Reduction in anisotropy of mechanical properties of coilable (α+β)-type titanium alloy thin sheet through simple heat treatment for use in next-generation aircraft applications

In order to reduce the anisotropy of mechanical properties of a coilable thin sheet of (α+β)-type titanium alloy, Ti-4.5Al-2Mo-1.6V-0.5Fe-0.3Si-0.03C (Ti9), for use in aircraft applications, duplex heat treatments were examined. In each duplex heat treatment, the first heat treatment controls the volume fraction of the primary α phase and orientation of the acicular α phase in (α+β) two-phase area between the primary α grains, whereas the second heat treatment stabilizes the α and β phases. The microstructure of the Ti9 sheet after the duplex heat treatment was analyzed by optical microscopy, pole-figure measurement through X-ray diffraction, and electron backscatter diffraction. The mechanical properties of the duplex heat-treated Ti9 sheet were evaluated by tensile tests.The Ti9 sheet was heat treated to obtain two different types of microstructures. A microstructure consisting of acicular α phase in the β grains was obtained by a first heat treatment above the β transus (1273. K) followed by water quenching and a second heat treatment at 973. K. A microstructure consisting of equiaxed primary α grains and an (acicular α+β) two-phase area between the primary α grains was obtained by heating below the β transus (1123-1223. K) followed by water quenching and a second heat treatment at 973. K. The volume fraction of the primary α grains decreased and the volume fraction of the acicular α phase with 12 variants increased instead of increasing first heat-treatment temperature, suppressing the alignment of the c axis of the α lattice parallel to the transverse direction within the rolling plane (T-texture formation) as a result. Anisotropy of the tensile properties can be decreased by increasing the first heat-treatment temperature because the T texture was decreased.