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Mixing instabilities during shearing in metals
02-01-2018
The dynamics of deformation during shearing in metallic multilayers have been captured using 3D X-ray synchrotron tomography. The observations include the formation of folds and vortices and delamination of layers. Numerical simulations show that metals behave as very viscous fluids. The process is similar to rock folding in geology.
We tend to think of common metals as rigid solids. This perspective is challenged here as new evidence shows that metals behave as very viscous fluids when subjected to shear deformation even at room temperature.
Interestingly, geologists already knew for quite a while that many large-scale geodynamical deformations taking place within the Earth over millions of years can be shown to follow fluidic behaviour. Indeed, they have been successfully using such an approach to explain phenomena such as rock folding and mountain building. Now, it has been demonstrated that such behaviour is not restricted to large dimensions and durations. In fact, even during short-period deformations and at the micrometre scale, metals behave similarly.
These findings were made possible by a novel experimental strategy that has enabled the morphological evolution within deforming solids to be captured in three dimensions for the first time. For this purpose, multilayers of copper/silver and copper/aluminium, representing a multiphase alloy, were created by alternately stacking individual 25-μm-thick foils of each material. Next, the multilayers were shear deformed using the high-pressure torsion (HPT) method (Figure 1a), whereby the sample is squeezed between two anvils while one anvil is rotating. This results in the formation of a disk-like sample.
During high-pressure torsion, each radial cross-section (i.e., plane with radius as its normal) is sheared to a slightly different extent due to the existence of a strain gradient along the radius of the disk. This causes a gradual evolution of morphology in the multilayer from its original phase-separated state to a relatively mixed-phase state, in the resulting sample disk. The interfacial and morphological evolution undergone by the stack could then be discovered by looking at successive cross-sections at different radii, corresponding to different levels of strain, using 3D X-ray synchrotron tomography at beamline ID19 (Figure 1b). This technique revealed a host of morphological events including delamination of the layers, formation of folds and rotating vortices in the multilayers (Figure 2), which were not considered in the previous atomic-scale mixing studies (e.g. [1]).
Figure 2. Morphological evolution of multilayers upon shearing acquired by 3D X-ray synchrotron tomography. (a–e) Selected snapshots of an Ag/Cu-multilayer that has been shear-deformed to a maximum strain of 393. The brighter phases are Ag. Here, the Cu layers predominantly delaminate before mixing takes place at the micrometre scale. (f–j) Selected snapshots of an Al/Cu-multilayer, shear-deformed to a maximum strain of 236. The brighter phases are Cu. Here, the Cu layers initially fold in a quasi-regular manner and subsequently evolve into periodic vortices, before mixing takes place at the micrometre scale. Note that in both sequences, the overall strain increases when going from the top to the bottom of each panel. The scale bar is 200 μm. The dark area within (e) is a crack that appeared after HPT deformation and during the cutting of the disk for tomography examination. See also: videos of morphological evolution during shearing: 1. in Ag/Cu, corresponding to (a-e) and 2) in Al/Cu, corresponding to (f-j). |
These new observations resembled many similarities to morphologies found in rocks. Hence, a computer model that was originally designed for mountain building processes was adapted to simulate the experimental conditions. The model identified the viscosity contrast (VC) and the stress exponent of the phases (n), as well as the strain rate (˙γ) as the key factors driving the microstructural evolution. Figure 3 shows the results of a simulation for a particular combination of (VC, n). In short, the results indicate that the same instability that causes kilometre-thick rock layers to fold on geological timescales is acting here at micrometre level.
The findings reveal the origin for the emergence of a wealth of morphologies in deforming solids. While the experiments were performed on multilayers under shear, the model is not limited to samples of this type and could be applied to any material system regardless of its morphology. This makes the model a versatile tool to study a broad range of materials and material processing techniques.
Principal publication and authors
Mixing instabilities during shearing of metals, M. Pouryazdan (a), B.J.P. Kaus (b), A. Rack (c), A. Ershov (d,e), H. Hahn (a,f), Nature Communications 8, 1611 (2017); doi: 10.1038/s41467-017-01879-5.
(a) Institute of Nanotechnology, Karlsruhe Institute of Technology (Germany)
(b) Institute of Geosciences and Center for Computational Sciences, University of Mainz (Germany)
(c) ESRF
(d) Institute for Photon Science and Synchrotron Radiation, Karlsruhe Institute of Technology (Germany)
(e) Laboratory for Applications of Synchrotron Radiation, Karlsruhe Institute of Technology (Germany)
(f) Joint Research Laboratory Nanomaterials at Technische Universität Darmstadt (Germany)
References
[1] Pouryazdan et al., Physical Review B 86, 144302 (2012).