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Thermodynamic evaluation of Al-2, 5 wt.%Mg-0, 7wt.%Li

Reduction of density, increase in elastic modulus and introduction of precipitation hardening by the formation of metastable δ’ (Al3Li) phase are considered to be three most important benefits of aluminium-lithium- magnesium (Al-Li-Mg) alloy production and utilization [1]. Reduction in density as well as enhancement of corrosion resistance, increases in stiffness and fracture toughness are most effectively done by adding Li . Furthermore, the Li addition to Al alloy will result in 6% increase in elastic modulus and 3% decrease in density for each percent of Li added up to 4%, respectively. Strengthening effect of Li addition is primarily realized as precipitation of the aforementioned metastable δ’phase as well as solid solution strengthening. Mg additions significantly influence strengthening by introducing new precipitates and changing the nucleation and behaviour of existing metastable δ’ phase. Besides that, Mg leads to the further density reductions. The solidification sequence and metastable δ’ phase behaviour, for the Al-2, 5 wt.%Mg-0, 7wt.%Li alloy, was obtained combining differential scanning calorimetry (DSC) and computer aided thermodynamic diagram calculation (CALPHAD) with optical and electron microscopy as well as energy dispersive spectrometry (EDS). According to the equilibrium phase diagram obtained by Thermo-Calc software support, solidification begins with development of primary α-Al dendrite network at 622°C. Since the Li possess the highest solubility in Al at 600°C, decrease in temperature will cause stabile δ (AlLi) phase formation at 368°C. Afterwards, Mg from the bulk α-Al and stabile δ react forming ternary T (Al2LiMg) phase through peritectoid reaction at 257°C. The equilibrium solidification ends with secondary eutectic Al3Mg2 precipitation at 138°C. However, in real solidification conditions, Mg has a more pronounced effect on the phase precipitation. Since Mg significantly reduces solubility of Li in α-Al, and therefore metastable δ’ phase precipitates after development of α-Al dendrite network. Afterward solidification sequence depends from Li/Mg ratio. For high Li/Mg ratio metastable δ’ phase becomes stabile δ, therefore avoiding formation of ternary T phase. In this case, since Li/Mg ratio is low, ternary T phase precipitates next, while the formation of secondary eutectic Al3Mg2 represents the end of solidification sequence. However, Scheil non- equilibrium calculation recognizes the existence of two phases: primary α-Al dendrites that start to form at 648°C, and Al3Mg2 which precipitate at 461°C. This deviation between equilibrium and real solidification conditions is confirmed by the results of EDS analysis. The presence of these phases is based on Al:Mg ratio, since EDS does not possess the ability of light element identification. Correlation of identified microstructural constituents thermodynamic stability by CALPHAD and DSC allows prediction of Al-2, 5 wt.%Mg- 0, 7wt.%Li alloy solidification sequence.

Al-2, 5wt.%Mg-0, 7wt.%Li alloy, lithium, magnesium, microstructure, solidification sequence

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