Hydrogenation treatment under several gigapascals assists diffusionless transformation in a face-centered cubic steel

The use of hydrogen in iron and steel has the potential to improve mechanical properties via altering the phase stability and dislocation behavior. When hydrogen is introduced under several gigapascals, a stoichiometric composition of hydrogen can be introduced for steel compositions. In this study, a face-centered cubic (fcc) stainless steel was hydrogenated under several gigapascals. When the steel was not hydrogenated, the microstructure after depressurization was an fcc with a hexagonal close-packed (hcp) structure. In contrast, the hydrogenation treatment resulted in a fine lath body-centered cubic (bcc) structure arising from diffusionless transformation. In particular, the bcc phase formed through the following transformation sequence: fcc → hcp → dhcp (double hexagonal close-packed phase) → bcc. That is, the use of hydrogenation treatment realized fine microstructure evolution through a new type of diffusionless transformation sequence, which is expected to be used in future alloy design strategies for developing high-strength steels.

Mechanical origin of martensite-like structures in two-dimensional ReS2

Martensite is a needle-shaped microstructure formed by a rapid, diffusionless transformation and significantly affects the mechanical properties of materials. Here, in two-dimensional ReS2 we show that martensite-like domain structures can form via a diffusionless transformation, involving small lattice deformations. By analyzing the strain distribution and topology of the as-grown chemical vapor deposition samples, we find that cooling-induced strain at the ReS2/substrate interface is responsible for the mechanical loading and is essential for martensite-like domain formation. Meanwhile, the effect of cooling rate, flake size and substrate on the microstructures revealed the mechanical origin of the transformation. The strain-induced lattice reconstructions are rationalized and possibly lead to ferroelastic effects. In view of the strong anisotropy in electronic and optical properties in two dimensional materials like ReS2, opportunities exist for strain-correlated micro/nanostructure engineering, which has potential use in next-generation strain-tunable devices.