Preparation And Pinning Effect Of In-situ Ti-TiBw Composites

Titanium matrix composites(TMCs)have broad application prospects in the fields of aviation,aerospace,and chemical engineering,due to their high specific strength,specific stiffness and excellent corrosion resistance.However,the inherent interfacial bond between the reinforcing phase and the matrix,and the matching of the thermal expansion coefficients of the TMCs,which leading to their strength and plasticity/toughness do not match,and the matrix will soften at high temperatures,resulting in a decrease in performance and service reliability,limiting their wide application.In order to solve the above problems,in this study,the powder metallurgy method(P/M)was used to prepared Ti Bw-enhanced and N solid solution cooperatively reinforced TMCs by the In-situ reaction between Ti and BN.On this basis,the prepared TMCs were annealed at different conditions.It was investigated that the effects of the reinforcement phases Ti Bw contents on the grain size of TMCs at different temperatures and times.The grain size,microstructure and mechanical properties were measured in order to analyed the grain growth kinetics,the influence of Ti Bw on the microstructure and mechanical properties and the pinning mechanism of grain size.The main research contents and conclusions are as follows:Prepared the Ti-Ti Bw composites by in-situ reaction of Ti and BN: The Ti and BN mixed powders were sintered at 1000 °C,an in-situ reaction between Ti and BN prepared the highly dense Ti Bw-reinforced and N-solution cooperatively reinforced TMCs.Microscopic observation indicated that Ti Bw is an elongated whisker and distributes at the grain boundary.Combining XRD,XPS and TEM analysis results,the N atoms generated by the decomposition of BN exist in solid solution in the Ti matrix,and the reaction has completed and the interface is clean.Effect of Heat Treatment on Grain Size of Ti-Ti Bw : Compared with pure Ti,with the increase of BN content,the formed Ti Bw content gradually increased,the matrix grain size gradually decreased,and Ti Bw significantly refined the matrix grain size.After 900~1200 ?heat treatment,Ti Bw significantly upgrated the grain coarsening temperature of the matrix.Before the roughening temperature,the grain size did not change significantly with the increase of annealing temperatures and times.At this time,the formed Ti Bw effectively inhibited grain growth.After the roughening temperature,the aspect ratio and morphology of the Ti Bw changed significantly,the effect of inhibiting grain growth is weakened,and the grain grow rapidly with the increase of annealing temperatures and times.The Pinning effect mechanism of Ti Bw on the grain size of Ti-Ti Bw : Compared with pure Ti,with the increase of the amount of BN added,the content of formed Ti Bw gradually increased,Ti Bw distributes at grain boundaries,hinders the grain boundary migration,and some grain boundaries are occupied by Ti Bw,which reduces the interfacial energy of the system.It will increase the interfacial energy by separating grain boundaries and whiskers,and lead to the increase of interfacial energy.The grain boundary migration rate will gradually decrease,and the activation energy required for grain boundary migration will gradually increase.When the long axis of Ti Bw is parallel to the grain boundary,with the increase of the aspect ratio and the Ti Bw content,the Ti Bw pinning effect on the grain boundary is more obvious.The Ti Bw at the grain boundary tends to be distributed parallel to the grain boundaries.Effect of Heat Treatment on Mechanical Properties of Ti-Ti Bw : Compared with pure Ti,the solid solution of N atoms in Ti matrix increases with the increase of BN addition,the hardness of samples increased gradually.After heat treatment at 900-1200 °C,the hardness of the matrix gradually increased with the increase of temperature,reaching the maximum at 1200 °C.Compared with pure Ti,with the increase of the amount of BN added,the tensile strength of the sample first increased and then decreased,and the elongation decreased.The fracture mode changes from toughness to brittle fracture.The strength and plasticity of the samples with 0.1wt% BN were 791 MPa and 13.6%,respectively.After heat treatment at 900 °C,the tensile strength increased by 36 MPa and the elongation did not change.In this temperature,Ti Bw has obvious pinning effect and plays the role of fine grain strengthening and Ti Bw loading.The strength and plasticity can be maintained at a good level.As the temperature continues to increase,the Ti Bw is coarsened and the pinning effect is reduced.At this time,the grain coarsening is severe,resulting in a decrease in the strength of the samples.