ENVIRONMENT AND MICROSTRUCTURE EFFECTS ON FATIGUE-CRACK FACET ORIENTATION IN AN AL-LI-CU-ZR ALLOY

The effects of environment, microstructure and texture on transgranula r fatigue crack facet orientation are established with electron-back s cattered pattern analysis and stereofractography for single grains in peak aged Al-Li-Cu-Zr alloy 2090. For vacuum, facets are near-{111} du e to fatigue Fracture through intense deformation bands with a complex planar-slip dislocation structure. Multiple facets in single grains a nd the tortuous crack path are caused by high shear stresses resolved on multiple slip systems. Low stress intensity range fatigue fracture in NaCl is transgranular and faceted, but not tortuous. Eighty-five pe r cent of the facets in unrecrystallized plate and 50% of the facets i n recrystallized sheet are within 10 degrees of a high index plane, on average {521}, subjected to high normal stresses. Such facets are inc onsistent with: (a) hydrogen-enhanced localized plasticity and {111} d ecohesion; (b) slip-locking with bisecting {100} cracking; (c) environ ment-enhanced alternate slip with {100} faceting; or (d) {100}/{100} d ecohesion. Environmental fatigue may be governed by faceted cracking a ssociated with hydrides or hydrogen embrittled dislocation cell walls. Copyright (C) 1996 Acta Metallurgica Inc.