Diffusion phenomena in nano-crystalline Cu produced by severe plastic deformation
Final Report Abstract
In recent research, technical methods were developed to strongly strain materials without changing their macroscopic shape. The resulting, so called severely plastically deformed (SPD) materials are distinguished by very small grain size in the order of 100 nm. Early experiments gave hope that a unique combination of strength and ductility may be achieved, but later attempts of confirmation were disappointing. SPD materials in general turned out to be rather brittle. In order to understand this problem to better detail, we proposed kinetic studies on the atomic transport along produced highenergy grain boundaries by accurate radiotracer method. To our surprise, the radiotracer experiments of outstanding sensitivity discovered an unexpected percolating porosity of about 1 ppm volume fraction in Cu and Cu-based SPD alloys. Still this is a very tiny volume fraction. But nevertheless, critical brittleness may originate from such porosity. Therefore understanding the discovered porosity becomes rather important should mechanical behavior be optimized. In view of this situation, we decided to focus the second funding period on a sensitive tracer analysis of the properties of the newly discovered percolating porosity. SPD processing has been performed applying Equal Channel Angular Pressing (ECAP) at the facilities of the research group of Prof. Y. Estrin (Monash University, Clayton, Australia). Cu materials of different purity levels ranging from 99.9 to 99.9998 wt.% were deformed. Radioactive tracer was dropped on to the surface and subsequently analyzed by micro-sectioning. Involved experiments were carried out, which included combined application of two tracer types and modification of the tracer source by addition of glycerin. The measurements verified unambiguously that the postulated percolating porosity does exist in ultrafine grained (UFG) Cu severely deformed via ECAP. Studies were devoted to the stability of this porosity under external actions including heat treatments with and without application of additional hydrostatic pressure and under different gas atmospheres. The goal was discovering conditions under which porosity-free nanocrystalline material may be produced by SPD. The spectacular result was found that even annealing at 800°C under purified Ar atmosphere does not remove the percolating porosity, whereas such heat treatment at only 150°C but under H2-atmosphere completely remove it. Also back pressure up to 200 MPa during ECAP can reduce but not completely avoid porosity after intensive straining. Concluding the experimental results, a model was developed to understand formation and stability of the percolating porosity. It is indicated that its formation requires a minimum atomic mobility to rearrange and condensate vacancies at triple junctions of the grain structure. In consequence, stability of the porosity is controlled by a sensitive quantitative balance among average pore size, total amount of free volume inside the sample, and applied external stress. Presumably, our findings will have important influence on the future development of ductile SPD materials. We were already able to suppress the porosity by controlled back pressure during deformation. In material obtained in this way, we could measure for the first time the transport kinetics of high energy grain boundaries properly, which were formerly masked by the dominating fast transport along the inner surfaces of the porosity.
Publications
- Grain boundary radiotracer diffusion of Ni in ultra-fine grained Cu and Cu-1wt.%Pb alloy produced by equal channel angular pressing. Materials Science Forum, 584-586, 380-386 (2008)
J. Ribbe, G. Schmitz, Y. Amouyal, Y. Estrin, S.V. Divinski
- Cracks in ultra fine grain copper produced by equal channel angular pressing. Phys. Rev. Lett. 102, 165501 (2009)
J. Ribbe, D. Baither, G. Schmitz, S.V. Divinski
- Diffusion in nanostructured materials. Defect Diffusion Forum, 289-292, 623-632 (2009)
S.V. Divinski
- Nano- and micro-scale free volume in ultrafine grained Cu-1wt%Pb alloy deformed by equal channel angular pressing. Acta Mater. 57, 5706-5717 (2009)
S.V. Divinski, J. Ribbe, D. Baither, G. Schmitz, G. Reglitz, H. Rösner, K. Sato, Y. Estrin, G. Wilde
(See online at https://dx.doi.org/10.1016/j.actamat.2009.07.066) - Network of ultra-fast transport channels in severely deformed nanocrystalline metals. J Applied Physics 106, 063502-1 (2009)
S.V. Divinski, J. Ribbe, G. Reglitz, Y. Estrin, G. Wilde
(See online at https://dx.doi.org/10.1063/1.3211966) - Ultra fast diffusion and internal porosity in ultra fine grain copper–lead alloy prepared by equal channel angular pressing. Scripta Mater. 61, 129-132 (2009)
J. Ribbe, D. Baither, G. Schmitz, S.V. Divinski
(See online at https://dx.doi.org/10.1016/j.scriptamat.2009.03.029)
