Influence of the Composition and Vacancy Concentration on Cluster Decomposition Behavior in Al–Si–Mg Alloy: A Kinetic Monte Carlo Study

Abstract

The influence of cluster composition and the addition of vacancies on the decomposition behavior of clusters during artificial aging in Al–Si–Mg alloys were analyzed according to the kinetic Montel Carlo model. Clusters with a balanced composition (Mg/(Mg + Si) = 0.5) were the most difficult to decompose. In addition, the cluster decomposition was slower when more vacancies were added to the cluster. Among Si, Mg, and vacancies, vacancies most significantly affect decomposition. The clusters with Mg/(Mg + Si) ≤ 0.4 strongly trap vacancies, which can be classified as hardly decomposable vacancy-rich clusters. The clustering behavior during natural aging and the effect of pre-aging were analyzed using the Kinetic Monte Carlo model. Pre-aging slows down cluster formation due to the lowered vacancy concentration. In addition, the overall composition of the clusters changes to easily decomposable clusters after pre-aging. Thus, not only is the number of clusters reduced but also the clusters are more easily decomposable when pre-aging is performed.

Molecular Dynamics Study on Crack Propagation in Al Containing Mg–Si Clusters Formed during Natural Aging

“…This phenomenon may be related to the rate at which Mg–Si clusters grow. As previously reported, the growth rate of Mg–Si clusters gradually slows as natural aging progresses, as inferred from the mechanical or electrical properties [ 6 , 7 , 8 ] and kinetic Monte Carlo simulation results [ 43 , 47 ]. However, the typical size of Mg–Si clusters is extremely small and could insignificantly increase the interaction between dislocations and Mg–Si clusters.…”

“…The size of the Mg–Si clusters increased as the natural aging time increased. We previously proposed that Mg–Si cluster growth, rather than nucleation, predominantly occurs after 30 min of natural aging [ 43 ]. That is, the effect of the gradually increasing Mg–Si cluster size reflects the effects of the natural aging time.…”

“…This study only addresses the situation where the Mg–Si cluster is close to the crack; however, there could be significant Mg–Si clusters further away from the crack. Furthermore, the chemical composition of the alloy [ 11 , 16 , 43 ], vacancy concentration [ 43 , 47 ], and dislocation density [ 43 ] all influence the growth rate of Mg–Si clusters. The tensile test was performed using an MD simulation at 0 K with a strain rate of 10 10 s −1 , which is an unrealistic condition for practical deformation.…”

“…Mg–Si clusters containing 15 and 189 solute atoms are comparable in size during the early [ 10 ] and sufficiently late (~1 year [ 9 ]) stages of natural aging in Al–Si–Mg alloys. The chemical composition of the Mg–Si cluster was selected to be Mg/(Mg + Si) = 0.5, which is the most stable for subsequent artificial aging [ 43 ]. Because the number of considered solute atoms in the Mg–Si clusters having radii of 4, 6, and 9 Å is odd, the composition cannot be Mg/(Mg + Si) = 0.5.…”