Copper‐alumina nanocomposites derived from CuAlO 2 : Phase transformation and microstructural coarsening

Abstract Partial reduction of bulk CuAlO 2 results in hierarchical structures wherein there are copper‐alumina regions with widely differing morphologies and scale. At the finest level, the distribution of the 2 phases is at the nanoscale. By means of atomic resolution STEM and electron diffraction, the nanocomposite regions were shown to consist of a dense array of metallic copper platelets dispersed in a matrix of θ‐Al 2 O 3 . The copper nanoplatelets were single crystal, and they all exhibited the same orientation relationship with the matrix, namely [110] Cu //[010] θ‐Al2O3 , (111) Cu //( ) θ‐Al2O3 . It was shown that the 2‐phase regions where the copper exhibited a significantly coarser, globular morphology, resulted from discontinuous coarsening. Interestingly, a change in the matrix phase from θ‐ to δ‐alumina was also observed as a result of the coarsening reaction. It is believed that in the nanocomposite regions, the θ‐alumina phase was stabilized by the lower interfacial energy between the copper (110) platelets and the matrix.