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 °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 °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 °C under substructure-invariant conditions. At 150 °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.