Microstructure Evolution And Mechanical Properties Of(Fe,Cr)₇C₃/Fe Surface Gradient Composite Produced By In Situ

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.Its reaction process,microstructure and phase composition were studied by the testing technologies of XRD,SEM,TEM and EDS.And the mechanism of Fe-Cr-C system in-situ reaction was discussed through classical kinetics theory.The hardness and elastic modulus of(Fe,Cr)₇C₃/Fe surface gradient composites were measured by the methods of nano-indentation.The relationship between surface hardness and elastic modulus of(Fe,Cr)₇C₃dense ceramic layer was discussed through the load-depth curve.The binding ability of(Fe,Cr)₇C₃dense ceramic layer and particles dispersion layer was discussed by the automatic scratch apparatus,then the critical load L_cwas determined.Through the observation of the scratch morphology,the elastic-plastic deformation mechanism and failure mechanism was analyzed.Experimental results show that:(1)The(Fe,Cr)₇C₃/Fe surface gradient composites was produced by casting at 1400?and heat treatment at 1000?with chrome board and gray cast iron as raw materials.The volume fraction of(Fe,Cr)₇C₃ ceramic decreases along the surface to matrix,and the grain size increases gradually.According to the characteristics of microstructure,the composite can be divided into dense ceramic layer,particles dispersion layer and less graphite area.(2)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 kinetics study of the Fe-Cr-C ternary system.(3)Nanoindentation measured the(Fe,Cr)₇C₃ dense ceramic layer the best fit load in the range of 100?300 m N,hardness reached 7.9 GPa and modulus of 240.7 GPa.With the load increases,the hardness and elastic modulus decreases at first and then increased.(4)Three change processes:micro cracks-cracks federation-broken flake,by combined the acoustic emission curve and the(Fe,Cr)₇C₃ dense ceramic layer scratch microstructure.Comprehensive assessment the binding properties of(Fe,Cr)₇C₃ dense ceramic layer,and analyzed the single stroke scratches failure mechanism.Conclueding the main failure mode of(Fe,Cr)₇C₃dense ceramic layer is cracking and spalling of the ceramic layer.