Synthesis And Microstructural Evolution Of TiAl/Nb Composites With Core-shell Structure

γ-TiAl based alloys have become advanced structural materials with promising applications in aerospace and automotive industries,due to their low density,high specific strength,high specific stifiness,excellent high-temperature oxidation resistance,superior creep resistance and other awe-inspiring comprehensive mechanical properties.However,the poor plasticity at ambient temperature seriously limits the industrial application of such alloys.In view of this,the core-shell architecture design strategy of ductile Nb shell wrapped with y-TiAl matrix phase is proposed to overcome the mechanical property defects of y-TiAl based alloys.The material preparation and microstructure evolution rules are systematically studied,which further broadens the lightweight high-performance structural material system.The preparation process of TiAl/Nb composites with core-shell structure,including the preparation of composite powders and the synthesis of bulk materials,was studied firstly.It is found that the core-shell composite powders of atomized yTiAl matrix being uniformly wrapped by fine Nb particles could be prepared by lowenergy ball milling technique,and its formation rate continuously increased with the prolongation of milling time.The peak value（0.926～0.975）could be reached when the ball milling was turned to 180rpm/168h.Continued ball milling would result in the breakage of the matrix powders and the shedding of Nb particles.The core-shell structural composites with controllable content of ductile phase could be prepared by hot pressing sintering（HPS）.By comparing the sintered microstructures obtained at different temperatures,it can be known that 1200℃ reactive sintering could not only ensure the metallurgical bonding between composite powders,but also guarantee the volumn fraction of ductile phase in the shell region.During HPS,the composite powders slided relatively to achieve dense arrangement under the action of load,and then realized metallurgical strong combination of core/shell and shell/shell interfaces via atomic diffusion crystallization nucleation,forming a dense core-shell structure microstructure in which the matrix particle phases were wrapped by honeycomb distributed Nb shell as well as resulting intermetallics.The room temperature strength and plasticity of the prepared TiAl/Nb composites are significantly improved in comparison to the traditional y-TiAl alloys,whose performance metric is 1.02GPa YS,2.44GPa CS and 32.9%FS.The synergetic increase of strength and plasticity is attributed to the extension of the second stage of strain hardening rate.The preferred deformation of Nb phase coordinated the sliding of the matrix phase,and then the proliferation of geometrically necessary dislocations maintained the continuity of the strain gradient,resulting in the continuous increase of the back stress and redistribution in the adjacent phase interfaces in turn,which stimulated the accommodation deformation of the intermediate phase and the matrix particle phase,thus weakening the effect of stress concentration.As a result,the continuous deformation ability and persistent strain hardening property were ameliorated synchronously.The TiAl-Nb diffusion reaction system was studied.The result shows that the thickness of diffusion reaction layer increased exponentially with the increase of annealing temperature and soaking time,which is mainly the result of vacancy mediated diaplacement diffusion of Ti,A1 and Nb principal atoms along the direction of decreasing chemical potential gradient.The phase transition path on Nb side was:β→δ→δ+σ→σ→B2_n→B2+γ_n,while the phase transition sequence on TiAl side was:γ→γ_ss→γ_ss＋α２→（γ+α₂）+（α₂+B2）→α₂+B2→B2+γ_n.The transition from y to α₂ could essentially be divided into two steps:γ→α and α→α₂.The crystal structure transformation of L1₀→HCP was through the 2Bδ shift of layer C relative to layer A under Shockley partials,followed by the unit cell expanding outward to a certain extent along the（11（?））and（110）orientations via Shockley partials and full dislocations,respectively.The local ordering of HCP structure into DO19 superstructure could be achieved by the fact that the atoms in layer A migrated inward along the（11（?）0）and（1（?）00）orientations,while the atoms in layer B sheared toward[1（?）00]orientation.Convex lenticular γ_n existed mainly in the form of twins in the B2 phase region.The fluctuation of solute atomic concentration in B2 produced high Ti、low Al and high Al、low Nb regions,the former was a spontaneous result of the stabilization of B2 structure and the latter afforded the compositional preparation for the nucleation of γ_n.In such process,the{110}close packed plane inevitably staggered defects such as vacancies and dislocations,which provided structure conditions for lattice reconstruction.Consequently,the B2 phase was transitioning to the y phase in the light of{111}_γ//{110}_B2,<1（?）0>_γ//<11（?）>_B2 orientation relationship.The formation of y twins was associated with the slip of the Shockley partials,driven by the release of internal stress caused by the distortion of the surrounding lattice.The aging temperature of 600℃ and 1000℃ was unfavorable to the precipitation of B2 precipitates.When aging at 800℃,the precipitation amount of nano B2 precipitates in γ regime and α₂ phase boundary increased with the holding time extending from 6h to 30h.This is because the nucleation and growth of the precipitates required the solute atoms in the vicinity to continuously aggregate to the defect areas such as the grain and subgrain boundaries,which led to the rearrangement and fluctuation of the atomic concertration in these areas,forming a precipitation area with a composition different from that of the parent phase.Since the solid solubility of Nb in γ phase was significantly higher than α₂ phase,the segregagtion of Nb atoms in γ phase was sufficient to achieve the nucleation condition of B2 precipitates,whilstα₂ phase had to rely on the segregation of Cr and Nb atoms at the phase boundary to induce the nucleation of such precipitates.The nucleation of y annealing twins could be induced by 9R and SF structures,the former needed to undergo γ→9R and γ→9R→γ_T transition.The internal stress induced Shockley partial dislocations to emit and slip from the close packed planes in different stacking periods in turn,thus realizing the shear of the atomic layer above the activation plane,leaving behind three ISFs,which thereby formed nine period stacking ordering（9R）structure.During ITB migration,when Shockley partial reacted with Frank partial to form full dislocation,9R recovered to y;when Shockley partial pairs and Frank partial were merged into full dislocation,9R changed to γ_T.Finally,ITB overlapped with y boundary to form CTB,resulting in the dislocation concellation of positive and negative signs.SF induced twinning nucleation and growth was essentially equivalent to the procedure of continuous generation and merger of SF.When a close packed plane emited Shockley partial,Aδ shear occurred in all atomic layers above such plane,and the resulting SF could be considered as a two-layer nanotwin.The subsequent emission of Shockley partial on the adjacent close packed plane caused the atomic layer above the plane to continue to shift Aδ,at which point ISF transformed into ESF,corresponding to a three-layer nanotwin.The twin successively thickened as Shockley partials were emited from other neighboring｛111}crystallographic planes.