Influence of indium trace addition on the microstructure and precipitation hardening response in Al-Si-Cu casting aluminum alloy

Thermodynamic calculations and experimental studies including transmission electron microscopy (TEM), atom probe tomography (APT), microhardness and specific electrical conductivity measurements, and high temperature uniaxial compression tests have been carried out in order to determine the influence of indium trace addition on the structure and precipitation hardening response in Al-Si-Cu based casting alloy. Analysis of the hardness curves for aging at 175 degrees C has revealed that the peak hardness of the In-containing alloy is about 22% higher compared to the In-free alloy (140 vs 115 HV) and is achieved in a much shorter time of aging (2 h vs 10 h). TEM analysis of the peak aged samples has revealed that much finer precipitates of the theta' phase with a higher number density are formed in the alloy with trace addition. Furthermore, fine spherical nanoparticles associated with theta' phase platelets are found in the In-containing alloy structure. Quantitative APT analyses have revealed that the number densities of theta'-phase precipitates and the spherical nanoparticles are equal at the beginning of aging, while in the peak aged state the number density of the theta'-phase is at least two times that of the spherical nanoparticles. APT analyses have also revealed that in the near-peak aged state more than half of copper (similar to 2.0 wt.%) is still dissolved in the aluminum matrix of the In-free alloy in comparison with only 0.2 at. % (similar to 0.46 wt. %) copper for the In-containing alloy. The results of uniaxial compression tests of the peak aged samples have shown that the 250 degrees C yield strength of the alloy with trace solute is about 1.5 times that of the base alloy (259 vs 174 MPa). Thus, indium trace addition catalyzes the decomposition process making it more effective and complete which is beneficial for increasing the alloy strength at low and elevated temperatures.