| The network structured titanium matrix composites?TMCs?exhibit high specific strength,stiffness,toughness,creep resistance,and elevated-temperature properties showing great potentials for aerospace applications.TMCs are also promising candidates of steels and superalloys for the sake of reducing weight by more than 40%.However,the low hardness and poor wear resistance of network architectured TMCs restrict the applications in anti-wear parts.The abrasion performance is expected to be improved by increasing the volume fraction of ceramic reinforcements,but the strength and plasticity tend to be sharply decreased.Therefore,obtaining good wear resistance and simultaneously keeping the high strength and toughness become the key prerequisite to the application in anti-wear components.In this work,the Ti64 powders,TiB2 powders,and graphite powders were used as raw materials to innovatively synthesize the high-content TMC coatings by the low-cost tungsten inert gas?TIG?cladding process with high efficiency,aiming at modifying the surface wear resistance of high-strength network structured TMC substrate.The single TiB?volume fraction of 20-50 vol.%?and hybrid TiB+TiC?TiB:TiC content ratio of 3:1,1:1,1:3?reinforced Ti64 composite coatings were designed and studied.Specifically,the microstructure characteristics and evolution mechanisms,interfacial orientation and properties,hardness and strengthening mechanisms,and wear performance have been analyzed and elucidated.It is revealed that with the ceramic content increasing from 20 vol.%to 50 vol.%,the TMC coating microstructures varied from a homogeneous distribution along depth direction to double-layered microstructures,which consist of coarse primary TiB prism,TiC dendrite,and fine eutectic ceramic phases at the coating upper zone through a dissolution-precipitation mechanism.Besides,the TiB prism was grafted with TiC building an intergrowth structure in the hybrid reinforced coatings.While at the nether region,the uniform and small-scaled rod-like TiB whiskers?TiBw?and spherical TiC particles?TiCp?were in-situ synthesized by the chemical reaction mechanism between liquid Ti and solid ceramics.The clean TiB/Ti interface was generated without chemical products,while a 90-140 nm thick Ti3AlC layer was formed via a peritectic reaction mechanism between partial TiC and matrix.Additionally,the Ti64 matrix grains were refined,and the content of supersaturated solid solution atoms was increased.Moreover,the heat input of TIG cladding process also has a significant influence on TMC coating microstructures.The lack of fusion and interfacial gap defects between coating and substrate appeared when the heat input was too low.By increasing that parameter,the quantity and volume fraction of coarse primary ceramic reinforcements were decreased,and the dilution rate was increased.With an optimized heat input of 162.7 J/mm,the strong metallurgical bonding between coating and substrate was achieved,accompanied by the retained network architecture in the heat affected zone?HAZ?.Subsequently,it is indicated that a novel Ti-TiB-TiC intergrowth phase was synthesized in hybrid TiB+TiC reinforced Ti64 coating after rapid melting and solidification.A detailed characterization told that TiC particles and matrix phases with a dimension of tens to hundreds of nanometers were embedded in TiB prism building that intergrowth phase.The interfacial orientation relationships of each phase boundary?PB?obtained by high resolution transmission electron microscopy?HRTEM?were revealed as follows:a)[2110]?-Ti//[111]?-Ti,?0001??-Ti//?110??-Ti,?0111??-Ti//?101??-Ti;b)[010]TiB//[2110]?-Ti,?001?TiB//?0111??-Ti;c)[010]TiB//[111]?-Ti,?001?TiB//?101??-Ti;d)[101]Tic//[2110]?-Ti,?111?TiC//?0110??-Ti;e)[112]TiC//[010]TiB,?220?TiC//?102?TiB.Additionally,first-principles modeling was employed to elucidate the interfacial atomic structures and properties.Revealing that the interfacial Ti-B ionic bonds contributed to the higher stability and bonding strength of B-terminated {201} TiB//{0001}?-Ti PB compared with other terminations.The corresponding formation energy???and work of separation(Wsep)were 1.442 J/m2 and 4.95 J/m2,respectively.The interfacial bonding strength was even higher than the internal strength of ?-Ti.As for TiC/?-Ti PB,the contacted facet was?111?TiC//?0110??-Ti,which exhibited higher Wsep with C-termination than Ti-terminated type.The ceramics presented a {220} TiC//{102}TiB PB with the ? and Wsep of 3.32 J/m2 and 5.78 J/m2.It is claimed that the strong B-C covalent bond was formed at the interface contributing to the interfacial bonding strength.The results show that the macro hardness of the TMC coatings was increased with the increase of reinforcement content and decreased with the increase of the heat input parameter.The 50 vol.%TiB/Ti64 coating fabricated at 162.7 J/mm exhibited a hardness of HRC 60 and 80.2%higher than that of the substrate?HRC 33.3?.Additionally,it is revealed that the primary TiB presented a feature of hardness:the base plane has a higher hardness of 1935.3 HV than that of the prismatic plane?1797.4 HV?,which is attributed to the B-B covalent bond arrangement in TiB crystals.Furthermore,the TiB-TiC intergrowth structure retained the large aspect ratio of TiB prism and exhibited superior load-bearing capacity with the microhardness as high as 2107.5 HV,is also higher than that of single TiB phase or single TiC dendrite.Owing to more supersaturated solid solution atoms,the microhardness of Ti64 matrix regions in hybrid TiB+TiC reinforced coating reached 606.5 HV,showing more improvement than the matrix hardness of single TiB reinforced coating?555.4 HV?.As a result,the maximum macro hardness of hybrid TMC coating was further increased to HRC 62.1.Meanwhile,the nanoindentation and microhardness results indicate that primary ceramic reinforcements with a large scale presented higher hardness and better load-bearing capacity in comparison to the fine eutectic phases.The strengthening mechanisms of the TMC coating are elucidated as the direct load-bearing reinforcing of primary ceramics,load-transfer strengthening of eutectic reinforcement,the grain refinement and solid solution strengthening effects of Ti64 matrix.The wear resistance at room temperature shows a similar trend as hardness,which was improved by increasing ceramic content or decreasing the heat input.The wear rate of 50 vol.%TiB/Ti64 coating fabricated at 162.7 J/mm was 2.65×10-4 mm3·N-1·m-1 showing a 60.9%reduction compared with the substrate(6.78×10-4 mm3·N-1·m-1).Additionally,the wear performance of hybrid TiB+TiC reinforced coating exhibited more improvement,in comparison to TiB/Ti64 coating,the wear rate of TMC coatings with a TiB:TiC ratio of 3:1,1:1,and 1:3 was further reduced by 71.5%,52.6%,and 72.3%,respectively.The network structured TMC substrate material was relatively soft,and the corresponding wear mechanisms include adhesive,abrasive,oxidation,and slight delamination wear.While the dominated mechanisms of TMC coatings are delamination,abrasive,and oxidation wear.The excellent wear performance of TMC coatings is attributed to the following aspects:the supporting and protection effects of ceramic reinforcements resisted scratching,which also pinned the transfer layer of oxides and wear debris offering cushion and lubrication effects.Moreover,the grain refinement and solid solution atoms of Ti64 matrix also contributed to improving the wear resistance.When the temperature increased to 500?,the wear performance of the TMC coatings decreased.Especially the?TiB+TiC?/Ti64 coating experienced a remarkable degradation,showing a higher wear rate than the TiB/Ti64 coating.It is revealed that the non-stoichiometric TixC?1<x?2?phase was synthesized in the?TiB+TiC?/Ti64 coating.The internal carbon vacancy site of TixC is easily occupied by oxygen resulting in rapid oxidation at elevated temperature,which further leads to the decrease of load-bearing capacity and wear resistance of TixC reinforcement itself.Consequently,the enhancing effect and abrasion performance of hybrid TMC coating were notably decreased. |
PDF Full Text Request