Microstructure And Mechanical Properties Of TC4-Mo_f/Al₃Ti Laminated Composites

Metal-Intermetallic Laminated(MIL)composites have drawn much attention due to their excellent mechanical properties.Among them,TC4-Al3Ti MIL composite is a striking one because it possesses numerous enviable advantages such as low density,high strength and modulus,making it a brilliant candidate for aerospace application.TC4-Al3Ti MIL composite is designed with an alternating brittle and ductile laminated structure to somewhat relieve the stress concentration of the Al3Ti phase during loading process.More importantly,this biomimicking design based on the principle of energy dissipation can weaken negative effects of the impact of raw microcracks on the mechanical properties of the materials.So favorable mechanical properties can be obtained.The method of laminated composite is a new way to prepare metal composites.However,the ductility and toughness of the sheet prepared by this method are still far behind those of traditional materials at room temperature,which limits the further development of this kind of composite.So,it is necessary to break through this bottleneck.Many researches reported that the addition of continuous fibers could effectively improve the mechanical properties of laminated composites.These fibers can not only improve the toughness of the composite material by preventing the stress concentration in the brittle Al3Ti matrix,but also can strengthen the matrix through its own superior performance.Thereby,superior comprehensive mechanical properties can be achieved.In this paper,continuous Mo fibers which has excellent mechanical properties at room temperature are introduced into Ti-Al layered composite materials using vacuum hot pressing sintering technology,and the microstructure and mechanical properties of this composite are studied in detail,and the interface effect is used to achieve the perfect combination between the fiber and the matrix,thereby overcoming the material’s performance shortcomings and making the material the overall toughness index meets the application requirements.By optimizing the structural design of composite materials,combined with a variety of microscopic characterization methods,in-depth research on the microstructure and mechanical properties of composite materials can provide reliable theoretical guidance and sufficient data support for the structural design,production and application of such materials.By studying the growth kinetics of Ti-Al and Mo-Al systems,it is found that the growth kinetics index of Ti-Al and Mo-Al compounds are close to 1 and 0.5 respectively,and the reaction mechanisms are interface reaction and intragranular diffusion,respectively.Combined with the design parameters,680℃ is selected as the holding temperature for preparing Mo fiber reinforced laminated composites.Then,in order to increase the volume fraction of the reinforced fiber,the diameter of the fiber is selected and its arrangement is optimized.In addition,a thin layer protection method was employed.By adding a thin Ti foil between the Al layer and Mo fiber,the reaction between Mo fiber and Al liquid was delayed,the time of MoAl combination reaction was reduced,the volume fraction of Mo fiber in the composite was increased,and the strength and toughness of the composite were further improved.Using the optimized process parameters,The composites with varied Ti layer volume fraction,fiber diameter and wire arrangement direction were successfully prepared.Meanwhile,TC4-Al3Ti composites containing no fiber with the same structural parameters were prepared for compare as well.The microstructure and phase composition of the composites were analyzed by various testing methods,and the interface between Mo fiber and matrix was especially studied.The results show that Al3Ti is the only product of Ti-Al intermetallic,while there are four kinds of Mo-Al compounds,which are nested around the fiber.Based on the thermodynamic calculation of Mo-Al solid-liquid interface reaction and combined with the interface microstructure,the formation sequence of Mo-Al intermetallic under this condition is inferred as follows:Al12Mo(outermost layer)→Al8Mo3(innermost layer)→Al4Mo→Al5Mo.The tensile,compressive and three-point bending tests of the composites were carried out under varied loading directions.The strengthening and toughening ability and mechanism of the fiber under different loads and different loading directions were studied in depth.Considering the influence of reinforcing fiber on the properties of the composite under various loads,the optimum structural parameters of TC4-Mof/Al3Ti were determined as the volume fraction of Ti layer of 33%and the diameter of original fiber of 250μm.the tensile strength and failure strain of the corresponding sample were 617MPa and 3.8%respectively,which were increased by 29.1%and 42.9%when the load was parallel to the fiber direction.Combined with the fracture morphology of the composites,the energy loss per unit area in the fracture process of the intermetallic layer was calculated quantitatively.According to the diameters of different original fibers(180μm and 250μm),the energy consumption per unit area is 4.10 and 4.42 times that of fiber-reinforced composite materials,respectively.And the influence factors and mechanism of fiber reinforcement on the energy consumption in the fracture process of the composites were analyzed.The Mo fiber reinforced composites were annealed.Through further interface reaction,the proportion of ductile phase in the interface of the composites was increased,and the toughness of the composites was improved.After annealing,a new Ti-Al intermetallic phase with gradient distribution is formed at the Al3Ti/Ti interface:Al2Ti,TiAl and Ti3Al.With the increase of annealing temperature,the diameter of Mo fiber continues to shrink,Mo-Al compound gradually transforms into Mo-rich Al8Mo3,Mo element diffuses into the lattice of Ti-Al compound,and Al3(Ti,Mo)and(Ti,Mo)Al replacement solid solution phases are formed.When the annealing temperature exceeds 1100℃,a new phase AlMo3 appears at the interface between Mo and Al8Mo3.By comparing the mechanical properties of the composites under different loading modes after annealing treatment,the optimal annealing process of TC4Mof/Al3Ti is determined as 900℃ and annealing time of 60min.Under this process,the longitudinal bending strength of 33Ti-250FRI sample reaches 342MPa,which is 41.9%higher than that before annealing treatment.