Direct Measurement of the Metastable Liquid Miscibility Gap in Fe--Co--Cu Ternary Alloy System

The metastable liquid--liquid phase separation in undercooled Fe--Co--Cu ternary alloy melts (X_Cu=0.10--0.84; X_Co:X_Fe=1:3, 1:1 and 3:1) is investigated by differential thermal analysis in combination with glass fluxing technique. In almost every case, the undercooling of the homogeneous alloy melt was sufficient to reach the boundary line of the submerged miscibility gap. The differential-thermal-analysis signals indicate that this separation into a (Fe,Co)-rich liquid phase L₁ and a Cu-rich liquid L₂ is exothermic and proceeds until the rapid solidification of the L₁ phase occurs. At a given Cu concentration and with the increase of Co content, the phase separation temperatures decrease monotonically between the corresponding values of the boundary systems Fe--Cu and Co--Cu. The boundary lines of the miscibility gap, which are determined for the three quasi-binary cross-sections of the (Fe,Co)--Cu alloy system, show remarkably flat domes. The occurrence of the liquid phase separation shows an evident influence on the subsequent γ-Fe(Co,Cu)→α-Fe(Co, Cu) solid phase transformation.