Ab initio interface configuration determination for β″ in Al–Mg–Si: Beyond the constraint of a preserved precipitate stoichiometry

Abstract

The precipitate-host lattice interface configuration stabilities for the β'' phase in the Al-MgSi alloy system have been examined using density functional theory. Usually, the supercell based calculations underlying such studies assume a preserved precipitate stoichiometry. Relaxing this assumption in the present work, we highlight significant results that may easily be overlooked when the stoichiometry constraint is operative. Our main findings are the following: (i) we reject the often proposed Mg5Si6 composition for β''. (ii) We stress the possibility of significant Mg-Al compositional disorder for the more likely β''-Mg5Al2Si4. Finally, (iii) we propose the existence of more than one locally stable interface configuration for the coherent β''/Al interfaces. It is argued that quantifying the solute atom diffusivity near the interface as well as within β'' is fundamental to addressing the importance of points (ii) and (iii).

Comparative in Mechanical Behavior of 6061 Aluminum Alloy Welded by Pulsed GMAW with Different Filler Metals and Heat Treatments

“…The solubilization heat treatment T4 promoted the change in the morphology of aluminum matrix from dispersed acicular precipitates (typical of β"-phase) to a morphology of strips and rods precipitates (typical of β-phase). The reduction in hardness after the solubilization heat treatment and the change in morphology of the precipitates is also an indicative of the thermal dissolution of β"-phase clusters to β-phase [32,33]. In the literature, the solubilization thermal treatment has been performed to 6061-T6 aluminum alloy followed by natural ageing during up to 24 h: the dissolution of β" precipitates, the diminishing oh hardness from 120 HV to 90 HV and the change in precipitates morphology from acicular to rods were reported [20].…”

A First-Principles Study of the Cu-Containing β″ Precipitates in Al-Mg-Si-Cu Alloy

“…For the β phases, the formation enthalpies and lattice parameters of Mg 4 Al 3 Si 4 , Mg 5 Al 2 Si 4 , Mg 6 AlSi 4 , and Mg 5 Si 6 were computed for each of the models of the crystal structures available in the literature [17,18,21], allowing a critical assessment of the validity of the models. Figure 1 shows four atomic models of the β without Cu.…”

“…However, according to recent experimental and theoretical studies, the composition of β would fluctuate around Mg 5 Al 2 Si 4 [19][20][21][22]. Furthermore, the most recent density functional theory (DFT) calculations inferred very minor formation enthalpy differences for β -Mg 5+x Al 2−x Si 4 (−1 < x < 1) [21]. These results indicate that the composition of β phase in Al matrix may change under certain conditions.…”

“…The monoclinic β phase was originally proposed to have the composition of Mg 5 Si 6 [17]. However, according to recent experimental and theoretical studies, the composition of β would fluctuate around Mg 5 Al 2 Si 4 [19][20][21][22]. Furthermore, the most recent density functional theory (DFT) calculations inferred very minor formation enthalpy differences for β -Mg 5+x Al 2−x Si 4 (−1 < x < 1) [21].…”

Enhanced nucleation and precipitation hardening in Al–Mg–Si(–Cu) alloys with minor Cd additions

“…The generic precipitation sequence of ternary Al-Mg-Si alloys is proposed as follows [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]: Supersaturated solid solution (SSSS)  atomic clusters  Guinier-Preston (GP) zones  β"  β', U1, U2, B'  β, Si Among these phases, β" phase (Si4Mg5-xAl2+x (x=0, 1)) is the most effective strengthening phase in Al-Mg-Si alloys [2,9,10,12,13,[20][21][22][23][24][25][26]. It has a needle shape with a C-centred monoclinic structure (C2/m, 𝑎 = 1.516 nm, b = 0.405 nm, c = 0.674 nm, β = 105.3°, see Figure 1(a)) [2,12,13,[27][28][29]. GP zones are also believed to have a contribution to the strength, but to a lesser extent [6,23].…”