Investigation On Corrosion Fatigue Behavior Of TA15 Alloy In 3.5%NaCl Salt Spray

TA15 alloy is a near-αtitanium alloy of high aluminum equivalency. It is an excellent candidate for aerospace applications owing to its good comprehensive mechanical properties and processing properties such as forging, heat treatment and welding. Since the atmospheric environment contain a lot of salt spray, all sorts of aircraft structure are inevitable in salt spray corrosion environment. So it has great practical significance to study the corrosion fatigue properties of TA15 alloy in salt spray.In this paper, the corrosion fatigue properties of TA15 alloy in 3.5% NaCl salt spray were investigated at different stress ratio (R=-1, R=0.06 and R=0.5). It is shown that the cyclic deformation characteristic of TA15 alloy in the salt spray was dependent on the applied stress ratio R. It is showing cyclic softening feature in the condition of R=-1. When the stress ratio R=0.06, it is exhibiting cyclic stabilization phenomenin. While in the condition ofR=0.5, it is expressing cyclic hardening characteristic. On the basis of statistical analysis for experimental results, we obtained the equations describing the S-N curves and p-S-N curves at different stress ratio. In salt spray condition, the influence of average stress to corrosion fatigue life could be regarded as Soderberg equation. The Sm-Sa-N curved surface and the p-Sm-Sa-N curved surface with different survival rate describing the relationship among average stress, stress amplitude and fatigue life were established.Meso-mechanical properties for two different metallographic structure of TA15 alloy before and after corrosion fatigue was studied by "Depth-Sensing Indentation (DSI)" technique. On the basis of experimental results for original specimen, we learned that the micro-hardness of a global structure is higher than theβlamellar structure. By contrast the data before and after corrosion fatigue, we known that the plasticity and strength ofαglobal structure is higher than theβlamellar structure. And the toughness ofβlamellar structure is higher than the a global structure. The meso-mechanical parameters forαglobal structure were damaged obviously. Theβlamellar structure has more ability to resist corrosion fatigue thanαglobal structure.SEM observations of the fracture surface indicate that the corrosion fatigue fracture of TA15 alloy appears multi-fatigue crack initiations characterized mainly by the cleavage morphology. In the crack growth zoon, the fracture morphology shows a character of the dependent change of deformation resistance on the stress ratio.