Strengthening-Toughening Mechanism Of Ti/Al₃Ti Metal-intermetallic Laminated Composite

With the development of the aviation,aerospace and the civilian fields,the requirement on the performance of materials have been higher.Recently,the Ti/Al₃Ti metal-intermetallic laminated?MIL?composite as a lightweight structural material has received extensive attention.However,Al₃Ti exhibits poor room temperature plasticity and low high temperature strength,limiting its practical applications.Aim to toughen the Ti/Al₃Ti MIL composite,continuous fiber is introduced for improving the toughness of the brittle Al₃Ti phase that dominates the overall mechanical properties of the MIL composite in this work.A novel continuous Al₂O₃ fiber reinforced Ti/Al₃Ti MIL composite?CCFR-MIL?was fabricated by“Foil-Fiber-Foil”technique.The microstructure and performance of the CCFR-MIL composite were investigated and fiber strengthing-toughening mechanism in the composite was studied in detail.The results show that the centerline is always formed in the intermetallic layer as a result of oxides and impurities accumulating,and the Al₂O₃ fibers distribute along the centerline.Besides,metallurgical reaction at the interface between Al₂O₃ fiber and Al₃Ti dose not occur during the process.Moreover,the CCFR-MIL composite possesses excellent properties,such as low density,high modulus and multicomponent.Additionally,during deformation,the interfacial debonding between fiber and intermetallic,pull-out and splitting of Al₂O₃ fibers occur,which improve the strength and ductility of the CCFR-MIL composite.Nevertheless,the mechanical properties of the CCFR-MIL composite are limited under the loading parallel to the layers due to the existence of the centerline.In order to eliminate the intermetallic centerline,a new kind of composite was fabricated on the basic of CCFR-MIL composite,namely,continuous ceramic Al₂O₃ and shape memory alloy NiTi hybrid fibers reinforced Ti-Al metal-intermetallic laminated composite?CSMAFR-MIL?.The microstructure characteristics and performance of the CSMAFR-MIL composite were analyzed and the synergistic strengthening-toughening mechanism of Al₂O₃and NiTi fibers in the composite was studied.The experimental results indicate that the NiTi fibers are exhausted and the intermetallic centerline is significantly eliminated.Meanwhile,the bonding of Al₂O₃ fiber and intermetallic is improved without interfacial reaction.The inhomogeneous intermetallic layer is formed mainly containing Al₃Ti,Al₃Ti_0.8V_0.2.2 and Al₃Ni intermetallics.In addition,the density and elastic modulus of the CSMAFR-MIL composite are increased slightly compared to those of the CCFR-MIL composite.Moreover,the compressive properties of CSMAFR-MIL composite are close to those of CCFR-MIL composite under the loading perpendicular to the layers.Nevertheless,the strength and ductility of the CSMAFR-MIL composite are obviously increased when the loading is parallel to the layers,because the debonding along the centerline dose not occur in the CSMAFR-MIL composite.Besides,the main fracture mode of the Al₂O₃ fibers is fiber pulling-out and breakage,which can also improve the mechanical properties of the CSMAFR-MIL composite.The investigation on the microstructure evolution of CSMAFR-MIL composite show that during the fabrication process,the interfacial reaction between NiTi and Al occurs prior to that occurs between Ti and Al,and?Al₃Ti+Al₃Ni?zone forms around the NiTi fiber firstly.With the increasing of temperature and reaction time,a thin(Al₃Ti+Al₃Ti_0.8V_0.2)zone forms between Ti and Al,and at the same time the oxides or impurities are pushed to the reaction front by the moving liquid-solid interface.It is noteworthy that the thickness of?Al₃Ti+Al₃Ni?zone grows much faster than that of(Al₃Ti/Al₃Ti_0.8V_0.2)zone.As a result,when the reaction interface moving away from?Al₃Ti+Al₃Ni?zone meets the corresponding interface moving from(Al₃Ti+Al₃Ti_0.8V_0.2)zone,oxides or impurities accumulate at the reaction front of(Al₃Ti+Al₃Ti_0.8V_0.2)zone may disperse in the intermetallic layer instead of forming the intermetallic centerline.Electron Backscattered Diffraction?EBSD?analysis is carried out to discuss the microstructure of the CSMAFR-MIL composite.The results show that the intermetallics with fine grain microstructure exist near the interface between layers,on the contrary,the coarse intermetallic grains are far from the interface.Nevertheless,the aspect ratio of intermetallic grain is not changed with the grain size,and most of the intermetallic grains are equiaxed.In addition,compared to Al₃Ti crystal,it is easier for Al₃Ni crystal to slip along the glide plane.Moreover,the stress concentration appears in the interface not only between layers,but also between fiber and intermetallics.Therefore,interfacial debonding easily occurs during loading.The discussion of the comprehensive performance and damage evolution of CCFR-MIL and CSMAFR-MIL composites include three aspects:tensile test,three-point bending test and fracture toughness test.The results of tensile test show that strength and plasticity of the CCFR-MIL composite are higher than those of the Ti/Al₃Ti MIL composite.The main fracture mechanisms of the CCFR-MIL composite contain delamination,debonding along the centerline,brittle fracture of Al₂O₃ fibers and interfacial debonding between fiber and intermetallic.After optimization design,the tensile strength and plasticity of CSMAFR-MIL composite can be further improved.Furthermore,during fracture,debonding along the centerline dose not occur in CSMAFR-MIL composite,and fiber pull-out is the major fracture mode of Al₂O₃ fibers.Besides,the bending strength and energy absorption characteristic of the CSMAFR-MIL composite are the most excellent among the these composites,and the composites fail layer-by-layer in the course of the bending test.The results of fracture toughness measurement shows that the fracture toughness of CSMAFR-MIL composite is the highest.As for the mechanical properties of CCFR-MIL and CSMAFR-MIL composites,the advantages of CSMAFR-MIL composite are obvious,so it has a promising prospect in the application.Furthermore,a method has been presented to improve fiber reinforcing effect in this paper,i.e.post-hot isostatic pressing?HIP?treatment for CCFR-MIL composite.The results indicate that HIP treatment can effectively improve the density and elastic modulus of CCFR-MIL composite.Furthermore,the strength of CCFR-MIL composite is slightly reduced,and the plasticity of CCFR-MIL composite can be obviously increased ascribed to HIP treatment.