Nanosteels

Steels are generally nanostructured alloys. However, on this specific website I present some recent developments which make use of well-designed nano-precipitate systems. 
Most of the examples are based on the maraging (= martensite + ageing) concept and some examples make use of fine precipitates in a ferritic matrix. 

 

Ultrastrong steel via minimal lattice misfit and high-density nanoprecipitation
doi:10.1038/nature22032
Ultrastrong steel via minimal lattice mi[...]
PDF-Dokument [6.2 MB]

High-performance steels are required for lightweight design strategies, savety, high strength structure design 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 superior 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. In this report we present a novel and quite 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 10^24 per
cubic metre) and small size (about 2.7 ± 0.2 nanometres). 

Nature 2017: Three-dimensional reconstruction of an atom probe tomography dataset confirming the B2 nature of the precipitates with full lattice coherence. Nature 2017: Three-dimensional reconstruction of an atom probe tomography dataset confirming the B2 nature of the precipitates with full lattice coherence.

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. 

Nature 2017: High-resolution HAADF STEM images and three-dimensional reconstruction of an atom probe tomography dataset confirming the B2 nature of the precipitates with full lattice coherence. Nature 2017: High-resolution HAADF STEM images and three-dimensional reconstruction of an atom probe tomography dataset confirming the B2 nature of the precipitates with full lattice coherence.
Nanoprecipitation in an Fe-19Ni-xAl maraging steel triggered by the intrinsic heat treatment during laser metal deposition
Acta Materialia 129 (2017) 52-60
Acta Mater 2017 Additive Manufacturing M[...]
PDF-Dokument [2.4 MB]

Owing to the effect of the layer-by-layer build-up of additively manufactured parts, the so deposited material experiences a cyclic re-heating in the form of a sequence of temperature pulses. In the current work, this “intrinsic heat treatment (IHT)” was exploited in this project to induce the precipitation of NiAl nanoparticles in an Fe-19Ni-xAl (at%) model maraging steel, a system known for rapid clustering. We used Laser Metal Deposition (LMD) to synthesize compositionally graded specimens. This allowed for the efficient screening of effects associated with varying Al contents ranging from 0 to 25 at% and for identifying promising concentrations for further studies. Based on the existence of the desired martensitic matrix, an upper bound for the Al concentration of 15 at% was defined. Owing to the presence of NiAl precipitates as observed by Atom Probe Tomography (APT), a lower bound of 3e5 at% Al was established. Within this concentration window, increasing the Al concentration gave rise to an increase in hardness by 225 HV due to an exceptionally high number density of 10^25 NiAl precipitates per m^3, as measured by APT. This work demonstrates the possibility of exploiting the IHT of the LMD process for the production of samples that are precipitation strengthened during the additive manufacturing process without need for any further heat treatment.

Atom Probe Tomography dataset obtained on a maraging steel synthesized by additive manufacturing: Precipitates visualized by drawing an isoconcentration surface at 15 at% Al: Kürnsteiner et al. / Acta Materialia 129 (2017) 52 Atom Probe Tomography dataset obtained on a maraging steel synthesized by additive manufacturing: Precipitates visualized by drawing an isoconcentration surface at 15 at% Al: Kürnsteiner et al. / Acta Materialia 129 (2017) 52
Core-shell nanoparticle arrays double the strength of steel
Scientific Reports | 7:42547 | DOI: 10.1038/srep42547
Scientific Reports 7 - 42547 Core-shell [...]
PDF-Dokument [2.2 MB]

Manipulating structure, defects and composition of a material at the atomic scale for enhancing its physical or mechanical properties is referred to as nanostructuring. Here, by combining advanced microscopy techniques, we unveil how formation of highly regular nano-arrays of nanoparticles doubles
the strength of an Fe-based alloy, doped with Ti, Mo, and V, from 500 MPa to 1 GPa, upon prolonged heat treatment. The nanoparticles form at moving heterophase interfaces during cooling from the high-temperature face-centered cubic austenite to the body-centered cubic ferrite phase. We observe MoC and TiC nanoparticles at early precipitation stages as well as core-shell nanoparticles with a Ti-C rich core and a Mo-V rich shell at later precipitation stages. The core-shell structure hampers particle coarsening, enhancing the material’s strength. Designing such highly organized metallic core-shell nanoparticle arrays provides a new pathway for developing a wide range of stable nano-architectured  engineering metallic alloys with drastically enhanced properties.

Scientific Reports: Atom probe tomography on nanoprecipitates in a ferritic Nanohiten Steel. Scientific Reports: Atom probe tomography on nanoprecipitates in a ferritic Nanohiten Steel.

 

Acta Mat. 2011, 59, p. 364