Microstructure Regulation And Thermal Deformation Mechanism Of Bimodal Sized Particulates Reinforced Titanium Matrix Composites

With the rapid development of aerospace technology,the demand for lightweight,high-strength and heat-resistant alloy structural materials for high-speed aircraft and high-performance engines is increasing.Titanium matrix composites based on near-?titanium alloy have high specific strength,high specific modulus and excellent high temperature strength,which can significantly improve the service temperature of the materials.However,the addition of hard ceramics phase reinforcement reduces the plastic rheological properties of composites,making the processing and forming of composites more difficult.In addition,near-?titanium alloy is sensitive to processing parameters,and its processing window is narrower.Isothermal forging process can effectively reduce the material deformation resistance,uniform structure and improve material utilization.The development of isothermal forging process for titanium matrix composites provides a broader prospect for the application of composites.It is important to study the thermal deformation behavior of composites for the formulation of isothermal forging process.In this paper,?TiB+La₂O₃?/Ti composites were prepared by in-situ technology.Two heat treatment processes,furnace cooling process and quenching and reheating process,were designed.The changes of microstructure and mechanical properties of?TiB+La₂O₃?/Ti composites after heat treatment were studied.The effect of heat treatment on the two-scale reinforcement was also discussed.Different initial microstructures were obtained by furnace cooling and quenching and reheating.Isothermal compression tests were carried out at different deformation temperatures.The effects of initial microstructures on thermal deformation behavior and microstructural evolution of composites were studied.The main conclusions are as follows:?1?The forged?TiB+La₂O₃?/Ti composites are mainly equiaxed primary?phase and a small amount of beta transformation structure.The matrix structure of composites during furnace cooling process is mainly lamellar?phase,while during quenching and reheating process,the matrix structure of composites is mainly fine needle?phase.Heat treatment has little effect on the morphology of TiB and La₂O₃.The hardness of the composites increases with the increase of the treatment temperature,but the tensile properties of the composites at room temperature are poor.The fracture mode of the composites is cleavage fracture according to the analysis of the tensile fracture surface.?2?In the early stage of isothermal compression,there is obvious work hardening,and the rheological stress-strain curve of the composites decreases after reaching the peak stress.Peak flow stress is sensitive to temperature.Peak flow stress decreases with the increase of temperature.Initial microstructures have a significant effect on the microstructural evolution of composites during hot deformation.A few microstructures spheroidize after deformation,and the rest are fined lamellar structures.?3?There are two distributions of?phase in the composites,parallel and intersecting,when the sample is furnace cooled to 920?.During the subsequent deformation,parallel lamellar?phases did not spheroidize,and a small number of intersecting?phases spheroidized.The mechanism of spheroidization was similar to that of grain boundary separation model.After quenching and reheating to 920?,the lamellar structure of the composites was refined.During the subsequent deformation,dynamic recrystallization of the fine lamellar structure resulted in a large number of refined equiaxed grains.?4?Micron TiB and sub-micron La₂O₃ particles play an important role in the evolution of microstructures during thermal deformation.High-density dislocations accumulate around the two-scale reinforcement,which hinder the movement of dislocations during deformation,strengthen the work hardening of composites and improve the rheological stress of composites.In addition,the high density dislocations around the reinforcement can provide a large number of nucleation particles and promote the dynamic recrystallization of the?phase.