| In order to improve room-and high-temperature properties of titanium matrix composites,(Ti5Si3+Ti Bw)/Ti6Al4 V composites with two-scale network structure were designed and fabricated by low energy milling,reaction hot pressing technique as well as solid solution and precipitation,basing on the system of Ti6Al4V-Ti B2-Si in the present work.Phase identification of composites was conducted by X-Ray diffraction(XRD).The microstructure of as-sintered and heat treated composites,and the morphology characteristics of composites after accelerated creep and oxidation tests were investigated using optical microscope(OM),scanning electron microscope(SEM),electron backscatter diffraction(EBSD)and transmission electron microscope(TEM).The mechanical properties at room and high temperatures of the as-sintered and heat treated composites were studied.The influence of heat treatments on the microstructure and properties of composites was analyzed.The anti-creep and anti-oxidation mechanisms of composites were studied.The microstructure analysis shows that Ti Bw were in-situ synthesized around the Ti6Al4 V matrix particles,and formed the first-scale network structure.Ti5Si3 particles precipitated in the ? phases around the equiaxed ? phases,and formed the second-scale network structure.The formation process of Ti5Si3 particles is as follows: 1)Si particles could dissolve and Si element diffused into the matrix.Meanwhile,in-situ synthesis,silicides were formed around Ti matrix particles by in-situ reaction between Ti and Si at 1000 oC.2)high-temperature solid solution;the silicide dissolved gradually at above 1000 oC and the diffusion rate of element was accelerated,then Si diffused into the ? phases at above 1200 oC.3)exsolution and precipitation;the solubility of Si element in the ?-Ti decreases with decreasing temperature during furnace cooling,and the fraction of ? phases decreases due to the phase transition(???).The microstructure characteristics of Ti5Si3 particles can be adjusted by controlling the sintering parameters and Ti Bw fractions.The quantity of Ti5Si3 particles in ? phases increased significantly with increasing sintering temperature and holding time.With increasing Ti Bw fractions,the quantity of Ti5Si3 particles near Ti Bw reinforcements increased.The mechanical properties results exhibit that the room-temperature ultimate tensile strength and elongation of the(4vol.%Ti5Si3+3.4vol.%Ti Bw)/Ti6Al4 V composites reach 1180 MPa and 5.0%,respectively.The ultimate tensile strength at 550 oC,600 oC and 650 oC are 770 MPa,610 MPa and 510 MPa,respectively.Compared with Ti6Al4 V alloy and 5vol%Ti Bw/Ti6Al4 V composites at 600 oC,the strength of composites with two-scale network structure increased by 56% and 11%.The analysis of fractographs and crack propagation process show that partial Ti Bw breaks and the cracks follow the first-scale network region.No crack of nano-Ti5Si3 particles was observed,due to their fine characteristic.The first-scale network structure made contribution to the enhancement of room-temperature yield strength of composites by the refinement of matrix microstructure.The introduction of second-scale network structure also improved yield strength by solution strengthening.Furthermore,the two-scale network structure effectively increased the deformation compatibility of composites.To further improve the mechanical properties of(Ti5Si3+Ti Bw)/Ti6Al4 V composites,heat treatments were carried out to adjust Ti5Si3 particles and matrix characteristics.The fraction of Ti5Si3 particles in the vicinity of first-scale Ti Bw reinforcements decreased with increasing quenching temperatures.The fraction of Ti5Si3 particles increased with increasing aging temperatures.The morphology of Ti5Si3 particles evolved from needlelike to elliptic.When the quenching temperature was elevated to 1200 °C,the Ti5Si3 particles dissolved and re-soluted into the matrix.While,fine Ti5Si3 particles precipitated in matrix after solution and aging treatments.The high temperature(600 °C)tensile strength of(4vol.%Ti5Si3+3.4vol.%Ti Bw)/Ti6Al4 V composites could reach 880 MPa through the heat treatment of 1200 °C/40 min/WQ+600 °C/5 h/AC.Compared with Ti6Al4 V alloys,the creep resistance of(Ti5Si3+Ti Bw)/Ti6Al4 V composites was significantly improved.The creep rate remarkably reduced,even reached one order of magnitude.Moreover,the rupture time of composites increased by 20 times furthest.The superior creep resistance was attributed to two-level microstructures and two-scale reinforcements.The Ti Bw reinforcements distributing around Ti6Al4 V matrix contributed to creep resistance primarily by blocking grain boundary sliding,while the Ti5Si3 particles mainly by hindering phase boundary sliding.During creep deformation,the diffusion rate of Si within the ? phases to the ?/? interface regions was facilitated by high temperature and high stress.Therefore,the precipitated nano-Ti5Si3 particles with smaller size further enhanced the creep resistance greatly.Compared with Ti6Al4 V alloy,the oxidation resistance of(4vol.%Ti5Si3+3.4vol.%Ti Bw)/Ti6Al4 V composite is superior at 700 oC and 800 oC.However,the oxide scales of(8vol.%Ti5Si3+3.4vol.%Ti Bw)/Ti6Al4 V composite remained intact and flat without spallation after oxidized at 800 oC for 100 h.This composite showed the highest oxidation resistance.The oxidation resistance of composites was enhanced drastically,which resulted from the refining of Ti O2 particles,thin and dense Al2O3 and Si O2 mixed scale and oxide scale fastened by reinforcements.The oxidation rate of composites decreased with increasing Ti5Si3 fractions,and increased with increasing Ti Bw fractions.The first-principles calculation showed that O atom prefers to absorb on the Ti surfaces,while,it is difficult for O atom to absorb on the ?-Ti/Ti5Si3 interface.Ti5Si3 decreased the adsorption ability of oxygen and played a significant role in impeding the adsorption of O on Ti surfaces.The stronger bonding interaction between O and Si atoms is beneficial to improving oxidation resistance. |
PDF Full Text Request