The Effect of Gd Content on α-Grain Growth in γ-TiAl Based Alloys

In this work, the effect of Gd alloying on the microstructure, α-grain growth kinetics, and recrystallization processes in the single–phase α-phase field was investigated using two alloys based on gamma titanium aluminide Ti-47.1Al-1.8Nb-0.5Zr-0.3V-0.001Gd (0.001Gd) and Ti-46.9Al-1.6Nb-0.5Zr-0.5V-0.03Gd (0.03Gd). It was shown that an increase in the Gd content from 0.001 at. % to 0.03 at. % led to a significant decrease in the size of lamellar (α2 + γ)-colonies from 260 to 150 μm and the interlamellar spacing from 450 to 230 nm in the as-cast state. The decrease in the average colony size is associated with an increase in the volume fraction of Gd oxide particles and the additional formation of Gd aluminide particles/interlayers, while the decrease in the interlamellar spacing is due to the retardation of the α2→γ transformation kinetics with an increase in the Gd content. During heating to the α- phase field, the alloy with a higher Gd content exhibits slower grain growth, which is associated with an increase in the volume fraction of Gd oxide particles, which restrain the movement of grain boundaries. Theoretical calculations of Zener pinning pressure (PZ) and Zener limiting grain size (DZ) confirmed the experimental results. In the Ti-46.9Al-1.6Nb-0.5Zr-0.5V-0.03Gd alloy, the retarding force was significantly higher, and the limiting grain size was smaller compared to the alloy with a lower Gd content.