Research On Microstructure And Mechanical Properties Of TC18Ti Matrix Composite During Heat Treatment And Isothermal Compression

The research is financially supported by973Program. The objective is to developnew kind of Ti matrix composite for C919Plane Program. TC18Ti matrix compositesreinforced with trace TiB and TiC are fabricated by in situ synthesis method in thiswork. The as cast ingots are subjected to thermo mechanical processing and heattreatment.The research emphases are the microstructural characteristics, which have not beenreported in previously published works, as well as the effects of trace reinforcements.It is revealed that the spheroidization of primary α is not the result of precipitation.The spheroidized α is formed during its growth. Moreover, the splitting of α duringheat treatment is observed. It is favorable for the spheroidization of α when thedislocation density in α is low. Also is low density dislocation the precondition for thesplitting of α. α may separate during its spheroidization process. This means that βâ†'αphase transformation and αâ†'β can concur under the heat treatment. Thespheroidization of α can accelerate the splitting of α. The essential reason for themicrostructure phenomena is that α has non coherent interface. Since α has thetendency to be spheroidized, typical bimodal structure and equiaxed structure can notbe formed. Owing to the difference in the extent of spheroidization, α phase presentsmorphology diversity.The trace TiB and TiC have significant effects on refining microstructure duringheat treatment. However, the decrease of β grain size gradually decreases withincreasing the reinforcements. It is suggested that the increase rate for the nucleationrate drop faster than that for the Zener drag. This is the primary reason for theabove mentioned microstructure phenomenon when the temperature of heat treatmentexceeds β transus temperature. Since the growth of α is restricted by β, the refinementof α phase presents similar tendency as that of β. When the temperature of heattreatment is lower than β transus temperature, the reason is that the increase rate of thedriving force for boundary migration of β grain drops slower than that of the pinningforce on boundary migration. Since α are evenly dispersed in β, and ripening plays acritical role in the growth of α. The sizes of α in the alloy and composites show littledifference. According to assessment, the microstructure refinement plays anidentically important role in improving the strengths of the composites comparing with load bearing of TiB and dispersion strengthening of TiC.It is revealed that the primary reason for the retardation to the spheroidization of αis the pining effects of the segregation of alloying elements on dislocation motion.The spheroidization rates of α in the composites are higher than that in alloy. Tracereinforcements accelerate the spheroidization rate of α by accelerating diffusion andpining boundary migration of β grain. However, reinforcements often induce thesegregation of alloying elements, and thus retard the spheroidization of α. Therefore,the spheroidization rate of α does not obviously increase with increasing thereinforcements.The trace reinforcements have significant effects on accelerating thespheroidization of α, refining β phase and promoting microstructure homogeneityduring isothermal compression. The specimens are subjected to heat treatment beforeisothermal compression. When the initial microstructure of the composite specimen isα β structure, α in the composite is almost spheroidized during the compression. Theeffects of the trace reinforcements on promoting microstructure homogeneity duringisothermal compression can be summarized to three points. First, the acceleration ofthe spheroidization of α, second, the load bearing of TiB, third, the refinement of βphase. Since α grain has non coherent interface, the spheroidization mechanism in thiswork is different from that reported in previously published works.It is proved by this work that TC18/(TiB+TiC) is advanced in design and hastremendous application value. The investigations in this work establish the favorablebase for industrial applications of TC18Ti matrix composite.