Abstract
This paper investigate the thermophysical properties of the liquid as well as the rapid solidification of Al5.0Mg0.2Sc alloys with 3.0 and 6.0 Li wt% by the discharge crucible, and Impulse Atomization techniques respectively. The discharge crucible method, allowed a simultaneous determination of density, surface tension and viscosity as a function of the temperature. While the density and surface tension are found to decrease with Li content, the viscosity increases due to short-range ordering occurring in intermetallic phases. In order to determine their characteristic temperatures, the alloys powder generated by Impulse Atomization were investigated using a Differential Scanning Calorimeter, while microstructural observations and chemical analyses were conducted using scanning and transmission electron microscopy with energy dispersive X-ray spectroscopy. To study the influence of Li on the microstructural phase formation in these alloys, diffraction patterns analyses in transmission electron microscopy, as well X-ray diffraction using synchrotron measurements were carried out. The conducted measurements and microstructure observations revealed the precipitations of Al3Sc and Al2MgLi phases in both alloy compositions. In addition, AlLi precipitates were observed in Al5.0Mg0.2Sc6.0Li alloy, which is in agreement with the phase diagram.
Highlights
AlLi
Mg decreases the solubility of Li in the Al matrix, leading to an enhancement of precipi tation in the Al–Mg-Li system, but the interfacial energy of the pre cipitates is not changed significantly, which is similar to the diffusivity of Li in Al [1,10]
The phases after atomization (Fig. 8) were identified with the use of the phase diagram presented in Fig. 3, and using data presented in the literature [48,52,53,56,59]
Summary
Al alloys (series 2xxx, 5xxx or 7xxx) are attracting renewed interest due to the need for lightweight materials with improved mechanical properties for the aerospace and automotive industries [1,2]. In order to obtain stable nanoscale precipitates, phase nucleation and diffusion-controlled growth steps are required [3] Coherency between these strengthening precipitates and the matrix is crucial for mechanical properties, incoherent precipitates result in reduced strength of the material. Their study indicates that Mg segregation occurs at the perfectly coherent α-Al/ Al3Sc hetero-phase interface, where it is kinetically trapped since Mg is insoluble in Al3Sc [4] This segregation of Mg leads to a substantial reduction in the anisotropy of the α-Al/Al3Sc interfacial free energies, which was confirmed by HREM observations [4]. In the case of a Sc addition to aluminium alloys, this element contributes to significantly improving the strength by forming nanoscale coherent L12Al3Sc precipitates [2,4,8] from a supersaturated α-Al solid solution. Mg decreases the solubility of Li in the Al matrix, leading to an enhancement of precipi tation in the Al–Mg-Li system, but the interfacial energy of the pre cipitates is not changed significantly, which is similar to the diffusivity of Li in Al [1,10]
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