Tailoring variant selection and corrosion behavior of electron beam welded metastable β-titanium alloys by ω-assisted α phase refinement via two-step aging

This work designs a two-step aging treatment for the 20 mm-thick electron beam welded Ti-35421 (Ti-3Al-5Mo-4Cr-2Zr-1Fe) joint, and focuses on studying the intermediate phase transformation and α variant selection evolutions induced by different heating rates via interrupted heating experiments and advanced characterization method of high-energy synchrotron radiation X-ray diffraction. In the as-welded state, the fusion zone consists of coarse β columnar grains and nano-scale athermal ω precipitates. The two-step aging successively at 350 °C and 550 °C promotes the dissolution of ω phases and the refinement of the α precipitates, yielding a uniform distribution within the β matrix. During the low-temperature aging, nano-scale isothermal ω phases precipitate, and such ω/β interfaces may provide favorable nucleation sites for nano-scale needle-like α precipitates. The heating rate between the two stages has a significant impact on the final microstructures, that is, a slower rate (5 °C/min) leads to strip-like vertical α variants, and finer α precipitates, whereas rapid heating results in longer as well as more aligned α precipitates. The optimally double-aged joint achieves superior tensile strength of 1078.64 MPa and elongation of 5.26% (higher than those of as-welded joint by 14.8% and 48.2%, respectively), the lowest corrosion current density of 74.69 nA/cm 2 , and the highest polarization resistance of 548.55 kΩ·cm 2 , attributed to its refined and homogeneous microstructure with α vertical variants.