Lattice image studies on the intervariant boundary structure and substructure of Cu-Zn-Al 18R martensite

Abstract

The boundaries between 18R martensite plates in a Cu-Zn-Al alloy, which join martensite variani each plate group and are designated as the A:C and A:D type intervariant boundaries, have been studied in a lattice imaging mode. The details of the atomic configurations across these interfaces are discussed. It is found that the A:C boundary is a straight coherent twin boundary matching exactly the (128)_18R planes of the adjoining martensite variant plates. The A:D boundary, although generally planar, does not match a particular crystallographic plane of a variant plate, but rather has a tendency to wander and form curved segments, except in the regions where occasional 2H structural faults internal to one of the variants, meet the 18R structure in the variant on the other side of the boundary Nonbasal stacking faults are observed as well, with a faulting plane, (128), which is crys tallographically equivalent to the A:C type intervariant plane. This nonbasal faulting is considered to be a consequence of cross-slipping of partial dislocations related to the basal plane faults.