Microstructural and Chemical Rejuvenation of a Ni-Based Superalloy
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NI-BASED superalloys are widely used in power generation for gas turbine components due to their excellent high-temperature performance. These components are subjected to an aggressive combination of elevated temperature and high stress during service, which causes significant microstructural degradation.[1–4] This microstructural deterioration includes coarsening of the strengthening Ni3Al precipitates (c¢) to form rafts and can also lead to local chemical segregation which can promote the formation of topologically close-packed phases (TCPs) and dendritic stress.[5–7] This microstructural and chemical degradation deteriorates the mechanical properties of the material, which can reduce their service life and ultimately lead to component failure. During a gas turbine engine’s life, it undergoes a maintenance schedule that includes a periodic service and refurbishment procedure. Within this refurbishment procedure, microstructural rejuvenation of a blade may be carried out in an attempt to restore a partially ZHIQI YAO, Research Student, MARK A.E. JEPSON, Lecturer in Metallurgy and Microscopy, and RACHEL C. THOMSON, Professor of Materials Engineering, are with the Department of Materials, Loughborough University, Loughborough, Leicestershire, LE11 3TU, U.K. Contact email: [email protected] CRAIG C. DEGNAN, Senior Materials Engineer, is with Uniper Technology Limited, Technology Centre, Ratcliffe-on-Soar, Nottingham, NG11 0EE, U.K. Manuscript submitted May 8, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
degraded microstructure to its pre-service condition through the application of specific heat treatments in an attempt to recover some additional service life from the component. In the case of the CMSX-4 alloy used for power plant applications, the predicted lifetime of the blade alloy is governed by chemical and mechanical degradation processes such as oxidation, thermal fatigue, and creep, but in some circumstances the degradation of the microstructure through the rafting of c¢ particles must also be considered, especially when attempting to rejuvenate the microstructure.[8–11] Rafting is a process whereby, under the influence of stress and temperature, the optimum cube-shaped c¢ particles elongate to form continuous rafts separated by widened Ni-matrix (c) channels. The direction of rafting depends on the direction of the applied stress with respect to the crystal direction within the material and on the sign of the c/c¢ lattice misfit.[12,13] Generally, the direction of rafting is perpendicular to applied tensile stresses[13] although this has been found to vary as a function of c¢ content in the alloy.[14] As rafting is a diffusion-based process, its rate is affected by the temperature of exposure and to the magnitude of stress applied.[15] The strength of Ni-based superalloys is highly reliant on the size and distribution of c¢ with fine and well-dispersed particles favorable for creep performance. Therefore, the formation of rafts is considered a softening process which deteriorates the high-temperature mech
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