Microstructure And Properties Of TiBw/TA15 Composites Prepared By Spark Plasma Sintering

By introducing high strength ceramic phase into titanium alloy matrix,the bottleneck of the service temperature limitation for titanium alloys will be overcomed owing to the effective combination of toughening effect for the matrix and strengthening effect for the reinforcements.As a developed powder sintering technology,spark plasma sintering is expected to achieve a good metallurgical bonding between titanium alloy particles and reinforcements in a shorter time and at a lower temperature,and thus obtain the TA15 titanium alloy and TiBw/TA15 composites with improved properties.In this dissertation,the TA15 titanium alloys and TiBw/TA15 composites were fabricated using spark plasma sintering under different sintering parameters.The microstructures and mechanical properties of the sintered samples were investigated.The evolution of microstructure and the strengthening mechanism of mechanical properties were further analyzed.Moreover,the high-temperature cyclic oxidation experiments of the samples prepared at the optimal sintering parameter were carried out,and the high-temperature oxidation resistance mechanism of TiBw/TA15 composites was revealed.Comparing the microstructures and mechanical properties of TA15 titanium alloys sintered at different sintering parameters,the optimal sintering temperature,holding time and pressure of the spark plasma sintering for TA15 titanium alloy powders were determined to be 900?,5 min and 50 MPa,respectively.For the sampes sintered at the optimal sintering parameters,the room and high-temperature?500??yield stresses were 936.4MPa and 544.1MPa,the room and high-temperature?500??ultimate compression stresses were 1641.3MPa and 1004.6MPa,and the plastic strains at room and high-temperature were 25.9%and 32.4%,respectively.In addition,the microstructures were mainly affected by the sintering temperature,while the holding time and pressure had weak effects.When the sintering temperature was 900?,the microstructures were composed of equiaxed?phase,thick?laths and a small amount of intergranular?phase.With the sintering temperature rising to 1000?and above,the microstructures became typical Widmanst?tten microstructure.The rapid fabrication of TiBw/TA15 composites with network morphology was realized by SPS process at 1100?for 10 min.The room and high-temperature?600??yield stresses of the composites sintered at 1100?for 10 min were 1172.5 MPa and616.3 MPa,respectively.Compared with the TA15 titanium alloy fabricated under the optimal parameters,the room-temperature yield stress of the composite was increased by 25.2%,and the high-temperature yield stress was 72.2 MPa higher than that of TA15titanium alloy tested at 500?.The mechanical properties of the composite showd that the maximal service temperature of TA15 titanium alloys could be elevated by at least100?.It was found that the addition of TiB₂ particles had no significant effect on the rapid densification of TA15 titanium alloy powders during SPS process.The main factor affecting the aspect ratio of TiBw was the sintering temperature.The average aspect ratio of the TiBw for the composites sintered at 1100?,1200?and 1300?were 11.08,7.41 and 7.40,respectively.The holding time mainly affected the average diameter of TiBw,but the effect on the average apect ratio was not obvious.The influence of TiBw on the modification of microstructure for titanium alloy matrix was mainly manifested in the following aspects:?a?the pinning effect of TiBw on the high temperature initial?grain boundary,?b?providing effective nucleation sites for equiaxed?phase,?c?refining the width of?colony,i.e.from 102.5?m of TA15 titanium alloy to 17.1?m of TiBw/TA15 composites when sintered at 1100?,and?d?the microstructure of the TiBw-lean regions were composed of basket-weave structures and some small sized?colony,instead of Widmanst?tten microstructure.The equiaxed primary?phase at the TiBw-rich boundary regions of TiBw/TA15composites could improve the compatible deformation ability and the small sized?colony distributed in TiBw-lean regions was beneficial to reduce the dislocation pile-up and inhomogeneity of deformation in the micro-regions.Furthermore,the basket-weave structures around the small sized?colony could prevent the propagation of micro-cracks in the microstructure.The above mentioned factors contributed to exerting the plasticity of the TiBw/TA15 composites.The improvement of strength for TiBw/TA15 composites could be attributed to several aspects:?a?the interface between in-situ reacted TiBw and the matrix bonded well and no brittle phases such as oxide formed at the interface,which effectively provided the load-bearing and transfer effects of TiBw,?b?dislocation strengthening induced by TiBw at the TiBw-rich boundary regions;?c?as the average aspect ratio of TiBw was larger than the critical value?¹.95?,the TiBw reinforcments showed similar strengthening effect.The reaction of oxidation for alloy elements in TA15 titanium alloy and TiBw/TA15composite could proceed spontaneously in the cyclic oxidation temperature range of873-1073K,based on the thermodynamic calculation.The effective activation energy Q of the TA15 titanium alloy and TiBw/TA15 composites was determined to be 306.938KJ/mol and 341.819 KJ/mol at the temperatures of 973-1073K,respectively.The experimental results indicated that the introducing of TiBw increased the critical energy required for oxidation of TA15 titanium alloy.With the increase of oxidation temperature,the cross sections of the oxide scales for the two materials exhibited layered microstructure,consisting of an outer layer?TiO₂ phase and Al₂O₃ phase?,an inner oxide layer?mainly TiO₂ phase?,and a transition layer.It was determined that the growth of TiO₂/Al₂O₃ layer was mainly controlled by outward diffusion of aluminum and titanium elements,while the growth of inner TiO₂ layer was mainly controlled by oxygen ingress diffusion for both materials at higher oxidation temperature.The reason that the TiBw/TA15 composite exhibited better oxidation resistance than TA15 titanium alloy at high-temperature was revealed.The TiBw reinforcements distributed at the boundary of three-dimensional spatial network structure and there existed lattice defects in TiBw-rich boundary regions.These lattice defects could provide a large number of nucleation sites for the oxide during the oxidation process of TiBw/TA15 composite.Then,numerous nucleations of oxides effectively reduced the spacing between oxide particles and promoted the formation of complete and continuous oxide scales at the TiBw-rich boundary regions during oxidation,which could effectively hinder the diffusion of elements,thus decrease the oxidation rate of the composite and improve the oxidation resistance.