Formation Mechanism And Properties Of Vc_x/Fe Based Surface Composites By In Situ Reaction Synthesis

In the modern industrial field,with the rapid development of science and technology,higher requirements are placed on the performance of various materials,and single-performance steel materials have been difficult to meet.The controllability of the structure and properties of surface-enhanced composites makes them widely used in aerospace,automotive and metallurgical fields,it is a very promising new composite material.In order to improve the strength of iron-based alloy surface,this paper uses high-purity vanadium plate and gray cast iron as raw materials,and uses the intrinsic reaction process to prepare VC_x/Fe based surface composite layer with sandwich structure.The in-situ reaction process,the phase composition,microstructure,growth kinetics,crystal structure and particle morphology of the iron-based surface vanadium carbide composite layer were studied.The growth mechanism of the composite layer was revealed.Furthermore,the hardness and fracture toughness of the formed vanadium carbide composite layer was evaluated.The results show that the heat-treatment temperature is 950°C,1000°C,1050°C respectively,and time for 1～5 h.The phase composition of reaction forms formed the vanadium carbide ceramic layer reinforced iron matrix surface composite layer is mainly V,V₈C₇,V₂C andα-Fe.The microstructure of the surface composite layer always shows a uniform and dense structure.With the increase of heat-treatment temperature and the holding time,the layer thickness of the vanadium carbide gradually increases,and the composite layer structure is composed of V₈C₇layer and V₂C layer from the iron matrix to vanadium plate.The three-dimensional microscopic morphology of the vanadium carbide powder extracted by the chemical solution is agglomerated spherical particles and a short rod-like shape.The heat-treatment temperature of the in-situ reaction process ranges from 950°C～1050°C.In the vanadium carbide layer,VC,V₂C,V₈C₇,Fe₃C phases may be formed.Therefore,the vanadium carbide layer by the reaction can be spontaneously formed.The stability of several carbides is V₂C,V₈C₇,VC,Fe₃C.The thickness of the carbide layer ranges from 16.62±0.16μm to 81.47±0.63μm under the different process parameters.The growth kinetics of composite the layer follows a parabolic rule and the calculated activation energy value.The growth of the vanadium carbide layer is in a variety of grain shapes with the different shapes and sizes.The grain size is smaller near the substrate,and the grain size is relatively larger in the region away from the substrate.The microhardness,elastic modulus of elasticity and fracture toughness of the vanadium carbide cermet layer were evaluated using an indentation testing.The vanadium carbide ceramic composite layer undergoes plastic deformation.The minimum elastic modulus is 254.041 GPa and the microhardness is 9.67 GPa which is2-3 times for the hardness of the iron matrix.The hardness of the V₂C layer in the composite layer is less than that of the V₈C₇layer.The values of nano hardness and elastic modulus of V₂C are 22.16 GPa and 372.84 GPa,and that of V₈C₇is 32.42 GPa,and 481.79 GPa.With the prolongation of the holding time,the hardness of the vanadium carbide composite layer becomes more stable as the density of the layer increases.The fracture toughness value of V₈C₇in the vanadium carbide composite layer is in the range of 2.41 to 4.41 MPa·m^1?2.The indentation morphology is generally a regular shape.There are cracks generated on the each corner of the indentation.As the increasing load,the projected area of the indentation the length of crack generated increase.In addition,the crack size and crack width also increase.