Micro- and Macromechanical Properties of Fully Densified Nanocrystalline Metals
Final Report Abstract
For the first time, the fundamentals of plastic deformation in fully dense bulk nanocrystalline (nc) Pd and Pd-10 at. % Au alloy with a mean grain size between 10 and 30 nm were investigated over the temperature range between 4.2 and 300 K. Experiments were focused on mechanical tests using new testing equipment for miniaturized specimens with stress state conditions, strain rate and temperature as the variables. Additionally a new approach was applied to study mechanical behaviour of nc alloy in very broad strain range using “instrumental high pressure torsion (HPT)”, a technique allowing to register the materials response during HPT. As a result of the fulfillment of the project, following results were obtained: 1. A new method of nanocrystalline samples preparation was developed. Combination of inert gas condensation and high pressure torsion allows to obtain samples of Pd and Pd-Au alloys with equiaxed grains and a mean grain size as small as 10-15 nm, with homogenous and almost textureless microstructure, of very high purity, and with only 2% of residual porosity. The igc+hpt procedure is the only processing method allowing to produce truly nc bulk materials without any drawbacks for scientific investigations. 2. The nc Pd and Pd-10 at.% Au alloy demonstrate very unusual for metallic polycrystalline materials mechanical behavior. Despite very high strength and good ductility observed in compression test, with ultimate strength eight times higher than that in their coarse grained counterparts, they manifest compression – tension anisotropy and fail in elastic range when tested in tensile mode. Fractography investigations as well as in situ in TEM tensile test of free standing Pd film had shown that fracture occurred along grain boundaries, which - therefore opposite to cg polycrystals - weaken the material, instead of making its stronger. 3. The nc Pd-Au alloy demonstrates an extended (up to 4% at room temperature) microplasticity stage with high strain hardening exponent. Plastic deformation has notable thermally activated character, which leads to the increase of the applied stress with decreasing deformation temperature. At cryogenic temperatures the microplastisity strain range shrinks and strain hardening exponent decreases, which was interpreted as a change of the deformation mechanism from grain boundary mediated one to dislocation slip. 4. Ex-situ texture measurements conducted using synchrotron radiation had shown that dislocation slip only slightly contributes to plasticity of nc Pd-Au alloy, as samples remained textureless after being deformed by shear up to strain γ = 1. Only further straining to γ ≈ 15 led to the formation of a very weak texture. Therefore we proposed that in nc state deformation propagates mostly along grain boundaries. Direct observation of grain boundary sliding was made in Pd with a mean grain size of 150 nm during in situ SEM compression test, when whole grains emerged from the surface as it was confirmed by EBSD measurement. On the macroscopic scale deformation was localized in shear bands. In some respect the mechanical behavior of nc Pd and Pd-Au alloys has some similarities with that of amorphous materials, namely compression/tension asymmetry and propensity to shear banding.
Publications
- Can we use kinetic analysis for investigation of deformation processes in nanocrystalline materials? Proceedings of ICSMA-15 in Dresden, Germany, August 2009. Journal of Physics: Conference Series 240 (2010) 012138
Yu. Ivanisenko, L. Kurmanaeva, K Yang, H.-J. Fecht
(See online at https://doi.org/10.1088/1742-6596/240/1/012138) - Grain refinement and mechanical properties in ultrafine grained Pd and Pd – Ag alloys produced by HPT. Material Science Engineering A 527 (2010) 1776
L. Kurmanaeva, Yu. Ivanisenko, J. Markmann, C. Kübel, A. Chuvilin, S. Doyle, R.Z. Valiev, H.-J. Fecht
(See online at https://doi.org/10.1016/j.msea.2009.11.001) - Information on deformation mechanisms in nanocrystalline Pd-10% Au inferred from texture analysis. Journal of Material Science 45 (2010) 4571
Yu. Ivanisenko, W. Skrotzki, R. Chulist, T. Lippmann, L. Kurmanaeva, H.-J. Fecht
(See online at https://doi.org/10.1007/s10853-010-4552-y) - Mechanical behaviour and in situ observation of shear bands in ultrafine-grained Pd and Pd-Ag alloys. Acta Materialia 58 (2010) 967
K. Yang, Yu. Ivanisenko, A. Caron, A. Chuvilin, L. Kurmanaeva, T. Scherer, R.Z. Valiev, H.-J. Fecht
(See online at https://doi.org/10.1016/j.actamat.2009.10.013) - Work hardening and inherent plastic instability of nanocrystalline metals. Physica Status Solidi RRL 4(5-6) (2010) 130
L. Kurmanaeva, Yu. Ivanisenko, J. Markmann, K. Yang, H.-J. Fecht, J. Weissmüller
(See online at https://doi.org/10.1002/pssr.201004095) - Cooperative grain boundary sliding and shear banding at high strains in ultrafine grained and nanocrystalline Pd alloys NanoSPD 5, Nanjing, China, March 21-25, 2011. Materials Science Forum 667-669 (2011) 649
Yu. Ivanisenko, H.-J. Fecht
(See online at https://doi.org/10.4028/www.scientific.net/MSF.667-669.649) - Texture development in a nanocrystalline Pd–Au alloy studied by synchrotron radiation. Scripta Materialia 66 (2012) 131
Yu. Ivanisenko, W. Skrotzki, R. Chulist, T. Lippmann, L. Kurmanaeva
(See online at https://doi.org/10.1016/j.scriptamat.2011.10.004) - First Direct In Situ Observation of Grain Boundary Sliding in Ultrafine Grained Noble Metal. Advanced Engineering Materials
K. Yang, H.-J. Fecht, Yu. Ivanisenko
(See online at https://doi.org/10.1002/adem.201300413) - New experimental insight into the mechanisms of nanoplasticity. Acta Materialia 61 (2013) 7271
W. Skrotzki, A. Eschke, B. Jóni, T. Ungár, L.S. Tóth, Yu. Ivanisenko, L. Kurmanaeva
(See online at https://doi.org/10.1016/j.actamat.2013.08.032) - Observation of shear band formation in nanocrystalline Pd–Au alloy during in situ SEM compression testing. Journal of Material Science 48 (2013) 6841
Yu. Ivanisenko, T. Werz, A. Minkow, J. Lohmiller, P.A. Gruber, A. Kobler, L. Kurmanaeva, H.-J. Fecht
(See online at https://doi.org/10.1007/s10853-013-7490-7) - Variation of the deformation mechanisms in a nanocrystalline Pd - 10 at.% Au alloy at room and cryogenic temperatures. International Journal of Plasticity 60 (2014) 40
Yu. Ivanisenko, E.D. Tabachnikova, I.A. Psaruk, S.N. Smirnov, A. Kilmametov, A. Kobler, C. Kübel, L. Kurmanaeva, K. Csach, Y. Mishkuf, T. Scherer, Y. A. Semerenko, H. Hahn
(See online at https://doi.org/10.1016/j.ijplas.2014.04.011)
