The Role of Oxidized Carbides on Thermal-Mechanical Performance of Polycrystalline Superalloys
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The Role of Oxidized Carbides on ThermalMechanical Performance of Polycrystalline Superalloys PARASKEVAS KONTIS, ZHUANGMING LI, MIKAEL SEGERSA¨LL, JOHAN J. MOVERARE, ROGER C. REED, DIERK RAABE, and BAPTISTE GAULT Oxidized MC carbides which act as main crack initiation sites in a polycrystalline superalloy under thermal-mechanical fatigue (TMF) conditions at 850 C were studied. Microstructural observations in the TMF tested specimens were compared to findings from bulk samples exposed isothermally in air at 850 C for 30 hours in the absence of any external applied load. Carbides were found to oxidize rapidly after exposure at 850 C for 30 hours resulting in surface eruptions corresponding to oxidation products, from where micro-cracks initiated. Plastic deformation due to volume expansion of the often porous oxidized carbides led to high dislocation densities in the adjacent matrix as revealed by controlled electron channeling contrast imaging. The high dislocation density facilitated the dissolution kinetics of c¢ precipitates by segregation and diffusion of chromium and cobalt along the dislocations via pipe diffusion, resulting in the formation of soft recrystallized grains. Atom probe tomography revealed substantial compositional differences between the recrystallized grains and the adjacent undeformed c matrix. Similar observations were made for the TMF tested alloy. Our observations provide new insights into the true detrimental role of oxidized MC carbides on the crack initiation performance of polycrystalline superalloys under TMF. https://doi.org/10.1007/s11661-018-4709-x The Author(s) 2018
I.
INTRODUCTION
THE thermal-mechanical fatigue (TMF) performance of nickel-based superalloys is a critical parameter in designing new superalloys with superior lifetime and performance.[1] Superalloys subjected to non-isothermal conditions, akin to actual service conditions, provide more representative insights into crack initiation sites and deformation mechanisms as compared to isothermal tests.[2] In particular, in the case of single-crystal superalloys, fracture along deformation twins and bands of sheared c¢ precipitates, which are the main deformation mechanisms, has been reported.[3,4] In addition, deformation twins and grain boundaries were found to act as crack initiation sites in polycrystalline
PARASKEVAS KONTIS, ZHUANGMING LI, DIERK RAABE, and BAPTISTE GAULT are with the Max-Planck-Institut fu¨r Eisenforschung, Max-Planck-Str. 1, 40237 Du¨sseldorf, Germany. Contact e-mail: [email protected] MIKAEL SEGERSA¨LL and JOHAN J. MOVERARE are with the Engineering Materials, Department of Management and Engineering, Linko¨ping University, 581 83 Linko¨ping, Sweden. ROGER C. REED is with the Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK. Manuscript submitted March 1, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
superalloys.[5–7] Nevertheless, there is a growing body of evidence that oxidized MC carbides and the associated recrystallization serve as the main crack initiation site
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