Renaissance of alloys: nanocrystalline bimetals

We propose the simulation-based design of a novel class of lightweight alloys with unrivaled mechanical properties and thermal stability. Our ambition is to combine just two metals with the oldest metallurgical concept (grain boundary control of mechanical properties) to beat the most modern superalloys in their high temperature use. Unlike many existing nanostructured alloys, our solution has a remarkable feature - it eliminates grain coarsening almost up to melting point.
Recent thermodynamic analysis of nc-metal alloys indicates that minority species tend to segregate at grain boundaries, which allows a decrease of the grain boundary (GB) energy, and thus a reduction of the grain boundary mobility. Under such conditions, nc-metal alloys with a positive enthalpy of grain boundary segregation minimize the Gibbs free energy at a certain grain size, and attain a thermodynamically stable nanostructure. Recent work on binary systems has shown that thermodynamically stable nanocrystalline alloys generally require a large enthalpy of grain boundary segregation relative to enthalpy of mixing, or, otherwise, a reasonably negative grain boundary interaction energy.
To screen stability of nanocrystalline alloys, we will use recently developed thermodynamic methodologies aimed at identifying elements and relative chemical compositions allowing a nanocrystalline state to occupy a relative minimum of the Gibbs free energy. In this project, we will focus only on promising combination of lighter elements (e.g. Ti, Mg, Al, Zr, Nb). First objective is thus to identify the most promising alloy based on simulation. Then, we will prepare such alloy and test it. We will use magnetron sputtering, which allows extending the solid solubility limit and refining the grain size down to the nanometre range. A combinatorial sputtering approach (i.e. chemical gradient in horizontal axis) speeds up the evaluation of structure, thermal stability and mechanical properties. Standard pre and post-annealing structural analysis (particularly elemental distribution across grain boundaries by EDX/EELS) will be coupled with in-situ observation of doping element segregation at grain boundaries in transmission electron microscope. The latter approach sheds light on initial diffusion, which will in turn be used to improve input into simulations. Mechanical properties and creep will be evaluated by nanoindentation including in-situ high temperature testing in a protective atmosphere at temperatures up to 800C.
The main result should be identification of possible alloys by model and validation of their preparation route and functional performance.

This project bridges the gap between deep theoretical and experimental aspects of nanograin metal alloys and aims to develop a novel class of materials with properties superseding existing material limits at high temperature. If successful, it opens a new path to design alloys for highly demanding applications. In the aerospace sector, a flagship of UK industry, novel alloys may represent a step change in design and efficiency. Let's consider a standard passenger plane, the Airbus A310. Present peak temperature in its jet engine is limited to 1150C. An increase of 50C in top working temperature translates into fuel savings and a reduction of environmental load (equivalent to 33 tons of CO2 equivalent per year); a similar CO2 reduction would be expected when new alloys replace landing gears and part of the aircraft body. A high density of interfaces is typically beneficial for radiation resistance (self-healing of defects); moreover, interfaces can accommodate helium (PI is very active in this area) and thus eliminate its detrimental effect on the material (swelling and blistering). Nanostructured alloys are resistant to fatigue, and another output of this work can be nanostructured alloy for dental implants (Ti, Zr, Nb are biocompatible).
The major impact and benefit, though, is in the simple composition of proposed alloys. Its structure comprises almost identical nanograins (pure metal or solid solution of two metals) separated by segregated doping element and is very simple to simulate in terms of mechanical properties, oxidation or corrosion. Atomistic simulations, such as ab initio or molecular dynamics, can be very powerful tools to further investigate the proposed binary alloys. Finally, novel alloys eliminate the need to use critical raw materials and promise complete recyclability, which is one of the major weaknesses of superalloys.