COST 507 - Repositório Aberto da Universidade do Porto

An Experimental Investigation of the Cu-Li-Mg Phase Diagram
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Abstract
Lazar Rokhlin*, Marko Hämäläinen", and Tatyana Dobatkina*
*Baikov Institute of Metallurgy, Moscow, Russia
"Helsinki University of Technology, Helsinki, Finland
This research forms a part of the COST 507 program on the Cu-Al-Mg-Zr-Li system. The
research included the investigation of the part of the Cu-Li-Mg ternary sub-system aiming
mainly to check and construct the phase diagram in the area near a proposed compound of
the LigMg6oCu32 composition [76Mel]. The investigation did not confirm formation of this
ternary compound. As result of the investigation, one polythermal section of the phase
diagram was constructed. One invariant four-phase equilibrium at 430 °C in the system was
discovered. The phase relationships in the Mg corner of the system in solid state were
determined.
1 Introduction
The overall system under study is the Cu-Al-Mg-Zr-Li system. This system is quite
complicated and its constitution may be understand from the knowledge of its sub-system
constitutions. The ternary Cu-Li-Mg system is one of these sub-system. Some years ago one
ternary compound of the LigMg6oCu32 composition was proposed in the system [76Mel].
Nevertheless, its existence was not confirmed reliably. The present work was directed
mainly to study the Cu-Li-Mg phase diagram in the concentration area near to this
composition and to check this compound formation.
2 Experimental Techniques
The phase diagram was investigated by study the alloys along the polythermal section for 64
at.% Mg which ran near the proposed ternary compound consisting of 32 at.%Cu, 8 at.%Li,
and 60 at.%Mg. Besides, some alloys close the proposed ternary compound were study, as
well. The starting materials were Cu 99.996 wt.%, Li 99.8 wt.%, and Mg 99.96 wt.%
purity. The alloys for the investigation were prepared by melting in alumina crucibles under
flux consisting of 75%LiCl + 25%LiF or 50%KC1 + 50%LiCl. The heating was performed
in the electrical resistivity furnace. Chemical analysis showed that Mg and Li losses during
melting were insignificant. The alloys were poured into ingots of 15 mm in diameter. The
pieces of the ingots were used for differential thermal analysis and the structure studies by
microscopy and X-ray methods. The structure was studied in the as cast condition and after
annealing at 370 °C for 96 hours followed by quenching into cold water. Differential
thermal analysis was performed using the apparatus TAG-24 of the "Setaram" firm under
He atmosphere or the own equipment under above mentioned fluxes. X-ray investigations
were performed from the plane samples using the Russian X-ray register DR.ON-3M with
Fe and Cu radiation.
3. Results of the Investigation
The polythermal section for 64 at.% Mg constructed is shown in Fig. 1. It shows
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