Bi-continuous metal matrix composites

Bi-continuous alumina/aluminium composites were made by infiltrating an alu mina preform which had the structure of a reticulated ceramic foam. The low density preforms were prepared from a polyurethane suspension of alumina p owder which was pyrolysed and sintered after foaming. Higher density prefor ms consisted of ceramic foams with open cells. All these preforms were infi ltrated with 6061 aluminium alloy using a modified squeeze easter fitted wi th a vacuum system and fine control of speed and pressure. The microstructu re of the preform fitted an established relationship between the ratio of w indow diameter to cell diameter (k) and void volume fraction (V-p). Low k f oams were infiltrated fully but on cooling below the solidus, interfacial d ebonding took place due to differential thermal contraction. This was overc ome by modifying the processing conditions. High k foams which had high fra ctional porosity, retained sound interfacial bonding. The composites posses s higher elastic modulus than conventional MMCs with a homogeneous reinforc ement distribution at a given volume fraction. The loss of electrical condu ctivity is negligible in the lower volume fraction range because of the thr ee dimensionally continuous aluminium phase. The experimental results are c ompared with a number of theoretical predictions. (C) 2001 Elsevier Science B.V. All rights reserved.