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The effects of temperature on the mechanical properties and thermal expansion of two discontinuously reinforced aluminium composites have been determined over the range 300–100 K. Silicon carbide particulate-reinforced 2009 and 6092 aluminium alloys were studied by tensile testing, in which both longitudinal and transverse strains were recorded, and by thermal expansion measurements. The test results clearly show that cooling to 100 K induces plastic flow in the aluminium alloy matrices due to the thermal expansion difference between aluminium and silicon carbide. At very low temperatures, the linear region of the stress–strain curve is greatly reduced or eliminated and the Poisson's ratio, ν, increases. For the higher yield strength 2009 matrix composite, ν increases from a room-temperature value of 0.28 to 0.35 at 100 K. For the lower-strength 6029 matrix composite, ν increases from a room-temperature value of 0.33 to a value of 0.5 at 100 K. A Poisson's ratio of 0.5 is the value characteristic of plastic flow in an incompressable material. Changes in yield strength, Young's modulus and thermal expansion with decreasing temperature are also consistent with thermally induced plastic flow in the composite matrix.