Al-Zn-Mg alloys with different precipitate sizes were investigated to determine the influence of the precipitate size on the flow stress and dislocation density change during tensile deformation. The dislocation density was measured using in-situ X-ray diffraction at the SPring-8 synchrotron radiation facility with a time resolution of about 2 s. In region II with rapid dislocation multiplication, from under-aging to peak aging, the dislocation density increased with increasing aging time. Under over-aging conditions, the amount of dislocation multiplication in region II decreased with increasing aging time. Even in region III, the increase in dislocation density with plastic deformation was the largest for the peak aging conditions. However, the amount of work hardening was small and the contribution of dislocation hardening to the strength of the material was minimal. For over-aging conditions, the increase in dislocation density in region III was smaller than for the other regions, but the amount of work hardening was relatively large. It is considered that the influence of the dislocation density on work hardening is determined by the effectiveness of precipitates as obstacles to dislocation motion.

Nanoclusters formed in Al-Mg-Si alloys affect the aging behavior of the alloys depending on the formation temperature. In the present study, first-principles calculations were carried out to evaluate the two- and three-body interactions between Mg, Si atoms and vacancies in the Al matrix and estimate the effect of local bond structures on the formation of nanoclusters. Monte Carlo simulations were subsequently performed to investigate the stable structure of nanocluster formed in Al-0.95 mass pct Mg-0.81 mass pct Si alloy. We found that the Mg-Si bond and SiVac bond were stable in the Al matrix. The result showed that the solute atoms are easy to aggregate with other types of solute atoms and that Si atom had a strong attractive interaction with a vacancy. Furthermore, Mg-Si-vacancy three-body bond was more stable than Mg-Si two-body bond and Si-vacancy two-body bond in the Al matrix. The results indicate that the nanoclusters in the Al matrix were thermally stabilized by the stable bonds between solute atoms and vacancy. Thus, the electronic structure calculations suggested that inner bonds within a nanocluster played a significant role in not only the thermal stability but also the formation and growth behavior of nanoclusters during aging at low temperatures.

In the present study, we investigated the effects of Cu or Ni additions on the precipitation of intermetallic phases in the heat-resistant aluminum alloy with a ternary composition of Al-5Mg-3.5Zn (mol%). The quaternary alloys with 1 mol%Cu or 1 mol%Ni content were solution-treated at 480 °C and subsequently aged at 300 °C for different periods. Both Cu and Ni additions have a slight effect on the age-hardening of these alloys at 300 °C. The added Cu element partitioned into not only T-Al₆Mg₁₁Zn₁₁ phase but η-Zn₂Mg phase precipitated in the α-Al matrix. The observed Cu enrichment in the precipitates of T phase indicated high stability of T phase in the Al-Mg-Zn-Cu quaternary system, which was different from the results of thermodynamic calculations using the existing database. The added Ni element enhanced the formation of fine Al₃Ni phase located at grain boundaries and slightly influenced the precipitation of T phase in grain interior. These results provided new insights to design novel heat-resistant Al alloys using the Al-Mg-Zn-Cu-Ni system.

In this study, effects of heterogeneous nucleation site particles on density, microstructure and hardness were studied in the pure aluminum samples additively manufactured by laser-based powder bed fusion (PBF-LB) method with a small amount of TiC particles. It was found that higher density of the sample was found by addition of small amount of TiC particles. It was also found that the sample containing 1.0 vol% TiC particles has the most refined microstructure. In addition, the samples fabricated with 0 vol% and 0.1 vol% TiC particles have <101> texture along building direction, whereas <100> texture was observed for the samples with 0.5 vol% and 1.0 vol% TiC particles. As the volume fraction of TiC particles increases, the hardness of the samples was also increased. Higher hardness in the samples with TiC particles was mainly caused by not reinforcement effects of TiC particles but refinement of grain structure by heterogeneous nucleation.