Formation of the A15 phase in thin-film Nb-Al diffusion couples

Abstract

Single-crystal and polycrystalline thin-film Nb-Al diffusion couples have been prepared by evaporation at pressures of 10−11–10−10 Torr. Sapphire, single-crystal MgO, polycrystalline Al2O3, and polycrystalline Nb were used as substrates. These couples were annealed in situ at temperatures between T=780 and 950 °C for 30 min, in order to study the formation of the A15 phase. Based on resistivity data, between 800 and 880 °C, the A15 phase was formed only on oxide substrates. The relative A15 content was higher in polycrystalline couples than in single-crystal couples. Between 880 and 950 °C the A15 phase was also obtained in couples deposited on Nb substrates. X-ray data confirmed that the temperature of the A2 (bcc) to A15 structure transformation was lower in couples where the Nb-Al solid solution was in contact with an oxide substrate. X-ray photoelectron spectroscopy indicated the occurrence of surface reactions between the oxide substrate and Nb or Al. Auger depth profiles of the Nb/Al ratio were similar in couples on all substrate types studied and could not be correlated with A15 phase formation. The A15 phase having Tc’s corresponding to the upper (Al-rich) boundary of the A15 phase stability field appeared to form locally at defects, grain boundaries, and other imperfections. Alternative explanations of the experimental findings are discussed.