Field Assisted Activated Sintering,Microstructure Regulation,Strengthening And Toughening Of TC4 Titanium Alloy

TC4 titanium alloys,one kind of dual-phase titanium alloy composed of?-phase and?-phase at room temperature,were widely used in the advanced equipment such as national defense,aerospace,marine vehicle industries,and the civilian areas for example biomedical,automotive,petrochemical industries due to its high specific strength,high melting point,corrosion resistance and good biocompatibility.The constant developments of these applications require higher comprehensive mechanical properties of titanium alloys.Therefore,in this thesis,focusing on the key points of controlling of the fine phase and microstructure in TC4 titanium,aiming at solving the mismatch of strength and toughness,we optimized the preparation technology,phase and microstructure.The researches on structure regulation and mechanical properties were also carried on TC4 titanium.Cryomilling and field-assisted activated sintering were used for material preparation.OM,SEM,TEM and EBSD were used for the characterization of structure and phase.Vickers hardness tester and universal testing machine were used for the evaluation of mechanical properties.The comprehensive control technology and mechanism between the preparation technology,phase,structure and properties of titanium alloys were obtained through the research.The results would promote the progress of solving the key scientific puzzles in the design and control of advanced structural materials.First of all,the TC4 titanium alloys with high relative density were synthesized via field-assisted activated sintering technology at relative low sintering temperature and rapid sintering rate.A relative density of 98.2%was achieved by the sintering temperature of 900 ^oC,with a temperature rate of 100 ^oC/min and dwelling time of 3min.With the increase of sintering temperature from 900 ^oC to above 1000 ^oC,the microstructure of the sintered TC4 titanium changed from basket structure to lamellar structure.The grain size and orientation distribution of basket structure TC4titanium were uniform and the grain boundary volume fraction was small,and there was collation phenomenon between grains.Alternately distributed alpha-clusters were caused by the lamellar structure of TC4 titanium,and the intergranular volume fraction was high.The test results of mechanical properties showed that,the sintered TC4 titanium with basket structure possessed the high compressive yield strength of1028 MPa and high strain of 34.93%,while the sintered TC4 titanium with lamellar structure possessed higher compressive yield strength of 1096 MPa.Secondly,fine-grained structures of TC4 titanium powder was synthesized via cryomilling,and the densification of TC4 titanium was realized via field-assisted activated sintering.The results showed that the average grain size of raw material powders was refined to 32 nm after cryomilling for 4.5 h.With the sintering temperature of 900 ^oC,temperature rate of 100 ^oC/min,and dwelling time of 3 min,the alpha clusters of the sintered blocks was equiaxed and obviously refined with a equiaxed grain size of 1-2?m.The main reasons of the formation and maintenance of fine crystal structure were ascribed to the relative low sintering temperature and high density dislocations introduced into the powders during cryomilling.The?phase mainly distributed in the grain boundary of?phase.The Fe atoms introduced by cryomilling act as one of the stable elements of?phase and the solid solution occurred between Ti and Fe,leading to an increase of?phase content.The compressive strength of TC4 titanium alloy was promoted to 1795 MPa by the fine grain,high density grain boundary and dislocation introduced during grain refinement process.Subsequently,dual-scale structures in TC4 titanium alloys were obtained by introducing coarse grains into fine grain structure.Results showed that the grain size in fine grain region was equiaxed in range of 1 to 2.5?m,the grain size in coarse grain region was lamellar in range of 18.5 to 24.5?m.The Vickers hardness and compressive strength of materials were increased with rising content of fine grains.When the ratio of fine grains and coarse grains was 3:1,the dual-scale TC4 titanium alloy obtained more balanced mechanical properties,with strength of 1544 MPa and fracture strain of 22.19%.Dual-phase structure enhanced the strength through dislocation and fine grain strengthening.Coarse grain structure hindered the propagation of cracks,and provided more space for the movement of dislocations,resulting in proving of strengthening and toughening of dual-scale TC4 titanium alloy.Finally,TiB and TiC ceramic reinforce particles were introduced into the TC4matrix by in-situ reaction between B₄C as the reactant and Ti element in titanium alloy.The results showed that,the reacted TiB particles in the form of whiskers have a misfit degree with titanium alloy matrix of 5.39%,and TiC particles were equiaxed.The grain boundaries between ceramic particles and TC4 matrix were clean and well combined.The in-situ reacted ceramic particles diffusely distributed in the basket TC4 matrix.Suggesting that,ceramic phases inhibited the growth of matrix grains and avoided the coarse structures,leading to basket microstructure.With the increasing content of ceramic phases,the compressive strength of TC4 titanium increased,and the compressive yield strength was improved to 1537 MPa with 9 vol.%B₄C content.The bigger particles promoted the formation of dimples in the process of deformation,and controlled the expansion of dimples by the closed boning with TC4 titanium.The smaller particles usually increased the deformation resistance of materials by entangling dislocations and producing dislocation plugging,leading to an enhancement on deformation resistance.