Simultaneously enhancing strength and ductility of Ti-6Al-4V alloy with the hierarchical structure via a novel thermal annealing treatment

A novel solution and aging (STA) heat treatment was proposed to improve the strength and ductility of Ti-6Al-4V alloy simultaneously. Effects of the novel heat treatment on microstructure and mechanical properties of the Ti-6Al-4V alloy prepared by spark plasma sintering (SPS) were studied in this work. The crystalline structure and microstructure of both the as-fabricated Ti-6Al-4V alloy and the STA Ti-6Al-4V alloy were observed using X-ray diffraction (XRD) technique, optical microscope (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), respectively. The XRD patterns indicated that the two Ti-6Al-4V samples have the similar crystalline structure with α phase and β phase. However, the results of OM images suggested that the microstructure of Ti-6Al-4V alloy was transformed from the Widmanstätten structure to a hierarchical structure which included the basket-weave structure and the Widmanstätten structure after the STA heat treatment. Apart from the above-mentioned microstructure, the hierarchical structure also exhibited specifically some fine dispersed β surrounded by some equiaxed α. Moreover, the results of the room temperature tensile tests revealed that the STA heat treatment could improve the strength and ductility of the Ti-6Al-4V alloy simultaneously. Based on the observation of TEM images, the increment of the elongation of the STA Ti-6Al-4V alloy can be attributed to the effect of annealing twins and equiaxed α, and the total strength increments should be caused by the dispersion strengthening effect of the dispersed β.