Mechanisms Of Strengthening And Toughening And Microstructure Evolution Under Non-Equilibrium Solidification Of TiAl Alloy Synergistically Reinforced By V/Cr And In-Situ Nano-TiB₂

TiAl alloys,as intermetallic compounds with the properties of both metals and ceramics,have the advantages of low density,high specific strength,outstanding resistance to oxidation and creep,and are considered to be an important alternative to titanium and nickel-based high-temperature alloys in aero-engines.However,the lack of high-temperature strength and room-temperature plasticity of TiAl alloys limit their application scope.In order to enhance the room temperature plasticity and high temperature strength of TiAl alloys to expand the application field and application temperature of TiAl alloys,TiAl alloys have been strengthened by means of in-situ ceramic particles.However,the enhancement of high-temperature strength of TiAl alloy reinforced by ceramic particles is not as effective as the enhancement of roomtemperature strength.This is due to the existence of ceramic particles at room temperature leading to a significant increase in grain boundaries and dislocations within the TiAl matrix,which can improve the room temperature mechanical properties of TiAl due to the dislocation-dislocation and dislocation-grain boundary interactions as dislocations slip at room temperature.However,at high temperature,dislocations move mainly in a way of climbing and grain boundaries tend to slip.The dislocation-dislocation and dislocation-grain boundary interactions disappears,and thus the enhancement effect of ceramic particles at high temperature is weaker than at room temperature.How to synergistically strengthen TiAl alloys through nanostrengthening phases and solid solution elements to achieve simultaneous improvement of room temperature and high temperature properties has important research value.When the alloying element V/Cr is added to solid-solution strengthen the TiAl alloy,the existence of nano-ceramic particles is conducive to the solid-solution of alloying elements,which is conducive to hindering the climbing of dislocations through solid-solution atoms and strengthening the high-temperature stability of grain boundaries through nano-particle pinning boundaries.The synergistic strengthening of alloying elements and nano-ceramic particles to simultaneously improve the room temperature and high temperature mechanical properties of TiAl alloys is an innovative research work.In this paper,V element and Cr element will be added on the basis of in-situ nano-TiB₂ particles to improve the room and high temperature mechanical properties of TiAl alloy.The influence and mechanism of in-situ nano-TiB₂ particles and V/Cr elements on the microstructure,room temperature and high temperature mechanical properties of TiAl alloys were studied.The main innovations are as follows:1.Based on comparative analysis of the microstructure evolution and microstructural regulation characteristics of V/Cr elements and nanoscale TiB₂ particles synergistically reinforced TiAl alloy,the microstructural evolution mechanism of TiAl alloy synergistically reinforced by V/Cr elements and nanoscale TiB₂ particles is revealed.In γ-TiAl,both V and Cr atoms tend to occupy Tiatomic sites and undergo strong d-d hybridization with Tiatoms,forming Ti-V and Ti-Cr bonds with high bond energies.However,the Ti-V bond in Ti₁₄Al₁₆V₂ is stronger,while the Ti-Cr bond in Ti₁₅Al₁₆ Cr and Ti₁₄Al₁₆Cr₂ are stronger,and both are stronger than the Ti-V bond in Ti₁₄Al₁₆V₂.The Cr element reduces the B/G value of γ-TiAl,which is conducive to improving ductility.Nano-TiB₂ particles aggregate at grain boundaries and form a network structure,reducing the grain size of the TiAl matrix by 77%.After adding V and Cr elements,most of the V and Cr elements are solid-solved in the γ-TiAl and α2-Ti3 Al phases of the TiAl matrix,while a small portion segregates at grain boundaries.However,compared with V elements,more Cr elements tend to segregate at grain boundaries,indicating lower solid solubility of Cr elements.2.Based on the analysis of mechanical properties of TiAl alloy enhanced by V/Cr and nano-TiB₂ particles at high temperature and room temperature,the effect of synergistic strengthening of V/Cr elements and nano-TiB₂ particles on the mechanical properties of TiAl alloy at room temperature and high temperature was revealed.The room temperature yield strength and ultimate compressive strength of the TiAl alloy strengthened only with in-situ nanoTiB₂ increased to 849 MPa and 1887 MPa,respectively,which were 19.9% and 7.5% higher than that of the TiAl alloy.At 600 °C,the yield strength（589 MPa）,ultimate compressive strength（1545 MPa）,fracture strain（32.2%）and product of strength and plasticity（34.8 GPa·%）of 4 vol.% TiB₂/TiAl increased by 16.4%,6.7%,5.9% and 14.5%,respectively.Under the synergistic effect of in-situ nano-TiB₂ and V elements,the room temperature yield strength,ultimate compressive strength and product of strength and plasticity of TiAl composites increased to 1059 MPa,2213 MPa and 22.9 GPa·%,respectively,compared with 4 vol.% TiB₂/ TiAl increased by 24.7%,17.3% and 9.0%,compared with TiAl alloy,increased by 49.6%,27.6% and 8.5%,respectively;the yield strength and ultimate compressive strength at 600 °C were increased to 876 MPa and 1612 MPa,respectively increased by 73.1% and 10.2% compared with TiAl.Under the synergistic effect of in-situ nano-TiB₂ and Cr elements,the room temperature yield strength,ultimate compressive strength,fracture strain and product of strength and plasticity reached 1121 MPa,2188 MPa,15.8% and 25.9 GPa·%,which are 32.0%,16.0%,6.0% and 23.3% higher than that of 4 vol.% TiB₂/TiAl,respectively.Compared with TiAl,the yield strength,ultimate compressive strength and product of strength and plasticity of 4 vol.% TiB₂/TiAl-3Cr increased by 58.3%,24.7 and 22.7%.The Cr element improves the yield strength more obviously,and the V element improves the ultimate compressive strength more obviously.At the same time,the Cr element also improves the room temperature plasticity.3.Based on the analysis of deformation behavior and fracture characteristics at room temperature and high temperature,the strengthening mechanism and toughening mechanism of V/Cr elements and in-situ nano-TiB₂ particles synergistically strengthened TiAl alloy were revealed.The strengthening mechanisms at room temperature are grain refinement strengthening,thermal mismatch strengthening and Orowan strengthening of nano-TiB₂ particles,and solid solution strengthening of V/Cr elements.Among them,the thermal mismatch strengthening of nano-TiB₂ particles and the solid solution strengthening of V/Cr elements contribute the most.The high-temperature strengthening mechanisms include thermal mismatch strengthening of nanoTiB₂ particles pinning grain boundaries and solid solution strengthening of V/Cr elements（reducing vacancies,forming Ti-V/Cr bonds with higher bond energy than Ti-Tibonds,prevent dislocations from climbing）.The room temperature toughening mechanism is that nano-TiB₂ particles and V/Cr elements cause grain boundary strengthening,grain refinement,crack scattering bifurcation and crack deflection leading to an increase in the crack propagation path.Nano-TiB₂ particles have a significant effect on improving high-temperature toughness,but the high-temperature plasticity of TiB₂/TiAl composites is reduced because V/Cr elements seriously hinder the dislocation climbing,which reduces toughness.