The effect of Ruthenium addition on the microstructure and mechanical properties of TiAl alloys

The high temperature compression behavior of TiAl-Ru alloys was studied at different temperatures and strain rates. Ru was found to have a strong strengthening effect on TiAl alloys. However the Ru addition amount was limited by its low solubility in gamma-TiAl and alpha 2-Ti3Al, and the detrimental effect of excessive ternary phase precipitation. Furthermore, the melting temperature decreases when Ru >= 0.6 at.% as the alloy composition approaches a ternary eutectic point. The strengthening mechanism is discussed and two separate mechanisms are proposed, viz, solid solution strengthening and refined colony strengthening. Intergranular cracks were found in the alloys with low Ru or no Ru addition, but were barely detected as Ru content increased to above 0.6 at.%. It was suggested that Ru showed a beneficial effect on both strength and ductility of TiAl alloys due to the refined colony size. Three-point bend test results showed that the Ru addition can also improve the room temperature ductility of TiAl alloys. Hot workability was increased according to the compression tests. Thermal-mechanically treated TiAl-Ru had much smaller grain size than the heat-treated samples due to dynamic recrystallization. But it did not show superior strength in the compression test compared to the heat-treated samples. The Zener-Hollomon parameter was calculated from the compression strength of heat-treated TiAl-Ru alloys. Its relationship with dynamic recrystallization and hot work is discussed. The mechanical properties of TiAl-Ru alloys are compared with TiAl-Nb samples and demonstrate a promising combination of strength and ductility. (C) 2011 Elsevier Ltd. All rights reserved.