Superlattice alloys have an atomically close-packed and ordered structure. The strong chemical binding and low atomic mobility make them very attractive to high-temperature structural applications in a range of engineering fields such as aerospace, automotive, gas turbine engine, and many other industries. However, the highly ordered crystalline structure makes them brittle. The research team led by Professor Liu has discovered a new approach via fabricating multicomponent superlattice alloys with disordered interfacial nanolayers to resolve this dilemma. The findings have been published in the prestigious scientific journal Science under the title “Ultra high-strength and ductile superlattice alloys with nanoscale disordered interfaces” [1].
According to conventional wisdom, adding trace amounts (0.1 to 0.5 atomic percent (at. %)) of boron substantially improves tensile ductility by increasing grain-boundary cohesion, but when more than 0.5 at. % of boron were added, this traditional approach would not work well. However, the team came up with the idea to add excessive amounts of boron to the multi-component alloys, and the results were to their surprise. By increasing the boron concentration to 2.5 at. %, the synthesized alloy has an ultra-thin disordered interfacial nanolayer along the grain boundary. The ultra-thin layer contains multiple principal elements with disordered atomic structures that prevent brittle intergranular fractures. The general structure of superlattice alloys is made of individual crystalline areas known as “grains”. The brittleness in these alloys is generally ascribed to cracking along their grain boundaries during tensile deformation. Such superlattice materials have ultra-high strengths of 1.6 gigapascals with tensile ductilities of 25% at room temperature, which makes them a lot more ductile than expected.
In addition, the team also discovered that the increase in grain size was negligible even after 120 hours of heating at temperatures of 1050°C. Most traditional structural materials suffer from thermally driven structural instability because of rapid grain growth at high temperatures. As a result, the strength of these materials decreases quickly, severely limiting their applications. We believe that the nanolayer is pivotal in suppressing growth in grain size and maintaining its strength at high temperatures. The thermal stability of the disordered nanolayer will render this type of alloy suitable for high-temperature structural applications.
The discovery of this disordered nanolayer along the grain boundaries in the alloy will positively impact the development of high-strength materials in the future and may open a pathway for further optimization of alloy properties.
The ultra-thin disordered layer at the grain boundaries is about 5 nm thick [1].
Reference
1. Yang, T, Zhao, YL, Li, WP, Yu, CY, Luan, JH, Lin, DY, Fan, L, Jiao, ZB, Liu, WH, Liu, XJ, Kai, JJ, Huang, JC & Liu, CT 2020, 'Ultrahigh-strength and ductile superlattice alloys with nanoscale disordered interfaces', Science (New York, N.Y.), vol. 369, no. 6502, pp. 427-432.
2. Yang, T, Zhao, YL, Tong, Y, Jiao, ZB, Wei, J, Cai, JX, Han, XD, Chen, D, Hu, A, Kai, JJ, Lu, K, Liu, Y & Liu, CT 2018, 'Multicomponent intermetallic nanoparticles and superb mechanical behaviors of complex alloys', Science (New York, N.Y.), vol. 362, no. 6417, pp. 933-937.
3. Zhang, T, Huang, Z, Yang, T, Kong, H, Luan, J, Wang, A, Wang, D, Kuo, W, Wang, Y & Liu, C-T 2021, 'In situ design of advanced titanium alloy with concentration modulations by additive manufacturing', Science, vol. 374, no. 6566, pp. 478-482.