Microstructure observations in rare earth element Gd-modified Ti-44 at% Al

Rare earth element Gd (0.15 at%) was added to Ti-44 at% Al to refine the microstructure. Almost all of the Gd atoms were found to form an oxide phase which was tentatively identified as Gd2TiO5. In the as-cast ingot the oxide phase possessed a complicated “finger” shaped morphology and formed a networked structure between the primary crystals. This morphology was broken down in the subsequent heat treatment at 1350°C to become discrete particles in the form of particulate, platelets or rods, often with a faceted appearance, although clusters of the particles reminiscent of their as-cast morphology were common. The distribution of the particles was improved by hot pressing which appeared to reduce the particle size and de-cluster them. Although the particles were not actually pinning the grain boundaries, their distribution seemed to have a significant effect on lamellar grain sizes after heat treatment. The addition of Gd appeared also to promote the formation of lamellae during cooling.