Crystallographic Theory and Mechanism of the Polymorphic β → α Transition in a Zirconium Single Crystal

A crystallographic theory of the polymorphic bcc → hcp transition in a zirconium single crystal has been formulated. According to the theory, a mechanism of the transition includes the main parameters of the martensitic transformation, namely, the lattice strain and the strain in an invariant lattice. The package of parallel plates that are formed in the course of the transformation is considered as a single martensite crystal. It has been established that the deformation of martensite upon this transition (with an invariant lattice) is aimed at restoring the shape of the initial section of the bcc phase (prototype), rather than at obtaining an invariant plane. The calculation shows that shape restoration in the basal plane of the hcp phase is mainly caused by the action of two systems of prismatic sliding. The sliding is concentrated in narrow layers of localized shear, which leads to the formation of a lamellar structure of the package. The shape restoration along the c axis occurs due to the pyramidal sliding.