The Compositional Dependence of the Microstructure and Properties of CMSX-4 Superalloys
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THE outstanding high-temperature mechanical performance of Ni-based single-crystal superalloys, in particular, their superior creep resistance, makes them favorite materials for turbine blades in aero engines.[1] Their interesting properties are largely dictated by the high volume fraction (up to 70 pct) of ordered, L12 structured c¢ precipitates, which are coherently embedded in a solid-solution-strengthened c matrix (fcc structure). During the high-temperature creep process above 900 C, the creep behavior strongly depends on the microstructural evolution. Specifically, in the early creep stage of so-called negative misfitting alloys, initially adjacent cuboidal c¢ particles coalesce and turn into platelike structures, which are normal to the stress direction. This microstructure of alternating platelets of c and c¢ phases is called the rafted structure. The HAO YU and SYBRAND VAN DER ZWAAG are with the Novel Aerospace Materials Group, Faculty of Aerospace Engineering, Delft University of Technology, 2629 HS Delft, The Netherlands. WEI XU is with the Novel Aerospace Materials Group, Faculty of Aerospace Engineering, Delft University of Technology, and also with the State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China. Contact e-mail: [email protected] Manuscript submitted July 10, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
lamellar structure is rather stable and essentially blocks the dislocation bypassing mechanism, resulting in a low creep rate during the secondary creep stage. In the later stage of creep, the destabilization of the rafted microstructure occurs through a topological inversion of c/c¢ microstructure, resulting in a steep increase in the creep rate. Hence, the creep kinetics can be linked to the process of microstructural evolution, and it is of great importance to identify the main microstructural evolution mechanisms and to incorporate their effects as a function of service conditions. The characterization of microstructural evolution during the creep process is of utmost importance in detecting the desirable mechanical properties of Ni single-crystalline superalloys. Hence, since the 1990s, a large number of models have been derived to describe and investigate the progress of c¢ morphology evolution. Most of them focus on the dynamics of dislocation behavior, exploring the balance between dislocation multiplication and interaction with precipitates, as well as dislocation annihilation and recovery.[2–6] Other models focus on probing the driving force of the anisotropic coarsening of precipitates.[7–10] Essentially, the microstructural characteristics for Ni superalloys are determined by three factors: c¢ precipitate volume fraction, isotropic coarsening, and directional coarsening, also called rafting. There are two main parameters in describing the rafted microstructure, i.e., the
microstructural periodicity k and the c channel width x. The k parameter characterizes the global coarsening of the c/c¢ microstructure, since the rafted c/c¢ comp
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