(a) Electron backscatter diffraction (EBSD) phase image showing the lamella microstructure of layered austenite grains embedded in tempered martensite matrix. (b) The dislocation structures in martensite as enlarged in transmission electron microscopy (TEM) image. (c) TEM image showing the elongation of dislocation cell structure after the 8% tensile strain. (d) TEM image confirming the transformation of metastable austenite to martensite after 16% tensile strain. Credit: The University of Hong Kong
Automotive, aerospace and defence applications require metallic materials with ultra-high strength. However, in some particular high-loading structural applications, metallic materials shall also have large ductility and high toughness to facilitate the precise forming of structural components and to avoid the catastrophic failure of components during service. Unfortunately, increasing strength often leads to the decrease in ductility, which is known as the strength-ductility trade-off. Eg. ceramics and amorphous materials have are not ductile, although they have great hardness and ultra-high strength. To simultaneously increase both strength and ductility of metallic materials using conventional industrial processing routes is both of great scientific and technological importance and is yet quite challenging in both the materials science community and industry sectors.
A Hong Kong-Beijing-Taiwan mechanical engineering team led by Dr Huang Mingxin from the University of Hong Kong (HKU) has recently developed a Super Steel (also called D&P Steel as it adopted a new deformed and partitioned (D&P) strategy) which addressed the strength-ductility trade-off. Its material cost is just 1/5 of that of the steel used in the current aerospace and defence applications.
Steel has been the most widely used metal in the history of humankind and can be produced with much higher efficiency than any other metallic materials. A strong and ductile steel has been sought since the beginning of Iron Age. It is very difficult to further improve the ductility of metallic materials when their yield strength is beyond 2 Gigapascal (GPa). A breakthrough steel – the Super Steel – which has been developed by an HKU-led HK-Beijing-Taiwan team, achieves a high ductility, above the yield strength of 2GPa.
In addition to the substantial improvement of tensile properties, this breakthrough steel has achieved the unprecedented yield strength of 2.2 GPa and uniform elongation of 16%. Additionally, this breakthrough steel has two advantages:
~Low raw-materials cost
The raw materials cost of the D&P steel is only 20% of the maraging steel used in aerospace and defence applications. The chemical composition of this breakthrough steel belongs to the system of medium manganese (Mn) steel, containing 10% manganese, 0.47% carbon, 2% aluminium, 0.7% vanadium (mass percent), and the balance is iron. No expensive alloying elements have been used exhaustively but just some common alloying compositions that can be widely seen in the commercialized steels.
The second advantage is that this breakthrough steel can be developed using conventional industrial processing routes, including warm rolling, cold rolling and annealing. This is different from the development of other metallic materials where the fabrication processes involve complex routes and special equipment, which are difficult to scale-up. Therefore, it is expected that the present breakthrough steel has a great potential for industrial mass production.
Compared with the widely used automotive steels as well as the steel used in aerospace and defence (maraging steel), the D&P steel demonstrated a much yield strength but maintaining a much better ductility (uniform elongation). The D&P steel also outperformed the nanotwinned (NT) steel which was also developed by the same HKU research team led by Dr. Huang Mingxin in 2015. Additionally, the developed D&P steel demonstrated the best combination of yield strength and uniform elongation among all existing high-strength metallic materials. In particular, the uniform elongation of the developed D&P steel is much higher than that of metallic materials with yield strength beyond 2.0 GPa. http://www.hku.hk/press/news_detail_16681.html
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