| TiAl-based alloy is an intermetallic compound with low density,high melting point,high specific strength and excellent oxidation resistance.It has potential for application in aerospace.However,the inherent brittleness of TiAl-base alloy makes it more difficult to process,which also limits its industrial application.In order to toughen TiAl-based alloys,researchers have proposed ?-TiAl-based alloys in which ?-stabilizing elements is added to guarantee the primary phase is the ?-phase,and take use of the orientation relationship of ?-phase,?-phase and lamellar to control the orientation of the lamellar orientation and makes it parallel with the direction of the load in the solidified structure.Peritectic reaction of ?-TiAl alloy during the solidification process has a great influence on the solidification path and microstructure in room temperature.In this paper,TiAlbased alloys with and without peritectic reaction were prepared by the composite cold crucible directional solidification method,quenching and DSC testing were also conducted.The primary phases,solidification paths and phase transition temperatures of the two TiAl-based alloys during solidification were investigated.The solidification microstructure characteristics of the TiAl-based alloys under different solidification parameters were observed and the process of their evolution was proposed The mechanical properties of the directional solidified specimens were tested and the fracture mechanism of the specimens was proposed.In this paper,the TiAl-based alloys with two components were analyzed by quenching and DSC,it was found that both of their primary phases were found to be ?-phase;DSC curve analysis shows that the phase transition of ?2??,?2??,B2??,??? and ??? reacted in the solid phase transition temperature range.And the solidification path were obtained as L?L+?????+???+B2+???+?2+B2+? ??2+B2+?.Two compositions of TiAl-based alloys were prepared using the composite cold crucible method at different cooling rates,and samples with directional characteristics were obtained.The samples of Ti43.5Al6V1 Cr alloy showed a severe banded structure.It tends to form periodic banded structure at low cooling rates,and tends to form a single banded structure at high cooling rates.The structure of Ti45Al2Cr2 Nb alloy consist of the columnar grain zone and the final solidification zone and is more homogeneity along the axial direction.The columnar grain in the sample gradually taper and increase in inclination as the cooling rate increases.The room temperature structure was observed,the Ti43.5Al6V1 Cr alloy was found to consist of(?2/?)lamellar,independent ? and B2 phases,while Ti45Al2Cr2 Nb alloy are essentially full-lamellar with only a few independent ? and B2 phases.The microstructure characteristics of the orientation sample and the solidification path of the TiAl-based alloy was combined,it is concluded that the formation of the B2 phase is caused by the enrichment of the ?-phase stable element.The B2 phase in the Ti43.5Al6V1 Cr alloy is caused by ? segregation during the ??? transformation,and is mainly distributed between lamellar cluster.The B2 phase in the Ti45Al2Cr2 Nb alloy is caused by S segregation during the L?? transformation,and is mainly distributed on the segregation zone within the grain.Analysis of the mechanical properties of the directional solidified specimens of the two alloys revealed that all of the thickness of lamellar,the content of the B2 phase and the Y2O3 particles have an influence on their properties.According to the sample fracture scanning photos,the mechanism of the lamellar,B2 phase and Y2O3 particles in the fracture process were analyzed.It was found that the resistance is larger when the crack penetrates through the lamellar cluster,while the resistance at the boundary between the B2 phase and the matrix is smaller.And the Y2O3 particles were found to have the effect of promoting crack propagation. |
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