Microstructural Evaluation of Boron Free and Boron Containing Heat-Treated Ti-35Nb-7.2Zr-5.7Ta Alloy

The microstructure of Ti-35Nb-7.2Zr-5.7Ta (TNZT) and Ti-35Nb-7.2Zr-5.7Ta-0.5B (TNZTB) alloys under different heat treatment conditions has been analyzed. The solution-treated and water-quenched TNZT sample consists mainly of beta phase with a very small amount of fine athermal omega precipitate. Precipitation of alpha can be observed when solution-treated samples are directly aged at 580 degrees C for 8 h. The microstructure of the samples subjected to single-stage aging at 300 degrees C or 400 degrees C consists of omega precipitates in equiaxed beta grains. Second stage aging at 580 degrees C for 8 h after first stage of aging at 300 degrees C or 400 degrees C results in the replacement of omega precipitates by secondary alpha. In all of these samples, the amount of omega or alpha phase was very small, and therefore they could not be detected by X-ray diffraction studies. However, analysis of selected area diffraction patterns obtained from transmission electron microscopy studies confirms their presence. The addition of boron leads to the formation of dispersed precipitates of TiB in the beta matrix of the TNZT alloy and also refines the beta grains in the microstructure. However, other microstructural features of the TNZTB alloy are similar to those of the TNZT alloy.