Controllable Preparation And Mechanism Study Of Ti/Al₂O₃ Composites Doped With Metal And Rare Earth Oxide

Ti/Al2_O3 cermet composite is an advanced composites,which has extensively applied in fields of aerospace,chemical,mechanical and electronic information etc.Here Ti/Al2_O3 cermet composites were fabricated by using vacuum hot pressing sintering technique.The sintering synthesis mechanism was investigated as well as the behavior of interface element diffusion,and the sequence of phase formation etc.Meanwhile,the element diffusion coefficient of interface reaction layer was solved out,and the reaction kinetic equation was also set up from the dynamic view.The sintering mechanism and diffusion model of Ti/Al2_O3 cermet composite under hot pressing conditions were perfected.Metal elements Nb,nano Ni and rare earth oxide,such as CeO2,Y2O3,were introduced into the composite system to control the interface reaction of Ti/Al2_O3 and inhibit the phase growth of Ti3Al and formation of strong bonding interface,accordingly optimize the interface structure and phase composition.The interface reaction,microstructure evolution and control mechanism of Ti-Al2O3 were investigated through XRD,SEM,EPMA,WDS,EDS to optimize the material composition and preparation process,accordingly obtain the Ti/Al2_O3 composites with excellent properties.Interfacial reaction products of Ti/Al2_O3 composites were studied when the sintering temperature was 1350?,1400?,1450?,1500?.The results showed that Al resolved from AI2O3 reacted with Ti to yield intermetallic compound of Ti3Al and TiAl,and the Al2TiOs was produced by reaction of Al,Ti and O.The Ti3Al phase produced in Ti/Al2_O3 composites increased gradually,while the TiAl phase dropped off with increasing the sintering temperature.Ignoring the effect of O and Ti solid solution on Gibbs free enthalpy,the thermodynamic calculation of Ti/Al2_O3 interfacial reaction products was carried out through the Raoul's law and Henry's law.The calculating result showed that ?GTi3Al<?GTiAl<0,which means that two intermetallic compounds could form spontaneously at high temperature,in contrast,the Ti3Al phase is easier to generate.The Gibbs free energy of Ti3Al decreased gradually with increasing temperature,while the free energy of TiAl increased gradually.Consequently,the higher the temperature was,the more Ti3A1 phase was produced.Performance test results indicated that sintering temperature has a significant effect on the relative density,flexural strength and fracture toughness of Ti/Al2_O3 composites,and their maximum values were 96.13%,329MPa and 5.1 MPa m1/2,respectively.The influence of metal doping Nb and nano Ni on interface phase composition,structure and properties of composite was also studied.The research results expressed that the phase composition and interface structure of Ti/Al2_O3 composites could be effectively enhanced by doping Nb and nano Ni.The AlNb2 phase was generated in the interface of Nb doped Ti/Al2_O3 composite,by comparison,the Al3Ni,Ni2Ti and Al2Ni4O were produced in the interface of Ti-Al2O3 after the incorporation of nano Ni.Two kinds of metal doping elements suppressed effectively the violent interfacial reaction between Ti and Al2O3,and reduced the formation of brittle Ti3Al phase,and promoted the interface structure and composition of the composites.The impacts of Nb and nano Ni content on properties of Ti/Al2_O3 composites were investigated,and the optimum content of Nb and nano Ni were identified as 1.5vol.%and 1vol.%,respectively.On this basis,the influence rule of sintering temperature and holding time on properties and interface structure of metal doped Ti/Al2_O3 composites was analyzed.The result demonstrated that the performance of the composites was improved significantly by promoting sintering temperature and prolonging holding time.When the sintering temperature was 1450? and the holding time was 90min,the Nb doped Ti/Al2_O3 composite has the best performance with the relative density of 99.26%,flexural strength of 410.29MPa,and fracture toughness of 6.89MPa-m1/2,and 1icrohardness of 16.8GPa·m1/2,respectively.When the sintering temperature was 1450? and the holding time was 60min,the nano Ni doped Ti/Al2_O3 composite has the best performance with the relative density of 99.24%,flexural strength of 425.77MPa,and fracture toughness of 7.94MPa-m1/2,and microhardness of 17.17GPa·m1/2,respectively.This is principally because the metal doping could refine the grain size of Ti/Al2_O3 composites,bring down the porosity and inhibit the interfacial reaction between Ti and Al2O3.The fracture mode would be changed when the material fracture,meanwhile fracture energy could be consumed by crack bridge and deflection during the propagation of crack,and finally improve the mechanical properties of the composites.The interface structure and phase composition of Ti/Al2_O3 composite could be effectively improved by doping CeO2,and new compounds of CeAlO3 and CeAl11O18 were generated at the interface.The value of free energy ?G of the interface reaction products was calculated,and their relationship was ?GAl2TiO5<?GCeAl11O18<?GCeAlO3<?GTi3Al<?GTiAl.Therefore,Al2TiO5,CeAlO3 and CeAl11O18 were easier to form in the sintering process,which effectively prevented the diffusion of Al phase to Ti phase,and improved the interface structure as well as the mechanical properties of the composites.The effect of CeO2 content on microstructure and properties of the composites was investigated.The results showed that the mechanical properties achieved the best effect when CeO2 content was 2.5vol.%.On this basis,the relationship between sintering temperature and material properties was considered,the results showed that the relative density,flexural strength,fracture toughness and microhardness reached the maximum values of 98.9%,482.6MPa,6.87MPa·m1/2 and 16.3GPa,respectively when the sintering temperature was 1500?.The effects of Y2O3 content and sintering temperature on the properties of Y2O3 doped Ti/Al2_O3 composite were investigated.The maximum values of relative density,flexural strength,fracture toughness and microhardness were 462.6MPa,6.57MPa-m1/2,99.2%and 16.6GPa,respectively.Due to the incorporation of CeO2,some abnormal grain CeAlO3 and CeAl11O18 generated in the Ti/Al2_O3 composite,which not only filled some part of pores inside the composite material,but also the shaped tabular grains with high aspect ratio played a part in strengthening and toughening effects like the whisker or staple fiber.Consequently,more fracture energy would be absorbed by those tabular grains with higher pullout energy,and the mechanical properties of the composites would be significantly improved.With doping of Y2O3,the formation of Ti3Al phase would be hampered because of the YAl and YAl2 generated in the composite interface,and the interface structure was optimized.The fracture mode of the composites has transferred from an intergranular one to a mixture of intergranular and transgranular,in this case,the crack path was deflected and prolonged to consume more fracture energy.Consequently,the mechanical properties of materials were improved.The Ti/Al2_O3 interfacial reaction diffusion mechanism at vacuum hot pressing conditions was analyzed from the dynamic view,and the reaction kinetics of Ti/Al2_O3 interface equations was built,also the effects of CeO2,Y2O3,Nb and Ni nanoparticles on Ti-Al2O3 interface reaction control mechanism were studied.The effects of sintering temperature and holding time on the effect of the Ti-Al2O3 interfacial reaction layer thickness were investigated.The results showed that when the sintering temperature is 1250?,the average thickness of the interfacial reaction layer was 63.1?m,with the increase of sintering temperature,surface densification degree increased,and the interfacial reaction layer thickness increases exponentially.When sintering temperature was 1500?,the average thickness of the interfacial reaction layer was 123.4?m.The interface diffusion coefficient of each element was calculated by empirical formula,the diffusion coefficient of Al element was significantly higher than that of O and Ti elements,and with the increase of sintering temperature,the diffusion coefficient of Al element increased significantly by exponential law,the formation of interfacial reaction layer is mainly controlled by the diffusion of Al element.On the basis of empirical formula of the thickness of the reaction layer and time relationship,the relationship between undoped Ti-Al2O3 interfacial reaction layer thickness and sintering temperature,the holding time was calculated.The interface reaction kinetics equation of Ti/Al2_O3 by linear fitting is,d=182.5exp?-6.6 ×104/RT?t0.48.CeO2,Y2O3,Nb and nano Ni reduced the diffusion element coefficient of Ti-Al2O3 interface,especially for the Al element.At the same time,doping improves the interfacial reaction activation energy of Ti-Al2O3.Comparing with the undoped samples,activation energy of the composite doping with CeO2,Y2O3,Nb and nano Ni increased by 94.24%,2.27%,2.58%and 30.3%.The composition of interfacial reaction layer was changed due to reaction of Ce,Y,Nb and Ni elements.By comparison,the suppressed effects of CeO2 on the interfacial reaction of Ti-Al2O3 was best.The reason was that the melting point of CeO2 is low,Ce4+ has a smaller ionic radius,which are more likely to react with Al and O from Al2O3,accordingly reduces the diffusion distance and the concentration of Al element.