coarsening rates and producing a small negative lattice misfit[7] and, consequently, improves the creep properties of superalloys at elevated temperatures.[8,9] However, it is well known that high amounts of Re degrade the creep and rupture properties in superalloys due to the formation of topological close-packed (TCP) phases induced by the low diffusivity of Re in the Ni matrix[10] and, consequently, there is an upper limit for the alloying content of Re.[11] In addition to this, we report new discovery of the detrimental effect of Re associated with the formation of stray grains. Stray grains in superalloys form grain boundaries and cause in-service failure because they are favorable locations for the initiation of cracks.[6,12] It has been suggested that the stray grains may be formed by nucleation from the corner wall of a platform region,[4,6,12,13] remelting of dendrite fragments[14,15] and heterogeneous nucleation of grains from TiN particles ahead of the solidification front.[16] Figure 1(a) shows a turbine blade examined in this study. During turbine blade manufacture, the cast blades are removed from the runner system, solution heat treated, and then aged to develop the required c/c¢ microstructure. These blades are then etched to allow for a visual inspection of any grain defects which may be on the surface of the component. While this is a rare occurrence, any high-angle grain boundary is a weak area within the c
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