Continual mechanical twinning: Part I: Formal description

Abstract

A model-independent description of mechanical twinning is presented. It is shown that slip, which may be induced concurrent with twinning, serves as an energy dissipation process. The relative case of this dissipation governs the characteristics of the total deformation process and predicts phenomena such as a negative strain-rate sensitivity and a positive temperature dependence for the deformation. Details of the description are tested by experiments on several cubic metals and appropriate experimental support from subsequent papers is quoted here. The alloy Fe-25 at. % Be is of particular interest because it deforms almost exclusively by twinning; moreover, when in an ordered state the alloy can untwin spontaneously even under opposing stresses. The formal description of continual mechanical twinning can be made sufficiently general to account in detail for such events, as well as the more standard observations of work hardening, orientation effects, etc. The existence of a critical resolved shear stress for twinning (which must, however, depend on the state of the material through dislocation mobility) is observed and forms an integral part of the description. No particular model for twin nucleation is necessary in the formal development since most results are adequately explained without specifically accounting for nucleation.