Study On Microstructure Control And Mechanical Behaviors Of In-situ（Ti,Nb）B/Ti₂AlNb Composites

Ti₂AlNb alloys have been regarded as the preferred high-temperature structural material for the weight reduction of aerospace engines,and have been expected to replace nickel-base superalloys in the 600-750℃ temperature range.However,insufficient high-temperature mechanical properties limit the further application of the alloys.In this study,（Ti,Nb）B/Ti₂AlNb composites were fabricated by ball-milling and subsequent spark plasma sintering（SPS）in order to improve the mechanical properties of Ti₂AlNb alloys.The effects of raw powder state,sintering parameters,（Ti,Nb）B contents,and hot forging on the microstructure and mechanical properties of（Ti,Nb）B/Ti₂AlNb composites were systematically studied using SEM,TEM,EBSD,and mechanical properties tests.The structure and properties of（Ti,Nb）B/matrix interfaces were unveiled.The synthesis mechanism of in-situ（Ti,Nb）B reinforcement and the preferred precipitation mechanism of the α2 phase（Ti3Al compound,P63/mmc）in the prior particle boundaries（PPBs）area were revealed.Besides,the strengthening and toughening mechanism of as-forged（Ti,Nb）B/Ti₂AlNb composites were clarified.Two preparation processes,i.e.blended elemental powder metallurgy（BEPM）and pre-alloyed powder metallurgy（PAPM）were adopted to prepare in-situ（Ti,Nb）B/Ti₂AlNb composites.The results illustrated that the composites fabricated by LaB6 and Ti₂AlNb powders with the ball milling parameters of 250 rpm/8 h and the sintering parameters of 1250℃/20 min/45 MPa had the best mechanical properties.When compared with the Ti₂AlNb based alloy,the 3.2 vol%（Ti,Nb）B/Ti₂AlNb composite prepared by this preparation process had an 11.8%and 9.2%increase in yield strength at 25 and 650℃,respectively.The（Ti.Nb）B reinforcement formed in the composite possessed a B27 crystal structure and was generated by the chemical reactions between LaB6 and Ti-Al-Nb compound/solid solution.The starting temperature of the in-situ reaction was in the 850-1030℃ temperature range.The Nb-rich area that appeared during the sintering promoted the formation of transition phases（Ti,Nb）3B2 and Bf-（Ti,Nb）B.According to HRTEM results,the（Ti,Nb）B reinforcement maintained three preferred crystallographic orientation relationships with Ti₂AlNb matrix phases,which could be written as:[1120]α2//[010]（Ti,Nb）B and（1100）α2//（100）（Ti,Nb）B;[111]B2//[010]（Ti,Nb）B and（112）B2//（100）（Ti,Nb）B;[110]O//[010]（Ti,Nb）B and（110）O//（100）（Ti,Nb）B.The（Ti,Nb）B/matrix interfaces,i.e.（1100）α2/（100）（Ti,Nb）B,（112）B2/（100）（Ti,Nb）B and（110）O/（100）（Ti,Nb）B interfaces were all coherent.The first-principle calculation results displayed that the bonding strength（adhesion work）of these three interfaces were in order of（112）B2/（100）（Ti,Nb）B（5.86 J/m2）<（110）O/（100）（Ti,Nb）B（6.08 J/m2）<（1100）α2/（100）（Ti,Nb）B（6.41 J/m2）,and the difference in bonding strength was caused by the difference in the lattice distortion of the matrix phases.On the three interfaces,the B atoms on the side of the reinforcement chemically bonded with the Ti,Al,and Nb atoms on the side of the matrix phases through covalent bonds.The differences in microstructure and mechanical properties of the composite prepared by BEPM and PAPM were studied.The composite fabricated by pre-alloyed powder metallurgy had a lower oxygen content and α2 precipitated content when compared with the composite synthesized by blended elemental powder metallurgy,and the reinforcement presented a three-dimensional quasi-continuous network distribution,which ensured that the PAPM composite had more excellent mechanical properties.After that,it was studied that the influence of sintering parameters on the microstructure and mechanical properties of PAPM composites.The results demonstrated that the sintering parameters mainly affected the mechanical properties by changing the microstructure of the prior particle boundaries（PPBs）area;increasing the aspect ratio of the（Ti,Nb）B reinforcement and reducing its size was favorable to improve the mechanical properties of the composite when the continuous distribution of brittle phases in the PPBs area was avoided.By the further analysis on the microstructure of PPBs area,it was found that the preferred precipitation of the brittle α2 phase in the PPBs area which was caused by the formation of（Ti,Nb）B and the promotion of（Ti,Nb）B on the α2 heterogeneous nucleation was one of the reasons for the PPBs premature failure during high-temperature tensile tests.Hot forging at the temperature of B2 single-phase region and subsequent annealing at the temperature of the B2+O dual-phase region could suppress the α2 preferential precipitation,reduce the angle between the PPBs length direction and the radial direction,and promote（Ti,Nb）B short fibers to be aligned along the radial direction.All of those avoided the PPBs premature failure when tensile along the radial direction at high temperature,leading to the improvement of the mechanical properties of the composite.The tensile strength of the as-forged 3.2 vol%（Ti,Nb）B/Ti₂AlNb composite at 650℃and 750℃ was 24.7 and 29.5%higher than that of the as-sintered composite.After hot forging,the O phase formed the[100]o//compression direction fiber texture and（Ti,Nb）B short fibers generated the[010]（Ti,Nb）B//radial direction fiber texture,which caused the microstructural anisotropy.The anisotropic microstructure resulted in the anisotropy of the mechanical properties of the as-forged composite.According to quantitative analysis,the strengthening mechanisms of the as-forged（Ti,Nb）B/Ti₂AlNb composite,in the order of strengthening effect,were matrix texture strengthening>strengthening caused by matrix microstructure refinement>（Ti,Nb）B texture strengthening.