Brittle fracture in polycrystalline Ir-0.3 pct W

Abstract

Polycrystalline iridium fails by brittle intergranular fracture at room temperature. We have previously shown that this fracture is intrinsic because segregation of impurity elements to the grain boundaries was not found. We examined the structure of grain boundaries in Ir-0.3 pct W by transmission electron microscopy and found numerous ledges in the boundaries. In most boundaries the ledges aligned with the {111} crystallographic planes in one of the grains. In addition to ledges we observed numerous microscopic cracks near the edges of the thin foils. Most cracks occurred along twin boundaries. However, a substantial number of cracks that were not associated with twins also aligned with {111} planes in the grain matrix, establishing the {111} planes as secondary cleavage planes; {100} are the primary planes. The ease of cleavage along {111} planes makes the predominantly {111} oriented ledges ideal sites for crack initiation and propagation along grain boundaries, thereby explaining the intrinsic nature of intergranular fracture in iridium.