Papers by Author: Hong Xiang Zhai

Abstract: In this paper, a new type of Ti₃SiC₂/Cu composites with the volume fractions of 30% Ti₃SiC₂ particle was prepared by hot pressing and vacuum sintering respectively. The effects of sintering temperature and holding time on the density, resistance and Vickers hardness of Cu-30vol%Ti₃SiC₂ composite were investigated. The results show that the mechanical properties of the composites prepared by hot pressing are better than that prepared by vacuum sintering. The relative densities of Cu-30vol% Ti₃SiC₂ composites are rather high in suitable sintering conditions. It achieved 100% for the composites prepared by hot pressing at 930°C for 2h, and 98.4% for the composites prepared by vacuum sintering at 1250°C for 1h. At the same time, the maximum Vickers hardness reached 1735MPa at 900°C by hot pressing. The resistance and Vickers hardness of the composites decreased with an increase in sintering temperature, whereas the density increased. Scanning electron microscope (SEM) and energy-dispersive spectroscopy (EDS) were used to observe the microstructure of the composites. The relationship between microstructure and mechanical properties was discussed.

Abstract: A high purity of Ti₂AlC powder has been synthesized by pressureless sintering a mixture of Ti-Al-TiC-Sn (Sn as an additive) powders. Four recipes with different mole ratios of Ti-Al-TiC-Sn were examined at sintering temperature from 1400°C to 1480°C. A high purity of Ti₂AlC powder can be obtained by sintering all these four recipes at temperature 1450°C for 10 min in an Ar atmosphere. The synthesis of Ti₂AlC on this large mole ratio scale of starting materials is associated with the evaporation of Al at high temperature and the structure stability of Ti₂AlC. From the X-ray diffraction analysis, a reaction path for the Ti₂AlC formation is proposed. Scanning electron microscopy was also used to characterize the samples.

Abstract: A series of new sub-micro-layered Ti3C2/(Cu-Al) cermets were prepared by in-situ hot-extruding a mixture of Ti3AlC2 and Cu powders, and some properties of these materials were tested. These cermets have quite high fracture strength and electric conductivity, due to the strong combination between Ti3C2 and (Cu-Al), and a special network microstructure formed by the (Cu-Al) phase surrounding the sub-micro-sheet layered Ti3C2 phase. The in-situ hot-extruding after pressless sintering can effectively eliminate pores contained in (Cu-Al) phase, and accelerate the diffusing of Cu towards the interlayer between Ti3C2 layers, so the fracture strength and electric conductivity are increased. With increasing the content of the ceramic phase, the strength of the cermets can be further increased while the ductility is reduced.

Abstract: A cold-extruding and temper means was developed in order to densify the Ti3AlC2 toughened Cu matrix composites with a lower content of Ti3AlC2 ceramic. The Ti3AlC2/Cu samples, with 10%, 15% and 20% of Ti3AlC2 in volume ratio, ware prepared by pressless sintering a mixture of copper and Ti3AlC2 powders, after which were formed by the cold isostatic compaction. The sintered samples ware extruded at room temperature, and then tempered at 950oC. Such treatment was performed twice for obtaining a more remarkable densifying effect. Changes in microstructures were observed by SEM after every extrusion and temper. The results show that particle size of Ti3AlC2 was reduced more than 50% after secondary extrusion and temper, and the particles distribution became more uniform in compared with the untreated samples. As a result of the microstructural change, the densities of the Ti3AlC2/Cu samples were increased about 5 %, and defects such as pores and microcracks were almost entirely slaked.

Abstract: Highly pure and dense bulk Ti2AlC was prepared by hot-pressing a mixture of the “312” phase Ti3AlC2 powders, and the element Ti and Al powders. Different ratios of the starting materials and different sintering temperature were attempted in order to obtain a highly pure and dense bulk Ti2AlC sample. Phase analysis and microstructures observing were performed by using by XRD, SEM as well as an X-ray fluorescence spectrometer. The results show that a nearly full dense bulk Ti2AlC sample can be prepared at 1300°C and 30MPa for 30 minutes in argon atmosphere. A dominant mechanism to form the “211” phase Ti2AlC can be attributed to the directly connecting between Al and Ti6C octahedron, which is as an intermediate phase in the Al-rich liquid mediator during the hot pressing.

Abstract: Several Ti3C2-Cu(Al) cermets were prepared by pressureless sintering or by in-situ hot-extruding a mixture of Ti3AlC2 and Cu powders, their microstructure and properties were investigated. The Al of Ti3AlC2 was dissolved and diffused into liquid Cu, forming Ti3C2-Cu(Al) cermet. Due to strong combination between Ti3C2 and Cu(Al), and special network microstructure formed by Cu(Al) phase surrounding sub-micro-sheet layered Ti3C2 phase, the cermets have quite high fracture strength and electric conductivity. Properties of cermets fabricated by in-situ hot-extruding were further increased.

Abstract: The dynamic process of crack initiation and propagation in a SiC/BN-Al2O3 laminated composite was observed in situ by scanning electron microscopy. During a bending test with a single-edge notched-beam specimen, an interfacial crack first initiated in the interlayer near the notch tip, after which a through-thickness crack formed in the matrix layer at the notch tip. After the through-thickness crack had grown across the first matrix layer, it was deflected by the next interlayer and again became an interfacial crack. Interfacial cracks and through-thickness cracks were generated alternately until the composite failed. The load-displacement plot of the laminated composite exhibited several peaks, each caused by one propagation of a through-thickness crack. The toughening mechanisms of the laminated composite included crack deflection, interfacial cracking, and through-thickness branch cracking.

Abstract: Joining of Cu/Ti3AlC2 cermet by an argon-arc welding technique without filler was firstly investigated. The results show that the Cu/Ti3AlC2 cermet can be joined firmly. The joining strength at room temperature was measured to be 851 MPa after optimization of the welding parameters with 2.6 A/mm2 for arc current density, 5 s for arc time, 10.8 kPa for applied pressure and 12 V for arc voltage. The microstructure in welded zone shows that fine TiCx particles uniformly dispersed in a network structure of Cu-Al alloys. This feature endows the Cu/Ti3AlC2 cermet with the high joining strength.

Abstract: The Cu/Ti3AlC2 composites were fabricated by pressureless sintering a mixture of Ti3AlC2 and copper powders. Their microstructures and properties were investigated. It was found that the molten Cu accelerating the decomposition of Ti3AlC2, inducing the interfacial exfoliation to generate, and forming a sub-micro-layered structure making up of TiCx layers and Cu-Al alloy layers within a Ti3AlC2 grain. The flexural strength of the composites is reduced with the increase of the volume content of Ti3AlC2 from 50 % to 90 %. The highest flexural strength reaches to as high as 915 MPa. The fracture mode was changed from ductile to brittle with increase in the content of Ti3AlC2. The higher flexural strength can be attributed to a stronger interface bond between TiCx and Cu-Al phase. The electrical resistivity and Vickers hardness of the composites were also measured.

Abstract: In this study, free Ti/Si/Al/C powder mixtures with molar ratio of 3:0.4:0.8:1.8 were heated in Argon with various schedules, in order to reveal the possibility for the synthesis of Ti3Si0.4Al0.8C1.8 solid solution powder. X-ray diffraction (XRD) was used for the evaluation of phase identities of the powder after different treatments. Scanning electron microscopy (SEM) was used to observe the morphology of the Ti3Si0.4Al0.8C1.8 solid solution. XRD results showed that predominantly single phase samples of Ti3Si0.4Al0.8C1.8 was prepared after heating at 1400oC for 5 min in Argon and the lattice parameters of Ti3Si0.4Al0.8C1.8 lay between those of Ti3SiC2 and Ti3AlC2.