High strength and thermal stability of bulk Cu/Ta nanolamellar multilayers fabricated by cross accumulative roll bonding

Atomically ordered bimetal interfaces typically develop in near-equilibrium epitaxial growth (bottom-up processing) of nanolayered composite films and have been considered responsible for a number of intriguing material properties. Here, we discover that interfaces of such atomic level order can also emerge ubiquitously in large-scale layered nanocomposites fabricated by extreme strain (top down) processing. This is a counterintuitive result, which we propose occurs because extreme plastic straining creates new interfaces separated by single crystal layers of nanometer thickness. On this basis, with atomic-scale modeling and crystal plasticity theory, we prove that the preferred bimetal interface arising from extreme strains corresponds to a unique stable state, which can be predicted by two controlling stability conditions. As another testament to its stability, we provide experimental evidence showing that this interface maintains its integrity in further straining (strains > 12), elevated temperatures (> 0.45 Tm of a constituent), and irradiation (light ion). These results open a new frontier in the fabrication of stable nanomaterials with severe plastic deformation techniques.

https://www.pnas.org/content/pnas/111/12/4386.full.pdf

Emergence of stable interfaces under extreme plastic deformation

Mechanical strength and electrical conductivity are the most important properties of conducting metallic materials used in electrical engineering. Today, there is a growing need in this field for innovative conductor materials with improved properties. Meanwhile, the main issue is that high electrical conductivity and high strength are usually mutually exclusive due to physical nature of these properties. Alloying of pure metals results in significant increase of their mechanical strength, whereas electrical conductivity dramatically drops due to the scattering of electrons at solutes and precipitates. Recent studies have shown that intelligent nanostructural design in Al, Cu, and their alloys can improve combination of high mechanical strength with enhanced electrical conductivity. It was demonstrated that mechanical strength and electrical conductivity of these materials are primarily controlled by their microstructure, of which grain size, morphology of second phases, and their distribution, as well as dislocation structure, are the most important parameters. Rapid development of the state-of-the-art methods for the microstructural characterization at nano- and atomic scale has allowed a deeper insight into microstructure–properties relationship. The approach of intelligent nanostructural design of Al and Cu alloys has even enabled to increase the material strength with simultaneous improvement of its electrical conductivity. In this case, recent works on nanostructuring alloys by severe plastic deformation are of special interest, which gives rise to fundamental questions dealing with new mechanisms of strength and electrical conductivity as well as innovation potential of practical application of nanostructured materials. These issues are considered and discussed in the present progress article.

Nanostructured Al and Cu alloys with superior strength and electrical conductivity

Structure, shear resistance and interaction with point defects of interfaces in Cu–Nb nanocomposites synthesized by severe plastic deformation

Plastic deformation can effectively produce nanostructured metals and alloys in bulk or surface-layer forms that are suitable for practical structural or functional applications. Such nanostructured materials are porosity-free and contamination-free, and therefore they are ideal for studying fundamental mechanisms and mechanical properties. In this article, we first give an overview of the principles of grain refinement by plastic deformation and an introduction to the reported processing techniques. Then the four most-developed and promising techniques will be described in detail: equal-channel angular pressing, high-pressure torsion, accumulative roll bonding for bulk nanostructured metals, and surface mechanical attrition treatment for nanostructured surface layers.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769400009180

Processing of nanostructured metals and alloys via plastic deformation

Effects of size and geometry on the plasticity of high-strength copper/tantalum nanofilamentary conductors obtained by severe plastic deformation

The recent developments in the fields of processing, mechanical characterisation as well as simulation allow for a deeper study of the relationships between the microstructure and the mechanical properties of nanostructured materials (thin films and bulk nano materials). The understanding of size effect on the elastic and plastic deformation mechanisms is of both fundamental and practical interests in particular for the optimisation of industrial applications. Understanding of the relationships between their fabrication, microstructure, and mechanical properties is also fundamental. In particular, one of the key factors is related to the deformation mechanisms responsible of the elevated mechanical properties of such materials. In situ structural studies during deformation process allow quantifying the strain stress relationship of nanomaterials in the elastic and plastic regimes in relation with the microstructure since the spacing d of the material is used as a strain gage. Due to the very small dimensions of the crystallites, very intense sources are necessary such as neutron reactors for bulk nanomaterials and synchrotrons for nanostructured thin films and bulk nanomaterials. For more than a decade, our group has developed in situ tensile testing in diffractometers at the laboratory using rotating anodes as X-ray sources or at dedicated beam lines of synchrotron and Neutron facilities such as LURE (France), ESRF (France), PSI (Switzerland), ALS (USA or in a near future SOLEIL (France). A close connection has been made recently with this last facility through an ANR-Pnano project called Cmonano. In addition, local analyses are done using nanoindentation for intrinsic mechanical properties and atom probe tomography for structural characterisation. Atomistic and molecular dynamics simulations are completing the instrumental study. This paper will give an overview of the subject considering recent experiments on nanometric multilayers (WCu) and thin films (MoCr) encountered in micro electronic applications as well as bulk CuNb and CuTa nanocomposites processed by severe plastic deformation.

V. Vidal

Papers

Effects of size and geometry on the plasticity of high-strength copper/tantalum nanofilamentary conductors obtained by severe plastic deformation

Small scale mechanical properties of polycrystalline materials: in situ diffraction studies