Study On Fabrication Of Mo_f/TiAl Composites And Their Deformation And Fracture Behavior

High-temperature structural materials play a key role on the realization of therevolutionary development of the aerospace propulsion system. Among them,TiAl-based alloys have become very competitive ones, because they have theadvantages of low density, high strength, good oxidation resistance and excellentcreep resistance. Currently, they have been applied to engine blades, turbine,exhaust valves and other components. However, the rapid development of theaerospace industry proposes higher demands on engine materials. In order to obtaina broader development and meet harsher service conditions, TiAl-based alloys needto further improve the fracture toughness at room temperature and increase the hightemperature strength. In the present work, toughening and strengthening of theTiAl-based alloy were realized by reinforcing it with continuous Mo fibers. TheMo_f/TiAl composite were successfully fabricated by powder slurry casting andvacuum hot pressing. Effects of the fabrication process parameters on themicrostructure and mechanical properties of the composites were studied; thedeformation and fracture behavior of the composites were observed and analyzed,and the toughening and strengthening mechanism of the composites were obtained.In this paper, the pre-coupling of the matrix powders and fibers wereconveniently and effectively realized by improved powders slurry casting process.Qualified preforms were prepared by powder slurry with a composition of10gPMMA,70ml acetone and60g Ti and Al powders. The prepared preforms are denseand void-free with uniform fiber distribution and homogeneous dispersion of Ti andAl powders. The composites were eventually obtained by degassing and vacuum hotpressing of the preforms.In the prepared Mo_f/TiAl composites, the fibers distribute uniformly and haveno obvious change in microstructure after being hot pressed, which maintain themorphology of columnar crystals with high aspect ratio; the matrices mainly containTiAl phase and a small amount of Ti3Al phase. Increasing the hot pressingtemperature and prolonging the hot pressing time can benefit the densification andmicrostructural uniformity of the matrices but exacerbate the interface reaction. There are two continuous reaction layers at the interface of matrix and fibers, whichare δ-(Mo,Ti)3Al phase with columnar grains distributing in a radial way andβ’-(Mo,Ti)A1phase with coarse equiaxed grains, respectively, from the Mo fiber tothe matrix. Nano-indentation test shew that δ phase has higher nano-hardness thanthat of β’ phase. The shear strength of the composite interface is higher than367MPa and debonding position is located at the Mo/δ interface. Study on thegrowth dynamics of interface phases indicates that both the interface phases growparabolicly and their grow rates increase with the temperature increasing.The mechanical properties of the composites are influenced by the fabricationprocess parameters, composite degassed at380℃for1hour and hot pressed at1100℃for1hour has the optimal properties. The composite reinforced byunidirectional fibers has a longitudinal and transverse bending strength of735.4MPa and249.3MPa respectively, and the former is slightly affected by theloading direction. Bending strength of the composite reinforced by orthogonalfibers decreases to374.4MPa. In the temperature range of700-1000℃，bendingstrength of the composites reinforced by unidirectional and orthogonal fibers firstlyincrease and then decrease, and they all reach the maximum value762.9MPa and564.4MPa at800℃.Compressive yielding strength of the composites decrease with the testingtemperature increasing, and it is slightly higher in the longitudinal direction than inthe transverse direction. The way of fiber distribution has little affect on it.Toughness of the composites reinforced by unidirectional fibers is as high as23.55MPa·m1/2, which is about50%higher than that of the matrix TiAl alloy.Under the co-effect of the brittle matrix and ductile fibers, the deformation andfracture of the Mof/TiAl composite show different way from that of thehomogeneous materials. The deformation and fracture two courses intertwine withthe change of the load carried by the matrix and fibers. Under room temperaturebending and tension in the longitudinal direction, the deformation and fracturecourses of the composite can be divided into three stages. In the first stage, thedeformation of the whole composite comes from the jointly elastic deformation ofthe matrix and fibers and they carry the load together; in the second stage,deformation of the matrix and fibers mismatch, many cracks initiate and propagatein the matrix in order to match the deformation of the composite, and so the load carried by the matrix decreases, while the fibers continue to be deformed elasticallyand the load carried by them increase; in the third stage, the matrix fails due to thesaturation of the cracks in it, so the fibers carry the load alone and deformation ofthe composite is provided by the elastic and plastic deformation of the fibers, andthe composite fractures after the fracture of the necked fibers. Crack initiates in thematrix and can propagate along the TiAl grain boundaries and across the Ti3Algrains. When meeting the δ and β’ two interface phases, cracks can directly goacross them but be prevented by the fibers, the two interface phases deform andcrack together with the matrix. Mo/δ、δ/β’ and β’/TiAl interface keep intact beforethe plastic deformation of the fibers, but after that, Mo/δ interface debonds becausethe brittle δ phase and ductile cannot deform jointly. Mo fibers are insensitive tocracks, so the crack in the matrix cannot propagate across the fibers. The fibersfracture when the stress reaches the tensile strength.At high temperatures, bending deformation and fracture of the composite showdifferent way with that at room temperature. Because of the improvement of theductility and plasticity, the matrix can match the whole deformation of thecomposite not by cracks but by deformation. After the necking and fracture of thefibers, the composite fractures quickly due to the insufficient load carrying abilityof the matrix.Under longitudinal compressive load, fibers curve greatly under the greatestshear stress with a45°angle to the longitudinal direction without fracture, and thematrix among them breaks with the interface Mo/δ debonding. Under normalcompressive load, the broken surface of the composite due to shear stress is parallelthe fiber axial and has many steps. Under high temperatures, the toughness andplasticity of the composite increase, the composite just deform plastically withoutrupture.The strengthening and toughening mechanisms have been revealed bycomprehensive analysis of the mechanical properties, deformation and fracturebehavior of the composite. The balance between the high matrix density, goodinterfacial bonding and less fiber damage is a guarantee of the maximum increasingof the composite strength. The toughening mechanisms of the composite include thebridging, plastic deformation, pull-out and debonding of the fibers and thedeflection of the cracks. Among them, the plastic deformation of the fiber makes the Mo fiber have higher toughening effect than that of the brittle ceramic fibers.