SHS Metallurgy of Composite Materials Based on the Nb–Si System

Abstract

Composite materials (CMs) based on niobium with functional and alloying additives (Si, Hf, Ti, Al, etc.) have prospects for industrial approval in aviation propulsion engineering. The authors previously showed that such CMs can be synthesized in an autowave mode (combustion mode) using highly exothermic mixtures of Nb₂O₅ with Al, Si, Hf, and Ti. It was found that hafnium actively participates in the reduction of Nb₂O₅, which complicates its introduction into the CM. This study is directed at investigating the possibility to synthesize Nb-based composite materials with a high Hf content using methods of centrifugal SHS metallurgy. It is shown in experimental investigations using a centrifugal installation under the effect of acceleration of 40 g that the replacement of active Hf by its less active compounds Hf–Al or Hf–Ti–Si–Al in the composition of the Nb₂O₅/Al mixtures makes it possible to transfer the combustion of the mixture from the explosion-like mode into the steady-state combustion mode. The content of Hf in the CM increases with an increase in the size of Hf–Al granules from 0–40 to 160–300 μm from 1.3 to 3.8 wt %. The introduction of Hf–Ti–Si–Al granules with a particle size from 1 to 3 mm into the initial charge makes it possible to form cast CMs based on niobium silicides with a Hf content up to 8.1 wt %. The integral composition and distribution of base and impurity elements in structure components of cast CMs, as well as their phase composition, were determined using electron microscopy and X-ray phase analysis. CMs with the maximal Hf content (8.1 wt %) contain three structural components: (1) the base, which includes Nb, Si, and Ti; (2) intergrain boundaries containing Nb, Ti, and Al; and (3) inclusions based on hafnium oxide. Three phases are revealed in the X-ray diffraction pattern of the CM, notably, solid solutions based on Nb and Nb5Si3, as well as a minor amount of Nb₃Si.