Acta Materialia 50 (2002) 3093–3104 www.actamat-journals.com The production of AlN-rich matrix composites by the reactive infiltration of Al alloys in nitrogen S. Swaminathan, B. Srinivasa Rao * , V. Jayaram Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India Received 18 June 2001; received in revised form 28 February 2002; accepted 4 March 2002 Abstract Aluminium nitride (AlN)–Al matrices reinforced with Al 2 O 3 particulate have been fabricated by reactive infiltration of Al–2% Mg alloy into Al 2 O 3 preforms in N 2 in the temperature range of 900–1075 °C. The growth of composites of useful thickness was facilitated by the presence of a Mg-rich external getter, in the absence of which composite growth is self-limiting and terminates prematurely. Successful growth of composites has been attributed to the reduction in residual oxygen partial pressure brought about by the reaction with oxygen of highly volatile Mg in the getter alloy. The microstructure of the matrix consists of AlN-rich regions contiguous with the particulate with metal-rich channels in-between, thereby suggesting that nitridation initiates by preferential wicking of alloy along the particle surfaces. The increase in nitride content of the matrix with temperature is consistent with hardness values that vary between ~3 and 10 GPa.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: AlN-rich matrix; Infiltration; External getter; Nitridation 1. Introduction The production and mechanical characterisation of Al 2 O 3 –Al matrix composites by directed melt oxidation, originally described by Newkirk et al. [1], has been extensively reported in the last 15 years [2]. A related process, with much greater flexibility of microstructural control, is the press- ureless infiltration of aluminium alloys in nitrogen- * Corresponding author. Present address: FB Materialwis- senschaft FG Nichtmetallisch-Anorganische, Werkstoffe Pet- ersen Strasse 23, Darmstadt 64287, Germany. Fax: +49-6151- 16-6314. E-mail address: bsrao@ceramics.tu-darmstadt.de (B. Srini- vasa Rao). 1359-6454/02/$22.00  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6454(02)00103-9 containing atmospheres. At low temperatures (750–950 °C for Al–5% Mg) the product is pre- dominantly a metal matrix composite (MMC) through capillarity-driven infiltration [3], while at higher temperatures infiltration is accompanied by significant nitridation [4] leading to the production of aluminium nitride (AlN)-rich matrices. This ability to vary the relative proportions of ceramic to metal over a range of 0–80% is absent in the oxide version of the process in which the volume fraction of alumina lies generally in the range of 70–85% of the void space. Of the two extremes that are achievable in matrix composition, the vast majority of published work is confined to the Al- matrix composites, i.e. matrices produced by low temperature infiltration. While microstructures of