How chemical boundaries make stronger steel

An international research team led by Hao Chen of Tsinghua University has bolstered steel’s physical properties by using its chemical boundaries. Chemical boundaries are interfaces where a material maintains its crystal structure but shifts its elemental composition. For example, steel could abruptly increase its concentration of alloying elements, such as cobalt, nickel, or manganese. These chemical boundaries stand in contrast with phase boundaries (changes in crystal structure) and grain boundaries (borders of distinct crystallites within a polycrystalline material). Yet all these boundaries play a role in strengthening steel. The researchers start with a standard steel specimen and heat it to 800 °C in about 10 s. This scorching creates sharp, submicroscopic chemical boundaries. As the metal cools, it settles into different phases, but these phases must now squeeze themselves between the sharp chemical boundaries and larger grain boundaries. This creates micro- and nanostructures in the final product, which can nearly double the strength of the original steel without sacrificing flexibility (Sci. Adv. 2020, DOI: 10.1126/sciadv.aay1430). Although the general heating approach is not new, notes John W. Morris Jr. of the University of California, Berkeley, who was not involved in the study, this implementation and its results are very impressive.