Effect Of Hot Treatment On Microstructure And Corrosion Resistance Of NAB Alloy

Nickel aluminum bronze alloys have good mechanical properties and corrosion resistance,and are widely used in marine engineering,aerospace and other fields.The casting defects such as shrinkage cavity,dendritic segregation and composition segregation in cast microstructure of nickel aluminum bronze alloys can be eliminated by hot working.However,alloys in hot working state have various complex phase compositions,and selective corrosion occurs due to the difference in electric potential of each phase.In this thesis,the corrosion behavior of hot worked nickel aluminum bronze alloys was studied,and the corrosion behavior of the alloys was simulated and verified using the first-principles calculations based on density functional theory.According to the corrosion behavior,different heat treatments were used to regulate the phase composition of the alloys,and the structure-activity relationship between the microstructure and corrosion resistance was established.The macroscopic performance was evaluated using electrochemical tests and corrosion weight loss tests,and the microstructure was characterized using SEM and XRD.The main research results are summarized as follows:（1）The corrosion behavior of heat-processed nickel aluminum bronze alloys mainly includes two processes:selective corrosion and pitting corrosion.Lamellarκ_iii phase andκ_iphase with high Fe and low Al content have a negative impact on the corrosion behavior of the alloys.In the neutral solution containing chlorine,a layer of corrosion product film was formed on the surface of the alloy which becomes denser with the increase of corrosion time,which could reduce the corrosion rate of the alloy.（2）Based on the first-principles calculations,the atomic structure models ofαphase andκphase were constructed,and the work function and surface energy of different crystal planes were calculated by using the plate model.It was found that the work function ofαphase was higher than that ofκphase,which leads to phase selective corrosion of nickel aluminum bronze alloys.Atomic force microscopy was used to measure surface potentials in different phases,further verifying the accuracy of first-principles calculations.（3）The microstructure of the alloy was changed by heat treatment to obtain the annealed,normalized,quenched,quenched 450℃aging state and quenched 550℃aging state microstructure.The phase composition of the alloy varied greatly under different heat treatment states.The extruded state,annealed state and normal state containing"α+κ_iii"eutectoid structure orβ′phase reduced the overall corrosion performance,and its corrosion performance was poor due to the local corrosion caused by the pitting corrosion of containing moreκ_i phase.When quenching at 550℃,the pitting corrosion was the most serious,and more holes were formed to corrode the tissues around the holes,resulting in the worst corrosion resistance;quenched 450℃aging state and quenched state of local corrosion and overall corrosion were slight,so the corrosion performance was the best.