Influence of processing defects on the measured properties of Cu–Al2O3 composites: A forensic investigation

The measured mechanical and physical properties of copper–alumina (Cu–Al2O3) composites are influenced by the defects induced during the manufacturing of the test samples. In this paper we shall demonstrate how the fundamental knowledge of the influence of the amount and type of micro-cracks on the mechanical and thermal properties of copper–alumina composites can be used as a forensic tool in identifying the defects induced during the manufacturing process of the test samples. This process involves several steps; the preparation of ingots from commercial powders by hot iso-static pressing (HIP), followed by their machining, hot extrusion, heat treatment, and if necessary further machining to make specimens of desired shape. Each step in this manufacturing process introduces some defects, so that the mechanical and physical properties measured using these samples, such as the Young modulus, the thermal conductivity and the coefficient of thermal expansion can be very different from the expected values, despite the fact that the density of the composite measured on the same samples is very close to its theoretical value. As a result open porosity is not the likely cause and it is reasonable to suspect that closed pores (e.g. micro-cracks) in the copper matrix and at the interfaces of the alumina inclusions formed during the multi-stage manufacture of test samples are responsible for the differences in the measured and expected mechanical and thermal properties.