Plasma electrolytic oxidation(PEO)is carried out on 6061 Al-alloys in a weak alkaline electrolyte containing NaOH,Na2SiO3 and NaCl.Centered on the correlation of composition and structure,analyses by means of X-ray diffration(XRD),scanning electron microscope(SEM)and energy dispersive spectrometry(EDS)are conducted on the specimens,which have been PEO-treated under hybrid voltages of different direct current(DC)values(140-280 V)with constant alternate current(AC)amplitude(200 V).Attention is paid to the composition,properties and growth mechanism of oxide layers formed with hybrid voltages.Moreover,the main effects of DC value are discussed.Ceramic layers with a double-layer structure which combines hard outer and soft inner layers are found to be consist ofα-Al2O3,γ-Al2O3 and mullite.With the DC values increasing,the growth of the ceramic layers tends to have increasingly obvious three-stage feature.

CoCrFeNi HEAs have better ductility, while the strength and corrosion resistance need to be further improved, while metal materials for deep-sea operations put forward the requirement of excellent mechanical properties and very high corrosion resistance; however, CoCrFeNi HEAs have been less studied for the trade-off between mechanical properties and corrosion resistance. Therefore, the present study utilized the laser melting deposition (LMD) technique to fabricate a series of (CoCrFeNi)Ti_x (x = 0.2, 0.4, 0.6, 0.8, 1.0 at.%) HEAs and systematically investigated the influence of Ti content on the alloy’s microstructure, phase composition, mechanical properties, and electrochemical performance. The research findings revealed that as the Ti content increased, the alloy gradually transformed from a single face-centered cubic (FCC) phase to an FCC and body-centered cubic (BCC) dual-phase structure. The addition of Ti induced a transition in the alloy’s microstructure from an equiaxed to a dendritic morphology, accompanied by grain refinement. Energy dispersive spectroscopy analysis confirmed the uniform distribution of Ti within the alloy. The hardness of the alloy increased significantly with the increase in Ti content, reaching 804.5 HV when the Ti content was 1.0 at.%, which was 4.13 times higher than the Ti-free alloy. The tensile and compression test results showed that the (CoCrFeNi)Ti_x alloy with a Ti content of 0.4 at.% exhibited the best overall mechanical performance. The electrochemical test results indicated that the addition of Ti effectively enhanced the corrosion resistance of the alloy, with the 0.4 at.% Ti-containing alloy exhibiting the optimal corrosion resistance. This study provides a strong theoretical and experimental foundation for the design of high-performance CoCrFeNi-based HEAs.