The Metallography and Superconductivity of Niobium-Rich Nb–Al–Ge Alloys

Abstract

Four alloys of nominal composition Nb _x (Al_0.75Ge_0.25), where x = 3·8, 3·3, 2·9, and 2·4 were prepared by arc-melting. The microstructures of the alloys consisted of dendritic solid solution, A15 phase and two minor phases—one specifically rich in aluminium, the other in germanium. Superconducting transition-temperature measurements, micro-hardness tests, and electronprobe microanalyses were carried out on each alloy before and after heat-treatment at 1023 K. Transition temperatures for the onset of superconductivity increased from ∼ 19 to 20·00–20·34 K as a result of heat-treatment, while the aluminium and germanium content of the A15 phase increased slightly owing to the partial decomposition of the solute-rich phases. Despite this increase in solute content, the A15 phase of each alloy was found to be niobium-rich relative to the stoichiometric composition. For heat-treated alloys, relationships appear to exist between T _c and A15-phase micro-hardness and, possibly, between T _c and A15-phase e/A ratio.