Investigating the Dislocation-Driven Micro-mechanical Response Under Non-isothermal Creep Conditions in Single-Crystal S
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Investigating the Dislocation-Driven Micro-mechanical Response Under Non-isothermal Creep Conditions in Single-Crystal Superalloys C. SCHWALBE, J. CORMIER, C.N. JONES, E. GALINDO-NAVA, and C.M.F. RAE The creep responses of the superalloy CMSX-4 under thermal cycling conditions (900 C to 1050 C) and constant load (r0 ¼ 200MPa) were analyzed using TEM dislocation analysis and compared to the modeled evolution of key creep parameters. By studying tests interrupted at different stages of creep, it is argued that the thermal cycling creep rate under these conditions depends on the creation of interfacial dislocation networks and their disintegration by the c¢-shear of dissimilar Burgers vector pairs. https://doi.org/10.1007/s11661-018-4764-3 The Author(s) 2018
I.
INTRODUCTION
SINGLE-CRYSTAL nickel-based superalloys are primarily used as turbine blade materials for aeronautical and power generation applications due to their unique resistance under high-temperature creep conditions. In combination with their preferential casting direction and the absence of grain boundaries, the creep life is enhanced due to a bi-phasic microstructure of harder c¢-precipitates (in an Ni3Al L12-ordered arrangement) coherently embedded in a softer c-matrix (disordered fcc). This microstructure has been shown to exhibit maximum creep resistance at a 70 pct c¢-volume fraction at low temperatures.[1] At higher temperatures, due to the increasing solubility of the c-phase with temperature, the c¢-phase gradually dissolves, being absent above 1280 C in the case of the alloy CMSX-4.* *CMSX-4 is a registered trademark of Cannon-Muskegon Corporation.
Lower precipitate fractions have been shown to decrease the creep resistance of the alloys.[2] This is
particularly relevant when analyzing alloys under in-flight conditions as these experience short spikes of high temperature during the take-off step. The micro-mechanical response under non-isothermal conditions has been shown to reduce the lifetime of an alloy relative to its isothermal creep life.[2,3] By varying the non-isothermal exposure cycles in the temperature range of 1050 C to 1150 C, Giraud et al.[4] concluded that one crucial factor for the lifetime reduction is accredited to the development of rafts. This observation is at odds with the well-established beneficial role of c¢-rafting under isothermal conditions.[5] This paper seeks to clarify the role c¢-rafting plays under non-isothermal creep conditions at the lower temperature range of 900 C to 1050 C. Particular attention will be given to the dislocation mechanisms driving damage accumulation[6] and the role of interfacial dislocation networks[2,7] under cyclic creep conditions. This study is aimed at understanding, quantifying, and ultimately modeling the dislocation response under non-isothermal creep.
II.
EXPERIMENTAL PROCEDURES
A. Materials
C. SCHWALBE, E. GALINDO-NAVA, and C.M.F. RAE are with the Department of Materials Science and Metallurgy, University of Cambridge/Rolls-Royce University Technology Partnership, 27
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