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

Abstract

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 (1273K) followed by water quenching and a second heat treatment at 973K. 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 (1123–1223K) followed by water quenching and a second heat treatment at 973K. 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.