Applied Composite Materials 4: 375-392, 1997. 375 (~) 1997 Kluwer Academic Publishers. Printed in the Netherlands. In Situ Monitoring of Thermally Cycled Metal Matrix Composites by Neutron Diffraction and Laser Extensometry MARK R. DAYMOND* and PHILIP J. WITHERS Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, U.K. (Received 10 January 1997; accepted 5 March 1997) Abstract. A novel stroboscopic neutron diffraction data collection system has been developed. In combination with scanning laser extensometry this has been used to investigate the thermal cycling behaviour of SiC short fibre reinforced AI matrix composites. Three-dimensional unit cell finite element models have been produced, incorporating matrix deformation both by creep and plasticity. Comparison of the experimental results with model predictions has allowed conclusions to be drawn about the deformation processes which dominate at different parts of the thermal cycle. Key words: neutron diffraction, thermal cycling, internal stress, thermal stress, relaxation, exten- sometry, stroboscopic, metal matrix composite, residual stress. 1. Introduction The addition of a dispersed reinforcing phase, typically ceramic particles, to a metal alloy matrix has proved to be an effective way of producing materials of high specific stiffness [1]. The two phases of such a metal matrix compos- ite (MMC) usually have widely differing coefficients of thermal expansion. As a result, when a MMC undergoes a change in temperature, the differences in thermal expansion cause misfit strains, and thus introduce internal stresses. Even for small thermal excursions these internal stresses can exceed the room tem- perature yield stress in the vicinity of the particles, particularly close to stress concentrations such as the reinforcement comers. This can cause localised yield- ing around the reinforcements. Because they are regenerated each cycle, these stresses become especially important under thermal cycling fatigue. In combina- tion with relatively small applied loads they can result in superplastic deformation of the composite to large strains (> 300% [2]). There is thus considerable interest in the investigation of the thermal cycling behaviour of MMCs [i, 3, 4], both with respect to their use in changing temperature environments [1] and to the exploitation of thermal cycling-induced superplasticity for the forming of MMC components [5]. * Currently at MLNSC, Los Alamos National Laboratory, New Mexico 87545, U.S.A.