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.

/pdf/high-tensile-ductility-in-a-nanostructured-metal-siwm61h9tg.pdf

High tensile ductility in a nanostructured metal.

Hall-Petch relation and boundary strengthening

Dynamic and post-dynamic recrystallization under hot, cold and severe plastic deformation conditions

This article presents an overview of the developments in stainless steels made since the 1990s. Some of the new applications that involve the use of stainless steel are also introduced. A brief introduction to the various classes of stainless steels, their precipitate phases and the status quo of their production around the globe is given first. The advances in a variety of subject areas that have been made recently will then be presented. These recent advances include (1) new findings on the various precipitate phases (the new J phase, new orientation relationships, new phase diagram for the Fe–Cr system, etc.); (2) new suggestions for the prevention/mitigation of the different problems and new methods for their detection/measurement and (3) new techniques for surface/bulk property enhancement (such as laser shot peening, grain boundary engineering and grain refinement). Recent developments in topics like phase prediction, stacking fault energy, superplasticity, metadynamic recrystallisation and the calculation of mechanical properties are introduced, too. In the end of this article, several new applications that involve the use of stainless steels are presented. Some of these are the use of austenitic stainless steels for signature authentication (magnetic recording), the utilisation of the cryogenic magnetic transition of the sigma phase for hot spot detection (the Sigmaplugs), the new Pt-enhanced radiopaque stainless steel (PERSS) coronary stents and stainless steel stents that may be used for magnetic drug targeting. Besides recent developments in conventional stainless steels, those in the high-nitrogen, low-Ni (or Ni-free) varieties are also introduced. These recent developments include new methods for attaining very high nitrogen contents, new guidelines for alloy design, the merits/demerits associated with high nitrogen contents, etc.

Recent developments in stainless steels

https://material.karlov.mff.cuni.cz/people/janecek/studenti/FyzMetNano/Estrin_AM61_2013_782.pdf

Extreme grain refinement by severe plastic deformation: A wealth of challenging science

High angle boundaries formed by grain subdivision mechanisms

Microstructure and strength of nickel at large strains

Geometrically necessary boundaries, incidental dislocation boundaries and geometrically necessary dislocations

Scaling of microstructural parameters: Misorientations of deformation induced boundaries

The evolution of dislocation structures in polycrystals with increasing strain is described within a framework of grain subdivision by dislocation boundaries and high angle boundaries. The evolving microstructures are characterized with emphasis on morphology and the changes in the misorientation angle across and spacing between deformation induced boundaries. Also the relationship between the slip pattern and the deformation microstructure is analyzed based on experimental observations showing a correlation between grain orientation in a polycrystal and the deformation microstructure. Next the changes in structural parameters are analyzed over a large strain range using a scaling hypothesis and the principle of similitude. The observations and analyses lead to a discussion of the effect of microstructural parameters on the flow stress behavior. Plastic deformation of metals has been a long lasting interest of Ali Argon and we are very glad on this occasion to thank him for many inspiring papers, scientific discussions and friendly companionship.

D.A. Hughes

Papers

High angle boundaries formed by grain subdivision mechanisms

Microstructure and strength of nickel at large strains

Geometrically necessary boundaries, incidental dislocation boundaries and geometrically necessary dislocations

Scaling of microstructural parameters: Misorientations of deformation induced boundaries

Microstructural evolution and hardening parameters