The corrosion behavior of Al-Cu-Mg alloys, specifically 2024 alloy (nominally, in weight %, 4.4 Cu, 1.5 Mg, 0.6 Mn), is thought to depend on heterogeneous Cu and Mg distribution through the existence of segregation-dependent local electrochemical cells at the corrosion interface. Few nanospectroscopy measurements of segregation have been made for this or similar alloys. These alloys are precipitation hardenable. The primary precipitating phases are S and the well known Θ(CuAl2) and their metastable intermediates. TEM analysis of aged alloys in this subgroup showed that the orthorhombic S phase (a=4.0Å, b=9.25Å, c=7.15Å) occurred as a thin plate type variant, called S´, within matrix grains and as larger monolithic particles on grain boundaries. Intragranular pricipitate particle densities were heterogeneous particularly near grain boundaries, indicating that strong segregation was present that would result in local electrochemical cells where grain boundaries and large precipitates intersected the alloy surface.
HRTEM and nanospectroscopy are used to analyze the structure and chemistry of heterophase interfaces and grain boundaries.