The Importance of Diffusivity and Partitioning Behavior of Solid Solution Strengthening Elements for the High Temperatur
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MODERN nickel-base superalloys contain many alloying elements to improve the mechanical properties and to increase the operating temperatures for higher efficiency. Their microstructure consists of c¢-precipitates (L12-structure), which are coherently embedded in a soft c-matrix (A1-structure). The creep behavior of superalloys is a key factor regarding the lifetime of a turbine blade. In addition to the microstructural characteristics like the c¢-volume fraction, c¢-size and c¢-morphology, the mechanical properties mostly depend on the solid solution hardening of the c-phase, especially in the high-temperature/low-stress regime (e.g., 1373 K S. GIESE, A. BEZOLD, M. PRO¨BSTLE, S. NEUMEIER, and M. GO¨KEN are with the Friedrich-Alexander-Universita¨t ErlangenNu¨rnberg (FAU), Department of Materials Science & Engineering, Institute I: General Materials Properties, Martensstr. 5, 91058 Erlangen, Germany. Contact e-mail: [email protected] A. HECKL is with the Friedrich-Alexander-Universita¨t ErlangenNu¨rnberg (FAU), Department of Materials Science & Engineering, Institute II: Materials Science and Engineering for Metals (WTM), Martensstr. 5, 91058 Erlangen, Germany, and now also with the Schaeffler Technologies AG & Co. KG, Industriestraße 1-3, 91074 Herzogenaurach, Germany. Manuscript submitted June 19, 2020. Accepted September 10, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
(1100 C)/140 MPa).[1–4] At small strains deformation occurs mainly in the c-channels, since the required stress for cutting the c¢-phase by dislocations is not high enough.[5,6] To strengthen the c-phase, the alloying elements must preferentially partition to the c-matrix and should have a low interdiffusion coefficient in the Ni solid solution. Alloying elements can be divided into two c0 =c groups depending on their partitioning coefficient ki , which is given as follows: 0
c0 =c ki 0
cc ¼ ic ; ci
½1
where cci and cci represent the atomic fractions c of an element i in the c¢- and c-phase, respectively. Elements like Co, Cr, Ru, Re, Mo and W are enriched in the c-phase (k < 1) and stabilize it because of the similar size of their atomic radii compared to Ni, whereas the c¢-phase is stabilized by Al, Ta and Ti (k > 1).[7,8] To correlate the concentration and diffusivity of solid solution strengtheners in the c-phase with the creep properties at high temperatures, Zhu et al. introduced a model for creep deformation that includes the calculation of the effective diffusion coefficient.[9] This approach was also used in the study by Proebstle et al. to explain the beneficial effect of the optimized partitioning behavior of W on the creep strength of a series of
Ni-base superalloys.[10] Besides the element W, a variety of different other alloying elements was added during the last 60 years of alloy development of Ni-base superalloys to study their impact on the creep strength. It was found that W, Mo and especially Re can act as effective solid solution strengtheners and therefore have a beneficial effect on the mechanical properties of turbine bla
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