Study On Preparation And Damage Behavior Of Al-based Amorphous Coating

Al-based amorphous alloys,discovered in the 1960s as a kind of low-density material,have a higher specific strength and possess long-range disorder and short-range order structures.They exhibit high hardness and excellent corrosion and wear resistance due to no grain boundaries and dislocations,which has attracted wide attention from many scholars at home and abroad.For most Al-based amorphous alloys,kinds of rare earth elements were necessary for high glass formation ability(GFA),which increased the cost and restricted the engineering application.Based on the simple Al Ni Ti ternary system,this paper has completed the systematic research work on material simulation calculation,composition design and optimization,coating preparation and performance characterization.The main work is shown as follows.Different kinds of compositions(Al_90-xNi₁₀Ti_x,x=0,3,6,9 and 12)based on Al Ni Ti ternary system were calculated by ab initio molecular dynamics(AIMD).Al₈₁Ni₁₀Ti₉ alloy possessed the high fitted glass transition temperature,580?,which was based on the change of cell volume with temperatrue.The diffusion rate of atoms decreased with the increase of Ti content.This trend conformed to the principle that the larger the atomic mass,the slower the diffusion rate.Thus,it showed that the change of Ti content did not cause the coordinated atom movement.Changes in Ti content caused the first peak of the pair distribution function to form a left shoulder peak to varying degrees.The main reason for this was that changes in Ti content caused changes in the position and intensity of the peaks of the partial pair distribution functions for Ni-Al and Ti-Al.Analysis of the coordination number and chemical short-range order showed that the Al-Ti affinity was higher than the Al-Ni affinity.This showed that the Al₃Ti phase precipitated easier than the Al₃Ni phase during the crystallization process.The statistics of the different atom-centred Voronoi polyhedra(VPs)showed that the Al-centred VPs had a small proportion of<0,0,12,0>icosahedra,which are favourable for increasing the amorphous formation ability,and were mostly deformed icosahedra.The Ni-centred VPs had no icosahedral structure.The Ti-centred VPs had a higher proportion of<0,0,12,0>.Nevertheless,due to the limited Ti atom content,it did not significantly favour the glass formation ability.Five symmetry analyses of the alloy showed that the tendency to glass was higher at Ti contents of 9 and 12 at.%.An overall analysis of the effect of Ti content on the glass formation ability showed that Al₇₈Ni₁₀Ti₁₂ had a better GFA in terms of kinetic and compositional undulations,and Al₈₁Ni₁₀Ti₉ was considered to have a better GFA in terms of atomic local structural stability.To address the problem that T_g of Al-based amorphous alloys,especially those without rare earth elements,cannot be measured,an experimental method was designed to evaluate the glass formation ability of different component alloys:ribbons with different compositions were prepared under the same condition by the single-roller melt-spinning method;the amorphous content of the alloy was calculated based on the ribbons'XRD;higher amorphous content means better glass formation ability.The effect of Ti and Ni contents on the glass formation ability was summerized by experimental methods,which illustrated the reliability of the simulation results and the composition with the best glass fomation ability was Al₈₁Ni₁₀Ti₉.According to the DSC of the Al₈₁Ni₁₀Ti₉ amorphous ribbon,ribbons were annealed at different temperatures and the precipitated phases under different annealing processes were determined according to the XRD characterisation.It was found that the maximum hardness of the annealed ribbon reached up to 625 HV₁₀₀ due to the hardening effect of the?-Al nanocrystals on the alloy.Based on the optimized composition,the powder of Al₈₁Ni₁₀Ti₉ was prepared by the vacuum atomisation method.There was no amorphous phased in the powder due to the low glass formation ability of the Al-based alloy and the low cooling rate during the atomized process.Three kinds of spraying technology were used to generate Al-based amorphous coatings which were plasma spraying(PS),flame spraying(FS)and high velocity air flame spraying(HVAF).The results showed that an amorphous phase was formed in the coating due to the powder remelted and then rapid cooled down during the plasma spraying process and the amorphous content was up to 49.76%;meanwhile,the high flame flow velocity during the spraying process ensured the coating density and the porosity is 3.1%,slightly higher than that of HVAF sprayed coatings and significantly lower than that of FS sprayed coatings;the PS sprayed coating possessed the highest hardness of 422 HV₁₀₀ due to the dese structure and high amorphous content.Friction tests on 6061 Al alloy,PS sprayed coating,FS sprayed coating and HVAF sprayed coating showed that the PS sprayed coating had the highest wear resistance and the wear rate was 1/4 of that of 6061 Al alloy.The wear mechanism is mainly delamination accompanied with oxidation wear.A kind of vacuum sealing process was used to improve the Al-based coating's corrision resistance:epoxy resin was used to fill the pores to delay the corrosion caused by porosity.From potential dynamic polarization curves of 6061 Al alloy,sealed coating and unsealed hole coating,it was found that the corrosion current densities were from top to bottom 6061 Al alloy,unsealed coating and sealed coating;corrosion potential were from top to bottom sealed coating,unsealed coating and 6061 Al alloy.This indicated that the amorphous alloy material itself had higher corrosion resistance,due to the homogeneity of the amorphous phase and the high passivation activity of the nanocrystals.Meanwhile,that the coating treated by the sealing process reduced the exposure to corrosive media around the defects,further enhancing the corrosion resistance.Comparing the impedance spectras of the samples of 6061 Al alloy,sealed and unsealed coatings with different immersion corrosion times,it can be seen that the impedance of all samples first decreased rapidly and then plateaued as the erosion time increased.The initial impedance of the sealed coating was significantly higher than that of the 6061 Al alloy,indicating that the sealing process improved the initial corrosion resistance of the coating,but due to the limited sealing depth,the effect of the sealing treatment diminished under long-term erosion conditions.Based on the salt spray corrosion results,it can be more intuitively illustrated that the sealed coating has superior corrosion resistance than 6061 Al alloy under short time corrosion conditions(within 200 h).Combined with SEM morphological analysis and XPS corrosion product analysis it can be found that:as crystal phases with a lower corrosion potential easily concentrated near the pores in the coating,so they was more prone to corrosion,mainly in the form of pitting;the defect-free areas in the coating,due to the homogeneity of the amorphous phase and the high activity of nanocrystals,possessed relatively high corrosion resistance,and their corrosion forms were mostly based on cracking;Ti in the coating can diffuse through Al³⁺vacancies into the outer layer of the coating during corrosion,improving the stability of the passivation film and participating in the corrosion reaction.