Microstructure And Mechanical Properties Of TiAl-based Laminated Composite Sheets Strengthened And Toughened By Periodic Titanium Alloy Layers

Ti-Al based alloys have the advantages of low density,high melting point,high specific strength,and excellent capacity,which are expected to replace nickel-based superalloys in the aerospace and automotive industries.Due to the intrinsic brittleness and poor work ability of Ti-Al intermetallic compounds,these disadvantages limit their applications.Therefore,by effectively coordinating and balancing the contradiction between strength and toughness,and realizing the synergistic improvement of strength and toughness,is the key to promote and expand the application of TiAl based alloys.Inspired by the multi-scale and multi-level structure of shells,the design of bionic structures has become an effective way to improve the performance of TiAl-based alloys.Based on the composite technology and bionic structure design concept,using Ti₂AlC as the reinforcing phases and Ti-6Al-4V foils as the toughening layers,the TC4/?Ti₂AlC-TiAl?based composite sheets were prepared by spark plasma sintering technology.The phase composition,interfacial layer structure,laminated structure,crack propagation characteristics,fracture morphology,and room temperature mechanical properties of the composite sheets were studied.The strengthening and toughening mechanism of the laminated composite sheets were revealed,and finally the preparation technology prototype of the laminated composite sheets was presented.The composite layer of the laminated composite sheets was mainly composed of Ti₂AlC,?2-Ti3Al,?-TiAl and a small amount of TiC phases.An obvious laminated structure obtained.The interface reaction product between the composite layer and the toughened layer with good interfacial bonding was Ti₃Al phase.The Ti₂AlC reinforced phases in the composite layer with a network structure were mainly distributed at the grain boundaries.Without Ti₂AlC,the composite sheets with five titanium alloy layers had the best performance.The flexural strength and fracture toughness in the direction perpendicular to laminated structure were 455.74 MPa and 21.65 MPa·m^1/2,respectively.And the flexural strength and fracture toughness in the direction parallel to laminated structure were 499.19MPa and 15.82 MPa·m^1/2,respectively.When the theoretical content of Ti₂AlC was 10 wt%,the composite sheets with five titanium alloy layers containing through-holes in the direction perpendicular to laminated structure had the best performance,the flexural strength and fracture toughness were 287.08MPa and 14.94 MPa·m^1/2,respectively.When the theoretical content of Ti₂AlC was 30 wt%,the composite sheet presented nearly isotropic characteristic,the fracture toughness in the direction perpendicular to laminated structure and parallel to laminated structure was 15.76MPa·m^1/2 and 14.11 MPa·m^1/2,meanwhile,the corresponding flexural strength was 241.42MPa and 239.03 MPa,respectively.When the theoretical content of Ti₂AlC was 20 wt%,the flexural strength and fracture toughness of the composite sheet with seven titanium alloy layers in the direction perpendicular to laminated structure achieved the maximum values of 564.86 MPa and 39.15MPa·m^1/2.When the loading direction was parallel to the laminated structure,the flexural strength and fracture toughness of the composite sheet with 10 wt%Ti2AlC reached the maximum values of 725.64 MPa and 23.51 MPa·m^1/2.When the theoretical content of Ti₂AlC was 10 wt%,the composite sheets with seven titanium alloy layers containing through-holes had the best performance.When the loading direction was perpendicular to the laminated structure,the flexural strength and fracture toughness were 351.87 MPa and 27.97 MPa·m^1/2.When the loading was parallel to the laminated structure,the flexural strength and fracture toughness were 436.52 MPa and 23.36MPa·m^1/2.When the theoretical content of Ti₂AlC was 20 wt%,the laminated composite sheet presented nearly isotropic characteristic.The fracture toughness in the direction perpendicular to laminated structure and parallel to laminated structure was 19.86 MPa·m^1/2and 20.38 MPa·m^1/2,respectively.the flexural strength in the direction perpendicular to laminated structure and parallel to laminated structure was 370.55 MPa and 363.73 MPa.The TC4 titanium alloy layers played an important role in energy dissipation,likewise changed the crack propagation path.The cracks mainly propagated along the interface between the composite layers and the toughening layers,whereafter deflected into the composite layers.When introducing the through-hole structure,the cracks propagated along the edge of the through-holes into the adjacent composite layer.In the expansion process,mixed crack propagation characteristics such as bridging,crack tip bifurcation,deflection appeared.The existence of Ti₂AlC reinforced phases hindered the crack growth,which reduced the driving force of crack propagation,and then the crack tip happened passivation.And the Ti2AlC particles were mainly pinned at the grain boundaries of the matrix,which restrained the growth of the?-TiAl/?₂-Ti₃Al grains and realized refinement strengthening effects.In a word,the biomimetic laminated composite structure design effectively realized the strengthening and toughening effects,which provided new ideas for the development of multi-scale and multi-stage structure TiAl-based alloys with high strength and toughness.