Microstructure Evolution And Mechanical Properties Of In Situ TC4 Matrix Composites In Isothermal Extrusion

Titanium matrix composites with good mechanical properties and excellent high temperature durability,has a prospect application in high technology field for aerospace,advanced military and so on.In situ discontinuously reinforced titanium matrix composites(DRTMCs)has become a key research field in recent years for its compatibility with the traditional titanium alloy preparation technology.However,the plasticity of DRTMCs is inevitably decreased due to the reinforcement when the most properties are improved.The deformation can simultaneously improve its strength and plasticity,but it is difficult to carry out for the high ratio of yield strength to tensile strength which makes its deformation very sensitive to the processing.The deformation temperature drops down sharply for a large temperature difference between die and material when the material deformed by the traditional forging,rolling and extrusion.The deformation of DRTMCs should be implemented under the condition of high strain rate and many passes with a small amount of deformation per pass are required.Therefore,the inhomogenous deformation and severe frature of reinforcements appeares in deformed DRTMCs which limited the improvement of its plasticity.In this paper,an isothermal extrusion technique of DRTMCs was proposed to eliminate the temperature difference between the material and the die and to provide a strong three direction compressive stress during deformation.It really improved the deformability of the DRTMCs to overcome the shortcoming of the exsisting deformation method and the DRTMCs were extruded by the new method.The effects of the main extrusion parameters,die angles,extrusion ratio and temperature,on the microstructure evolution and mechanical properties were studied thoroughly.Based on the study of the microstructure evolution of the TC4 alloy and the composites with various content and type of reinforcements,the mechanism of the deformation was revealed.The main research contents and conclusions are as follows:1)A variety of composites with different reinforcement types and contents were successfully fabricated by in-situ method.The effects of the type and content of reinforcements on the microstructure of the composites were studied.The addition of the reinforcements significantly refines the primary ? grains and the lamellae a of the composites,and prevents the formation of the thick continuous grain boundary a phase.The cluster phenomenon appeares with the increase of the content of the reinforcements.The plasticity is decreased dramatically for the formation of coarse TiC and lamellar TiB in the 10 vol.%(TiB+TiC)/TC4 composites.The reinforcements promote the dynamic recrystallization of the matrix and the deformation of the matrix comes more homogenous than that of TC4 alloy.2)TEM,HREM and SEM are used to study the evolution of microstructure during isothermal extrusion.The reinforcements and phase interface of ?/? hinder dislocation motion of a phase and promote the continuous dynamic recrystallization in a phase.A fine equiaxed grain with a thickness equal to the thickness of the lamellae is formed in the original lamellae.The reinforcements and phase interface of ?/? hinder dislocation motion of a phase and promote the driving force of alpha phase precipitation.The nanometer needle like a phase is precipitated under a low temperature deformation or during the cooling process after deformation.3)The EBSD are used to study the crystal orientation evolution of the alpha phase during isothermal extrusion.The mechanism of texture formation in a phase is revealed.The (?) slip in basal and cylindical surfce is the main deformation mode during extrusion and forms the strong fiber texture of<10(?)0>?RD.The (?)+(?) slip and (?) slip act crossover is a supplement deformation mode and forms a weak basal texture of cylindrical texture.The reinforcements weaken the fiber texture of<10(?)0>?RD and enhance thefiber texture of<11(?)0>? RD,which improve the plasticity of the composites.4)The effects of isothermal extrusion deformation on the morphology and distribution of the reinforcements were studied.It is revealed that there are three kinds of fracture mechanisms of TiB reinforcement in the deformation,which are rotational fracture,compression cracking and velocity mismatch tensile fracture.The rotation fracture and compression cracking usually occur in the early stage of diameter reduction deformation.The fracture surface is easier to combine with the matrix in consequent deformation.The velocity mismatch tensile crack usually occurs in the extrusion deformation near the exit stage.Harmful to the mechanical properties of composites due to the poor combination between the matrix and the fractured surfaces of reinforcements.It is necessary to adopt the lower strain rate to decrease the velocity difference between the front and end of the reinforcements and the fracture of the reinforcement would be reduced5)The effect of isothermal extrusion parameters on the mechanical properties of the composites was studied.The mechanism of strengthening and toughening of composites is expounded.The strength and elongation of the composites were improved by isothermal extrusion deformation at the same time.The strengthening of composites mainly comes from the bearing stress of reinforced TiB,dislocation strengthening and strengthening of continuous dynamic recrystallized fine grains.In this paper,the isothermal extrusion technology of titanium matrix composites was proposed and developed.The strength and plasticity of titanium matrix composites were improved simultaneously after extrusion.The strong plastic organic matching of titanium matrix composites was realized.The law that the improvement of the metal matrix composites strength will inevitably lead to the decline of its plasticity are broken.It also provides a new idea for the preparation of high strength and high toughness Composites.The strength of the TC4 matrix composites was more than 1100MPa with an elongation over 15%.The strength and ductility of the TC4 titanium alloy are significantly higher than that of the national standard.It provides a strong technical support for the application of this material in aerospace field.New material identification and technical standards developed by the aerospace sector in October 2016.It is expected to obtain large-scale applications in key areas of the country.