Processing and Mechanical Properties of Alumina Reinforced Copper-Aluminum Composites

Copper-based composites reinforced with alumina particles are produced by two different pro- cesses, namely gas pressure driven infiltration of a porous alumina preform and a two-step in-situ process developed in the course of this work. The resulting composites are characterized for their microstructures and mechanical properties. It is shown that during gas-pressure infiltration of angular alumina particles with liquid copper- aluminum alloys at 1300◦C, rounding of the alumina particles occurs. Rounding is quantified by measurements of local curvature, and shown not to be found after infiltration of the same particles with pure copper at 1150◦C. Transport pathways necessary for rounding are examined by analysis of the thermodynamics and kinetics of the copper-aluminum-alumina system, drawing from the literature on wetting of alumina by copper and its alloys. Analysis leads to two different possible explanations for the phenomenon: (i) the presence of aluminum in the melt increases the equilibrium oxygen concentration and thus accelerates volume diffusion or (ii) segregation of aluminum in the liquid alloy to the alloy-alumina interface causes accelerated interfacial diffusion. The pressure infiltration of four different copper-aluminum alloys with varying titanium content (Cu8wt%Alxwt%Ti with x=0, 0.2, 1, 2) into preforms of two types of alumina particle leads to sound, pore-free composites reinforced with 56-59 vol% alumina. Depending on the titanium content of the matrix alloy, a new phase appears at the interface between the alumina particles and the metallic matrix; this phase was identified as the mixed oxide Ti3(Cu,Al)3O. A very thin (5-10 nm) titanium-rich layer between alumina and the ternary oxide is responsible for a mechanically strong interface between the copper-alloy matrix and alumina. Peak mechanical properties in these composites were obtained by infiltration of equiaxed Al2O3 particles with a Cu8wt%0.2wt%Ti alloy. Their measured properties displayed a Young’s modulus near 235 GPa, a density of 5.5 g/cm3, a tensile strength and elongation of 600 MPa and 1% respectively, together with a compressive strength near 1100 MPa reached at 9% strain. Copper matrix alumina composites are also produced via a process combining in-situ formation of alumina and deformation processing. Elemental powders are uniaxially compacted and subjected to a two-step heat treatment that leads to the formation of alumina films in a copper-aluminum solid solution matrix. During deformation processing, the composite is consolidated and the alumina films are broken up into particles of average diameter 1-2 μm. Further hot working in the form of upsetting and extrusion further refines the alumina particles and improves their distribution, which improves the composite mechanical properties. Cu7wt%Al matrix composites reinforced with 26 vol% alumina particles were obtained by this process. In optimum processing conditions of this work, these composites have a tensile strength of 846±44 MPa and a tensile ductility of 2.2±0.8%, with likely room for significant further improvement of these properties after more extensive deformation processing.