Effect Of Hot Rolling Process On Microstructure And Properties Of Nickel Aluminum Bronze

Nickel Aluminum Bronze?NAB?is widely used for marine equipment due to the high mechanical properties and excellent corrosion resistance.However,on the one hand,defects such as shrinkage porosity,composition segregation,and inhomogeneous microstructure in the cast alloys are harmful to the mechanical and corrosion properties of NAB alloys.On the other hand,both the maritime silk road construction and the national marine strategy put forward very high requirements for the corrosion resistance of NAB alloys.Hot deformation is one of the important plastic deformation process to eliminate casting defects and homogenize microstructure,which significantly improves the properties of NAB alloy.In this thesis,as-cast nickel aluminum bronze?Cu-9Al-10Ni-4Fe-1.2Mn,all in wt.%?were annealed and then hot-rolled.Different hot deformation degree was utilized.The effects of annealing,hot rolling and different deformation degree on the microstructures and properties of nickel aluminum bronze were studied by X-ray fluorescence,optical microscope,scanning electron microscope,transmission electron microscope,electron probe microscopic analyzer and X-ray diffraction,respectively,to reveal the influence mechanism.The work aims to develop the nickel aluminum bronze with good comprehensive properties.The microstructures and properties of the as-cast,as-annealed and hot-rolled NAB alloys were studied.It was found that the microstructure of the as-cast NAB alloy contains?phase,?_III phase and some coarse NiAl strips.The as-cast NAB alloy exhibits the lowest strength and hardness,while the highest elongation.Annealing introduced a large number of lath-shaped?_V phases to precipitate in the?phase,and the size of the eutectoid?_III increased,which results in the increase in the strength and hardness while the decrease in elongation through the second phase strengthening.The interfaces between the uncorroded?phases and their surrounding corrosion products increased,the chloride ions can easily enter along these interfaces,resulting in the severe damage of the film.However,after further hot rolling,the original continuous?phases(lamellar?_III phase and lath-shaped?_V phase)were spheroidized,dispersed and decreased in content;dislocations and twins were also formed.Compared to the as-annealed alloy,further hot rolling simultaneously increased the strength,hardness and elongation of the NAB alloy through deformation strengthening and the second phase strengthening,where the yield strength was increased by⁸4%.The interfaces between the uncorroded?phases and the surrounding corrosion products became discontinuous caused by the spheroidization of?phases,reducing the corrosion of the substrate by the chloride ions via these interfaces.As a result,the corrosion resistance of the hot-rolled NAB sample was greatly improved.Therefore,hot-rolled NAB alloy exhibits an optimum combination of mechanical properties and corrosion resistance.The ultimate tensile strength,yield strength and elongation are 883 MPa,659 MPa and 15.2%,respectively.The mass loss of hot-rolled NAB alloy is 1.86 g/m² after immersion in the 3.5 wt.%NaCl solution for 28 days.The microstructure and corrosion resistance of hot-rolled NAB alloys with different deformation degree were studied.It was found that the increase in the deformation degree did not change the type of phases,but changed the morphology and content of phases in the NAB alloys.The microstructure of the NAB alloys contain?phase,?_III phase and?_V phase.When the deformation reached 40%,most of the?_III phase and?_V phase in the alloy were broken and spheroidized,whereas there are still a small number of?_III phase and?_V phase that retain the original morphology?lamellar and lath-shaped?,and the?_V phases were densely distributed in the?phase.As the deformation increased to 85%,the?phase became slender along the rolling direction,and the spheroidization degree of?_III phase and?_V phase increased,with a redeaced number of?_V phase.In addition,the corrosion resistance of the NAB alloy in 3.5 wt.%NaCl solution was improved with the increase of deformation degree.The mass loss of NAB alloy was decreased from 2.61 g/m²?40%?to 1.76 g/m²?85%?after immersion in the 3.5 wt.%NaCl solution for 28 days,the corrosion resistance of NAB alloy was improved.This is because the spheroidization degree of?phases(?_III and?_V phases)in NAB alloy increased with the increase of deformation degree,which made the morphology of?phase become discontinuous.In addition,the content of?phases was also reduced,so that the interfaces between the uncorroded?phases and the surrounding corrosion products was reduced and became discontinuous,reducing the corrosion of the substrate by the chloride ions via these interfaces.As a result,the corrosion resistance of the NAB alloys was improved.