Calculation And Simulation On The Initial Stage Of Ageing Evolution In As-cast Al-Si-X Alloys

The solid solution hardening efficiency of different alloying elements such as Cu, Mg,and Si on Al solid solutions were investigated according to the chemical bonding of Al solidsolutions in this thesis by using the first-principles. The cluster formation behaviors wereanalyzed according to the interactions between particles (solutes, vacancy) in Al-Si-Cu,Al-Si-Mg and Al-Si-Cu-Mg alloys. The Kinetic Monte Carlo (KMC) method was applied tosimulate the distribution of time-dependent solute in Al-Si base alloys. The precipitationbehavior and property of precipitates in ? and ? sequences were studied by means offirst-principles calculations.The interactions between particles (solutes? vacancy) were studied by means offirst-principle calculations to obtain the binding energies and stable structures of complexes.Density of State (DOS) shows strong interaction between Cu and Al atom, Mg and Al atomand weak interaction between Si and Al atom. A quantitative understanding on the chemicalbonding of Al107X1solid solutions can be gained by analyzing the electron LocalizationFunction (ELF). The order of solid solution hardening efficiency of Cu, Mg and Si in Al maybe understood according to the increased bonding strength and range of Al107X1solidsolutions.The coherent energy of Si phase with face centered cubic lattice structure and thesimulation parameter were calculated by first-principles. The simulation results of clusterformation behavior in Al-Si alloy show that Si clusters with hundreds of Si atoms may form intens seconds. This is consistent with the conclusions that Si phase precipitates quickly.The analysis of clusters formation behavior according to the interactions betweenparticles (solute, vacancy) in Al-Si-Cu alloy shows that Si-rich clusters and Cu-rich clusterscould form. The formation of Si clusters is rapid due to the powerful occupying to vacancy bySi atoms. Clusters contains Si and Cu atoms may appear and with higher Si and less Cucontent. Si-Cu clusters may transform to Si phase. The KMC simulation results show that Siclusters form and Cu atoms have no impact on the formation of Si clusters in Al-Si-Cu alloyscompared to Al-Si alloy. Si-Cu clusters appear with less Cu atoms and Cu-rich clusters appearlater. It is consistent with the calculation. The precipitation behavior and property of precipitates in ? and ? phase were studied by means of first-principle calculations. ? phasehas stronger heat stability according to the analysis of formation enthalpy and DOS. The bondpopulation analysis results show that the covalent bonding ability of ? ' phase is stronger, thegreater the ability of resistance to deformation, the higher the hardness of the alloy, hardeningefficiency may be better when ? ' phase exists rather than ? phase. Alloy hardness valuesdecline when ? ' phase transforms to ? phase may be relation to the differences of propertybetween them.The analysis of cluster formation behavior on the basis of the interactions betweenparticles (solute, vacancy) in Al-Si-Mg alloy shows that similar to Al-Si-Cu alloy Si clusterscould form quickly.(Si, Mg)-rich clusters also form quickly because Mg atom could diffuserapidly and has a stronger tendency to bind with Si atom than with Mg atom. The KMCsimulation results show that Si-Mg clusters form and Mg atom promotes the formation of (Si,Mg)-rich clusters compared to Al-Si alloy.(Si, Mg)-rich clusters provide the componentfoundation for the ? precipitation sequence. Formation enthalpy calculations of ?' phase showthat Mg9Si5phase is the most stable ?' phase structure. The thermodynamic stability issuccessively increased from GP zone, ?'' phase, ? ' phase and to ? phase. With theimprovement of aging temperature, the precipitates will sequentially appear. It is consistentwith the aging precipitation sequence supersaturated from solid solution (SSS)-GP zone-?''-?'-?.Evolution behavior of clusters and subsequent precipitation in Al-Si-Cu-Mg alloy withexcess-Si and high Cu/Mg ration during aging were studied by means of three DimensionalAtom Probe (3DAP).3DAP experiment results show that dot-like (Si, Mg)-rich clustersincluding Si-Mg and Si-Mg-Cu clusters appear in alloy aged at175?for0.25h, which isconsistent with the first-principles calculation that the strong binding between Si atoms, Si andMg atoms, Si, Mg and Cu atoms and clusters with corresponding component can exist stably.The KMC simulation results show that Si-Mg clusters and Si-Mg-Cu clusters form. Thenumber of clusters is more and the size of clusters is smaller.3DAP results show that at0.25to4h clusters continue transition to Q' phase (Al4Cu2Mg8Si7) and small Cu-rich clusters appear at4h.