Study On Preparation And Mechanicial Properties Of In Situ Titanium Carbide TiC Reinforced Iron Matrix Composites

In the present work,to enhance the strength of gray cast iron matrix,the titanium carbide reinforced-iron matrix composites were prepared via an the wire composite-and-in situ reaction process,using gray iron and high-purity titanium wire as raw materials.The phase composition,chemical composition,microstructure and crystallographic structure of the titanium carbide reaction layer and the titanium carbide particles were characterized by the advanced X ray diffraction(XRD),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),transmission electron microscopy(TEM)and electron backscatter diffraction(EBSD)technologies.Moreover,in order to determine the phase of possible appearance and the sequence of their stability,the thermodynamics of Ti-C-Fe ternary system were calculated.The growth rate constant and activation energy of the formed titanium carbide layer were investigated by means of the classical kinetics theory.The hardness,elastic modulus and fracture toughness of the titanium carbide layer were obtained by measurements,and the fracture behavior also was analyzed.The obtained results are shown as following:(1)The titanium carbide reinforced-iron matrix composites were fabricated by the wire composite-and-in situ reaction process.At the same temperature,with the increasing reaction time,the thickness of TiC layer is increased gradually.The region near titanium wire has iron content less than that of the region close to the gray cast iron.The Fe accumulates in the grain boundary of titanium carbide particles.By analyzing the surface of sample that has been corroded by using the concentrated hydrochloric acid with a long time,the result indicates that the corrosion resistance of the region near the gray cast iron in the titanium carbide layer is smaller than the region near titanium wire due to the low density of TiC particles.The TiC particles that have fell off from the reaction layer are mostly agglomerated and their present the spherical or quadrangular morphology.Through EBSD detection,TiC phase,FeTi phase and Fe₂Ti phase are found,and these phases distribute in the whole titanium carbide reaction layer.(2)The thermodynamic calculation result shows that the Gibbs free energy of reaction of TiC,Fe₂Ti,FeTi and Fe₃C phases are all below 0 at 1323-1423K.Therefore,these four phases are all thermodynamically stable.Under the premise of stable existence,the smaller Gibbs free energy corresponds to the higher stability.Therefore,the sequence of stability of several phases is TiC,Fe₂Ti,FeTi,Fe₃C.(3)The thickness of the reaction layer was analyzed at the temperature of 1323K,1373K and 1423K for 10min,30min,60min,90min and 120min,respectively.It is found that the growth of the titanium carbide layer obeys the classical kinetics theory d²=Kt.By plotting and fitting the data of titanium carbide layer thickness d and in situ reaction time t,the growth rate constants at 1323K,1373K,and 1423K are caculated to be 6.26×10^-9cm²/s,1.46×10^-8cm²/s,6.5×10^-8cm²/s.The diffusion activation energy(358.27kJ/mol)of the formed layer could be obtained by further calculation.(4)In the composites,the hardness of the titanium carbide reaction layer increases with the decreasing load value,which conforms to the normal indentation size effect.With regard to the reaction layer,it has the highest hardness value of about 30.98 GPa,which is seven times bigger than that of the grey cast iron matrix.Moreover,the elastic modulus of the reaction layer gets a value of 464.71 GPa.At the applied loads of 100gf,200gf,300gf,500gf,1000gf,the cracks,appearing at the surface of the reaction layer,are determined to be median crack system.Based on the crack type and appropriate equation,the calculated fracture toughness is 0.74-1.71MPa·m^1/2.In this paper,the fracture behavior of the reaction layer mainly includes the crack deflection,crack bridging,crack branching and crack pinning.