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

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 simi...