Non-Equilibrium Grain Boundaries in Severely Deformed Nickel: Existence, Structure and Diffusion Properties
Final Report Abstract
The project was devoted to a detailed investigation of the atomic structure and the kinetic properties of grain boundaries in ultrafine grained Ni produced by equal channel angular pressing (ECAP) and their impact on the micromechanical (tensile / creep conditions) properties. With respect to the grain boundary structure in SPD-processed materials with UFG size, we conclude that: • Non-equilibrium grain boundaries exist in UFG materials and these specific grain boundaries possess an increased free energy density, increased width, high density of dislocations (full or partial) associated with the near-boundary region and correspondingly large residual microstrain. • The structural width of non-equilibrium grain boundaries is significantly smaller than 10 nm, if the rotational component of the strain gradient across the interface is used as a measure of the width. It reaches a value of 1.5 to 2.0 nm being still twice as large as the width of relaxed high-angle grain boundaries in annealed coarse-grained materials. The shear components of the strain field reveal similar values of the grain boundary width. Systematic measurements by the radiotracer technique discovered a hierarchic nature of internal interfaces which are developing as a result of strong dislocation activity during SPD processing and, presumably, of a localization of plastic flow. Both “conventionally fast” as well as “ultra-fast” short-circuit diffusion paths were observed in SPD processed materials, with the latter being embedded in a network of grain boundaries akin relaxed high angle GBs as they exist in annealed coarse-grained materials. The following processes/phenomena contribute to the non-equilibrium state of grain boundaries in SPD processed materials: • abundant vacancies and vacancy-like defects in interfaces produced by severe deformation; • redistribution of the related excess free volume, release of local strains/stresses • chemical effects (ordering) may be important in alloys and compounds affecting the atomic redistribution and retarding e.g. the stress/strain relaxation; • segregation can be especially important in alloys involving even 2D compound formation along interfaces. DSC and dilatometry curves are compared and a good agreement was reached. The following main aspects are to be highlighted. • The concentration of deformation-induced vacancies in ECAP-Ni is about 10^-4 when deformation is performed at room temperature via the route BC4 and it is decreased with increasing temperature of deformation. • The grain boundary expansion in Ni equals to 0.039 ± 0.015 nm. • SPD deformation increases the free volume related to grain boundaries corresponding to an excess of the grain boundary expansion by more than 5 to 10%. A phenomenological model was proposed that, in analogy to the so-called “lock-in” model for heterophase interfaces assumes that the SPD-induced state of high-angle GBs is characterized by a network of 'open channels' with highly delocalized excess free volume that is still confined to these 'open channels'.
Publications
- Grain boundaries in ultrafine grained materials processed by severe plastic deformation and related phenomena, Mater Sci Eng A 540 (2012) 1 – 12
X. Sauvage, G. Wilde, S.V. Divinski, Z. Horita, R.Z. Valiev
(See online at https://doi.org/10.1016/j.msea.2012.01.080) - Effect of annealing on percolating porosity developed in ultrafine grained copper produced by Equal Channel Angular Pressing, Acta Mater 61 (2013) 5477-5486
J. Ribbe, G. Schmitz, D. Gunderov, Y. Estrin, Y. Amouyal, G. Wilde, S.V. Divinski
(See online at https://doi.org/10.1016/j.actamat.2013.05.036) - Effect of back-pressure during Equal Channel Angular Pressing on deformation-induced porosity in copper, Scripta Mater 68 (2013) 925 – 928
J. Ribbe, G. Schmitz, H. Rösner, R. Lapovock, Y. Estrin, G. Wilde, S.V. Divinski
(See online at https://doi.org/10.1016/j.scriptamat.2013.02.034) - Evolution of Ni structure at dynamic channel-angular pressing, Mater Sci Eng A 585 (2013) 281-291
V.V. Popov, E.N. Popova, D.D. Kuznetsov, A.V. Stolbovsky, E.V. Shorohov, G. Reglitz, S.V. Divinski, G. Wilde
(See online at https://doi.org/10.1016/j.msea.2013.07.043) - Percolating porosity in ultrafine grained copper processed by highpressure torsion, J Appl Phys 114 (2013) 183509
M. Wegner, J. Leuthold, M. Peterlechner, D. Setman, M. Zehetbauer, R. Pippan, S.V. Divinski, G. Wilde
(See online at https://doi.org/10.1063/1.4829705) - Effect of pinning by orientation gradient on the thermal stability of ultrafine grained Ni produced by Equal Channel Angular Pressing, J Appl Phys 115 (2014) 113503
S.V. Divinski, G. Reglitz, M. Wegner, M. Peterlechner, G. Wilde
(See online at https://doi.org/10.1063/1.4867416) - Grain boundary and triple junction diffusion in nanocrystalline copper, J Appl Phys 116 (2014) 093514
M. Wegner, J. Leuthold, M. Peterlechner, X. Song, S.V. Divinski, G. Wilde
(See online at https://doi.org/10.1063/1.4893960) - Effect of heat treatment on diffusion, internal friction, microstructure and mechanical properties of ultrafine grained nickel severely deformed by equal channel angular pressing, Acta Mater 82 (2015) 11-21
S.V. Divinski, G. Reglitz, I. Golovin, M. Peterlechner, G. Wilde
(See online at https://doi.org/10.1016/j.actamat.2014.08.064) - On the processing pathway dependence of microstructure evolution during severe plastic deformation – Nickel as a case example, Adv Eng Mater 17 (2015)
D. Prokoshkina, A. Moros, G. Reglitz, G.P.Dinda, D.D. Kuznetsov, V.V. Popov, A.V. Stolbovsky, E.V. Shorohov, S.V. Divinski, G. Wilde
(See online at https://doi.org/10.1002/adem.201500174) - Combined volumetric, energetic and microstructural defect analysis of ECAP-processed Ni, Acta Mater 103 (2016) 396-406
G. Reglitz, B. Oberdorfer, N. Fleischmann, J.A. Kotzurek, S.V. Divinski, W. Sprengel, G. Wilde, R. Würschum
(See online at https://doi.org/10.1016/j.actamat.2015.10.004)
