Strength evaluation of α and β phases by nanoindentation in Ti-15Mo alloys with Fe and Al addition

The strengths of the alpha precipitate and the beta matrix were evaluated by nanohardness in the Ti-15Mo-1Fe and Ti-15Mo-3Al alloys and compared to those of the Ti-15Mo alloy. The a phases with similar size (a long axis of a few micrometres and a short axis of a few hundred nanometres), distribution and volume fraction were obtained in three alloys by adjusting the aging temperature. Analyses by SEM-EDS confirmed that Fe and Mo were enriched in the beta phase and depleted in the alpha phase, while Al was enriched in the alpha phase and depleted in the beta phase. Tensile tests were carried out, and the tensile strength was shown to be higher in the Ti-15Mo-1Fe and Ti-15Mo-3Al alloys than in the Ti-15Mo alloy. The nanohardness measurements indicated that the alpha phase was softer than the beta phase in both Ti-15Mo-1Fe and Ti-15Mo alloys, while it was harder in the Ti-15Mo-3Al alloy. The increased tensile strength was mainly caused by the strength of the Fe enriched beta phase in the Ti-15Mo-1Fe alloy and by the strength of the Al enriched alpha phase in the Ti-15Mo-3Al alloy.