Controlled Preparation And Strengthening-Toughening Mechanisms Of(TiC+α-Fe)_p Dual-Scale Structure Reinforced Iron Matrix Composites

Architecture design is an effective way to solve the problem of the inversion relationship between strength and plasticity/toughness of steel/iron matrix composites(SIMCs).However,the obvious stress concentration at the reinforcement/matrix macroscopic interface and the low bonding strength of the reinforcement phase/metal microscopic interface in the reinforcement("two types of interfaces" problem)are the bottlenecks restricting the full play of the synergistic effect of strengthening and toughening.Aiming at this issue,(TiC+α-Fe)_p dual-scale structure reinforced iron matrix composites((TiC+α-Fe)_p,DS-IMCs)were obtained by near-eutectic temperature hot pressing sintering method according to the architecture design based on the nonsteady state diffusion principle and in situ reaction.The strength and ductility/toughness were fully coordinated and balanced through the macroscopic gradient structure interface and the microscopic coherent/semi-coherent interface.The(TiC+α-Fe)_p DS-IMCs was prepared by adjusting the composition of raw material powder,preparation parameters,and post-processing methods.The microstructure evolution and formation mechanism of(TiC+α-Fe)_p DS-IMCs,as well as the effects of reinforcement characteristics parameters and heat treatment parameters on the microstructure,properties,and the strengthening-toughening mechanisms of composites,were investigated.The results are as follows:(1)The change of carbon content in the raw material powder is the key factor affecting the evolution of the reinforcement from the Ti@TiC_p core-shell structure to the(TiC+α-Fe)_p dual-scale structure.When the carbon content of the raw material power is in the range of 0.4%-1.9%,the reinforcement in the composites is mainly of Ti@TiC_p core-shell structure.Moreover,with the increase in carbon content,the size of the Ti core gradually decreases,and the thickness of the TiC shell layer gradually increases.The average size of the equiaxed TiC increases from～250 nm to～8 μm from the matrix to the core/shell interface.When the carbon content of the raw material powder is 2.4%and 2.9%,the reinforcement is the(TiC+α-Fe)_p dual-scale structure.From the matrix to the center of the dual-scale structure,the TiC particles evolve from micro-nano-scale equiaxed crystals to micro-scale columnar crystals,then micro-scale equiaxed crystals,and finally,submicro-scale equiaxed crystals.(2)The carbon concentration gradient and solid-state phase transition are the main reasons for the formation of the(TiC+α-Fe)_p dual-scale structure and its macroscopic gradient structure and microscopic coherent/semi-coherent interface.Along the carbon diffusion direction,the concentration of carbon atoms decreases in gradient,so that the nucleation rate of TiC particles gradually decreases,and the growth rate and coarsening rate increase gradually,resulting in a gradual increase in the size of TiC particles and the formation of a gradient structure.At the same time,there are two main mechanisms for the in-situ formation of TiC particles:(ⅰ)C atoms enter the octahedral vacancies of the body-centered cubic structure β-Ti lattice by diffusion and undergo a diffusion-type solid-state phase transformation to form TiC,namely,β-Ti+C→TiC(solid-state phase transition mechanism Ⅰ);(ⅱ)C atoms diffuse into the solid solution β-Ti(Fe),and undergo a solid-state phase transition in a diffusion-segregation manner to form TiC and α-Fe phases,namely,β-Ti(Fe)+C→TiC+α-Fe(solid-state phase transition mechanism Ⅱ).With the reaction’s progress,TiC particle formation’s leading mechanism gradually changes from solid phase transition mechanism Ⅰ to solid phase transition mechanism Ⅱ,and the corresponding growth rate and coarsening rate of TiC also changed.(3)The mechanical properties of(TiC+α-Fe)_p DS-IMCs can be fine-tuned by adjusting the size and volume fraction of the dual-scale structure reinforcement and the heat treatment parameters.When the size of the(TiC+α-Fe)_p dual-scale structure reinforcement is reduced from 90-135 μm to 3-9 μm,the composites’ strength and plasticity/toughness are simultaneously improved.Moreover,with the increase of the volume fraction of(TiC+α-Fe)_p dual-scale structure reinforcement from 10%to 50%,the probability of aggregation of the reinforcements increases,and the strength of the composites first increases and then decreases.In contrast,the plasticity/toughness gradually decreases.Furthermore,when the heat treatment parameters are 850℃-30 min+225℃-2 h,the optimal matching of the reinforcement/matrix interface structure is obtained.The hardness,tensile strength,elongation,compressive strength,and compressive strain of the composites reached the optimum values,which were 57.8 ± 0.5 HRC,1655 ± 20 MPa,8.6 ± 0.3%,2703 ± 80 and 16.3 ± 1.5%,respectively.(4)The synergistic coupling of the "macroscopic interface" debonding mechanism and the"double dislocation packing" dislocation strengthening mechanism is the main reason for the simultaneous improvement of the strength and plasticity/toughness of(TiC+α-Fe)_p DS-IMCs.The toughening mechanisms of(TiC+α-Fe)_p DS-IMCs mainly include the "macroscopic interface" debonding mechanism,the microscopic interface induced crack deflection and bridging mechanism,the reinforcement and matrix synergistically induced crack deflection,crack passivation and crack bridging mechanism,the "step-wise" fracture mechanism of TiC particles,and the toughening mechanism in the matrix.The strengthening mechanisms of(TiC+α-Fe)_p DS-IMCs mainly include the "double dislocation packing" dislocation strengthening mechanism,load transfer strengthening mechanism,martensitic transformation strengthening and fine grain strengthening mechanism after the heat treatment,and architecture strengthening-toughening mechanism.In particular,the synergistic coupling effect of the "macroscopic interface" debonding mechanism and the "double dislocation packing" dislocation strengthening mechanism resulted in a synergistic improvement of the strength and plasticity/toughness of(TiC+α-Fe)_p DS-IMCs.In conclusion,based on the innovative idea of introducing the(TiC+α-Fe)_p dual-scale structure into steel/iron,the controllable preparation of(TiC+α-Fe)_p DS-IMCs is realized.This effectively improves the common bottleneck problems in which the macro/micro interface restricts the full play of the synergistic effect of strengthening and toughening of architectured SIMCs.Moreover,the matching strengthening-toughening mechanisms in(TiC+α-Fe)_p DS-IMCs are also illustrated.It provides the experimental basis and theoretical reference for the developing and applying of strength and toughness integrated SIMCs.