The structure of a grain boundary in a Zr-2.5 wt% Nb alloy

The structure of a grain boundary in a heat treated Zr-2.5 wt% Nb alloy has been studied by high resolution transmission electron microscopy. The orie ntation of the two grains, related by a mirror operation about (1 (2) over bar 10), lay within about 2.5 degrees of a (10 (1) over bar 1)[(1) over bar 012] twin. The boundary was faceted, with coherent terraces lying along th e (10 (1) over bar 1) planes in both grains, and steps parallel to the trac e of either (0002) or ((1) over bar 011) in the two grains. The steps were two, six, ten or fourteen planes in height, and were found to contain a ter minating half plane in lattice images formed by the (10 (1) over bar 1) pla nes. These observations are explained by making use of Pond's theory of the dichromatic pattern for the two grains in the observed orientation relatio nship.