Review on superior strength and enhanced ductility of metallic nanomaterials

a b s t r a c t Grain boundaries influence mechanical, functional, and kinetic properties of metallic alloys. They can be manipulated via solute decoration enabling changes in energy, mobility, structure, and cohesion or even promoting local phase transformation. In the approach which we refer here to as ‘segregation engineering’ solute decoration is not regarded as an undesired phenomenon but is instead utilized to manipulate specific grain boundary structures, compositions and properties that enable useful material behavior. The underlying thermodynamics follow the adsorption isotherm. Hence, matrix-solute combinations suited for designing interfaces in metallic alloys can be identified by considering four main aspects, namely, the segregation coefficient of the decorating element; its effects on interface cohesion, energy, structure and mobility; its diffusion coefficient; and the free energies of competing bulk phases, precipitate phases or complexions. From a practical perspective, segregation engineering in alloys can be usually realized by a modest diffusion heat treatment, hence, making it available in large scale manufacturing.

Grain boundary segregation engineering in metallic alloys: A pathway to the design of interfaces

This paper will review the grain growth in nanocrystalline materials with emphasis on the grain size stabilization that can result from solute additions. The grain growth in nominally pure nanocrystalline metals will be presented followed by descriptions of the stabilization of nanocrystalline grain sizes by kinetic approaches and thermodynamic strategies. The descriptions of nanocrystalline grain size by solute additions will be taken from the literature as well as from recent research in the authors’ laboratory. Examples of kinetic stabilization, which involves reduction of the grain boundary mobility, include second phase drag, solute drag, chemical ordering, and grain size stabilization. The thermodynamic stabilization, which is due to the lowering of the specific grain boundary energy by solute segregation to the grain boundaries, will be described for systems including Pd–Zr, Fe–Zr, Ni–W, Ni–P, and Co–P. Recrystallization during grain growth will be presented for the Ti–N system. Finally, a summary of alloys where nanocrystalline grain sizes can be maintained at annealing temperatures close to the melting point will be presented.

Stabilization of nanocrystalline grain sizes by solute additions

A free-energy function for binary polycrystalline solid solutions is developed based on pairwise nearest-neighbor interactions. The model permits intergranular regions to exhibit unique energetics and compositions from grain interiors, under the assumption of random site occupation in each region. For a given composition, there is an equilibrium grain size, and the alloy configuration in equilibrium generally involves solute segregation. The present approach reduces to a standard model of grain boundary segregation in the limit of infinite grain size, but substantially generalizes prior thermodynamic models for nanoscale alloy systems. In particular, the present model allows consideration of weakly segregating systems, systems away from the dilute limit, and is derived for structures of arbitrary dimensionality. A series of solutions for the equilibrium alloy configuration and grain size are also presented as a function of simple input parameters, including temperature, alloy interaction energies, and component grain boundary energies.

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Patrick Z. Wong

Papers

Grain-size stabilization in nanocrystalline FeZr alloys