Macroscopic and microscopic aspects of the deformation and fracture mechanisms of ultrafine-grained aluminum processed by hot isostatic pressing

A commercial purity aluminum nano-powder has been consolidated by the hot isostatic pressing technique. The bulk material, in addition to the unavoidable native Al2O3 phase, has a microstructure comprising a fraction of microcrystalline grains (>= 500 nm) that are embedded in an ultrafine-grained matrix (150 nm). True stress-true strain curves acquired from compressive tests at room temperature showed a rapid and brief hardening at the early stage of the deformation followed by a short stress plateau and linear work softening. The material yielded at 390 MPa - 10 times the flow stress of the coarse-grained counterpart material. The per cent reduction to rupture was about 20% mostly due to the presence of microcrystalline grains acting against crack propagation. In addition, it is shown that microcrystalline grains deform via dislocation-based mechanisms. while present evidence suggests that the matrix deforms by a cooperative grain boundary sliding. ((c)) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.