Creep of replicated microcellular aluminium

Open-pore replicated microcellular 99.99% pure aluminium is tested in tensile creep, varying the temperature from 150 to 450 degrees C, the applied stress from 0.15 to 0.5 MPa, and the relative density from 0.14 to 0.28. Tensile creep curves are of classical shape, with a well-defined secondary stage of steady-state creep, for all except a few samples that were tested at higher temperature and lower stress; these display signs of extensive oxidation along the pore surface. All other samples crept at 250 degrees C or above exhibit a steady-state creep rate with an activation energy of 141 kJ mol(-1), a stress exponent of 7.2 +/- 0.4, and a dependence on relative density to a power near -21. The substructure of microcellular aluminium crept in this regime consists in subgrains that straddle, as in a bamboo structure, individual struts making the foam. This observation, coupled with the stress exponent near 7.5, shows that fine-scale microcellular pure aluminium can creep >250 degrees C under substructure-invariant conditions. At 150 degrees C, creep data are more scattered and show a steeper dependence on applied stress, suggesting power-law breakdown. The Monkman-Grant correlation describes well the tensile failure of this material. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.