Research On Microstructure And Properties Of Titanium And Ti-6Al-3Nb-2Zr-1Mo Alloy Prepared By TiH₂ Powders As Raw Material

Titanium and its alloys are new structural materials developed in the 1950s,which have excellent physical and chemical properties.However,their application and promotion in the civil market are restricted by high production cost.The emergence of powder metallurgy technology has alleviated this dilemma to some extent.In recent years,more and more scholars have paid attention to the new technology of preparing titanium and its alloys by using Ti H₂powders,however,its performance still need to be enhanced to meet the demand of practical engineering application comparing with the casting and forging ones.therefore,through trying new forming methods,optimizing sintering process and introducing hot extrusion plastic deformation technology to improve the microstructure and mechanical properties of the samples becomes very necessary.The purpose of this paper is to fabricate pure Ti and Ti-6Al-3Nb-2Zr-1Mo alloy with high strength and toughness but low cost.An in-depth and systematic research on several aspects including the characterization of Ti H₂ powders,forming,sintering,hot extrusion and heat treatment process was conducted.The evolution of microstructures was analyzed through OM,XRD,SEM and TEM and so on.The main conclusions of this paper are as follows:(?)The decomposition of TiH₂ powder is an endothermic process,whose temperature range of dehydrogenation concentrated in 450?700?.(?)The density of TiH₂ green billet increases with the increase of forming pressure.Compared with room-temperature pressing,the green and sintering density of Ti H₂ were significantly increased by warm compaction.The higher the temperature of pressing,the biger the improvement was.Pure Ti with nearly full-density can be obtained at 500 MPa/200?,and its microstructure was composed of 90?120?m equiaxed?-Ti grains and uniformly distributed circular pores.At 500 MPa/150?,the yield and tensile strength and plasticity are 508 MPa,638 MPa and 14.6%,respectively.However,there is an inevitable risk of oxidation for the powders in the process of warm compaction.(?)With the increase of sintering temperature and the extension of holding time,the porosity of the material decreases,the roundness of the pores increases and the radius decreases;In the matrix of sintered pure Ti,the particles'boundary disappers and the grain grows up gradually;While,as for the sintered alloy,the content of equiaxed?-Ti decreases and the volume fraction of lamellar microstructure increases.Both the mechanical properties of pure Ti and alloy varied with different sintering processes in stages.The optimal performance of pure Ti can be obtained at 1250?/4 h,whose yield and tensile strength and plasticity are 466 MPa,580 MPa,22%,respectively.While the relative density,yield strength,tensile strength and elongation of the alloy is 97.5%,1004 MPa,1092 MPa and 3.4%at the same conditions.(?)The pore defects in the matrix were eliminated by hot extrusion process,which resulted in the preferential orientation and great refinement of grains.After hot extrusion,a bimodal microstructure was obtained for both pure Ti and Ti-6Al-3Nb-2Zr-1Mo alloy.At the same time,their mechanical properties are greatly improved compared with the sintered ones;For pure Ti,the DH-Ti H₂+HE sample got the most significant lifting,and its yield strength,tensile strength and elongation are 592 MPa,761 MPa and 31.5%,respectively;While,for the alloy,the mechanical properties of the SS-T6321 sample after hot extrusion are the best,with the yield and tensile strength reaching 1270 MPa and 1406 MPa,and the tensile plasticity still remaining 12.4%.It is found by calculation that fine grain strengthening are the main strengthening mechanisms for the hot-extruded pure titanium and alloy.(?)With the increase of annealing temperature,the content of initial?phase decreases while the volume fraction of?-transformed phase increases,and the secondary?phase gradually precipitates along the?-phase with a needle-like distribution.After heat treatment,the mechanical properties of the alloy are further improved.At the condition of 1000?/1 h AC,the best comprehensive mechanical properties were obtained,the yield and tensile strength of the alloy are 1310 MPa and 1456 MPa,respectively,and the tensile plasticity is 13.7%.