THE ISOTHERMAL SECTION AT 500-DEGREES-C OF THE Y-LA-MG TERNARY-SYSTEM

Ternary Y-La-Mg alloys have been studied by X-ray diffraction, optical microscopy, scanning electron microscopy, and electron probe microanalysis. Phase equilibria have been established in the isothermal section at 500 degrees C in the 50 to 100 at. pct Mg composition range. The sections (YxLa1-x) Mg (continuous solid solution), (YxLa1-x)Mg-2 (cF24-Cu2Mg type for 0 less than or equal to x less than or equal to 0.72 and hP12-MgZn2 type for 0.82 less than or equal to x less than or equal to 1 solid solutions), and YxLa1-xMg3 (0 less than or equal to x less than or equal to 0.47) have been especially studied. The approximate to YxLa1-xMg5 (0.35 less than or equal to x less than or equal to 0.77) ternary compound (cF440-GdMg5 type) has been found. A reinvestigation of selected regions of the binary La-Mg and Y-Mg systems proved necessary. The La5Mg41 phase, not previously reported, has been found by the annealing of samples at 600 degrees C. The YMg2 phase shows a wide homogeneity range up to 73 at. pet Mg. The ternary section has been compared with a simulated one that was created by placing sections of binary systems of Mg with different rare earth elements side by side. The results are related to the pseudolanthanide concept.