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

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

Grain boundary segregation leads to nanoscale chemical variations that can alter a material's performance by orders of magnitude (e.g., embrittlement). To understand this phenomenon, a large number of grain boundaries must be characterized in terms of both their five crystallographic interface parameters and their atomic-scale chemical composition. We demonstrate how this can be achieved using an approach that combines the accuracy of structural characterization in transmission electron microscopy with the 3D chemical sensitivity of atom probe tomography. We find a linear trend between carbon segregation and the misorientation angle ω for low-angle grain boundaries in ferrite, which indicates that ω is the most influential crystallographic parameter in this regime. However, there are significant deviations from this linear trend indicating an additional strong influence of other crystallographic parameters (grain boundary plane, rotation axis). For high-angle grain boundaries, no general trend between carbon excess and ω is observed; i.e., the grain boundary plane and rotation axis have an even higher influence on the segregation behavior in this regime. Slight deviations from special grain boundary configurations are shown to lead to unexpectedly high levels of segregation.

Quantification of Grain Boundary Segregation in Nanocrystalline Material

Experimental evidence of concurrent compositional and structural instabilities leading to ω precipitation in titanium-molybdenum alloys

In many materials, mechanistic understanding of material microstructure/property relationships requires knowledge of alloy structures at the atomic scale. This remains one of the main challenges of materials science. Historically, because of insufficient spatial resolution of available microstructural techniques, theories relating the role of alloying elements to materials properties were inferred from phenomenological studies. More recently, with the advent of techniques such as atom-probe tomography the spatial resolution limits have been dramatically improved. For instance, since the speculation by Cottrell and Bilby in 1949, it was generally believed that solute segregation at dislocations leads to strain hardening, but the direct proof came only recently when Blavette et al. (Nature, 1999) directly imaged a solute atmosphere at an edge dislocation, using atom-probe tomography. The recent progress in atom-probe tomography (both in dataset size and materials that can be analysed) enables atomic-scale studies of the structures of alloys and, more specifically, the analysis of solute behaviour which is a crucial issue in all areas of materials science. Clustering, ordering, site occupancy and solute/defect interactions are topics that are relevant to all classes of materials and directly affect the mechanical, electrical, magnetic, transport, etc., properties of materials. The real space information from atom-probe tomography provides a direct comparison with atomic simulations. Furthermore experimental data from the early stages of phase transformation can now be used to help validate the energetics used in atomistic modelling. This review will highlight: (1) the current limits of spatial and chemical resolution of atom-probe tomography and the resulting limits of data interpretation, (2) the data analysis tools that have been developed so far and the possible future paths for development and (3) examples where atom-probe tomography has provided mechanistic insight on the behaviour of solutes, in particular clustering, ordering and interactions with defects, and their effect on material properties.

Applications of atom-probe tomography to the characterisation of solute behaviours

Modern wide field-of-view atom probes permit observation of a wide range of crystallographic features that can be used to calibrate the tomographic reconstruction of the analyzed volume. In this study, methodologies to determine values of the geometric parameters involved in the tomographic reconstruction of atom probe data sets are presented and discussed. The influence of the tip to electrode distance and specimen temperature on these parameters is explored. Significantly, their influence is demonstrated to be very limited, indicating a relatively wide regime of experimental parameters space for sound atom probe tomography (APT) experiments. These methods have been used on several specimens and material types, and the results indicate that the reconstruction parameters are specific to each specimen. Finally, it is shown how an accurate calibration of the reconstruction enables improvements to the quality and reliability of the microscopy and microanalysis capabilities of the atom probe.

Advances in the calibration of atom probe tomographic reconstruction

Qualification of the tomographic reconstruction in atom probe by advanced spatial distribution map techniques

Atom-probe microscopy offers unprecedented insights on the subnanometer structure and chemistry of materials in three dimensions. The actual spatial resolution achievable is however still an uncertain parameter, as no comprehensive study has been undertaken to unveil the physics underpinning how key parameters impact the performance. Here, we present a comprehensive investigation of the in-depth and lateral resolution of the technique. We discuss methods to estimate the resolution and show a resolution better than 20 pm in-depth. Models to support our results were developed and are discussed in the present letter.

Origin of the spatial resolution in atom probe microscopy

Atom probe tomography observation of hydrogen in high-Mn steel and silver charged via an electrolytic route

This paper examines the extraction of structural information in the form of Radial Distribution Functions (RDFs) using Atom Probe Tomography (APT) data. These functions are generated in a highly efficient manner, thus allowing for the analysis of large data sets typical of APT. Experimental RDF calculations were performed for crystalline aluminium and a Mg65Cu25Y10 bulk metallic glass. For the pure aluminium sample, significant pair distance information was extracted, the quality of which was found to vary throughout the data set. Through a novel analysis procedure, the measured total RDF was used to map the local pair distance quality about each reconstructed atom. Surprisingly, the RDF quality maps indicated improved pair distance quality around poles and zone lines. In the case of the metallic glass, however, significant pair correlations were not discernible within the data set, despite short-range ordering being observed using TEM diffraction. The lack of correlations is thought to be associated with...

Daniel Haley

Papers

Advances in the calibration of atom probe tomographic reconstruction

Qualification of the tomographic reconstruction in atom probe by advanced spatial distribution map techniques

Origin of the spatial resolution in atom probe microscopy

Atom probe tomography observation of hydrogen in high-Mn steel and silver charged via an electrolytic route

Influence of field evaporation on Radial Distribution Functions in Atom Probe Tomography