| The development of titanium-based materials with improved mechanical characteristics(i.e.,increased specific strength,hardness,and corrosion resistance)has involved intensive applications in aerospace,defence and biomedical fields.However,the unaffordable price,inferior hardness and bad wear resistance restrict its mass usage and commercial success.In-situ particle reinforced titanium matrix composites have been proved to be able to overcome these obstacles.Newly developed approach such as spark plasma sintering,semisolid sintering and SLM 3D printing have been engaged in fabrication of high-performance titanium matrix composites,facing the dilemma of performance and cost.Blended elemental powder metallurgy(BEPM)has long been regarded as an effective method to to prepare titanium matrix composites in a cost-efficient way owing to near-net-shape forming.However,sintering of such BEPM compacts is characterized by poor densification,so hot-deformation processes are usually required to enhance the overall densification of the resulting composites,improving material performance to meet customer requirements,but reducing the cost-efficiency.This paper dedicates to form highly dense and homogeneous titanium matrix composites with hydrogen assisted BEPM apporoach in cooperation of simple blending,milling,press-and-sintering technologies.Taking advantages of specific characteristics of titanium hydride and reversible alloying role of hydrogen,two innovative approaches have been mentioned:Hydrogen assisted double press-and-sintering method and Hydrogen assisted sintering based on ball-milled powders and blending.Highly dense homogeneous titanium matrix composites with refined matrix and reinforcements were successfully prepared via above routes.Microstructure observation,XRD phase identification,dilatometric analysis,ONH impurity contents test and room temperature compressive/tensile test are comprehensively adopted to reveal sintering densificaion,strengthen mechanism and fracture mode.This paper has explored new approaches for low-cost fabrication of high-performance Titanium matrix composites.The research works and main conclusions are summarized as follows:(1)Starting with?88?m TiH2 and?10?m TiB2,in-situ Ti/Ti B composites with 8.4 vol.%and 16.8 vol.%reinforcements were prepared by Hydrogen assisted double press-and-sintering route.Hydrogenation enables the transformation of porous pre-sintered composites into low strength hydrogenated pre-alloyed(PA)powders.Boride clusters and coarse Ti B lamellar can be easily crushed and remixed during milling.During 2nd sintering stage,dehydrogenation of hydrogenated PA compacts has similar shrinkage behavior and sintering densification effect as pure Ti H2 compacts.Phase transformation induced formation of highly dense crystal defects in matrix,promoting diffusion and pore healing.The homogeneous distribution of refined reinforcements restricts the coarse growth of matrix grains during 2nd sintering.Highly dense Ti/Ti B composites with evenly distributed reinforcements were formed finally.Compared with pre-sintered compacts,2nd sintering of Ti/8.4 vol.%Ti B compacts reduced the porosity from 7.5%to 0.8%,while hardness and ultimate tensile strength increased from 260-270 HV,862 MPa to390-410 HV,1087 MPa correspondingly.(2)Varieties and particle distribution of starting additives influence the homogeneity,porosity level and hardness of pre-sintered composites.?5?m Boron powders or?5?m graphite particles can easily form agglomerations during blending,resulting in inhomogeneous distribution of partialy reacted reinforcements and residual porosity in sintered composites.Usage of Ti B2 and B4C in large size in starting blends always result in sea-urchin like agglomerations with residual porosity.Dramatic volume change of matrix during dehydrogenation,difference of elemental diffusion velocity and special growth mechanism of Ti B all take account for the microstructural inhomogenity after pre-sintering.Adverse effects of the raw reinforcing powder on microstructure uniformity,porosity,and mechanical characteristics of the pre-sintered in situ composites become negligible in final sintering stage via hydrogen assisted double press-and-sintering approach.Refinement in density,reinforcments distribution eables the improvement of hardness in fabrication of Ti B/Ti C/Ti B+Ti C reinforced Ti-based composites via the noted approach.(3)Nearly dense(Porosity<0.1%)Ti6Al4V/TiB composites were successfully prepared by hydrogen assisted double press-and-sintering approach,starting with?88?m Ti H2,?63?m MA and?10?m Ti B2.Hydrogenation and milling eanbles the transformation from inhomogeneous pre-sintered microstructure to brittle hydrogenated PA powders.The fine Ti B needles locating in grain borders and inside prohibit the growth of?grains during 2nd sintering stage and promote the formation of nearly equi-axiel?crystals.The strengthening mechanisms include porosity reduction,grain refinement,load-bearing transformation and increased impurity contents.Compared with pre-sintered compacts,2nd sintering of Ti6Al4V/10 vol.%Ti B compacts decrease the porosity from 6.8%to 0.7%,while hardness,compressive strength and ultimate tensile strength increased from 250-270 HV,1860 MPa and 1011 MPa to 430-440 HV,2006 MPa and 1314MPa correspondingly.(4)Ti6Al4V/5 vol.%TiB composites were prepared by Hydrogen assisted sintering based on ball-milled powders and blending route.Preliminary activation milling of ductile 45?88 HDH Ti powder and?10?m Ti B2 particles over 2–6 h are proved to be effective in refining additive particles,crushing additive agglomerations and increasing contact zones between matrix and additives.The use of Ti H2 provides the preservation of acceptable oxygen contents in sintered composites as well as the improvement of sintering densification.Cooperative use of Ti H2 and milled HDH-Ti particles as a titanium base of powder blends realize the regulation of porosity level,microstructure homogenization and mechanical properties.Highly dense(0.6%)and purified(oxygen content:0.374%)Ti6Al4V/5 vol.%Ti B composites were prepared with 4 h milling and 1:1 ratio between Ti H2 and HDH-Ti.The compressive strength(1923 MPa)and ultimate tensile strength(1183 MPa)are lower,but the elongation increased from 0%to 7.3%,when compared with mechanical test results of 2nd sintered composites.The composites prepared shows a mixed ductile-brittle failure mode at room temperature. |
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