The martensite phase of high-purity lithium

The low-temperature martensitic transformation of pure lithium is characterized by a complex phase diagram exhibiting the coexistence of several close-packed phases with the bcc matrix. On cooling, below the martensitic start temperature a long-range ordered 9R structure and a one-dimensionally disordered polytype phase are observed. On heating, an intermediate fcc phase appears in the hysteresis regime. Microstructure investigations, application of the crystallographic theory of martensite and thermal cycling during neutron scattering experiments together strongly indicate that fcc is the true thermal equilibrium phase. The 9R structure is a metastable phase which minimizes the lattice mismatch with respect to the bcc matrix and therefore is formed for geometrical reasons.