Effect of Indium Microaddition on the Structure and Strengthening of Binary Al–Cu Alloys

Abstract—Thermodynamic calculations performed with the Thermo-Calc software and experimental methods, in particular, the high-resolution transmission electron microscopy (TEM), are used to study the effect of indium microaddition on heat-treatment-induced changes of the phase composition and character of strengthening of the Al–1.5 wt % Cu and Al–3.5 wt % Cu alloys. The 0.1 wt % indium addition is shown to completely suppress the natural aging of the Al–3.5% Cu alloy. However, indium substantially intensifies the decomposition process of the aluminum solid solution (Al) during artificial aging; this determines the advantages of microalloyed compositions over the base alloys in both the time of reaching the peak strength and hardness. In particular, after aging to the peak strength, the hardness of the Al–3.5% Cu–0.1% In alloy is ~20% higher than that of the base alloy (124 HV against 105 HV), whereas the hardness of the Al–1.5% Cu–0.1% In alloy is more than 60% higher than that of the base Al–1.5% Cu alloy (59 HV against 37 HV). TEM analysis showed that the observed increase in the hardness during aging of the microalloyed compositions is due to the formation of substantially finer structure of aging products (the average linear size of the strengthening θ' phase decreases from 100 to 50 nm) along with the higher particle distribution density.