Effect of the ultrasonic surface rolling process and plasma electrolytic oxidation on the hot salt corrosion fatigue behavior of TC11 alloy

Abstract

The effects of the ultrasonic surface rolling process (USRP), plasma electrolytic oxidation (PEO), and their combined treatment (USRP + PEO) on the hot salt corrosion fatigue (HSCF) behavior of TC11 alloy are comparatively studied. The mechanism of improving the HSCF performance of TC11 alloy through the above surface strengthening techniques is revealed by analyzing the surface integrity and microstructure of the modified layer. The results show that the TC11 alloy exhibits high HSCF sensitivity, and fatigue limit reduces by 27.78 % under the condition of 0.1 mg/cm2 solid NaCl deposition at 500 °C compared to the sample without salt deposition. The mechanism of HSCF is that corrosion pits promote the initiation of fatigue cracks by causing local stress concentration on the sample’s surface. Meanwhile, hydrogen and hydride generated by hot salt corrosion also promote fatigue crack propagation. Compared with the base material (BM) sample under the condition of 0.1 mg/cm2 NaCl deposition, the HSCF limit increases by 10.26 % by USRP treatment. This is due to the combination of an increase in dislocation density, a decrease of β-phase content, and the introduction of compressive residual stress (CRS), improving the high-temperature fatigue resistance, corrosion resistance, and hydrogen damage resistance of TC11 alloy. PEO treatment increases HSCF resistance of TC11 alloy, and the HSCF limit increases by 47.44 % compared with the BM sample. This is mainly because the dense barrier layer (DBL) could effectively inhibit erosion by the chloride medium and inhibit hydrogen damage, which significantly improve the crack initiation resistance. The HSCF limit of the TC11 alloy further increases by 64.10 % by the USRP + PEO treatment, and the effect of improving the HSCF resistance is better than that of USRP or PEO treatment alone. This is attributed to the fact that USRP pretreatment increases the alloy’s erosion resistance to a corrosive medium by increasing the thickness of the DBL. Meanwhile, the synergistic effect of stable CRS and gradient nanostructure introduced by USRP significantly increases the crack initiation and propagation resistance of HSCF for TC11 alloys.