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The structural stabilities and thermal properties of η′ and η phase at high temperature and pressure were investigated via first principles calculations combined with quasi-harmonic Debye approximation (QHA). The calculated results illustrate η′ possess high formation enthalpy with lower structural stability at high temperature, which may transits to equilibrium phase η at temperature range of 510–520 K. The calculated thermodynamic properties indicate η exhibits better thermal stability than η′ at high temperature. The thermoelastic properties of η′ and η were estimated based on the temperature-dependent elastic constants Cij(T), and it is revealed the elastic moduli of η′ are larger than that of η though both two phases become deformable with elevated temperature. Moreover, the evaluated thermoelastic anisotropies suggest η′ and η are essentially elastic isotropic on {0001} plane, while η′ exhibit serious mechanical anisotropy and angular bonding characters on both {10-10} and {01–10} planes at high temperature.

Isothermal hot compression tests were conducted on the 6063 aluminum alloy using a Gleeble-3810 thermal simulator at four different strain rates (0.01, 0.1, 1, and 10 s–1) and five different temperatures (300, 350, 400, 450, and 500°C). Based on the constitutive relationship to process the experimental data, processing maps were produced to evaluate the efficiency of power dissipation (η) and identify regimes with flow instability. Processing maps were produced for two regions with relatively high η at a strain of 0.6, namely (300–320)°C/(0.01–0.02) s–1 and (400–500)°C/(0.01–1) s–1. There were also two unstable regions at (300–325)°C/(0.06–1.5) s–1 and (350–500)°C/(3–10) s–1. The microstructures observed by optical microscopy and electron back-scattered diffraction maps indicated that the deformed samples only underwent dynamic recovery under deformation conditions of (300–320)°C/(0.01–0.02) s–1, while at 500°C/0.01 s–1 the samples showed a high degree of dynamic recrystallization and then produced new equiaxed fine grains. At high strain rates, the precipitates had a pinning effect on dislocations, leading to stress concentration. Local plastic deformation occurred, resulting in thermoplastic instability. Therefore, the optimum processing conditions for the 6063 aluminum alloy determined from true stress-strain curves, processing maps, and microstructural analysis were 500°C/0.01 s–1. [ABSTRACT FROM AUTHOR]

Abstract—This study investigated the effect of deep cryogenic treatment (DCT) on the aging processes of Al–0.43Mg–1.27Si alloy. Differences in the mechanical properties and precipitation behavior between conventional treatment and DCT were analyzed by means of hardness test, tensile test, optical microscopy (OM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). DCT promoted the precipitation of Si phases, which is accompanied by an increase in hardness. The content of Si was improved from 0.84 to 1.08% and the hardness was increased from 46.11 to 48.89 HV. The temperature of precipitation of GP zones and β'' phase was lower, which accelerated precipitation of the β'' phase and then improved the aging hardening response of the alloy. [ABSTRACT FROM AUTHOR]

In this paper, the formation energies and elastic constants of -Al2O3, MgO and AlN in both rock salt (cubic) and wurtzite (hexagonal) structures were investigated by first principles calculations. The results show that the formation energy being −17.8, −6.3, −3.06 and −3.46 eV/formula unit for -Al2O3, MgO, AlN (rock salt) and AlN (wurtzite). It suggests that in the ground state, -Al2O3 is relatively more stable than MgO and AlN. The elastic properties for a polycrystalline in the ground state were calculated with the obtained elastic constants, the elastic properties reveal the rock salt structure AlN is the hardest particles among all the inclusions, and all of these inclusions are classified as brittle materials, which is detrimental to the ductile nature of aluminum matrix. The calculated anisotropy index shows that the AlN (wurtzite) and -Al2O3 have a lower degree of anisotropy compared with MgO and AlN (rock salt). The calculated results are in good agreement with the values of experimental and other works. [ABSTRACT FROM AUTHOR]