Papers by Author: Zhong Min Zhao

Abstract: Based on the technology of combustion synthesis in high-gravity field, the TiC-TiB₂ ceramics with refined microstructures have been fabricated by adjusting the technological parameter and proportioning of raw starting powders. The effects of particle size of raw powders on crystal growth and mechanical properties were studied in this paper. It found that the propagation rate and the combustion temperature can be improved by decreasing the grain size of raw powders, as well as the heat exchange and mass diffusion were enhanced too, so that the microstructure homogenization, densification and mechanical properties of the TiC-TiB₂ ceramics were improved accordingly. The highest density, relative density, Vickers hardness and fracture toughness of TiC-TiB₂ composites measured 4.07 g/cm³, 87.5%, 22.5 ± 1.2 GPa and 9.5 ± 0.8 MPa·m^0.5, respectively, since that the fine B₄C powder with particle size < 3.5 µm and the fine Ti powder with particle size < 38 µm were chosed as raw starting powders. The fracture behaviour of TiC-TiB₂ composites was dominated by the strong coupled toughening mechanisms of crack deflection and crack-bridging, and the high fracture toughness of the TiC-TiB₂ ceramics benefits mainly from the achievement of micro-nanocrystalline microstructure in the full-density solidified ceramic.

Abstract: A series of TiC-TiB₂ ceramics were prepared by combustion synthesis in ultra-high gravity field, the microstructure, fracture behavior and toughening mechanism of the composite ceramic are discussed. The maximum fracture toughness of 13.1 MPa · m^0.5 is achieved in TiC-66.7%TiB_2, very close to the values (10~15 MPa · m^0.5) of fiber-reinforced ceramic composite, contributed from the intensive coupled toughening mechanisms of crack deflection, frictionally crack-bridging and pull out by a large number of fine TiB₂ platelets and ductile phase toughing caused by a few Cr-Al metallic phases.

Abstract: Based on increasing the acceleration of high-gravity field from 500 g to 2500 g, a series of TiC-TiB₂ composites are prepared by combustion synthesis in high-gravity field, and microstructure modification, microstructure homogeneity and mechanical properties of TiC-TiB₂ composites are discussed in terms of liquid-liquid separation of TiC-TiB₂ and Al₂O₃ in high gravity field. XRD, FESEM and EDS results showed that by increasing the acceleration of high gravity field, Al₂O₃ inclusions in the ceramic decreased both in volume fraction and size due to the enhanced separation of TiC-TiB₂ liquid and Al₂O₃ liquid, while the enhanced Stokes flow induced by the enhanced liquid-liquid separation also promoted the constitutional homogeneity of TiC-TiB₂ melt, thereby bringing about the refinement of the solidified microstructures, finally, making Vickers hardness and fracture toughness of the composites increase with the acceleration of high gravity field.

Abstract: Based on taking combustion synthesis into high-gravity field to fabricate high-hardness bulk solidified TiC-TiB₂ composite, layered composite of solidified TiC-TiB₂ ceramic to Ti-6Al-4V alloy was obtained by the introduction of Ti-6Al-4V alloy plates at the bottom of graphite crucible. XRD, FESEM and EDS results showed that the layered composite was achieved in multi-scale (micron/submicron/micro-nanometer) and multi-level (TiC_1-x-TiB-TiB₂ + TiB₂-Ti-TiC_1-x-TiB + TiB₂-TiC_1-x-TiB-Ti + TiB-TiC_1-x-Ti + TiC_1-x-Ti + Ti) graded microstructure between the ceramic and Ti alloy, which was characterized by size and distribution of TiB₂ and TiB, thereby paving a new way for developing light-weight materials for special engineering.

Abstract: Based on taking combustion synthesis to prepare high-performance TiC-TiB₂ composite in high-gravity field, joint of TiC-TiB₂ with stainless steel and Ti-6Al-4V alloy were achieved respectively by the introduction of stainless steel and Ti alloy plates at the bottom of combustion system. Instant melting at surface part of stainless steel and Ti alloy, followed by atom diffusion between liquid TiC-TiB₂ ceramic and the molten metal was considered a reason for the achievement of the joint. Because of the presence of both coarsened Al₂O₃ inclusions nearby the ceramic and the partition layers of Al₂O₃ between the ceramic and the intermediate, shear strength of joint between the ceramic and stainless steel presented a low value of 120 ± 30 MPa. In contrast, because of the interdiffusion of C, B and Ti atoms between the liquid ceramic and the molten Ti alloy, ultrafine-grained microstructure develops in the ceramic nearby the joint, while the joint characterized by constitutional continuous gradient is also achieved from the ceramic to Ti alloy, resulting in high shear strength of 650 ± 25 MPa between TiC-TiB₂ ceramic and Ti-6Al-4V alloy.

Abstract: Micro-nanocrystalline microstructures characterized by TiB₂ platelets of the average thickness close to or smaller than 1 μm, were achieved in full-density solidified TiC-TiB₂ composites by combustion synthesis in high-gravity field. XRD, FESEM and EDS results showed that a large number of fine TiB₂ platelets were uniformly embedded in irregular TiC grains, a few Cr-Al metallic phases or in between those phases. The achievement of micro-nanocrystalline microstructures results from low-velocity faceting growth of TiB₂ crystal, high-velocity non-faceting growth of TiC solid and high diffusion rate of C relative to B in liquid TiC-TiB₂. The relative density, Vickers hardness, flexural strength and fracture toughness of TiC-TiB₂ composites measured 99.3%, 21.5 ± 1.5 GPa, 845 ± 35 GPa and 16.8 ± 1.5 MP • m^0.5, respectively. High flexural strength of the materials benefits mainly from the achievement of micro-nanocrystalline microstructure in the full-density solidified ceramic and high fracture toughness contributed from intensive coupled toughening mechanisms by a large number of fine TiB₂ platelets and a few Cr-Al metallic phases.

Abstract: By adjusting the mole ratio of C and B elements in combustion system, solidified TiC-TiB₂ composites with different TiB₂ mole fraction were achieved by combustion synthesis in high-gravity field. XRD, FESEM and EDS results showed that with increasing TiB₂ content, the matrix of TiC-TiB₂ composite ceramics transformed a number of fine TiB₂ platelets from the TiC spherical grains, and fine-grained even ultrafine-grained microstructures were achieved in solidified TiC-50mol% TiB₂ due to eutectic growth under rapid solidification of the ceramic. Properties showed that relative density, Vickers hardness and flexural strength of TiC-50mol%TiB₂ simultaneously reached the maximum values of 21.5 ± 1.5 GPa and 860 ± 35 MPa , whereas TiC-66.7mol%TiB₂ presented the maximum fracture toughness of 13.5 ± 1.5 MPa • m^0.5. FESEM fractography analyses of the ceramics exhibited a mixed mode of transcrystalline fracture of TiC spherical grains and intercrystalline fracture of TiB₂ platelets, and the tendency of intercrystalline fracture was obviously enhanced with increasing TiB₂ content to be 66.7 mol%, resulting in enhanced toughening mechanisms of crack deflection, crack-bridging and pull-out by fine TiB₂ platelets, thus, the highest flexural strength was achieved in TiC-50mol%TiB₂ due to the achievements of both fine-grained microstructures and high fracture toughness in the full-density solidified ceramics.

Abstract: By adding ZrO₂ and Y₂O₃ powder blend into the thermit, large bulk Al₂O₃/ZrO₂ (Y₂O₃) eutectics were prepared by combustion synthesis under high gravity, and the influences of high gravity on microstructures, crystal growth and properties of the materials were also discussed. The XRD patterns showed that the introduction of high gravity field did not change phase constitution of the ceramics, and the ceramic matrix was mainly composed of α-Al₂O₃, t-ZrO₂ and m-ZrO₂. SEM images and EDS analyses showed that with increasing high gravity level, the morphologies of the ceramic microstructures transformed from the cellular eutectics to the rod-shaped colonies, and volume fraction and aspect ratio of the rod-shaped colonies increased while the rod-shaped colonies were refined; as the high-gravity field was larger than 200g, the microstructures of composite ceramics developed as the randomly-orientated rod-shaped colonies with a symmetrical triangular dispersion of tetragonal ZrO₂ fibers of 300nm in the average diameter. Relative density, hardness, flexural strength and fracture toughness simultaneously reached the highest values of 98.6%, 18.6GPa, 1248MPa and 15.6MPa·m^1/2 as the maximum high-gravity level of 250g was achieved. The increases of relative density and hardness of the ceramics with the high-gravity level are attributed to the acceleration of gas-escape from ceramic melts and the elimination of shrinkage cavity in the ceramics. The increase in fracture toughness results from the enhancement of the coupled toughening mechanisms while the increase in flexural strength comes from the refinement of the microstructures, the decrease of critical defect size and the achievement of high fracture toughness.

Abstract: Large-bulk TiB₂-TiC composite ceramics were prepared by combustion synthesis under high gravity. XRD, SEM and EDS results showed TiB₂-TiC composites were mainly composed of the fine-grained microstructures of TiC matrix in which a large number of the fine TiB₂ platelet grains were dispersed uniformly, whereas there discontinuously dispersed the ε-carbides with the enrichment of Ti atoms, and a few of isolated, irregular α-Al₂O₃ grains and Al₂O₃-ZrO₂ colonies were also observed at the boundaries of the eutectic microstructures. The results of properties indicate that with increasing mass fraction of B₄C+Ti+C in combustion systems, the relative density and fracture toughness of TiB₂-TiC composites are all among 97%~99% and 6.5~7.1 MPa·m^1/2, respectively, and the Vickers hardness and flexural strength are increased gradually to the maximum values of 28.6GPa and 615MPa, respectively. The achievement of full-density TiB₂-TiC composites benefited from the design of full-liquid SHS products and the introduction of high-gravity field, and high hardness of the composite ceramics resulted from the absence of intermediate borides and the achievement of stoichiometric TiC phases due to rapid solidification, whereas high flexural strength of the composite ceramics benefited from the homogenization and refinement of the microstructures due to the rapid separation of the liquid oxides and the rapid coupled growth of TiB₂-TiC.

Abstract: By using combustion synthesis under high gravity, TiC-TiB₂ fine-grained composite ceramics with hypoeutectic, eutectic and hypereutectic microstructures were prepared through rapid solidification. XRD, FESEM and EDS results show that with increasing TiB₂ content, TiC-TiB₂ composite ceramics transform the microstructures consisting of fine TiB₂ platelets from ones composed of fine TiC spherical grains, whereas when TiB₂ content reaches 50mol%, the ceramics develop the eutectic microstructures that small aspect-ratio TiB₂ platelets were embedded in TiC matrix. Mechanical properties show that relative density, Vickers hardness and flexural strength of TiC-50mol%TiB₂ all reach the maximum values (respectively as 98.6 % ,18.4 GPa, 840 MPa) due to eutectic reaction during solidification, meanwhile TiC-50mol%TiB₂ has the maximum fracture toughness of 11.5 MPa∙m^0.5 due to the cooperative action of crack deflection, crack-bridging and pull-out toughening by fine TiB₂ platelets.