Localised cyclic plastic deformation on translamellar fracture surfaces ina P/M gamma-TiAl-based alloy

Large numbers of fine parallel steps are generated on translamellar fractur e surfaces during slow crack growth (da/dN less than or equal to 5.0 x 10(- 6) mm/cycle) under cyclic loading. These are seen only rarely during fast c rack growth (da/dN greater than or equal to 1.0 x 10(-4) mm/cycle) and are not seen during catastrophic fracture. These steps occur due to intense pla stic deformation as a result of cleavage on {111} planes along twin boundar ies and dislocation bands. Such action may promote an inherent resistance t o crack growth owing to higher energy dissipation. TEM examinations show th at the intense deformation structure consists of both microtwins and disloc ation bands if the lamellae are orientated to allow easy glide. Conversely, microtwin activity dominates when easy dislocation glide is prevented. Cra ck initiation and growth resistance are sensitive to such microscopic featu res within the deformation zone. Lamellar volume fraction, the proportion a nd distribution of gamma grains inside lamellae and lamellar interfacial st rength may all influence the local plastic deformation through their intera ction with underlying slip and twinning processes. (C) 1999 Acta Metallurgi ca Inc. Published by Elsevier Science Lid. All rights reserved.