| Titanium matrix composites(TMCs)reinforced with TiB whiskers(TiBw)arc the most promising high-performancc discontinuous reinforced titanium matrix composites(DRTMCs)due to high specific strength,excellent wear resistance as well as deformable processing.Howcver,in situ TiB whiskers reinforcement in composites are likely to coarsen at high temperatures and lose the pinning effect on the matrix resulting in a decrease in the strength of the composites at elevated temperatures,but the mechanism of coarsening TiBw is not yet clear in existing studies.Therefore,based on the preparation of TiBw-reinforced TMCs.this paper systematically studied the process of TiB whiskers decomposition,growth and coarsening through heat treatment,and further analyzed the coarsenm echanism.TiB2 powder(2.95 wt.%)and Ti powder were used as raw materials to fabricated Ti-TiBw composites by spark plasma sintering(SPS).The phase compositions and microstructure of Ti-TiRw composites were investigated,particularly the morphology,size and distribution of the TiB reinforcemt.On this basis,by rapidly cooling the Ti-TiBw composites,the effect of heat treatment conditions on the morphology and size of the TiBw reinforcement in the Ti-TiBw composites and coarsen mechanism of TiB whiskers at high temperature were investigated.Finally,the effect of annealing temperature on the microstructure and mechanical properties of Ti-TiBw composites was studied by different annealing treatment.Studies have shown that during the SPS sintering process,Ti and TiB2 completely react to form the TiB phase for the sintered Ti-TiBw composites,TiB mainly exhibits cluster morphology and distribute in primary particle boundary of the matrix.A small amount of single line needle-like TiB whiskers distribute in the Ti matrix.The in situ synthesised TiB whiskers have a 1327 orthorhombic crystal structure.The transverse section of the in situ TiB whiskers is a hexagon shape,which always consists of(100)、(101)and(101)crystallographic planes.The TiB whiskers always grow along the[010]direction.It is found that compared with the holding time,the heat treatment temperature is the most important factor for TiB whisker coarsening and TiBw mainly coarsen occurs in the temperature-raising stage.With the increase of rapidly cooling treatment temperature,both the radial and axial dirnensions of TiB whiskers increase,but the increase rate of radial dimension is greater than that of axial dimension,which means TiBw coarsening.During the rapidly cooling process,as the temperature rises,the decomposition of small-sized TiB whiskers,B atoms diffuse from small-sized TiBw to large-sized TiBw.Therefore TiB whiskers coarsen,the number of TiB whiskers decrease and the average size of TiB whiskers increaseThe statistical results of the aspect ratio of TiB whiskers in Ti-TiBw composites after annealing treatment are much higher than the critical aspect ratio of TiB whiskers.With the raise in annealing temperature,the aspect ratio of TiB whiskers of Ti-TiBw composites decrease first and then increase and the overall aspect ratio of TiB whiskers can be maintained at 12.With the raise in annealing temperature,the hardness of the composites after annealing treantmeng decrease first and then increase.Compared with sinterd Ti-TiBw composite(349 HV),the hardness of the composites with annealing treantmeng decreases slightiy.The yield strengths and tensile strength of sintered composites are 801 MPa and 872 MPa respectively,and the elongation is only 1.69%.Compared with sintered composite,the yield strength of annealed Ti-TiBw composites is slightly reduced and the tensile strength is slightly increased.With the increase of annealing temperature,the yield strength and tensile strength of Ti-TiBw composite fluctuate slightly.However,the elongation of Ti-TiBw composite increase significantly with annealing treatment increase.When the annealing treatment temperature is 1400℃,its tensile strength can reach 901 MPa,elongation can be increased to 6.7%. |
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