Thermal stability of Al2MgC2 and thermodynamic modeling of the Al–C–Mg system - Application to grain refinement of Mg–Al alloys

Abstract

In the scope of supporting the development of Mg-Al alloys and related materials, an experimental study coupled with a CALPHAD thermodynamic modeling of the Al-C-Mg system was conducted. The peritectic decomposition of Al2MgC2 to form Al4C3, graphite and a liquid phase was measured at 1559±10 K by DTA. In order to model the Mg solubility in Al4C3, DFT calculations were performed on the end-member phases Mg4C3, Al2Mg2C3 and Mg2Al2C3 and it was shown that Mg substitutes on the Al2 crystallographic site of the carbide structure. Based on recent literature data and a revised Al-C binary, a model description of the Al-C-Mg ternary is proposed for the first time. More specifically, it is used to calculate the liquidus projection and the phase formation sequence during Scheil solidification of a 91Mg-9Al wt% alloy inoculated with carbon. This work provides a convincing argument that Al2MgC2 is the nucleant responsible for the grain refinement of Mg-Al alloys inoculated by C.

Al2MgC2 and AlFe3C formation in AZ91 Mg alloy melted in Fe-C crucibles

“…1 in the Supplementary Information, where it can be seen that only the trigonal T2-Al2MgC2 structure is consistent with the experimental Kikuchi patterns from this phase. Furthermore, we note that T2-Al2MgC2 is the expected equilibrium phase in AZ91 containing impurity carbon from recent work on the thermodynamics of the Al-C-Mg system [13,40].…”

“…Emley [12] suggested that, during superheating, the walls of steel crucibles are decarburised resulting in carbon pickup to the Mg alloy, and that more carbon pickup occurs from new crucibles since they contain more carbon in the surface layers. When the carbon content exceeds the solubility limit of an AZ91 melt, carbides form and according to recent thermodynamic calculations the expected carbide in Mg-9Al-0.5C (wt.%) is Al2MgC2 [13].…”

“…In more recent studies [22,23], Al2MgC2 has been proposed to be the nucleant for grain refinement of Mg-Al alloys, although this phase was found [24] and synthesised [25] back in the 1990s. In the Al-C-Mg ternary system [13,25], Al4C3 and Al2MgC2 are both stable phases at 1000 K and Al4C3 is reported to be the equilibrium phase with Mg-Al liquid only when the Al content is more than 19 wt.%. This indicates that the nucleant for grain refinement of commercial Mg-Al alloys (normally ≤9 wt.% Al) by carbon inoculation is more likely to be Al2MgC2 ternary carbide instead of Al4C3.…”

Nano-Enhanced Phase Reinforced Magnesium Matrix Composites: A Review of the Matrix, Reinforcement, Interface Design, Properties and Potential Applications

“…For carbon-reinforced MMCs, the Al 2 MgC 2 phase is formed along the grain boundary with the increase in Al content. Deffrennes et al [25] obtained a 70Mg-19Al-11Cwt% sample in a Ta crucible and quenched at 1273 K for 240 h to obtain Al 2 MgC 2 crystal. A relatively accurate ternary system model was obtained by comparing the decomposition temperatures calculated by differential thermal analysis (DTA) and discrete Fourier transform (DFT).…”

“…Figure 1. The 91Mg-9Al wt% alloy was heated to 1000 k and modified by 0.5wt% C. The phase (a) linear and (b) logarithmic mass fraction images of the Scheil solidification model were obtained[25]. SEM morphologies of (c) Mg6Zn and (d) ZnO-0.5GO/Mg6Zn[27].…”