A Hong Kong-Beijing-Taiwan mechanical engineering team led by Dr Huang Mingxin from the University of Hong Kong (HKU) has developed a strategy to develop both high strength and high ductility in inexpensive, medium Mn (Manganese) steel. Its material cost is just one-fifth of that of the steel used in current aerospace and defense applications.
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 the control of martensitic transformation. The as-developed D&P steel possessed an ultra-high yield strength of 2.21 GPa and 2.05 GPa and uniform elongation of 16%.
The D&P strategy could apply to any other alloy with deformation-induced martensitic transformation and provides a pathway for development of high strength, high ductility materials, the researchers said in their paper in the journal Science.
Background. Automotive, aerospace and defence applications require metallic materials with ultra-high strength. However, in some particular high-loading structural applications, metallic materials also require 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. Simultaneously to 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.
Steels have been the most widely used metallic materials in history 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.
The D&P steel. The team used a medium Mn steel (10% Mn, 0.47% C, 2% Al and 0.7% V (wt.%)) for the D&P process. The raw materials cost of the D&P steel is only 20% of the maraging steel used in aerospace and defense applications. No expensive alloying elements were used exhaustively—just some common alloying compositions that can be widely seen in the commercialized steels.
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 (see DP780 and Q&P980) as 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 nanotwinned (NT) steel which was also developed by the same HKU research team led by Dr. Huang Mingxin in 2015.
|Tensile properties of the present breakthrough D&P steel compared with other high-strength steels, including maraging steel, nanotwinned (NT) steel, quenching and partitioning (Q&P980) steel and dual-phase (DP780) steel. Click to enlarge.|
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.
B. B. He, B. Hu, H. W. Yen, G. J. Cheng, Z. K. Wang, H. W. Luo, M. X. Huang (2017) “High dislocation density–induced large ductility in deformed and partitioned steels” Science doi: 10.1126/science.aan0177