An international team of researchers led by Hao Chen of Tsinghua University has developed a unique chemical boundary engineering (CBE) approach significantly to improve steel’s physical properties. In an open-access paper in Science Advances, the team reports that when applied to plain steels with carbon content of only up to 0.2 wt %, this approach yields ultimate strength levels beyond 2.0 GPa in combination with good ductility (>20%).

Although the study uses plain carbon steels, the CBE design approach is, in principle, applicable also to other alloys, according to the team.

Stronger steels with high ductility are crucial for solving key challenges in lightweight transportation and safe infrastructures, as evidenced by the incredible amount of 1.8 billion tons produced every year. High-strength steels, especially those with an ultimate tensile strength beyond 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. Yet, using a high carbon and doping content is not pertinent in construction steels due to weldability and cost constraints.

Microstructures with high lattice defect densities serve instead as a better route toward lean, affordable, and strong steels. Among these defect types, grain boundaries (GBs) and phase boundaries (PBs), which are planar discontinuities in metallic crystals, are particularly efficient in tuning the mechanical response of polycrystalline materials. Grain boundary engineering (GBE), e.g., modulating the quantity or arrangement of GBs/PBs, has been widely used to tailor the mechanical properties of advanced engineering materials. Yet, further enhancement of GB-related properties is limited by the instability (low thermal stability/high mobility) of these crystallographic planar interfaces when alloys are exposed to mechanical or thermal loads, causing, for instance, grain coarsening.

To expand the dimensionality of materials design, a not-yet fully explored type of planar defect, so-called chemical boundaries (CBs) is used here to architect novel microstructures that can act on the local phase transformation response of the material. CBs represent a very sharp chemical discontinuity inside a continuous lattice region … In our study, each CB is the residue of a former PB with its element partitioning retained upon removing the local change in crystal structure. Once formed, CBs act as strong barriers restricting subsequent phase transformations within ultrafine (submicron) domains. This methodology can result in a novel hierarchically heterogeneous microstructure consisting of martensite and austenite with nanoscaled laths and nanotwins, respectively, which can make it possible to achieve ultimate tensile strengths in excess of 2.0 GPa in combination with high ductility (>20%) in steels without high carbon content and/or doping expensive elements.

—Ding et al.

Chemical boundaries are interfaces where a material maintains its crystal structure but shifts its elemental composition. These chemical boundaries stand in contrast with phase boundaries (changes in crystal structure) and grain boundaries (borders of distinct crystallites 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 a standard austenite reversion treatment (ART) at the intercritical region (600 °C for 2 hours. The result was an ultrafine duplex microstructure consisting of equiaxed ferrite and metastable austenite, with mean grain diameters of 340 and 290 nm, respectively.

Transmission electron microscopy using energy-dispersive spectroscopy (TEM-EDS) and three-dimensional atom probe tomography (3D-APT) found significant Mn partitioning from ferrite to austenite, resulting in a significant amount of retained austenite. This partitioning causes a nanoscale discontinuity in the Mn concentration at the planar austenite/ferrite PBs.

The researchers then rapidly heated the processed steel (>100 °C/s) to the single-phase austenite region ( °C), followed by immediate quenching to ambient temperature. The fast heating results in the rapid elimination of all austenite/ferrite PBs and many GBs—i.e., there is a detachment between each PB and its associated chemical discontinuity.

As the metal cools, it settles into different phases; these phases must squeeze themselves between the sharp chemical boundaries and larger grain boundaries. This creates micro- and nano-structures in the final product, which can nearly double the strength of the original steel without sacrificing flexibility.

Microstructural evolution of the steel processed via the CBE strategy. (A) EBSD image quality map with superposed phase color map of the face-centered cubic (FCC) phase (red region) of the ART-processed steel, showing the equiaxed microstructure of austenite (γ) with ferrite (α), and (C) the ultrafine dual phase microstructure of γ and martensite (α′) of the CBE-processed steel. (B) Sketch of the microstructural evolution of the steels during ultrafast heating and quenching via the CBE strategy to illustrate the role of GBs, PBs, and CBs. Ding et al.

The current study demonstrates that CBE opens up alternative routes to achieve 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 temperatures by the mismatch between the sluggish Mn diffusion in austenite and fast migration of austenite/ferrite PBs. The extensive CBs can then restrain the martensitic transformation to submicron regions during subsequent quenching, resulting in an extremely fine martensite + austenite microstructure. The hard martensite network delays yielding, and the enhanced TRIP effect guaranties good ductility. The CBE method can be extended to other metallic systems and possibly be used as a surface treatment.

—Ding et al.

Resources

• 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) “Chemical boundary engineering: A new route toward lean, ultrastrong yet ductile steels” Science Advances doi: 10.1126/sciadv.aay1430