Investigation of metal foam formation by microscopy and ultra small-angle neutron scattering

Zinc foams were prepared by applying the powder compaction method which comprises mixing zinc powder with a small fraction of a blowing agent, compacting this mix and creating a foam by heat treatment above the melting temperature of the metal. The foaming process which normally leads to a volume expansion of some 500% was artificially stopped in very early stages by quenching. The resulting samples with porosities below 20% were then characterised by microscopy and ultra small-angle neutron scattering (USANS). Foam microstructures, pore morphologies, the locations of the blowing agent particles, pore and particle size distributions and total pore volume fractions were determined. Two mechanisms for pore formation could be identified by microscopy. The pore growth behaviour for different size regimes could be obtained from the USANS distribution curves, thus giving insight into the early stages of metal foam formation. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.