Nucleation and growth of α-Ti on TiB precipitates in Ti-15Mo-2.6Nb-3Al-0.2Si-0.12B

The microstructure was investigated of a beta-stabilized Ti-15Mo-2.6Nb-3Al-0.2Si-0.12B alloy at two different aging temperatures, 540 degrees C/8 h and 660 degrees C/8 h. In particular, the heterogeneous nucleation of alpha-Ti from TiB particles was studied at these aging temperatures. At the lower aging temperature, alpha-Ti precipitated as needle-like shapes on the TiB phase. In contrast, the higher aged sample exhibited globular alpha-Ti morphology around the TiB phase. This difference was rationalized in terms of the coarsening behavior of alpha-Ti around the TiB phase. Various orientation relationships were observed between these two samples. This difference is because of the precipitation of alpha-Ti on two different TiB planes. In addition, atom probe analysis confirmed the segregation of alpha and beta stabilizing elements to the respective phases. At the lower aging temperature, it was noted that silicon enriched the alpha-Ti/beta-Ti interface when the alpha-Ti/beta-Ti/TiB were all in contact. Upon alpha-Ti coarsening, silicon enrichment was observed at the alpha-Ti/TiB interface at the higher aging temperature.