An international team of researchers led by Hao Chen of Tsinghua University has developed a unique chemical boundary engineering (CBE) approach to dramatically improve the physical properties of steel. In an open access article in Scientists progress, the team reports that when applied to simple steels with a carbon content of up to 0.2% by weight only, this approach results in ultimate strength levels in excess of 2.0 GPa in combination. with good ductility (> 20%).
Although the study uses simple carbon steels, the CBE design approach is, in principle, also applicable to other alloys, according to the team.
Stronger steels with high ductility are essential to solve the main challenges of light transport and safe infrastructure, as evidenced by the incredible 1.8 billion tonnes produced each year. High strength steels, especially those with tensile strength greater than 2.0 GPa, generally require a high level of carbon [>0.4 weight % (wt %)] and / or expensive doping elements such as cobalt, nickel and chromium. However, the use of a high carbon and doping content is irrelevant in structural steels because of weldability and cost constraints.
Rather, microstructures with high lattice fault densities are a better path to lean, affordable and strong steels. Among these types of defects, grain boundaries (GB) and phase boundaries (PB), which are plane discontinuities in metal crystals, are particularly effective in controlling the mechanical response of polycrystalline materials. Grain boundary engineering (GBE), for example, modulating the amount or arrangement of GB / PB, has been widely used to tailor the mechanical properties of advanced engineering materials. However, the further improvement in GB-related properties is limited by the instability (low thermal stability / high mobility) of these crystallographic planar interfaces when the alloys are exposed to mechanical or thermal loads, causing, for example, magnification of the crystals. grains.
To extend the dimensionality of materials design, a type of planar defect not yet fully explored, called chemical boundaries (CB), is used here to design new microstructures that can act on the local phase transformation response of the material. The CBs represent a very clear chemical discontinuity within a continuous lattice region… In our study, each CB is the residue of an old PB with its element partition conserved upon removal of the local change in the crystal structure. Once formed, CBs act as strong barriers limiting subsequent phase transformations in ultra-fine (submicronic) domains. This methodology can result in a novel hierarchically heterogeneous microstructure composed of martensite and austenite with nanoscale slats and nanotunins respectively, which can achieve ultimate tensile strengths greater than 2.0 GPa in combination with ductility. high (> 20%) in steels without high carbon content and / or doping expensive elements.
—Ding et al.
Chemical boundaries are interfaces where a material retains its crystalline structure but changes its elemental composition. These chemical boundaries contrast with phase boundaries (changes in crystal structure) and grain boundaries (distinct crystallite boundaries within a polycrystalline material).
The researchers worked with a low carbon steel with a lean composition of 0.18C-8Mn (wt%). The material was first subjected to cold rolling and standard austenite reversion treatment (ART) in the intercritical region (600 ° C for 2 hours. The result was an ultra-fine duplex microstructure composed of equiaxed ferrite. and metastable austenite, with average grain diameters of 340 and 290 nm, respectively.
Transmission electron microscopy using energy dispersive spectroscopy (TEM-EDS) and three-dimensional atomic probe tomography (3D-APT) revealed significant separation of Mn from the austenite ferrite, resulting in a significant amount of d retained austenite. This partitioning causes a nanoscale discontinuity in the concentration of Mn at the level of the austenite / ferrite planar PB.
The researchers then quickly heated the treated steel (> 100 ° C / s) to the region of single-phase austenite (° C), followed by immediate quenching at room temperature. Rapid heating results in rapid removal of all austenite / ferrite PB and many GBs, i.e. there is a detachment between each PB and its associated chemical discontinuity.
As the metal cools, it settles in different phases; these phases must weave between the neat chemical boundaries and the larger grain boundaries. This creates micro- and nano-structures in the final product, which can almost double the strength of the original steel without sacrificing flexibility.
Microstructural evolution of steel treated via the CBE strategy. (A) EBSD image quality map with superimposed phase color map of face-centered cubic phase (FCC) (red region) of ART-treated steel, showing equiaxial microstructure of austenite (γ) with ferrite (α), and (VS) the ultrafine dual-phase microstructure of and martensite (α ′) of CBE-treated steel. (B) Sketch of the microstructural evolution of steels during ultra-rapid heating and quenching via the CBE strategy to illustrate the role of GB, PB and CB. Ding et al.
The present study demonstrates that the CBE opens up alternative avenues for obtaining unique microstructures other than via conventional GBE approaches, leading to ductile and strong steels without high carbon content and expensive doping elements. The CBs in this study are created at high temperature by the shift between the slow diffusion of Mn in austenite and the rapid migration of austenite / ferrite PB. Extended CBs can then restrict martensitic transformation to submicronic regions during subsequent quenching, resulting in an extremely fine martensite + austenite microstructure. The hard martensite network retards flow and the improved TRIP effect guarantees good ductility. The CBE method can be extended to other metal systems and possibly be used as a surface treatment.
—Ding et al.
Ran Ding, Yingjie Yao, Binhan Sun, Geng Liu, Jianguo He, Tong Li, Xinhao Wan, Zongbiao Dai, Dirk Ponge, Dierk Raabe, Chi Zhang, Andy Godfrey, Goro Miyamoto, Tadashi Furuhara, Zhigang Yang, Sybrand Van Der Zwaag, Hao Chen (2020) “Engineering of chemical frontiers: a new path towards lightweight, ultra-resistant but ductile steels”
Scientists progress do I: 10.1126 / sciadv.aay1430