Microstructure and Composition of Rare Earth-Transition Metal-Aluminium-Magnesium Alloys

The determination of the microstructure and chemical composition of La(0.7-x)Pr(x)Mg(0.3)Al(0.3)Mn(0.4)Co(0.5)Ni(3.8) (0 <= x <= 0.7) metal hydride alloys has been carried out using scanning electron microscopy (SEM), energy dispersive X ray analysis (EDX) and X ray diffraction analysis (XRD). The substitution of La with Pr changed the grain structure from equiaxial to columnar. The relative atomic ratio of rare earth to (Al, Mn, Co, Ni) in the matrix phase was 1: 5 (LaNi(5)-type structure). Magnesium was detected only in two other phases present. A grey phase revealed 11 at.% Mg and the concentration ratios of other elements indicated the composition to be close to PrMgNi(4). A dark phase was very heterogeneous in composition, attributed to the as-cast state of these alloys. The phases identified by XRD analysis in the La(0.7)Mg(0.3)Al(0.3)Mn(0.4)Co(0.5)Ni(3.8) alloy were: La(Ni,Co)(5), LaAl(Ni,Co)(4), La(2)(Ni,Co)(7) and AlMn(Ni,Co)(2). Praseodymium favors the formation of a phase with a PuNi(3)-type structure. Cobalt substituted Ni in the structures and yielded phases of the type: Pr(Ni,Co)(5) and Pr(Ni,Co)(3).