Bulk nanostructured materials from severe plastic deformation

Mechanical properties of nanocrystalline materials

Principles of equal-channel angular pressing as a processing tool for grain refinement

Nanocrystalline metals--with grain sizes of less than 100 nm--have strengths exceeding those of coarse-grained and even alloyed metals, and are thus expected to have many applications. For example, pure nanocrystalline Cu (refs 1-7) has a yield strength in excess of 400 MPa, which is six times higher than that of coarse-grained Cu. But nanocrystalline materials often exhibit low tensile ductility at room temperature, which limits their practical utility. The elongation to failure is typically less than a few per cent; the regime of uniform deformation is even smaller. Here we describe a thermomechanical treatment of Cu that results in a bimodal grain size distribution, with micrometre-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains. The matrix grains impart high strength, as expected from an extrapolation of the Hall-Petch relationship. Meanwhile, the inhomogeneous microstructure induces strain hardening mechanisms that stabilize the tensile deformation, leading to a high tensile ductility--65% elongation to failure, and 30% uniform elongation. We expect that these results will have implications in the development of tough nanostructured metals for forming operations and high-performance structural applications including microelectromechanical and biomedical systems.

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High tensile ductility in a nanostructured metal.

Using high-pressure torsion for metal processing: Fundamentals and applications

It is well known that plastic deformation induced by conventional forming methodssuch as rolling, drawing or extrusion can significantly increase the strength of metalsHowever, this increase is usually accompanied by a loss of ductility. For example, Fig.1 shows that with increasing plastic deformation, the yield strength of Cu and Almonotonically increases while their elongation to failure (ductility) decreases. Thesame trend is also true for other metals and alloys. Here we report an extraordinarycombination of high strength and high ductility produced in metals subject to severeplastic deformation (SPD). We believe that this unusual mechanical behavior is causedby the unique nanostructures generated by SPD processing. The combination ofultrafine grain size and high-density dislocations appears to enable deformation by newmechanisms. This work demonstrates the possibility of tailoring the microstructures ofmetals and alloys by SPD to obtain both high strength and high ductility. Materialswith such desirable mechanical properties are very attractive for advanced structuralapplications.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400060143

Paradox of strength and ductility in metals processed by severe plastic deformation

Microstructures and dislocation configurations in nanostructured Cu processed by repetitive corrugation and straightening

Equal channel angular pressing (ECAP) is an innovative technique that can produce bulk ultrafine-grained (UFG) materials in product forms large enough for structural applications. It is well known that ECAP route, defined by the sequence of orientations of the billets relative to the die during the iterative ECAP passes, significantly affects the microstructural development of the work piece. Studies reported in the literature have so far focused on fcc metals such as Al and Cu. In this work, we have studied the influence of ECAP routes on the microstructures and properties of hcp commercially-pure Ti. Three ECAP routes, conventionally defined as BA, BC and C, were used to process the Ti billets. Surface quality, microstructures, microhardness, tensile properties, anisotropy, and thermal stability were studied. The route BC is shown to be the best route for processing hcp Ti.

Influence of ECAP routes on the microstructure and properties of pure Ti

Corrosion resistance of ultra fine-grained Ti

In this paper, we report a new severe plastic deformation (SPD) technique, which combines equal channel angular pressing (ECAP) with Conform, to process ultrafine grained (UFG) materials in a continuous manner. ECAP in its original form can only process short metal bars and is labor intensive. Conform is a technique that has been used to continuously form metals into various shapes. By combining these two techniques, we were able to produce UFG structures in an Al wire and to significantly increase its strength.

Terry C. Lowe

Papers

Paradox of strength and ductility in metals processed by severe plastic deformation

Microstructures and dislocation configurations in nanostructured Cu processed by repetitive corrugation and straightening

Influence of ECAP routes on the microstructure and properties of pure Ti

Corrosion resistance of ultra fine-grained Ti

Continuous processing of ultrafine grained Al by ECAP–Conform