Microstructure of in situ Mg metal matrix composites based on silica nanoparticles Anita Olszówka-Myalska 1,a , Sam A. McDonald 2,b , Philip J. Withers 2,c , Hanna Myalska 1,d , Grzegorz Moskal 1,e 1 Silesian University of Technology, ul. Krasińskiego 8, 40-019 Katowice Poland 2 University of Manchester, Grosvenor Street, Manchester M1 7HS, UK a anita.olszowka-myalska@polsl.pl, b sam.mcdonalds@manchester.ac.uk, c philip.withers@manchester.ac.uk, d hamya@wp.pl, e grzegorz.moskal@polsl.pl Keywords: magnesium composite, Mg 2 Si, nanoparticle silica, differential scanning calorimeter, X- ray tomography Abstract. Metal matrix composites comprising a magnesium matrix and Mg 2 Si/MgO dispersoids obtained by hot pressing silica nanoparticle agglomerates and metal powder in a Degussa press were characterized. Two powder mixtures having weight proportions of Mg:SiO 2 of 10:0.3 and 10:1 were identically sintered. Their microstructures were characterized by optical microscopy and X-ray diffraction. The size and distribution of the Mg 2 Si and MgO dispersoids formed in situ were assessed as a function of the original nanosilica content. The behaviour of the composites under compression testing was assessed in 3D by X-ray microtomography using 225kV Nikon X-tek and 150kV Xradia MicroXCT scanners. This provided insights into composite strengthening mechanisms and matrix particle decohesion. Introduction Magnesium silicide Mg 2 Si because of its low density (1.91 kg/m 3 ), high melting point (1358 K), good Young’s modulus (120 GPa) and microhardness (100-600 HV), significant coefficient of thermal expansion (7.5x10 -6 K -1 ) [1] is a good candidate as a reinforcing phase in magnesium matrix composites. Further, it can be formed in situ as a reaction product between magnesium and silicon [2-5] or silica [5-8] by a range of technologies. The interphase bonding between Mg 2 Si and Mg is strong because it forms as a reaction product. However, final material properties depend on the dispersion of Mg 2 Si in the matrix which is a function of the constituent particulate stock, the consolidation process and subsequent processing. In the case of the silica precursor, various crystalline and amorphous powders feedstocks based on; commercial SiO 2 powder with a size range of 3-375µm [5,7], silica rice husks of 3.9-39.2µm [6] and flyash of 100µm [8] have been tried. The aim of this paper is to examine whether agglomerates of nanoparticles of amorphous SiO 2 are suitable as a precursor of the Mg 2 Si phase in magnesium matrix composite obtained by hot pressing, both in terms of initial microstructure and its influence on properties. Methods and results Material fabrication. The possible reactions between Mg and SiO 2 along with their Gibbs free energy evaluated using the HSC Chemistry 4.1 program are presented in Tab.1. These show a negative value of ∆G over the temperature range appropriate for magnesium powder metallurgy processes and confirm the formation of the Mg 2 Si intermetallic phase as the final reaction product. Solid State Phenomena Vol. 191 (2012) pp 189-198 Online available since 2012/Aug/24 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/SSP.191.189 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 157.158.128.218-28/08/12,09:46:54)