COST 507 - Repositório Aberto da Universidade do Porto

Figure 4 shows the assessed phase diagram with the fixed amount of 1 at.·;
magnesium using data from Kuznetsov [82Kuz].
82KUX
A Liquidus
LiqM + CuSIZrH
Uq»2 + Cu51ZrU + Cu5Zi
Upper line, Llq»2 + Fcc_Al + Cu5Zr
Lower line, Llq»2 + Fcc_A 1 + CuSZr
Fce.AKCu)
Cu53 r+Cu51Zrl. +L»ves_C15
0.05 0.10 0.15
MOLE FRACTION ZR
0.20
Figure 4. The assessed Cu-Mg-Zr phase diagram with 1 at.% Mg.
The assessed phase diagram agreed well with the liquidus data but not with the lower
boundary of the Liquid#2 + Fcc_Al + CusZr phases. The invariant temperatures were
995.0 K and 1007 K. The large positive value of the excess Gibbs energy parameter in
the Fcc_Al phase did not have a great influence on the lower limit of the phase region
liquid#2 + Fcc_Al + Cu5Zr mainly perhaps because the invariant phase boundary at
995.0 K was fixed.
Figure 5 shows the isothermal cut at 500 °C with the new τ phase (Cu 2 MgZr). In
Figure 5 the τ phase was in the equilibrium with the Laves_C15 phase with the
negligible solubility of zirconium. The τ phase was also in the equilibrium with the
phases Cu 5 |Zr| 4 , CugZr3 and CU[oZr 7 . Table 4 shows the optimized parameter of the
Gibbs energy of formation in the τ phase.
Table 4. The calculated Gibbs energy of formation in the τ phase at 298.15 K.
PHASE
Cu 2 MgZr (τ)
G
h,G
A
-72654.140
B*T
-
The τ phase was optimized without any exact knowledge of the melting point. The
calculated melting point was 1044.78 Κ in the equilibrium with the τ, Laves_C15 and
liquid#2 phases. The solubility of zirconium in the Laves_C15 phase was taken
negligible because there were no experimental data available concerning the
zirconium solubility in the Laves_C15 phase.
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