Low Cycle Fatigue Behavior of a Directionally Solidified Nickel-Based Superalloy: Mechanistic and Microstructural Aspect
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UCTION
DIRECTIONALLY solidified nickel-based superalloys are used to construct the hot section components like turbine blades and vanes of aero-engines.[1–5] During the service, the blade materials experience fluctuating thermal stresses, which accentuates the need to possess exceptionally good high-temperature low cycle fatigue (LCF) resistance.[6–9] The LCF deformation behavior of the blade material, therefore, has attracted interest in the past decades .[6,8–11] To prevent the premature replacement of such an expensive component and to avoid economic/human casualties, fatigue life prediction is of paramount importance. It is worth mentioning that the success of a life prediction scheme requires a proper consciousness of LCF damage micromechanisms at various temperatures.[12–15] Microstructural evolution during service exposure has a sizable effect on the LCF damage micromechanisms and fatigue life.[14–21] Therefore, it is extremely important to understand the microstructural alterations and damage mechanisms during service exposure. Despite all the efforts, a universally acceptable fatigue life prediction scheme is still scarce.[20–22] Some of the life R.K. RAI is with the AcSIR, CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India. Contact e-mail: [email protected] J.K. SAHU is with the CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India. N. PAULOSE and C. FERNANDO are with the Gas Turbine Research Establishment, Bangalore 560093, India. Manuscript submitted on August 1, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
prediction schemes work effectively for certain alloy systems, but not as well for others. The different forms of damage that originated in various alloy systems are the main culprit for it.[22–24] The chemistry of the directionally solidified (DS) nickel-based superalloy used in the present study evolved from that of MAR-M 247 alloy, which is designed for the turbine blade application.[1] However, to enhance the creep and LCF resistance required for the blade application, the chemical composition and heat treatment of the alloy was altered.[1] The tensile and creep deformation behaviors of the DS alloy were discussed by the authors recently.[25,26] Those studies[25,26] revealed that both tensile and creep deformation behaviors of the alloy are highly sensitive to the testing temperatures and microstructural stabilities. Non-uniform planar slip, involving precipitates shearing by a/2 h110i and a/3 h112i dislocation partials, was found to be the dominant deformation mechanism at the lower temperatures whereas homogeneous slip resulting from dislocation bypass mechanisms is dominant at the higher temperatures (T > 850 C). In addition to it, precipitate coarsening and oxidation occurring at a higher temperature also play a crucial role during deformation.[25,26] Given the dominating effect of temperature on the deformation behavior, the LCF damage mechanisms of the DS alloy were assessed under pertaining conditions. An in-depth understanding of LCF deformation, micromechanis
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