The Detriment of Coherency Strains to the Electrical Conductivity of Naturally-Aged B319 Al Alloy

Coherency strains that arise during natural aging can substantially affect the material properties of Al alloys. Yet, these time-dependent phase transformations must be precisely distinguished from the immediate results of quenching after solution treatment. In this study, automotive B319 Al castings with varying solidification rates and strontium contents were subjected to solution treatments at 500 °C for up to 48 hour. Their hardness and electrical conductivity were measured as-quenched and again after 7 days at ambient-temperature. The role of Cu segregation and growth was effectively isolated from the Cu dissolution and Si spheroidization processes that occurred during the prior high-temperature treatment. The evolution of coherent Cu zones in the Al lattice within 10 hour of natural aging was sufficient to reach stable, 20 to 30 pct improvements in alloy hardness, relative to the as-cast and as-quenched conditions. However, the increased coherency strains from natural aging were detrimental to conductivity, promoting 1 to 1.5 pct IACS reductions from the as-quenched values, independent of the casting prehistory. These insights provided a clear understanding of the mechanisms controlling mechanical properties and electron flow in B319 alloy, and they elucidated the relative magnitudes of solid solution and coherency strains. These observations are critical for improving Al alloy heat treatment procedures.