Scale Effect Of Titanium Boride Reinforced Titanium Matrix Composites Based On High-Energy Beam Additive Manufacturing

In-situ synthesized titanium boride whisker(TiBw)reinforced discontinuously reinforced titanium matrix composites(DRTMCs)present broad application prospects in major equipment and aerospace fields due to the excellent performance such as high specific strength,high temperature resistance and deformable processing.However,the size of the reinforcement in DRTMCs is affected by the difference of raw material state,preparation process and other factors,resulting in the structural defects caused by the difficulty in effectively controlling the TiBw diameter,aspect ratio and dispersion,which is the main reason for the strength-plastic inversion phenomenon(defined as "scale effect" in this paper),In view of this,this paper proposes to combine the traditional casting method with rapid solidification technology,use the two-stage rapid cooling process of electrode induction gas atomization(EIGA),selective laser melting(SLM)and electron beam powder bed melting(EBM),refine the TiBw to nano scale through active control,so as to broaden the control window of DRTMCs microstructure and properties.Thus,a new idea of composites preparation combining casting,gas atomization and high-energy beam additive manufacturing(SLM/EBM)is formed,which lays the foundation for the elucidation of the scale effect and the regulation of the structure and performance of DRTMCs.This paper firstly obtained TiBw-containing spherical titanium-based composite powder by EIGA using TiBw-reinforced titanium matrix composite ingots prepared by casting,and then used them to prepare nano-TiBw-reinforced DRTMCs with faster cooling rate in SLM/EBM.The formation pattern of quasi-continuous network structure and TiBw refinement mechanism under fast cooling conditions were clarified,and the corresponding toughening mechanism was also revealed.On this basis,the evolution of the distribution,interfacial structure,orientation relationship and fracture behavior of TiBw during its evolution from nanoscale to micron scale were systematically investigated to elucidate the coupled synergistic relationship between scale-microstructure-properties of the reinforcement,and finally reveal the scale effect of the reinforcement in DRTMCs based on the high-energy beam additive manufacturing and heat treatment conditions.The main findings of the study are as follows:Spherical Ti-TiBw composite powders were prepared by EIGA and their microstructure and properties were analyzed,the TiBw in composite powder is distributed at the grain boundary with micro/nano scale,and forming a quasi-continuous network structure in the matrix;the TiBw scale is significantly refined compared with the forged DRTMCs with the same volume fraction of TiBw.On this base,the ultrafine nano-TiBw reinforced DRTMCs was prepared by using the TiTiBw composite powder as feedstock via the fast-cooling rate of SLM(103-108 K/s).The TiBw scale and grain size were significantly refined during SLM,which makes the tensile strength can be reached to 869 MPa while maintaining 11.2%elongation.Meanwhile,compared with TMCs with the same TiBw content prepared by traditional powder metallurgy and forging methods,the tensile strength is increased by 28%and 61%respectively.Compared with SLM,EBM has a higher preheating temperature,which can effectively improve the cracking problem of SLM caused by thermal stress accumulation.The nano-TiBwreinforced TC4-based composites were prepared by EBM using the TC4-TiBw composite powder.TiBw is distributed at the grain boundaries in nanoscale and forms a quasi-continuous network structure with the matrix in composites.The tensile strength could reach 1121 MPa and the elongation could be maintained at 8.9%,which was 82%higher than that of the forged composites.The increase of strength is due to the formation of nanoscale TiBw,which greatly increases the number of reinforcements and interface,reduces the spacing,resulting the dislocation density increased and improves the deformation resistance of the composites.In addition,the nano-TiBw reduces the generation of stress concentrations during plastic deformation and inhibits the chance of cracks sprouting inside the reinforcement,providing a more durable load transfer effect.The exertion of matrix plasticity inside the network-like structure and the passivation of cracks are the main reasons for the increased elongation.The study on the scaling characteristic parameters of TiBw showed that the TC4-TiBw composites printed by EBM can still remain the network structure when heat treated at 800950℃.The growth of TiBw is controlled by the vacancy diffusion of boron atoms in TiBw,which increases its aspect ratio.When the heat treatment temperature is increased to 10001200℃,the diffusion coefficient of boron atoms is increased due to greater than the phase transition temperature,which makes TiBw diffuse into the crystal and show homogeneous distribution.At this temperature,the growth rate of TiBw along the radial direction is greater than that of the axial direction and coarsening occurs,this coarsening process follows the Ostwald ripening mechanism.In addition,the orientation and interface structure of TiBw also changed with the growth of the TiBw.Below the 950℃,there is no obvious orientation relationship between TiBw and Ti matrix;when the heat treatment temperature is higher than 1000℃,TiBw and α-Ti follow the orientation relationship:{0001} α-Ti//{001} TiBw,{1120} α-Ti/{010} TiBw,{1010} α-Ti//{100} TiBw.With the known orientation relationships,the(100)crystal plane of the TiB w column cross-section is bonded to Ti with a semi-coherent interface and coherent interface,while the(101)and(101)crystal planes of the cross-section are incoherent interfaces with the Ti matrix.The study of the scale effect of the reinforcement showed that the strength of TC4-TiBw composites showed a trend of increasing and then decreasing with the evolution of TiBw scale characteristic parameters,while the elongation showed an overall decreasing pattern.When TiBw is enriched in the grain boundaries at the nanoscale and forms a quasi-continuous network structure with the matrix,the strength of the composites is significantly improved due to the durable load transfer and fine grain strengthening effect of nano-TiBw.At the same time,the passivation of cracks by the plastic zone inside the network structure results in good elongation maintenance of the composites.When TiBw is coarsened to micron scale and uniformly distributed,the interfacial structure of TiBw and matrix evolves from semi-coherent interface to coherent interface,and the interfacial bond strength increases,so that cracks are difficult to be generated at the interface and transfer the stress to the reinforcement,which lead the cracks sprout and expand in the coarse TiBw,resulting in premature fracture of TiBw under low stress and loss of load transfer.Meanwhile,the coarse TiBw reduces the pining effect on the grains,which leads to a reduction in the strength of the composites.In addition,the uniform distribution of TiBw greatly divides the connectivity of the matrix,making the plastic zone compressed and the dislocation free range shortened,which eventually leads to the premature failure of the composites by low stress fracture.According to the research idea proposed in this paper,the size of TiBw is refined to nano scale through two-stage rapid cooling process,which significantly improves the serious problems caused by TiBw reinforced the strength-plastic of DRTMCs,and widens the performance regulation window.Meanwhile,the corresponding strengthening and toughening mechanism,as well as the scale effect of reinforcement in DRTMCs are clarified,which provides a new strategy for the preparation of high-performance DRTMCs.