Microstructure of a complex Nb–Si-based alloy and its behavior during high-temperature oxidation

Abstract

A in-situ composite Nb–Si–Ti–Hf–Cr–Mo–Al composite material alloyed with yttrium and zirconium is studied. The evolution of the structure–phase state of the alloy during oxidation under dynamic and isothermal conditions is considered on samples prepared by vacuum remelting and directional solidification. The phase composition and the microstructure of the alloy are examined by the methods of physico-chemical analysis, and the distribution of alloying elements in initial samples and the products of oxidation is estimated. Thermogravimetric experiments are performed on powders and compacted samples during continuous (in the range 25–1400°C) and isothermal (at 900 and 1100°C) heating in air. The directional solidification of an Nb–Si–Ti–Al–Hf–Cr–Mo–Zr–Y is found to cause the formation of an ultradispersed eutectic consisting of α-Nbss and γ-Nb5Si3ss cells. The as-cast sample prepared by vacuum remelting has a dendritic structure and contains Nb3Si apart from these phases. Oxidation leads to the formation of a double oxide layer and an inner oxidation zone, which retain the two-phase microstructure and the ratio of alloying elements that are characteristic of the initial alloy. Diffusion redistribution is only detected for molybdenum. The cyclicity of heating at the initial stage of oxidation weakly influences the oxidation resistance of the alloy.