Reactive spark plasma sintering (SPS) of nitride reinforced titanium alloy composites

Coupled in situ alloying and nitridation of titanium-vanadium alloys, has been achieved by introducing reactive nitrogen gas during the spark plasma sintering (SPS) of blended titanium and vanadium elemental powders, leading to a new class of nitride reinforced titanium alloy composites. The resulting microstructure includes precipitates of the δ-TiN phase with the NaCl structure, equiaxed (or globular) precipitates of a nitrogen enriched hcp α(Ti,N) phase with a c/a ratio more than what is expected for pure hcp Ti, and fine scale plate-shaped precipitates of hcp α-Ti, distributed within a bcc β matrix. During SPS processing, the δ-TiN phase appears to form at a temperature of 1400 °C, while only hcp α(Ti,N) and α-Ti phases form at lower processing temperatures. Consequently, the highest microhardness is exhibited by the composite processed at 1400 °C while those processed at 1300 °C or below exhibit lower values. Processing at temperatures below 1300 °C, resulted in an incomplete alloying of the blend of titanium and vanadium powders. These δ-TiN precipitates act as heterogeneous nucleation sites for the α(Ti,N) precipitates that appear to engulf and exhibit an orientation relationship with the nitride phase at the center. Furthermore, fine scale α-Ti plates are precipitated within the nitride precipitates, presumably resulting from the retrograde solubility of nitrogen in titanium.