Next-generation high-performance structural materials are required for lightweight design strategies and advanced energy applications. Maraging steels, combining a martensite matrix with nanoprecipitates, are a class of high-strength materials with the potential for matching these demands. Their outstanding strength originates from semi-coherent precipitates, which unavoidably exhibit a heterogeneous distribution that creates large coherency strains, which in turn may promote crack initiation under load. Here we report a counterintuitive strategy for the design of ultrastrong steel alloys by high-density nanoprecipitation with minimal lattice misfit. We found that these highly dispersed, fully coherent precipitates (that is, the crystal lattice of the precipitates is almost the same as that of the surrounding matrix), showing very low lattice misfit with the matrix and high anti-phase boundary energy, strengthen alloys without sacrificing ductility. Such low lattice misfit (0.03 ± 0.04 per cent) decreases the nucleation barrier for precipitation, thus enabling and stabilizing nanoprecipitates with an extremely high number density (more than 1024 per cubic metre) and small size (about 2.7 ± 0.2 nanometres). The minimized elastic misfit strain around the particles does not contribute much to the dislocation interaction, which is typically needed for strength increase. Instead, our strengthening mechanism exploits the chemical ordering effect that creates backstresses (the forces opposing deformation) when precipitates are cut by dislocations. We create a class of steels, strengthened by Ni(Al,Fe) precipitates, with a strength of up to 2.2 gigapascals and good ductility (about 8.2 per cent). The chemical composition of the precipitates enables a substantial reduction in cost compared to conventional maraging steels owing to the replacement of the essential but high-cost alloying elements cobalt and titanium with inexpensive and lightweight aluminium. Strengthening of this class of steel alloy is based on minimal lattice misfit to achieve maximal precipitate dispersion and high cutting stress (the stress required for dislocations to cut through coherent precipitates and thus produce plastic deformation), and we envisage that this lattice misfit design concept may be applied to many other metallic alloys.

Ultrastrong steel via minimal lattice misfit and high-density nanoprecipitation

Atom probe tomography is a powerful tool for the characterization of the size, morphology and composition of ultrafine features in a variety of materials. With the development of new forms of specimen preparation especially with focused ion beam milling systems, atom probe tomography should be extended to a wider variety of applications in nanotechnology.

Atom probe tomography

The technology of high‐resolution focused ion beams has advanced dramatically in the past 15 years as focusing systems have evolved from laboratory instruments producing minuscule current densities to high current density tools which have sparked an important new process: direct micromachining at the micrometer level. This development has been due primarily to the exploitation of field emission ion sources and in particular the liquid‐metal ion source. Originally developed in the early 1960’s as a byproduct of the development of electrostatic rocket engines, the liquid‐metal ion source was adapted for focused beam work in the late 1970’s, when it was demonstrated that submicrometer focused ion beams could be produced with current densities greater than 1 A cm−2. Ions can be produced with liquid‐metal ion sources from elements including Al, As, Au, B, Be, Bi, Cs, Cu, Ga, Ge, Fe, In, Li, P, Pb, Pd, Si, Sn, and Zn. In the past decade, focused ion beam systems with liquid‐metal ion sources have had a signific...

High‐resolution focused ion beams

The post-ionization of field evaporated ions: A theoretical explanation of multiple charge states

Fine-scale precipitation of the metastable Zn- and Mg-rich η′ phase and its precursors is essential for the mechanical properties of Al–Zn–Mg alloys. However, at present neither the precipitation sequence nor the structure and composition of the intermediate precipitate phases are completely clear. This paper deals with an investigation of precipitation in an industrial Al–Zn–Mg alloy at various stages of a conventional two-step ageing treatment at 100° and 150°C. Studies were performed using both transmission electron microscopy and atom-probe field ion microscopy. Transmission electron microscopy (TEM) analysis revealed two parallel precipitation paths; one involving formation and dissolution of the ordered GP (I) zones, the other involving formation of clusters (type II), having a different atomic arrangement compared to the Al-matrix, which transform to the η′ phase. Atom-probe study of the material after short time ageing at 100°C did not show any observable distinction between GP (I) and type II precipitates. In the peak-aged material the best classification of precipitates was obtained using their morphology (the cigar-like and the plate-like) because there was significant overlap in the range of total solute contents of each type of precipitate. Generally the Zn:Mg ratio in all observed types of precipitates was close to 1:1 and the total solute atom content increased with ageing time. Distribution of alloying elements in the precipitates and in the surrounding matrix is discussed.

Investigation of precipitation in an Al–Zn–Mg alloy after two-step ageing treatment at 100° and 150°C

It is shown that the final observed charge state of a field-evaporated ion can be explained by the occurrence of post-ionization whereby an ion being accelerated away from a metal surface in a strong electric field loses one or more electrons by tunneling into the substrate. Calculations using an approximate analytic formula derived from a simple model potential predict the probability of post-ionization for many different elements.

Post-Ionization of Field-Evaporated Ions

Ionisation distributions of field-ionised ions from liquid metal ion sources are calculated Various shapes for the emitter region, a range of currents and a range of electric field strengths at the liquid surface are considered Their effects on the energy deficit distribution of the ions is presented through a numerical calculation for gold and gallium

David R. Kingham

Papers

The post-ionization of field evaporated ions: A theoretical explanation of multiple charge states

Post-Ionization of Field-Evaporated Ions

Field ion energy deficit calculations for liquid metal ion sources

Field ionization theory: A new, analytic, formalism

Calculations of field ionization in the field ion microscope