Deformation twinning in Ni_2MnGa

摘要

Deformation twinning is investigated in the martensitic phase of a Ni_(46.75)Mn_(34)Ga_(19.25) (at.) alloy. X-ray and electron diffraction are used to establish the crystallography of the non-modulated tetragonal martensite, and transmission electron microscopy is employed to deduce the twinning parameters. It is convenient to define the twinning parameters with respect to a "monoclinic" unit cell, designated 2M: then K_1, η_1, K_2, and η_2 are (001), 100, (100), and 001 respectively. The Burgers vector of the active twinning disconnections is close to 1/6100 and the disconnections are associated with steps of height d_(002). These defects are expected to be highly mobile since their motion does not require atomic shuffling. It is shown that periodic arrangements of two layer twins produce modulated crystal structures, such as 14M.