Stress Corrosion Behavior of Near-β Titanium Alloy Ti-3Al-5Mo-4Nb-4Cr-2Zr with different phase compositions

Abstract:

Near-β titanium alloys for marine engineering applications may be suffered from stress corrosion cracking. The phase composition has a significant impact on stress corrosion behavior. Different phase compositions of a near-β titanium alloy, Ti-3Al-5Mo-4Nb-4Cr-2Zr, were obtained by undergoing various heat treatments. The microstructures were characterized by scanning electron microscope and X-ray diffraction. The electrochemical measurements and slow strain rate testing were carried out to investigate the effect of phase composition on stress corrosion behavior. The results indicate that the single β phase is obtained by solution treatment at β phase field (Pβ). After aged at 500℃ for 6h, fine secondary α phase with narrow spacing precipitate within β matrix (Pβ+fα). After aged at 650℃ for 6h, coarsened secondary α phase with wide spacing precipitate within β matrix (Pβ+cα). For the Pβ+cα, the corrosion potential and elevation angle of capacitive impedance arc as well as impedance modulus at 0.01Hz are relatively minimum, and the corrosion current density is relatively maximum. The Pβ+cα exhibits relatively poor corrosion resistance. The relatively better order is the Pβ+fα and Pβ. The stress corrosion susceptibility index order from high to low is Pβ+cα、Pβ+fα、Pβ. The fracture morphology exhibits a mixed characteristic with shallow dimples and micro-cracks as well as tear ridges and flat facets. The combination of Absorption Induced Dislocation Emission (AIDE) and Hydrogen Enhanced Localized Plasticity (HELP) is the main mechanism for stress corrosion cracking. The decrease of secondary α phase spacing has a beneficial effect on reducing the stress corrosion susceptibility.