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We investigate processing-microstructure relationships in the production of Al2O3 particle reinforced copper composites by solidification processing. We show that during production of the composites by gas-pressure infiltration of packed Al2O3 particle preforms with liquid Cu or with liquid Cu8wt%Al at either 1,150 or 1,300 A degrees C, capillarity-driven transport of alumina can cause rounding of the Al2O3 particles. We use quantitative metallography to show that the extent of particle rounding increases markedly with temperature and with the initial aluminum concentration in the melt. An analysis of the thermodynamics and kinetics governing the transport of alumina in contact with molten copper, considering both interfacial and volume diffusion, leads to propose two mechanisms for the rounding effect, namely (i) variations in the equilibrium concentration of oxygen in the melt as affected by the initial aluminum concentration, or (ii) segregation of aluminum to the interface with the ceramic.