Influence of Solid Solution Hardening on Creep Properties of Single-Crystal Nickel-Based Superalloys
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NICKEL-BASED superalloys are used as hightemperature materials in gas turbines since the 1940s. In order to achieve increased efficiency, the turbine entry temperature is raised steadily leading to higher material temperatures. The temperature capabilities of the alloys used increased over the years by improvements in alloy composition and processing route from wrought alloys to cast single-crystal alloys. The outstanding mechanical high-temperature properties result from the microstructure consisting of cuboidal-shaped L12-ordered c¢-precipitates coherently embedded in fcc nickel-solidsolution matrix.[1] Many investigations concentrate on optimizing the microstructural parameters like c¢-size dc¢, c¢ volume fraction fc¢, or misfit d, see, for example.[2,3] Modern single-crystal nickel-based superalloys often meet the values dc¢ = 0.45 lm, fc¢ = 70 pct, and d = (1…3) 9 103, which are considered as optimal,[4,5] i.e., precipitation strengthening is exploited to its optimum. Thus creep strengthening of the generally soft matrix by alloying with refractory elements is of high interest. One of the most potent solid solution forming elements for improving the creep resistance of the matrix is rhenium[6] which even led to classification of superalloys in generations according to their rhenium content.[7] The first generation contains no rhenium, the second generation has a rhenium content of 3 wt pct, ERNST FLEISCHMANN and CHRISTIAN KONRAD, Post-doctorals, RAINER VO¨LKL, Senior Scientist (Post-doctoral), and UWE GLATZEL, Professor, Chairman, are with the Metals and Alloys, University Bayreuth, Ludwig-Thoma-Str. 36b, 95447 Bayreuth, Germany. Contact e-mail: [email protected] JOHANNES PREUßNER, Senior Scientist, is with the Fraunhofer-Institut fu¨r Werkstoffmechanik IWM, Wo¨hlerstr. 11, 79108 Freiburg, Germany. ERNST AFFELDT, Senior Scientist, is with the MTU Aero Engines AG, Dachauer Str. 665, 80995 Munich, Germany. Manuscript submitted February 28, 2014. Article published online January 10, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
the third generation 6 wt pct rhenium, the fourth and higher generations also contain ruthenium. Despite the excellent creep properties, the development directions in recent times depart from exotic elements like rhenium and ruthenium because they are scarce strategic elements and are subjected to large price fluctuations.[8] Furthermore, rhenium and ruthenium tend to promote the formation of brittle phases that form after long-time exposure at high temperatures and thereby have detrimental effects on mechanical properties[9] and on oxidation behavior.[10] Currently, efforts are undertaken to develop rhenium-free alloys with similar or even better properties than rhenium-containing alloys. Therefore, it is important to know why a higher rhenium content leads to a better creep resistance and how big this effect is. In this paper, we quantify the effect of rhenium as solid solution forming element on the creep resistance of single-crystal matrix alloys. To our knowledge, there is no obse
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