Microstructure Evolution And Wear Resistance Of ?Fe,Cr?₇C₃/Fe Surface Gradient Composite Produced By In Situ

Particulate-reinforced metal matrix composites have been widely used in various applications,including aerospace,metallurgy,building materials,electric power,hydropower,mining and others due to their high specific strength,high wear resistance,and high temperature stability.At present,the fabrication of metal matrix composites?MMCs?with in-situ synthesized particulate reinforcements has gained a growing scientific attention.In this work,the?Fe,Cr?₇C₃/Fe surface gradient composites were fabricated by a new method,combining casting with in situ reaction process,using pure chromium plate and gray cast iron.The in situ reaction process,microstructural evolution,thermodynamics,the reaction layer growth dynamics of the?Fe,Cr?₇C₃/Fe surface gradient composites under the two temperature systems of liquid-solid and solid-solid were investigated.The microstructural evolution,fracture toughness and fracture mechanisms,friction and wear performance and wear mechanisms of the bulk ceramic layer fabricated in the reaction process have also been analyzed.The experimental results showed that:?1?In the Fe-Cr-C ternary system,the Gibbs free energy of chromium carbide decreased with the increase of temperature.The calculated ?G was negative,and the stable Cr3C2,Cr23C6,and Cr₇C₃ phases could be formed at the studied temperature range.The stability of carbides can be placed in the order:Cr3C2>Cr23C6>Cr₇C₃.From the Fe-Cr-C liquidus surface projection,it can be deduced that the Cr3C2 phase could not be formed in this study.?2?The liquid-solid diffusion reaction process and the formation mechanisms of the different reaction zones in the Fe-Cr-C ternary system were also studied according to the microstructural evolution during the holding time of heat treatment.The growth rate constants and activation energies of the different reaction zones were achieved through the growth dynamics study of the Fe-Cr-C ternary system.The microhardness of the?Fe,Cr?₇C₃/Fe surface gradient composites presented a gradient distribution from the surface to the iron matrix.The microhardness value of the I area was the highest?1210 HV0.1?and about six times higher than that of the iron matrix?214 HV0.1?.?3?The solid-solid diffusion reaction process and the formation mechanisms of the different reaction zones in the Fe-Cr-C ternary system were also researched according to the microstructural evolution during the holding time of heat treatment.The growth rate constants and activation energies of the different reaction zones were achieved through the growth dynamics study of the Fe-Cr-C ternary system.The microhardness of the?Fe,Cr?₇C₃/Fe surface gradient composites presented a gradient distribution from the surface to the iron matrix.The microhardness value of the I area was the highest?1484 HV0.1?and nearly seven times higher than that of the iron matrix?214 HVo.i?.?4?The wear resistance and wear mechanisms of the?Fe,Cr?₇C₃/Fe surface gradient composites were analyzed through the two-body abrasive wear test.The relative wear resistance of the areas I,II,III,and IV of the sample fabricated by the liquid-solid process were found to be 6.96,5.30,5.10 and 1.86,respectively,whereas the relative wear resistance of the areas I,II,III,and IV of the sample fabricated by the solid-solid process were found to be 42.18,25.93,15.07 and 1.48,respectively.The relative wear resistance of the bulk ceramic layer was the highest,and its wear mechanism was microcracking and microcutting.The extension direction of microcracks was vertical to the wear direction.?5?The microstructural evolution of the bulk ceramic layer fabricated at 1050°C for different holding times were analyzed.It can be concluded that the thickness of the bulk ceramic layer first increased and then decreased with the increase of holding time,and finally disappeared with the disappearance of the chromium plate.The bulk ceramic changed from the amorphous to amorphous-nanocrystalline with the increase the of holding time.The average size of nanocrystalline is 100 nm.The fracture toughness of the bulk ceramic increased with the increase of holding time,and its average value was about 6.33 vPa·mii2.The fracture mechanism conformed to the bend toughening model.The friction coefficient decreased with the increase of load.That is,under the loads of 5N,ION,15N,and 20N,the friction coefficients were determined to be 0.81,0.475,0.452 and 0.435,respectively.The wear mechanisms were considered as abrasive wear,adhesive wear and fatigue wear.