PHASE COMPOSITION, STRUCTURE, AND MECHANICAL PROPERTIES OF NIOBIUM-DOPED γ-TiAl  MATERIALS PRODUCED BY THE POWDER HYDRIDE TECHNOLOGY

Abstract

The effect of niobium on the structure, phase composition, and mechanical properties of γ-TiAl alloys were studied. The γ-TiAl alloys were doped with niobium within a solid solution; the amount of niobium in the alloys was from 2 to 10 at.%. Niobium was introduced as an Al₃Nb intermetallic, allowing a superfine powder mixture to be produced by high-energy grinding. A TiH₂ + Al₃Ti + Al₃Nb powder mixture was used to prepare the γ-TiAl alloys. This route minimized the Kirkendall–Frenkel effect in the Ti–Al system and prevented increase in porosity during sintering. Only TiAl and Ti₃Al phases were revealed in the sintered materials, indicating that niobium had dissolved in the existing phases. To achieve the desired phase composition in the alloy, the content of aluminum had to be increased to compensate for its partial loss through evaporation during sintering. The alloys with a lower aluminum content showed higher strength but lower ductility, both at room and elevated temperatures, because of a greater amount of the α₂ phase. Niobium doping reduced sintering shrinkage by 2–4% and inhibited the grain growth. The material with a low niobium content had greater strength and ductility at a sintering temperature of 1200 °C, when the grain size hardly changed. The grain growth was inhibited by niobium doping at a high sintering temperature of 1400 °C. The yield strength increased with the niobium content. The studied alloys exhibited satisfactory low-temperature strength and ductility, as well as high creep resistance at 700 °C. They showed little tendency for weakening and are therefore promising for high-temperature applications above 700 °C.