Nontrivial nanostructure, stress relaxation mechanisms, and crystallography for pressure-induced Si-I -> Si-II phase transformation

H Chen and VI Levitas and D Popov and N Velisavljevic, NATURE COMMUNICATIONS, 13, 982 (2022).

DOI: 10.1038/s41467-022-28604-1

Crystallographic theory based on energy minimization suggests austenite- twinned martensite interfaces with specific orientation, which are confirmed experimentally for various materials. Pressure-induced phase transformation (PT) from semiconducting Si-I to metallic Si-II, due to very large and anisotropic transformation strain, may challenge this theory. Here, unexpected nanostructure evolution during Si-I -> Si-II PT is revealed by combining molecular dynamics (MD), crystallographic theory, generalized for strained crystals, and in situ real-time Laue X-ray diffraction (XRD). Twinned Si-II, consisting of two martensitic variants, and unexpected nanobands, consisting of alternating strongly deformed and rotated residual Si-I and third variant of Si-II, form 111 interface with Si-I and produce almost self-accommodated nanostructure despite the large transformation volumetric strain of -0.237. The interfacial bands arrest the 111 interfaces, leading to repeating nucleation-growth-arrest process and to growth by propagating 110 interface, which (as well as 111 interface) do not appear in traditional crystallographic theory.

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