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Micro- and Macromechanical Properties of Fully Densified Nanocrystalline Metals

Subject Area Materials Engineering
Term from 2006 to 2014
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 19964558
 
Final Report Year 2015

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.

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