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An international group of researchers has managed to double the strength of steel by incorporating a planar array of core-shell nanoparticles that consist of a Ti-C core and a Mo-V shell. The researchers have combined a series of microscopy techniques, alongside a novel nanostructuring method, to not only increase the strength, but to see how the alloy formation occurs. The incorporation of such nanoparticles has increased the strength of the iron-based alloy from 500 Mpa to 1 GPa.
Materials which are strong often lack ductility and ductile materials are not strong. This is known as the strength-ductility trade-off. The mechanical properties within a material are governed by how movable the defect sites are within a materials internal structure. For metal-containing materials, the governing defects are generally present as linear dislocations in the lattice- a primary carrier of shear.
Traditional nanostructuring is a method that allows the internal defect sites within metal alloys to be manipulated, so that the strength of the material can be increased whilst maintaining its ductility. It is a widely-used method to produce nanocrystalline metallic materials with a large quantity of grain boundaries. However, it does suffer from the final material being metastable, modified fine-scale substructures and lack of scalable production.
However, an alternative approach to nanostructuring is to utilise nanometer precipitates that act as nanoparticles in a metallic matrix, which help to provide stability against linear dislocations. The yield strength, high-temperature strength and creep resistance can all be improved by such methods.
Many body-centred cubic (bcc) lattices can be improved by the incorporation of finely dispersed transition metal ions. The metallic ions only leave narrow spaces, so dislocations must curve around or cut through nanoparticles- an energy intensive process that increases the flow stress. The effect can be further enhanced using a regular planar array, i.e. along a crystallographic plane, rather than in a random array- where the random order can allow for dislocations to escape and induce shearing at lower stresses.
The researchers have developed a novel method that combines transformation and interphase phenomena, using highly-organised, high density and thermally stable core-shell nanoparticles composed of a Ti-C core and a Mo-V shell.
As the general theory suggests, the core-shell nanoparticles act as dislocation stabilisers by restricting their motion. The core-shell nanoparticles also adopt a planar array and shift their lattice parameters from a face-centred cubic (fcc) to a bcc lattice-type structure. The average spacing between the layers is 9-12 nm, with the 110 plane being the most stable. There is an average density of 6.8 x 1023 m-3 particles in each layer, with an interparticle spacing of only 3-10 nm.
The enhanced alloy is subjected to annealing phases which allows the alloy to be used in environments up to 640 °C. In addition to this, the regular ordering of nanoparticles doubles the strength of the enhanced steel material to 1 Gpa. The nanoparticles are also resistant to coarsing.
Whilst the materials strength is the centre of attention, it also exhibits a remarkable increase in ductility. The Youngs modulus of the alloy is increased from 400 MPa to 830 MPa through the incorporation of core-shell nanoparticles. In practical terms, this allows for an increase of elongation by 18-21% before the material fractures. This is also independent of the heat treatment time.
Whilst the main focus of the research is the alloys enhanced strength, other properties such as ductility are increased making the alloy much more efficient for many practical applications. The next stage now, and where it is expected to flourish, is in producing a wide-range of stable nano-architectured metallic alloys (with enhanced properties compared to current materials) to be used across many engineering industries.
Reference
Seol J-B., Na S-H., Gault B., Kim J-E., Han J-C., Park C-G., Raabe D., Core-shell nanoparticle arrays double the strength of steel, Scientific Reports, 2017, 7, 42457
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