Journal Article

Ultrastrong steel via minimal lattice misfit and high-density nanoprecipitation

Ponge, Dirk, Jiang, Suihe, Hirata, Akihiko, Wang, Hui, Chen, Honghong, Liu, Xiongjun, Wang, Yandong, Yao, Mengji, Lu, Zhaoping, Gault, Baptiste, Chen, Mingwei, Raabe, Dierk, Wu, Yuan

2017

Overview

A method of producing superstrong yet ductile steels using cheaper and lighter alloying elements is described, based on minimization of the lattice misfit to achieve a maximal dispersion of nanoprecipitates, leading to ultimate precipitation strengthening. Extreme precipitation makes superstrong steel Ultrastrong and yet ductile steels are important materials for the automotive and energy industries, among others. A key subgroup is the maraging steels, martensitic steels that have been aged by extended heat treatment. They acquire their strength from semi-coherent intermetallic precipitates. In this paper, maraging steels are described in which the expensive cobalt and titanium alloying elements are entirely replaced with lightweight and inexpensive aluminium. The resulting precipitates were produced in the steel at high density and with minimal lattice mismatch strain, leading to an impressive combination of very high strength (up to 2.2 gigapascals) and good ductility (about 8.2 per cent). The materials are characterized using a suite of high-resolution techniques, including atom probe tomography, HAADF STEM and synchrotron XRD. 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 1 , 2 , 3 . Their outstanding strength originates from semi-coherent precipitates 4 , 5 , which unavoidably exhibit a heterogeneous distribution that creates large coherency strains, which in turn may promote crack initiation under load 6 , 7 , 8 . 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 10 24 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.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/301/1023/1026 / 639/301/1023/303 / Aging / Alloys / Aluminum / Aluminum - chemistry / Chemical Precipitation / Cobalt / Cobalt - chemistry / Crack initiation / Deformation / Dental Alloys - chemistry / Design / Ductility / Elasticity / Fourier transforms / Humanities and Social Sciences / letter / Materials Testing / Microscopy, Electron, Scanning Transmission / Microstructure / multidisciplinary / Nanoparticles - chemistry / Nanoparticles - ultrastructure / Nanotechnology / Science / Steel / Steel - chemistry / Steel - economics / Synchrotrons / Tensile Strength / Titanium - chemistry / Tomography / Yield stress