Effect of Nb content on phase transformation, microstructure of the sintered and heat-treated Ti (10-25) wt.% Nb alloys

The Ti9Nb1, Ti8.5Nb1.5, Ti8Nb2 and Ti7.5Nb2.5 alloys were developed by powder mixing, compaction and sintering followed by heat treatment. Phase transformation on the sintered and heat-treated Ti (10-25 wt.%) Nb alloys were investigated using the X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The DSC analysis shows an endothermic peak at similar to 800 degrees C in Ti9Nb1 alloy. For Ti8.5Nb1.5 and Ti8Nb2 alloys, both endothermic peaks appeared with a slight variation similar to 660 degrees C while the Ti7.5Nb2.5 alloy occurred at 490 degrees C. All these endothermic peaks were reversible upon cooling and revealed exothermic peaks which confirm the alpha <-> beta phase transformation. These Ti-Nb alloys were all subject to heat treatment at 1100 degrees C (beta-zone). The FCC Ti-Nb phase was detected by the XRD analysis in as-sintered Ti7.5Nb2.5 alloy. Moreover, a BCC phase coexisted with the HCP in Ti9Nb1, Ti8.5Nb1.5 and Ti8Nb2 alloys. Optical microscopy analysis revealed the equiaxed grains composed of the light beta-phase segregating on the grain boundaries. The Ti9Nb1 has low Vickers hardness of all alloys while Ti8Nb2 and Ti7.5Nb2.5 alloys are harder due to high amount of Nb content. The EDS and Eltra techniques confirmed the chemical compositions and O contents. Generally, the densities of the Ti-Nb alloys increased with increasing Nb content.