Characteristics of the A/D type twin boundary in 18R martensite in a Cu–Zn–Al alloy

The A/D type twin boundary between 18R martensite plates in a Cu–Zn–Al alloy has been studied by both transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). It is curved in TEM and has irregularly serrated steps in HREM. The randomly distributed faults have no obvious influence on the boundary orientation. The boundary segments consist of respective basal planes of the two variants. Crystallographic analysis shows that the macroscopic curve results from the non-self-accommodation of transformation strain of the two variants. The calculated boundary energies based upon the model of a small-angle tilt grain boundary indicate that the deviation from the exact twin orientation and the irregularly serrated steps result from energy minimization.