Crystallography of the martensitic transformation in indium-thallium alloys

On cooling a single crystal of indium containing 20.75 atomic per cent thallium may transform martensitically by the migration of a single interface, covering the whole cross-section, from the high temperature face-centred cubic β-structure to the face-centred tetragonal α-structure. Applied external mechanical stress is able to reverse the direction of interface motion during transformation. The α-martensite consists of a stack of fine parallel twins which are sufficiently wide to be visible under an optical microscope. There are no longrange stresses, provided the twins have mean thickness ratio 1.6:1. By extension or compression the whole specimen may undergo a conversion into one or the other orientation by movement of the twin boundaries, leading to ferroplasticity or ferroelasticity. The geometry and crystallography of and the effects of external stresses on the β → α transformation have been calculated on the basis of the phenomenological crystallographic martensite theory. (author)