Herringbone Structure and Significantly Enhanced Hardness in W-Modified Ti₂AlNb Alloys by Spark Plasma Sintering

The W-modified Ti₂AlNb-based alloys synthesized at 1100 °C by spark plasma sintering were solution treated at 1300 °C for 4 h and then aged at 800–1000 °C for 1 h. The phase composition, microstructure evolution, and microhardness of the aged alloys are investigated in this study. A significant enhancement of hardness, ~ 750 HV, is obtained in the alloy aged at 900 °C, while that of the one without W addition is only ~ 470 HV. The hardness is also higher than that of common β-Ti and Ti–6Al–4V alloys. As the ageing temperature increases, the B2/O structure evolves from B2 + O colonies to Widmannstätten structure, followed by a “disordering to ordering” procedure. This process also involves the variation of the angle between adjacent O phase from 90° to 60°. Specifically, a herringbone Widmannstätten B2 + O structure is constructed in the alloys aged in the α₂ + B2 + O phase region, which is related to the diffusion of W and the substitution of W for Nb in the lattice of B2 or O.