Stress Corrosion Behavior of Near-β Titanium Alloy Ti-3Al-5Mo-4Nb-4Cr-2Zr with Different Phase Composition
Near-β titanium alloys for marine engineering may be suffered from stress corrosion cracking.The phase composition has a significant impact on stress corrosion behavior.Different phase composition of a near-β titanium alloy,Ti-3Al-5Mo-4Nb-4Cr-2Zr,was obtained by various heat treatments.The microstructures were characterized by scanning electron microscope and X-ray diffractometer.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 ageing at 500 ℃ for 6 h,fine secondary α phase with narrow spacing precipitates within β matrix(Pβ+fα).After aging at 650 ℃ for 6 h,coarsened secondary α phase with wide spacing precipitates 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 0.01 Hz 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+rα 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 in secondary α phase spacing has a beneficial effect on reducing the stress corrosion susceptibility.