Effect of Boron Or Carbon on the High Temperature Deformation Microstructure and Mechanical Properties of TiAl Alloy

Author:Li Ming Zuo

Supervisor:chen yu yong xiao shu long


Degree Year:2019





The high niobium containing TiAl alloys exhibit much more excellent high-temperature oxidation resistance,ultimate strength and creep resistance,as the high-temperature structural material above 900°C.However,much niobium addition could cause the coarse columnar crystal formation in the solidified microstructure,severeβsegregation around colonies boundaries.The nonuniform and coarse microstructure lead to the unstable mechanical properties,worsen the room-temperature plastic and fracture toughness of as-cast alloys.Therefore,the main research hotspots of high Nb-TiAl alloys focus on the preparation of alloys which own the refined and uniform microstructure,the excellent mechanical properties now.In this case,boron or carbon were added during the melting process to improve the solidified microstructure,mechanical properties and thermal deformation capacity of high niobium containing TiAl alloys.Meanwhile,the microstructure evolution mechanism and dynamic recrystallization mechanism during the high temperature deformation,as well mechanical properties and strengthening and toughening mechanism in the forged alloys,etc.were also studied in this paper.The effect of boron or carbon on the solidification microstructure and room temperature(RT)mechanical properties of Ti-43Al-6Nb-1Mo-1Cr alloys was studied.With boron or carbon content increasing,coarse columnar crystal gradually transformed into refined equiaxed crystal,βsegregation and Al elements segregation were reduced gradually in the solidified microstructure.When the boron content reached 1.0at.%,average grains sizes were refined from 584.94μm to 32.54μm,stripe shaped TiB formed around grain boundaries,the ultimate tensile strength(UTS)at room temperature increased from 479.69MPa to 681.32MPa,the RT fracture load was enhanced from 78.58MPa to 108.01MPa.With the carbon content increasing to1.0at.%,average sizes of(α2+γ)colonies decreased from 584.94μm to 30.00μm,long Ti3AlC precipitated among the microstructure,the UTS of asted alloys at room temperature increased from 479.69MPa to 730.11MPa,the RT elongation was enhanced from 0.19%to 0.51%,the RT fracture load increased from 78.58MPa to107.71MPa.The large constitutional supercooling and heterogeneous nucleation caused by boron or carbon addition were the refinement mechanisms in the microstructure.Fine-grains strengthening and secondary phase strengthening caused the dramatically increased strength at room temperature.TiB and Ti3AlC phase increased resistance of crack propagation to enhance fracture load.The high temperature tensile properties at 800°C,850°C and 900°C of Ti-43Al-6Nb-1Mo-1Cr alloys with different boron or carbon content were studied.Boron addition could enhance the ductile brittle transition temperature(BBTT)to 850-900°C.When the boron content reached 1.0at.%,the UTS of alloys at 800°C increased from534.85MPa to 656.12MPa,the UTS of alloys at 850°C increased from 507.36MPa to642.43MPa,the UTS of alloys at 900°C increased from 379.91MPa to 508.44MPa.With the carbon content increasing to 1.0at.%,the UTS of alloys at 800°C,850°C and900°C were enhanced to 756.14MPa,724.38MPa and 585.62MPa.0.5at.%carbon addition could increase the UTS and elongation at high temperatures together.With boron or carbon addition,theα2/γlamellae with solution strengthening and TiB or Ti3AlC obstacles could strengthen the alloys through the increased content and energy limit of dislocation pile-ups.Less slipping systems opened in TiB while dislocations could not slip through TiB phase.However,Ti3AlC phase could provide positions for dislocation and twins movement to support the plastic deformation.The effect of boron or carbon addition on the flow stress curves in different conditions,dynamic recrystallization(DRX)critical model,hot processing maps,thermal deformation behavior and microstructure evolution of Ti-43Al-6Nb-1Mo-1Cr alloys were studied.Boron or carbon addition decreased the peak stresses in slow strain rates(0.001s-1 and 0.01s-1)at higher temperatures,decreased the critical strain of DRX starting,reduced cracking and unstability during hot processing,quickened theγ→αphase transformation,promoted the superplastic deformation at 1250°C,expanded the stable region in hot processing maps.The refinement caused by boron or carbon could increase the volume fraction of colony boundaries with high density dislocations,TiB or Ti3AlC obstacles could both cause dislocation pile-ups and promote DRXed grains nucleation.With boron content increasing to 1.0at.%,the hot deformation activate energy increased to 669.46kJ/mol,unstable regions in the hot processing maps were narrowed and moved to higher and lower strain rates.Carbon addition enhanced the hot deformation activate energy to 695.88 kJ/mol,while unstable regions in the hot processing maps moved to lower strain rates.Based on the hot processing maps of Ti-43Al-6Nb-1Mo-1Cr-(B/C)alloys,suitable process parameters and multi-steps forging process were selected to process the alloys.The forged pies with excellent microstructure and mechanical properties were obtained,while the effect of boron or carbon addition on the properties of forged alloys was studied.Through the multi-steps forging process,the microstructure of Ti-43Al-6Nb-1Mo-1Cr-(B/C)alloys was refined dramatically while the UTS increased dramatically.Average grain sizes of forged Ti-43Al-6Nb-1Mo-1Cr alloys were refined to 12.63μm,while the UTS at room temperature and 800°C were enhanced to645.65MPa and 676.05MPa.Average grain sizes of forged Ti-43Al-6Nb-1Mo-1Cr-0.6B alloys were 10.32μm,while the broken and dispersed TiB could enhance the UTS at room temperature and 800°C to 843.51MPa and 729.00 MPa through dislocations and secondary phase strengthening.Averge grains sizes of forged Ti-43Al-6Nb-1Mo-1Cr-0.5C alloys were refined to 18.37μm.The DRXed phase not exceeding 5μm and dispersed Ti3AlC phase increased dislocation pile-ups contents and pinned the conlony boundaries and lamellaer interfaces,which caused the UTS at room temperature and 800°C increasing to 725.18MPa and 739.12MPa.