Papers by Author: Shi Bo Li

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: 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: 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: 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.

Abstract: Cu/Ti3AlC2 cermets prepared by pressless sintering a mixture of Ti3AlC2 and copper powders were investigated. It was found that the Cu/Ti3AlC2 possesses an unusual microstructure made up of sub-micro-sheet layered Ti3C2 and Cu-Al alloy within one Ti3AlC2 particulate. The fracture strength measured by the three-point-bending manner is increased but the deformation rate is reduced with increase in the volume content of Ti3AlC2 from 30 % to 90 %. The highest fracture strength reached to as higher as 983.9 MPa, corresponding to an extreme strain of 2.64 %. The fracture in mode was changed from brittle to ductile with reduce in the content of Ti3AlC2. The higher fracture strength can be attributed to a stronger interface bond between Ti3AlC2 and Cu-Al phase. A significant network feature formed by the Cu-Al alloy surrounding Ti3AlC2 particulates was observed from the fracture face.

Abstract: Polycrystalline bulk samples of ternary carbide Ti3AlC2 ceramics were fabricated by reactively hot-pressing a mixture of Ti, Al, and graphite powders with and without Sn additive. The effects of sintering temperature, time and addition of Sn on the purity, mechanical properties and microstructure of Ti3AlC2 were investigated. The result showed that the TiC content was strongly influenced by sintering temperature for the Ti3AlC2 samples without Sn additive, and the most suitable sintering temperature to create the lowest TiC content was 1400°C. The addition of Sn additive led to a distinct decrease in TiC content. The flexural strength of the testing materials had close relation with the TiC content and sintering time. A certain content of TiC second phase and longer sintering time were helpful to improving the flexural strength. The sample sintered at 1400°C for 2 h possessed the highest flexural strength.

Abstract: Ti2SnC has been fabricated from Ti, Sn and graphite elemental powders by mechanically activated low-temperature synthesis (MALS) technique. Superfine powders were obtained after milling the elemental powders for only 1 h with a charge ratio of 20:1. The mechanically alloyed powders were then pressureless sintered at different temperatures at Ar atmosphere for 0.5 h. High content of Ti2SnC was obtained at 950 oC, which is lower than the previously reported temperatures of above 1200 oC by sintering the conventional mixture powders. The microstructure shows that Ti2SnC grains with plate-like shape and smooth surface are less than 5 μm in size. The result demonstrates that the MALS is a novel method for the synthesis of Ti2SnC or other ceramic powders.

Abstract: A search for Ti3Si1.2-xAlxC2 (x=0~1.2) solid solution was undertaken using precise X-ray diffraction measurements. The samples covering the whole concentration range were studied. Except very ends of the concentration range, the samples contained two phases, identified as Ti3Si1.2-xAlxC2 solid solution and TiC respectively. Lattice parameter, a increased, c increased, c/a increased, and cell volume increased with the increasing of Al concentration.

Abstract: Microscale plasticity of Ti3SiC2 was investigated by Vickers hardness indentation. The surface layer of the hardness indentations was removed by acid solution to observe microstructure beneath the indentations, where a large number of bending, delamination and kinking grains were found. These features suggest that Ti3SiC2 is able to consume microdamage around the indentations. Numerous basal plane dislocations and stacking faults lying in Ti3SiC2 grains or accumulating at grain boundaries were observed. The basal plane dislocations play an important role in the microscale plastic deformation. The plasticity and damage tolerance for Ti3SiC2 at room temperature should be attributed to multiple energy absorbing mechanisms: grains bending, delamination, kink-band formation, and the basal plane slip, etc.

Abstract: An Al2O3 matrix composites, i.e. partially stabilized zirconia toughening alumina (ZTA) reinforced by SiC particle (ZTA-SiCP), was prepared by hot pressing (HP). Fatigue behavior of ZTA-SiCP under cyclic compressive loads was investigated on different loading conditions. The application of cyclic compressive loads to a notched specimen led to a stable crack growth along the notch plane in a direction normal to the far-field compressive axis. Irreversible damages in the main form of microcrack were induced at the stress concentration zone during compression loading, and it led to high residual tensile stresses ahead of the notch root upon unloading. Nucleation and growth of a model I fatigue crack were caused by the residual tensile stresses at the notch root. Along with propagation of the fatigue crack, a gradual decrease in crack growth rate was shown due to the crack closure caused by accumulating of debris particles within the wake of growing crack, and thus led to the crack arrested at last. The fatigue crack length was investigated as a function of notch length, the maximum compressive stress, stress range and load frequency.