ICAMS / Interdisciplinary Centre for Advanced Materials Simulation
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A joint APT/TEM approach to study co-deformation of amorphous/nanocrystalline nanolaminates

Date: 12.08.2014
Time: 11:00 a.m.
Place: IC 02-722

Wei Guo, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany

Amorphous metals permit ultimate mechanical strength and hardness. However, their macroscopic plasticity has been limited by the formation of catastrophic shear bands. In this work, multilayers composed of amorphous CuZr and nanocrystalline Cu layers have been synthesized with different layer thicknesses (100 nm Cu/100 nm CuZr, 50 nm Cu/100 nm CuZr, 10 nm Cu/100 nm CuZr). Nano-pillar compression tests showed that the strongest multilayered samples (100 nm glass layers and 50 nm Cu layers) exhibited a compressive yield strength of 2.57±0.21 GPa (matching the strength of pure CuZr metallic glass) and a true strain to failure >15%. This exceeded the linear rule-of-mixture of the mechanical properties of the two bulk constituents. As the Cu layer thickness increased from 10 nm to 100 nm, a deformation-mode transition from highly localized shear banding to preferential deformation of the Cu layers through an intermediate stage of co-plastic deformation of both amorphous and nanocrystalline layers was observed.

By probing the deformed structures with correlative atom probe tomography and transmission electron microscopy, we found that crystallographic slip bands in the Cu layers coincide with non-crystallographic shear bands in the amorphous CuZr layers. Dislocations from the crystalline layers drag Cu atoms across the interface into the CuZr layers. Additionally, crystalline Cu blocks are displaced into the CuZr layers by shear deformation. In these sheared and thus Cu enriched zones, the initially amorphous CuZr is transformed into an amorphous plus crystalline nanocomposite. This effect might impede catastrophic shear banding events and thus enable the design of “strong and ductile” crystalline-amorphous nanolaminate materials.

Furthermore, the deformation induced chemical mixing (mechanical alloying) was specifically investigated based on current multilayered structures. The chemical and structural evolution inside shear bands with different strain levels was traced by atom probe tomography and transmission electron microscopy. The initially pure Cu and amorphous CuZr layers were found to be chemically mixed via shear banding after reaching a critical shear strain, the strain value being dependent on the initial layer thickness. The shear bands can squeeze the neighboring Zr atoms into the Cu dislocation core structures in a Cu layer thickness of <5 nm, resulting in local atomic intermixing. In the end, the deformation induced crystallization will be discussed.

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