Experimental Investigation of Fe-Co-La System: Liquidus and Solidus Projections

Phase equilibria in the Co-Fe-La system were studied using differential thermal analysis (DTA), X-ray diffraction analysis, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Liquidus and solidus projections and a melting diagram for this system over the whole concentration range and a Scheil diagram for solidification were constructed. The ternary compound (Co,Fe)(17)La-2 (tau) (Th2Zn17-type structure) forms by peritectic reaction L + (gamma Co,Fe) + Co13La reversible arrow tau at 978 degrees C. This ternary compound is located along the isoconcentrate of 11 at.% La and extends from 45.8 to 79.3 at.% Co. The Co13La phase has the widest homogeneity region and dissolves up to 43.3 at.% Fe. The solubilities of Fe in Co5La, Co7La2, Co19La5, and Co3La2 were established to be 10.3, 9.2, 4.7, and 3.2 at.%, respectively. The solubility of Fe in Co1.7La2 and CoLa3 according to EPMA does not exceed 1 at.%. The maximum solubility of La in (gamma Co,Fe) and (alpha Co,Fe) is determined to be less than 1 at.%.