Effect of beryllium and non-equilibrium heat treatment on mechanical properties of B319.0 alloy with 1.0%Fe

Abstract

B319.0 hypoeutectic alloy is a heat treatable Al-Si-Cu-Mg system with nominal composition Al-6Si-3.5Cu-0.3Mg and about 0.1-1.2Fe. The purpose of this study was to determine the influence of Be and non-equilibrium heat treatment on the microstructure and mechanical properties of B319.0 alloys with a higher Fe (1.0 wt-%) content. Results indicate that Be additions can reduce the amount and average length of iron bearing phases, can transform some platelet like shapes (β-Al5FeSi) of iron intermetallic phase to comparatively harmless Chinese-script shapes (α-Al8Fe2Si), and can also refine the eutectic silicon. High temperature solution heat treatment(≥ 510°C) may not only completely dissolve the Al2Cu intermetallics, but may also cause dissolution and fragmentation behaviour in β-phase platelets and consequently reduce their volume fraction. Because the β-phase platelets are smaller in the Be-containing alloy under identical conditions, their dissolution behavior is more obvious than that in Be-free alloy. However, if deterioration of the mechanical properties is to be avoided, the solution temperature should be lower than the interdendritic melting temperature which resulted in an ultrafine eutectic phase on quenching. DSC and electrical conductivity analysis showed that the combined effect of Be additive and non-equilibrium heat treatment caused significant increase in the precipitation kinetics of B319.0 alloy. Fractographic analysis of tensile specimens revealed that fracture processes were mainly initiated by nucleation at β-Al5FeSi platelets as a result of their cracking and decohesion from the matrix. Therefore, adding Be to B319.0 alloy and optimising the solution heat treatment could result in fewer fracture initiating sites and greater precipitation of strengthening phases, with a consequent improvement in tensile properties.