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 degrees C to become discrete particles in the form of particulate, platelets or rods, often with a faceted appearance, although dusters 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. (C) 2000 Elsevier Science Ltd. All rights reserved.