LOCALISED CYCLIC PLASTIC DEFORMATION ON TRANSLAMELLAR FRACTURE SURFACES IN A P/M γ- TiAl-BASED ALLOY

摘要

Large numbers of fine parallel steps are generated on translamellar fracture surfaces during slow crack growth (da/dN ≤ 5.0 × 10~(-6) mm/cycle) under cyclic loading. These are seen only rarely during fast crack growth (da/dN ≥ 1.0 × 10~(-4) mm/cycle) and are not seen during catastrophic fracture. These steps occur due to intense plastic deformation as a result of cleavage on {111} planes along twin boundaries and dislocation bands. Such action may promote an inherent resistance to crack growth owing to higher energy dissipation. TEM examinations show that the intense deformation structure consists of both microtwins and dislocation bands if the lamellae are orientated to allow easy glide. Conversely, microtwin activity dominates when easy dislocation glide is prevented. Crack initiation and growth resistance are sensitive to such microscopic features within the deformation zone. Lamellar volume fraction, the proportion and dis- tribution of γ grains inside lamellae and lamellar interfacial strength may all influence the local plastic de- formation through their interaction with underlying slip and twinning processes.