Materials Research (Feb 2012)
Microstructure and transformation characteristics of a TiAlNb alloy with Cr and Mo addition
Abstract
The present study investigated the Ti-46Al-4Nb-2Mo and Ti-46Al-4Nb-2Cr (at.%) alloys, which were prepared by solution treatment at 1,400 °C for 30 minutes prior to being air-cooled. The alloys were subsequently reheated to 1350 °C for 60 minutes and were cooled to room temperature by oil or water quenching. The evolution of the microstructure in the alloys was investigated by detailed characterization of the massive-γ transformation of the heat-treated samples by optical microscopy and electron backscatter diffraction (EBSD). Lamellar structures consisting of and α2 phases with small amounts of β-phase were distributed along the grain boundary after the solution treatment process in both alloys. A massive-γ transformation from the α-phase field was observed for the samples that were cooled by water or oil quenching. After cooling in either oil or water, the massive-γ transformed structure of Ti-46Al-4Nb-2Cr without a lamellar phase was distributed on the α2-matrix. In addition, the massive-γ phases confirmed that the nucleation site was at the grain boundary of the α/α parent phase. Moreover, the volume fraction of massive-γ decreased as the cooling rate increased. The EBSD results demonstrated that the massive-γ transformation inherited the orientation of the lamellar-γ, and nucleation occurred around the parent α-matrix. The detailed observation orientation relationship by Kikuchi patterns and pole figures revealed a massive- transformation, and the α2-matrix had the associated orientation relationship from the γ-lamellar and parent α-matrix.
