Microstructure And Mechanical Properties Of Hybrid Reinforced Titanium Matrix Composites By Casting

Discontinuous reinforced titanium matrix composites have higher specific strength and specific stiffness than titanium alloys.It is one of the most important focus among the field of materials.Hybrid strengthening takes the physical/mechanical characteristics of two or more than two enhancements into account,so it promotes and makes up for each other.The hybrid effect among the multiple reinforcements improves several properties of the single reinforced material significantly.In this study,La₂O₃/Ti,?TiB+La₂O₃?/Ti,?TiC+La₂O₃?/Ti and?TiB+TiC+La₂O₃?/Ti titanium matrix composites are fabricated by vacuum consumable melting,heat processing and heat treatment with lanthanum doped powder,titanium diboride powder and graphite powder based on the matrix of pure titanium.Phase compositions of the pure titanium and titanium matrix composites,matrix structures and morphologies of the reinforcements before and after the forging are observed.Vickers hardness before and after forging are tested and a theoretical model of hardness is established to reveal the influence of different reinforcements on Vickers hardness.Fracture surface and longitudinal fracture near the fracture surface are observed to analyze the fracture mechanism of forged composites based on the tensile properties at ambient temperature and high temperature.The influence of the coupling effect among different reinforcements on the mechanical properties of the composites is investigated.The main conclusions are obtained as followed:?1?The matrix of as-cast material exhibits a near fully coarse alpha structure.La₂O₃ presents short fiber and dispersion distribution particles which correspond to primary phase and secondary precipitation phase.TiB presents short fiber and distributes randomly.Two morphologies of TiC are dendrite which corresponds to primary phase and equiaxed which corresponds to eutectic and secondary precipitation phase.The microstructure of forged pure titanium is equal axis alpha.However,the matrix of forged composites is composed of the deformed alpha microstructure.?2?The hardness of as-cast composites is increased by the solid solution enhancement caused by the formation of the hard phase reinforcement and the addition of the elements.The microstructure of matrix is improved and several casting defects are eliminated by hot forging.The effect of reinforcements on the dislocation of matrix composite deformation leads to the dislocation density in matrix higher than in single metal deformation,and the hardness of composites after hot forging is increased.?3?Tensile strengths of pure titanium,La₂O₃/Ti,?TiB+La₂O₃?/Ti,?TiC+La₂O₃?/Ti and?TiB+TiC+La₂O₃?/Ti are 334 MPa,625 MPa,840MPa,834 MPa and 868 MPa respectively.The elongations are 35.4%,21.5%,8.4%,2%and 1.1%respectively.Type of tensile fracture for pure titanium,La₂O₃/Ti and?TiB+La₂O₃?/Ti at ambient temperature is ductile.Type of tensile fracture of?TiC+La₂O₃?/Ti and?TiB+TiC+La₂O₃?/Ti at ambient temperature is brittle.The failure of composites at ambient temperature is the load-bearing fracture of the reinforced body.The strengthening mechanisms at ambient temperature are as follows:the load-carrying capacity of reinforcements,the dispersion strengthening of La₂O₃ particles and the reduction of the oxygen content in the matrix.?4?The sequences of yield strength and tensile strength at high temperature are as follows:?TiB+TiC+La₂O₃?/Ti>?TiC+La₂O₃?/Ti>?TiB+La₂O₃?/Ti>La₂O₃/Ti>pure titanium.The yield strength and tensile strength of the same material at high temperature are significantly lower than at ambient temperature.With the increasing of temperature,the tensile strength of material decreases approximately linearly.Type of tensile fracture for materials at high temperature is ductile,and there are a large number of dimples in the fracture.The fracture failure of composites at300?are different from each other.