Nanostructured metals are generally unstable; their grains grow rapidly even at low temperatures, rendering them difficult to process and often unsuitable for usage. Alloying has been found to improve stability, but only in a few empirically discovered systems. We have developed a theoretical framework with which stable nanostructured alloys can be designed. A nanostructure stability map based on a thermodynamic model is applied to design stable nanostructured tungsten alloys. We identify a candidate alloy, W-Ti, and demonstrate substantially enhanced stability for the high-temperature, long-duration conditions amenable to powder-route production of bulk nanostructured tungsten. This nanostructured alloy adopts a heterogeneous chemical distribution that is anticipated by the present theoretical framework but unexpected on the basis of conventional bulk thermodynamics.

Grain boundary segregation has been established through both simulation and experiments as a successful approach to stabilize nanocrystalline materials against grain growth. However, relatively few alloy systems have been studied in this context; these vary in their efficacy, and in many cases the stabilization effect is compromised by second phase precipitation. Here we address the open-ended design problem of how to select alloy systems that may be stable in a nanocrystalline state. We continue the development of a general "regular nanocrystalline solution" model to identify the conditions under which binary nanocrystalline alloy systems with positive heats of mixing are stable with respect to both grain growth (segregation removes the grain boundary energy penalty) and phase separation (the free energy of the nanocrystalline system is lower than the common tangent defining the bulk miscibility gap). We calculate a "nanostructure stability map" in terms of alloy thermodynamic parameters. Three main regions are delineated in these maps: one where grain boundary segregation does not result in a stabilized nanocrystalline structure, one in which macroscopic phase separation would be preferential (despite the presence of a nanocrystalline state stable against grain growth), and one for which the nanocrystalline state is stable against both grain growth and phase separation. Additional details about the stabilized structures are also presented in the map, which can be regarded as a tool for the design of stable nanocrystalline alloys. Atomistic simulations on nanocrystalline alloys show that structural stabilization is contingent upon the distribution and character of the solute atom. A certain minimum concentration of solute is often found to be necessary for grain size stabilization, as for various solute species in simulated copper [11][12][13][14][15][16][17]. The efficacy of different solute species is variable, and in some studies has been related to the size difference between solute and solvent atoms [11][12][13]. However, in these studies, the grain boundaries are manually decorated with solute atoms, which may represent artificial segregation states. There have been fewer simulation works on systems where segregation is thermodynamic (by, e.g., Monte Carlo methods) [16][17][18][19][20]. These suggest that equilibrium solute segregation lowers the grain boundary energy to varying degrees. Experimentally, a reduction in the propensity for grain growth in nanocrystalline materials has been observed in a variety of binary alloys [21][22][23][24][25][26][27][28][29][30][31]. There are many indications in experimental systems that there is a "preferred" grain size which emerges during processing which is closely linked to the solute content [2,24,30,32,33]; this is considered significant evidence for a thermodynamic contribution to stabilization. The grain size that is stable against coarsening is correlated to the solute concentration in these systems, but the system also often exhibits instabilities w...

Grain boundary segregation provides a method for stabilization of nanocrystalline metals -the selection of an alloying element that will segregate to the boundaries can lower the grain boundary energy, attenuating the driving force for grain growth. The segregation strength, relative to mixing enthalpy, of a binary system determines the propensity for segregation stabilization. This relationship has been codified for the design space of positive enthalpy alloys; unfortunately, quantitative values for the grain boundary segregation enthalpy exist in only very few material systems, hampering the prospect of nanocrystalline alloy design. Here we present a Miedema-type model for estimation of grain boundary segregation enthalpy, with which potential nanocrystalline phase-forming alloys can be rapidly screened. Calculations of the necessary enthalpies are made for ~2500 alloys and used to make predictions about nanocrystalline stability.Abstract Figure: Fig. 1

It is a new beginning for innovative fundamental and applied science in nanocrystalline materials. Many of the processing and consolidation challenges that have haunted nanocrystalline materials are now more fully understood, opening the doors for bulk nanocrystalline materials and parts to be produced. While challenges remain, recent advances in experimental, computational, and theoretical capability have allowed for bulk specimens that have heretofore been pursued only on a limited basis. This article discusses the methodology for synthesis and consolidation of bulk nanocrystalline materials using mechanical alloying, the alloy development and synthesis process for stabilizing these materials at elevated temperatures, and the physical and mechanical properties of nanocrystalline materials with a focus throughout on nanocrystalline copper and a nanocrystalline Cu-Ta system, consolidated via equal channel angular extrusion, with properties rivaling that of nanocrystalline pure Ta. Moreover, modeling and simulation approaches as well as experimental results for grain growth, grain boundary processes, and deformation mechanisms in nanocrystalline copper are briefly reviewed and discussed. Integrating experiments and computational materials science for synthesizing bulk nanocrystalline materials can bring about the next generation of ultrahigh strength materials for defense and energy applications.

Development of ultra-high strength and corrosion-resistant aluminum (Al) alloys is demonstrated by a combination of suitable alloying elements and processing technology able to cause extended solid solubility and nanocrystalline structure. Binary Al-transition metal (M: Cr, Ni, Mo, Si, Ti, Mn, V, Nb) alloys, produced by high-energy ball milling and subsequent cold compaction, have exhibited significantly high hardness and corrosion resistance compared to any commercial Al alloy. The cyclic potentiodynamic polarization tests revealed a significant improvement in pitting and repassivation potentials. X-ray diffraction analysis revealed the grain refinement < 100 nm and extended solid solubility. IMPACT STATEMENT High-energy ball-milled Al alloys, owing to excellent corrosion resistance and high hardness, are expected to be a new class of Al alloys and initiate a multidisciplinary research direction.