COHERENCY OF THE TRANSFORMATION STRAIN AT THE GRAIN BOUNDARY AND FRACTURE IN Cu-Zn-Al ALLOY

Abstract
Two types of Cu-Zn-Al bicrystal specimens having e/a≈1.4, one with the boundary parallel to and the other perpendicular to the tensile direction, were extended at room temperature. The morphological change near the boundary was continuously observed by a microscope with a VTR during the extension. For a particular orientation with respect to the tensile direction, the same variant crystal of β₁' martensite as that which would be stress-induced in an individual component crystal is produced with transformation strains which occasionally continue across the boundary. However, a different variant crystal from that in the individual component crystal is usually formed near the boundary depending on the orientation of the other grain. This variant has an orientation such that the strain fields coincide with those produced by a variant which is transformed simultaneously in the other grain near the boundary. Since the possible number of shear systems producing the intimate β₁' variants in both grains is limited, the fracture takes place often at the grain boundary. However, when α₁' martensites are formed at the boundary, as frequently observed, the fracture is suppressed to a great extent, because of increasing numbers of independent shear systems. Consequently, the degree of ease or difficulty in producing α₁' martensites is closely associated with the character of fracture in Cu-Zn-Al alloy. An extended work on fracture using polycrystalline specimens of different e/a values also supported the above results.