Segregation-Assisted Plasticity in Ni-Based Superalloys
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Segregation-Assisted Plasticity in Ni-Based Superalloys D. BARBA, T.M. SMITH, J. MIAO, M.J. MILLS, and R.C. REED Correlative high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy are used to study deformation-induced planar faults in the single-crystal superalloy MD2 crept at 800 °C and 650 MPa. Segregation of Cr and Co at microtwins, anti-phase boundaries (APB), and complex/superlattice extrinsic and intrinsic stacking faults (CESF/SESF and CISF/SISF) is confirmed and quantified. The extent of this is found to depend upon the fault type, being most pronounced for the APB. The CESF/SESF is studied in detail due to its role as a precursor of the microtwins causing the majority of plasticity under these conditions. Quantitative modeling is carried out to rationalize the findings; the experimental results are consistent with a greater predicted velocity for the lengthening of the CESF/ SESF—compared with the other types of fault—and hence confirm its role in the diffusion-assisted plasticity needed for the microtwinning mechanism to be operative. https://doi.org/10.1007/s11661-018-4567-6 Ó The Author(s) 2018
I.
INTRODUCTION
PLANAR faults such as anti-phase boundaries (APBs) and superlattice stacking faults (SSFs) of various flavors—intrinsic (SISF), extrinsic (SESF), or complex (CSF)—are of crucial importance to the deformation behavior of intermetallic compounds, and alloys which contain phases based upon them.[1–3] This is because dislocations, even when dissociated, cannot pass through the ordered lattice without their creation in one form or another. A corollary is a range of interesting but practically important plastic phenomena: anomalous yielding, a substantial strain hardening effect and an anisotropy of tensile/compressive behavior which is non-Schmidian.[4–6] The above applies particularly to the case of the Ni-based superalloys[7,8]—because of the presence of a significant fraction of the c0 -phase of ordered L12 crystallography. Well known is the substantial influence of the APB and its role in the temperature-dependence
D. BARBA is with the Department of Engineering Science, University of Oxford, Oxford, OX13PJ, UK. Contact e-mail: [email protected] T.M. SMITH is with the NASA Glenn Research Center, Cleveland, OH 44135. J. MIAO and M.J. MILLS are with the Center for Electron Microscopy and Analysis, The Ohio State University, Columbus, OH 43212 and also with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. R.C. REED is with the Department of Engineering Science, University of Oxford and also with the Department of Materials, University of Oxford, Parks Road, Oxford, OX13PH, UK. Manuscript submitted November 13, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
of the yield point which is positive rather than negative, at least until a temperature of approximately 800 °C is reached[9]; the accepted explanation is the anisotropy of the APB energy and in particular the role of cross-slip from f111g to the f001g
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