Design of Stable Nanocrystalline Alloys

Abstract

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.

“Bulk” Nanocrystalline Metals: Review of the Current State of the Art and Future Opportunities for Copper and Copper Alloys

“…In general, two approaches are used to reduce the velocity of a moving grain boundary (and stabilize the grain structure): 45 by kinetically hindering GB mobility or by thermodynamically lowering the GB energy through solute segregation (i.e., modifying either the kinetic parameter M or the thermodynamic driving force P, respectively). 24,40,[46][47][48][49][50][51][52][53][54][55][56][57][58] These two approaches are often referred to as the thermodynamic approach and kinetic approach. Because grain boundary mobility follows an Arrhenius behavior, kinetic approaches for reducing grain boundary mobility will eventually be overcome by temperature.…”

“…Recently, a number of models have been derived that look at these effects independently and the combined effect for systems in which both methods of stability could operate. [46][47][48][49][50][51][52][53][54][55][56][57][58][59] …”

“…51,57,58 As an example of a thermodynamic stability map, Fig. 7 shows a map for nanocrystalline Cu, in which the red and black dots denote the stabilizing and nonstabilizing solutes, respectively.…”

“…57 These different models use slightly different approaches to the analytical representations of grain boundaries (i.e., a more simplified grain boundary model 57 versus broken bond models and bond interaction energy distributions 50,51,59,101 ) but are driven largely by accessible materials properties-the enthalpy of mixing and the elastic or segregation enthalpy of the alloying element. In many cases, these models have been validated against TEM and/ or x-ray diffraction data of the stabilized grain size as a function of annealing temperature (e.g., Refs.…”

Hybrid nanostructured aluminum alloy with super-high strength

“…12 In addition, the non-equilibrium state and the poor thermal stability of the NC or MG Al alloys limit their use at high temperatures. 12,13 An alternative approach to preparing NC materials is to crystallize amorphous solids under appropriate heat treatment conditions. 12,[14][15][16][17] Although this provides a successful method to synthesize Al alloys with strengths approaching the maximum, it does not offer good control over the microstructure and resulting properties, which are essentially determined by the crystallization behavior of the amorphous precursors.…”