Fabrication And Properties Of Continuous SiC Fiber Reinforced Ti-Al₃Ti Laminated Composites

The aim of the present work is to improve the mechanical properties of intermetallic compounds by introducing metal layer and ceramic fiber reinforcements,and to investigate the microstructure evolution,mechanical behavior and failure mechanism of the novel Ceramic-Fiber-Reinforced Metal-Intermetallic-Laminated?CFR-MIL?composite.Ti foils,Al foils and SiC fibers were used to fabricate Ti-?SiC_f/Al₃Ti?CFR-MIL composite by vacuum hot pressing.Several methods were adopted to optimize the interface between SiC fiber and Al3Ti,including annealing treatment,nickel plating of fiber and titanium barrier protecting.In addition,microstructure evolution and reaction mechanism of Ti-?SiC_f/Al₃Ti?laminated composite were studied via scanning electron microscopy?SEM?,X-ray diffraction?XRD?and electron backscatter diffraction?EBSD?.The mechanical properties of Ti-?SiC_f/Al₃Ti?laminated composite were investigated by microhardness,nanoindentation,tensile test,compression test,bending test and fracture toughness measurement.The failure behavior was studied via observation on fracture surface.In light of this,the relationship among microstructure,fracture mechanism and mechanical properties were established.Ti-?SiC_f/Al₃Ti?laminated composite was prepared by vacuum hot pressing at 660?,which was comprised of alternating Ti layers and SiC_f/Al₃Ti fiber-reinforced-intermetallic compound layers?SiC fiber as reinforcement?.Al₃Ti was the solely newly-formed Ti-Al intermetallic compound in the as-fabricated composite.Additionally,together with the so-called centerline,SiC fibers distributed along the center section of Al3Ti layer and were bonded with Al₃Ti by metallurgical interaction and mechanical joint.There existed a trace amount of TiC and TiSi₂ formed at the SiC_f/Al₃Ti interfacial zone,as well as some voids.Besides,the layers were straight and compactly bonded with uniform thickness.Ti-?SiC_f/Al₃Ti?laminated composite exhibited superior strength and ductility than those of Ti-Al₃Ti laminated composite without fiber reinforcement.The failure of the Ti-?SiC_f/Al₃Ti?laminated composite contained the following process:1)the formation and propagation of brittle cracks in Al₃Ti layer;2)the cracks deflection and bifurcation ascribed to the arresting of Ti layer and SiC fiber,causing the delamination and fiber-debonding;3)the pullout and fracture of SiC fibers,fragmentation of Al3Ti,delamination of Ti/Al3Ti interface and the buckling or fracture of Ti.Moreover,Ti layers showed typical microvoid coalescence fracture mode and Al₃Ti exhibited brittle failure,including intergranular and transgranular fracture.Furthermore,the debonding,pullout and fracture of SiC fibers were observed obviously.All the above failure behaviors of the composite can increase the resistance to crack propagation,lengthen the path of crack growth and dissipate or absorb extra fracture energy.As a result,the mechanical properties of Ti-?SiC_f/Al₃Ti?laminated composite were improved effectively.Interfacial reactions occurred in Ti-?SiC_f/Al₃Ti?laminated composite due to the annealing treatment at 950?.TiC,TiSi₂,Al₄SiC₄ and Ti₂AlC were formed between SiC fiber and Al₃Ti,meanwhile,Ti₃Al,TiAl and Al₂Ti layers were formed between Ti and Al₃Ti,which was credited to the diffusion of Ti,Al,Si and C atoms at the interfacial zones.Furthermore,the annealed Ti-?SiC_f/Al₃Ti?laminated composite exhibited superior ultimate tensile strength and fracture elongation than those of the as-fabricated composite.Ti layer showed ductile fracture and Al₃Ti fractured with obvious cleavage steps and transgranular modes.Moreover,SiC fibers were cracked without fiber-pullout due to the strong interfacial bonding obtained during annealing process.Nickel-plated-fiber-reinforced metal-intermetallic-laminated?NPFR-MIL?composites were fabricated using nickel plated SiC fibers,in which dense interfacial reaction zones were formed between SiC fibers and Al₃Ti.The reaction zones were comprised of Al₃Ni intermetallic compounds and combined well with SiC fiber and Al₃Ti.In electroless nickel plated SiC fiber reinforced MIL composite,the irregular Al₃Ni zone surrounded SiC fiber.Unlike that,the Al₃Ni zone in nickel electroplated fiber reinforced MIL composite was arched and distributed at the left and right sides of SiC fiber.The different shapes and distributions of Al₃Ni phase caused the distinct mechanical properties of the NPFR-MIL composites and various failure modes of SiC fibers,including fiber debonding and pullout,as well as fiber breakage and splitting.Ti-?SiC_f/Al₃Ti?laminated composite was prepared using pure Ti foils as barrier layer between SiC fibers and Al foils to eliminate the influence of centerline on SiC_f/Al₃Ti interface.The morphology observation indicated that no centerline was found adjacent to SiC fiber in the composite.Compact interfacial bonding was obtained between SiC fiber and Al₃Ti without voids and crack.Furthermore,the debonding of centerline was not observed in this region in the tensile fractured composite,which was obviously different from that in the region without Ti barrier.These indicated that Ti barrier can effectively transfer the defective centerline from fiber reinforced intermetallic zone to another one.A new fiber-foil stacked method was proposed to increase the percentage of fiber reinforcement in Ti-?SiC_f/Al₃Ti?laminated composite.The morphology observation showed that there were five equidistantly arranged layers of SiC fiber in each Al₃Ti layer,the SiC_f/Al₃Ti interface was continuous and without void.In addition,the intermetallic was dense and uniform and the interface between Ti and Al₃Ti layers were tightly combined.Beside,a novel kind of CFR-MIL composite with double-scale layered structure was designed and prepared,in which there existed three small-scale Ti-?SiC_f/Al₃Ti?layered units between the adjacent TC4 alloy layers.The layered unit was composed of alternative pure Ti layers and SiC_f/Al₃Ti layers.In the mall-scale units,SiC fibers were boned well with the intermetallic Al₃Ti.Moreover,the thin pure Ti layer was tortuous and discontinuous,and the thick TC4layer was straight and continuous.Pure Ti layer was connected with the adjacent TC4 layer by intermetallic Al₃Ti.