Effect of aging temperature on microstructure and mechanical properties of a novel Ti-6121 alloy

The effect of heat treatment on the microstructure and mechanical properties of a novel α + β Ti-6.2Al-0.8V-2.25Mo-1.2Cr-0.5Si (wt%) alloy was investigated in this paper. Microstructure and crystalline structure of the alloys after different solution and aging treatments were identified by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated that a multi-scale microstructure was possessed in the heat-treated samples, which was composed of equiaxed primary α (α_p) phase and nano-scale acicular secondary α (α_s). After 900 ℃ solution for 1 h and 550 ℃ aging for 6 h, the alloy exhibited the optimal combination of strength and ductility (the ultimate tensile strength of ∼1387 MPa, the elongation of ∼11 %). Furthermore, the combination mechanism of strength-ductility of the sample under this condition was investigated. The samples before and after tensile fracture were analyzed using transmission electron microscope (TEM) and electron back-scattered diffraction (EBSD). The results showed that a large proportion of nano-scale acicular α_s phases precipitated from the β matrix can effectively prevent the slip of dislocations and enhance the strength of the alloy. In addition, the multi-scale microstructure can create the strain partition, which can mediate the strain incompatibility between the α_p and the transformed β (β_T) to obtain an excellent combination of strength and ductility.