Cerium Alloys

Cerium alloys readily with a large number of metals. Many of the alloys thus obtained are hard and brittle, differing therefore from pure cerium, which is soft and malleable. Accordingly, small particles are easily detached from the alloys by friction, and when the alloys are rich in cerium the heat produced is more than sufficient to raise the temperature of the particles above 160°, the ignition-point of cerium. Hence numerous alloys rich in cerium are pyrophoric. Some of them, e.g. iron-cerium alloys, are of commercial importance owing to this property, being utilised in the manufacture of automatic lighters, etc. The alloy of cerium, lanthanum, etc., known as " Mischmetall," is a valuable reducing agent.

Sodium alloys with cerium quietly, forming a hard and somewhat pyrophoric alloy that oxidises on exposure to air. Silver and gold alloy readily with cerium; the silver alloys are hard and brittle, but the gold alloys are fairly soft.

The copper alloys have been systematically examined and four compounds discovered, viz. Cu₆Ce (m.p. 935°), Cu₄Ce, Cu₂Ce (m.p. 820°), and

CuCe. Between 55 and 85 per cent. Ce the alloys are pyrophoric, and with more than 30 per cent. Ce they soon disintegrate.

Calcium and cerium alloys are white and very hard; they are stable in air, do not disintegrate, and emit brilliant sparks when struck. They may be prepared from the metals, or by reducing cerous fluoride with calcium.

Magnesium and cerium alloys are white in colour and extremely brittle. The alloys rich in cerium form excellent flashlight powders. Those containing 60-75 per cent, of cerium may be finely powdered, and form excellent reducing agents, e.g. for preparing vanadium from its oxide. Alloys containing more than 75 per cent, of cerium are difficult to powder without catching fire. The alloys have been systematically examined by Vogel, who has described the compounds Ce₄Mg (m.p. 632°), CeMg (m.p. 738°), CeMg₃ (m.p. 780°), and CeMg₉. The compound Ce₄Mg breaks up into Ce and CeMg at 497°; the compounds CeMg and CeMg₃ form solid solutions with one another. Zinc alloys with cerium almost explosively. The alloy is hard, brittle, and pyrophoric, but oxidises in the air. Cadmium behaves similarly to zinc. Mercury readily alloys with cerium. The amalgams, which are liquid at ordinary temperatures when only 1 or 2 per cent, of cerium is present, decompose water with the evolution of hydrogen and oxidise quickly in the air, spontaneous combustion readily occurring when 8-10 per cent, of cerium is present.

The system cerium-aluminium equilibrium diagram (Vogel). Solid phases; Ce along AB, Ce3Al along BCD, Ce2Al along DE, CeAl along EF, CeAl2 along FGH, βCeAl4 along HJ, αCeAl4 along JK, Al along KL.

Aluminium and cerium alloys have been studied by the thermal and microscopic methods. The equilibrium diagram, due to Vogel, is shown in fig. No solid solutions are formed. Five compounds exist, two only being stable at the melting-point: Ce₃Al (m.p. 614°), Ce₂Al, CeAl, CeAl₂ (m.p. 1460°), and CeAl₄. The compound CeAl₄ exists in two forms, the transition-point being 1005°. The form stable at ordinary temperatures is probably rhombic. Its density is 4.193. The cerium-aluminium alloys are stable to air and water unless the cerium is more than 75 per cent. The maximum hardness is 6, corresponding to CeAl₂. This compound is very resistant towards acids, but burns brilliantly.