| Nowadays,steel still occupied dominant position in the metal materials,due to its excellent performance and low price.However,the corrosion of steel results pretty loss,and it is very important to improve the corrosion resistance of steel.For a long time,hot dip zinc is significant method to protect steel as it is low price as well as zinc coating has very nice anti-corrosion.As we know,zinc will be interacted with other elements,and the new phases will be formed between coating and steel interface when steel is immersed in zinc bath.The microstructures evolution of these new phases have great influence on the physical properties of the zinc coating,especially,the steel has been exposed in external environment,and which often need to be friction or extruded,the interdiffusion and mechanical properties of zinc-based alloys are great important for materials design and service life for those novel anti-corrosion materials.Therefore,the motivation of this thesis is to investigate the diffusion characteristics and mechanical properties of Zn-Zr,Zn-Cr and Zn-Co systems.The method of solid to solid diffusion couples was used to obtain intermetallic compounds.Metallurgical microscopes and electron microscopes were used to observe the morphology and composition of the intermetallic compounds.In addition,nanoindentation was used to test the hardness and elastic modulus of the intermetallic compounds of the three binary systems.Through the above methods,this thesis obtains the following conclusions:(1)In the Zn-Zr diffusion couples,three intermetallic compounds(Zn3Zr,Zn39Zr5,Zn22Zr)have been determined through EPMA except Zn2Zr and Zn Zr.Through the fitting analysis of thickness of diffusion layer,it is found that the thickness of the two intermetallic compounds(Zn39Zr5,Zn22Zr)conform to the parabolic growth law.In addition,the diffusion rate of Zn39Zr5phase is the fastest at 325??350?and the thickness of the diffusion layer is the widest,the interdiffusion coefficient is 7.35×10-15m2/s,the diffusion rate of Zn22Zr phase is the fastest at 375??400?and the diffusion layer thickness is the widest,the diffusion coefficient is 6.82×10-14m2/s.The diffusion coefficient of the Zn-Zr system intermetallic compounds is fitted using the Arrhenius method,and the activation energy of Zn22Zr is the largest,which is 239.46 k J/mol.(2)The diffusion layer of the Zn-Zr diffusion couple was tested and analyzed with a nanoindentation tester.The results showed that the hardness of Zn3Zr,Zn39Zr5and Zn22Zr were 8.24±0.25 GPa,5.48±0.47 GPa and 2.37±0.62GPa,respectively as well as elastic modulus are 149.23±6.20 GPa,150.68±2.41GPa and 183.65±6.26 GPa,respectively.The hardness of Zn-Zr intermetallic compounds decrease with increasing Zn content.(3)In the Zn-Cr diffusion couples,two intermetallic compounds formed in the diffusion layer that is Zn13Cr and Zn17Cr.Due to the large diffusion activation energy and growth activation energy required for Zn13Cr,the growth of Zn13Cr is extremely slow,and it is not found that the intermetallic compound exhibits a parabolic growth over time.The pydiffusion code has been used to calculate the interdiffusion coefficients of Zn17Cr,Zn13Cr and Zn(hcp).The calculated results showed that the interdiffusion coefficients of Zn17Cr,Zn13Cr and Zn(hcp)increased with increasing annealing temperature.At 325?,350?,375?,and 400?,the interdiffusion coefficients of Zn13Cr,Zn17Cr and Zn(hcp)are 1.12×10-19m2/s,1.11×10-15m2/s,6.40×10-16m2/s,5.02×10-15m2/s;4.44×10-16m2/s,5.69×10-15m2/s,2.27×10-14m2/s,2.47×10-13m2/s;9.33×10-17m2/s,8.49×10-17m2/s,2.93×10-16m2/s,1.78×10-15m2/s,respectively.(4)The hardness of the Zn17Cr phase in the Zn-Cr diffusion couples is 3.01±0.11 GPa,and the elastic modulus is 165.78±6.68 GPa.Zn13Cr could not be tested due to its thin diffusion layer.(5)For the Zn-Co binary system,three intermetallic compounds were found as?2(Zn13Co),?1(Zn53Co7)and?(Zn21Co5).EPMA results showed that three intermetallic compounds are not linear compounds,the composition range of?2phase is 91.1 at.%?93.7 at.%,the composition range of?1phase is 88.5at.%?90.9 at.%and the composition range of?phase is 74.9 at.%?87.9 at.%.The diffusion coefficients calculated for?2,?1+?and the overall diffusion layer at 325?,350?,375?and 400?are:2.67×10-14m2/s,6.13×10-14m2/s,7.25×10-14m2/s,3.26×10-13m2/s;8.33×10-16m2/s,2.22×10-15m2/s,9.07×10-15m2/s,1.09×10-14m2/s;3.70×10-14m2/s,8.68×10-14m2/s,1.34×10-13m2/s,4.56×10-13m2/s,respectively?(6)Nanoindentation was used to obtain the mechanical properties of Zn-Co intermetallic compounds.The hardness of?2(Zn13Co)is 3.92±0.19 GPa,and the elastic modulus is 185.13±7.92 GPa. |
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