Steel alloys with ultrahigh strength (UHSSs) above 1,500 megapascals (MPa) are in great demand for infrastructure subject to extreme conditions, such as aircraft landing gears, and rocket engine cases. There are challenges presented by the need for combining increased strength and good ductility to prolong the steel’s service lifetime, saving energy and reducing emissions.
A research team led by LYU Zhaoping, Vice President of USTB and a professor at the State Key Laboratory for Advanced Metals and Materials at the University of Science and Technology Beijing (USTB), have designed a steel with strength up to 2,200 MPa, and ductility of over 8%, to be produced at a reduced cost.
The high strength in conventional maraging steels, a class of UHSSs, is due to the presence of semi-coherent precipitates, particles whose elastic energy comes from a crystallographic misfit through the addition of high-cost metallic elements such as cobalt and titanium. At the same time, the lattice mismatch between these precipitates and the surrounding steel matrix, can lead to strain, which can cause earlier cracking under load.
“The strength-ductility trade-off has been a long-lasting dilemma. When moving to structural applications, the cost is another important factor to be balanced,” says LYU.
LYU and his colleagues have been dedicated to improving the comprehensive performance of steels. They envisaged a steel with outstanding strength and maximum ductility for forming into any shape, with improved toughness and margin of safety in service.
After observing an experimental steel exhibiting a surprising performance, LYU and his team explored the strengthening mechanism. After years of research, the team has developed a pathway to this ultra-strong steel using high-density nanoprecipitation with minimal lattice misfit. They found that the evenly dispersed, fully coherent precipitates — their lattice structure was almost the same as that of the steel matrix — strengthen the alloys without sacrificing ductility.
“The steel alloy is hardened by a very dense distribution of nanosized aluminium-iron-nickel precipitates. We also enhance the ordering effect that creates back stresses, the forces opposing deformation,” says LYU. The team also replaced cobalt and titanium with aluminium, which is lighter and more economical.
The toughness, another importance index of the performance, is also significantly improved in the team’s experiments, but the mechanism for this not yet defined, according to LYU. This class of ultra-strong steel is in a pilot production run.
“This innovative design concept has proven applicable for other metallic alloys,” LYU points out. The same mechanism of coherent nanoprecipitation is leveraged in his other research topics, such as high-entropy alloys and TWIP steels.
“Despite hundreds of years of research in this subject, there are opportunities for innovation along with emerging technologies. For example, the new testing and inspection facilities such as three-dimensional atom probe tomography, can bring light to the microscopic world of structural materials, while computer science such as machine learning can make it to a macro level with powerful data processing to improve the alloy design processes. New application scenarios and requirements can also be the source of innovation,” LYU says.
