| Titanium(Ti)and its alloys have widespread applications due to their lightweighting,high specific strength,superior thermal stability and excellent corrosion resistance.Powder metallurgy(PM)offers an attractive forming technology for fabricating these alloys on new component design,near-net shaping manufacturing,microstructure optimization and product stability.However,interstitial oxygen(O)contamination and poor mechanical properties remain to be the critical issues for PM Ti parts.Thus,the oxygen-control methods of organic film oxygen-blocking for low-oxygen and high-activity HDH Ti powder,and chemical oxygenscavenging for cheap and high-oxygen HDH Ti powder were put forward.The highperformance in-situ particle reinforced Ti matrix composites were successfully fabricated by the PM pressureless sintering,combined with hot extrusion and heat treatment.A systematic study has been made on the effects of PCS and CaC2 addition on the oxygen-control mechanism,phase evolution,microstructure,roomtemperature mechanical properties,wear resistance,and high-temperature mechanical properties of Ti matrix composites.The main conclusions are as follows:The primary source of O contamination for the highly reactive HDH Ti powder mainly comes from the fabrication process in air.Hypothetically,the solutionassisted wet mixing process can coat the Ti particle with a continuous PCS polymer film,which can act as a barrier and halts O diffusion into Ti in fabrication process.The oxygen content of the fabricated Ti/PCS composites was reduced by about 1400 ppm compared with that of pure Ti.In other words,PCS can decompose during sintering and react with Ti matrix to in-situ form dispersed TiC particles,greatly refining the grain sizes.The average grain size of ?-Ti decreases from 100.5?m in pure Ti to 16.1 ?m in the Ti-3PCS composite.Moreover,the fabricated Ti3PCS composite exhibits a high tensile strength of 861 MPa,high yield strength of 754 MPa and reasonably large elongation of 10.8%,respectively.These values correspond to an increase of 56.8%,64.6%and 68.8%when compared to pure Ti properties.In addition,the in-situ PCS-derived TiC particles can increase material hardness,offer superior load transfer capability and possess high oxidation resistance,thus contributing to the excellent wear resistance of Ti/PCS composites.Adding the high-purity CaC2 oxygen-scavenger to solve the problem of resource degradation for the high-oxygen HDH Ti powder.A continuous oxide layer with a thickness of 9.3 nm exists in the raw high-oxygen HDH-Ti powder surface.The surface oxide layer is composed of TiO2,Ti2O3 and TiO respectively,and it begins to dissolve into Ti matrix between 700? and 800?.The incorporation of CaC2 can effectively react with the surface oxide layer to form micron-sized TiC and nano-sized CaTiO3 particles at 617-676?,prior to its active dissolution.The unique oxygen-scavenging effect by CaC2 addition produces a high strength and superior ductility for ?-Ti alloy.Under tensile loading,the Ti-0.4CaC2 composite exhibits a high ultimate tensile strength of 621 MPa,high yield strength of 508 MPa and excellent elongation of 29.3%,which are all higher than that of pure Ti sample.Meanwhile,this alloy has met the ASTM standard B988 for Grade 1-4 Ti alloys.The applicability of the oxygen-control methods for PCS and CaC2 addition to the widely used HDH Ti-6Al-4V(TC4)alloy powder was further studied,and found that their incorporation can also improve the strength and elongation of TC4 matrix composites.Moreover,hot extrusion can refine the grain sizes of TC4/PCS composites,and lots of strain accumulation near TiC particles induces dynamic recrystallization,promoting the formation of many fine a equiaxed grains.After solution treatment,primary ? and metastable ?' phases are formed in TC4/PCS composites.Then,the ?' phases decompose and transform into fine dispersed ?+?mixed phases and nano-sized ? particles after aging treatment.Combined with the high hardness and superior load transfer capability of TiC particles,both the roomtemperature and high-temperature strength are greatly increased.For example,the as-heated TC4-3PCS composite has a room-temperature tensile strength of 1405 MPa and high-temperature tensile strength at 600? of 671 MPa,much higher than the 947 MPa and 385 MPa for the as-sintered TC4 alloy.On the other hand,hot extrusion and heat treatment make TC4-0.2CaC2 composite possess superfine a equiaxed grain with the average grain size of only 2.21 ?m,much lower than the 23.08 ?m of the as-heated TC4 alloy.After solution and aging treatment,many fine dispersed ?+? lamellar phases are formed in the TC4-0.2CaC2 composite,satisfying a specific orientation relationship of(0002)?-Ti//(211)?-Ti and[1210]?-Ti//[113]?-Ti.The as-heated TC4-0.2CaC2 composite has a room-temperature tensile strength of 1301 MPa,yield strength of 1267 MPa and elongation of 22.4%,much higher than the ASTM standard B381 for forged Ti-6Al-4V alloy.This work provided a fabrication technology for high-performance Ti matrix composites using low-cost HDH Ti powder,promoting the engineering application of Ti materials. |
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