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. For example, ceramics and amorphous materials have negligible ductility, 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 and P) strategy) which addressed the strength-ductility trade-off. Its material cost is just one-fifth of that of the steel used in the current aerospace and defence applications.
Steels have been the most widely used metallic materials in the history of mankind and can be produced with much higher efficiency than any other metallic materials. Therefore developing a strong and ductile breakthrough steel has been a long quest since the beginning of Iron Age in mankind history. In particular, it is very difficult to further improve the ductility of metallic materials when their yield strength is beyond 2 Gigapascal (GPa). Excitingly, a breakthrough steel – the Super Steel — which has been developed by an HKU-led HK-Beijing-Taiwan team and published in the prestigious academic journal Science recently, is a successful attempt in realizing the above dream of achieving a high ductility above the yield strength of 2 GPa.
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:
1 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. Figure 1 compares the raw materials cost between the present D&P steel with other high-strength steels.
2 Simple industrial processing
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 steelsas well as the steel used in aerospace and defence (maraging steel), the D&P steel demonstrated a much higher 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.
A wide variety of industrial applications require materials with high strength and ductility. Unfortunately, the strategies for increasing material strength, such as processing to create line defects (dislocations), tend to decrease ductility. We developed a strategy to circumvent this in inexpensive, medium Mn steel. Cold rolling followed by low-temperature tempering developed steel with metastable austenite grains embedded in a highly dislocated martensite matrix. This deformed and partitioned (D&P) process produced dislocation hardening, but retained high ductility both through the glide of intensive mobile dislocations and by allowing us to control martensitic transformation. The D&P strategy should apply to any other alloy with deformation-induced martensitic transformation and provides a pathway for development of high strength, high ductility materials.
Process could be applicable to making stronger and cheaper Titanium
The new steel overcomes the challenge of creating martensite known to be an issue for compositionally similar steels that rely on a quenching and partitioning process. The high dislocation density in the D&P steel not only increases the yield strength by dislocation forest hardening but also enables a large ductility by the glide of existing mobile dislocations and by the controlled release of TRIP effect. The high dislocation density is the origin of the inverse strength-ductility trade-off. We expect this strategy will be useful in other systems with similar deformation-induced martensitic transformation mechanism such as titanium alloys. The D&P steel exhibits low raw materials cost as compared to the maraging steel while maintaining a comparable ultimate tensile strength. Therefore, by engineering dislocations, we simultaneously alleviate the economic concerns while achieving ultra-high strength.