Papers by Author: Lin Geng

Abstract: The two (Al2O3+TiB2+Al3Ti)/Al composites were fabricated from Al-B2O3-TiO2 and Al-B-TiO2 raw powders by reactive hot pressing, respectively. The microstructure of in situ two composites was analyzed by OM, SEM and TEM. The results showed that coarse Al3Ti blocks with several tens of micrometers size were formed during hot pressing. The equiaxed Al2O3 particulates and hexagonal TiB2 particulates with finer sizes were formed in the composites simultaneously. The microstructure formation mechanism of (Al2O3+TiB2+Al3Ti)/Al composites were discussed. The results showed that Al2O3 reinforcements were formed on the surface of TiO2 or B2O3 powder and TiB2 particles were formed on B or B2O3 powders. The formation of coarse Al3Ti block is result from continuous diffusion of Ti in liquid Al during reactive hot pressing. In addition, there are fine Al3Ti precipitates exist in the composite fabricated from Al-B-TiO2 powders. This contributes to the improved mechanical properties in terms of yield and ultimate stresses and Young’s modulus of the composite.

Abstract: It is attractive to use multi-wall carbon nanotubes (MWNTs), which has a high modulus and strength, to enhance the properties of metal matrix composites by hybrid strategy. In this paper, novel aluminum borate whisker (ABOw) and MWNTs hybrid composites were prepared by squeeze cast technique and the properties were investigated. The results show that hybrid preforms may be made by co-deposition of ABOw and MWNTs in a solution by wet method. MWNTs separate the ABOw from whisker-to-whisker contact and decrease the compressive deformation of the hybrid preforms during squeeze cast processing. MWNTs distribute along mainly in the grains of aluminum matrix, making the mechanical properties of the hybrid composites higher than singularly reinforced composite.

Abstract: The effect of temperature on strain softening behavior of composites with small misaligned whiskers is investigated. The results show that the temperature affects the matrix and whisker mechanical behavior and corresponding composite deformation behavior. With increasing temperature, the whisker rotation angle increases, but their breakage decreases. Meanwhile elevating temperature not only reduces the matrix flow stress and work hardening rate, but also decreases load transfer from the matrix to the whiskers and stress induced by the whisker rotation and breakage.It is found that during hot compression, strain softening behavior of composites decreases as temperature increases.

Abstract: SiCw/Al-18Si composites were prepared by squeeze casting technique. SiCw/Al-18Si composites were remelted before solidification. The effects of volume fraction of SiC whisker on solidification behavior of SiCw/Al-18Si composites were investigated by means of differential scanning calorimetry (DSC) technique and microstructure observation. DSC results indicated that the start solidification temperature and primary silicon peak temperature decreased gradually and the degree of supercooling increased with the increasing of SiC whisker content. SiC whisker and Sr decreased the average size of Si phases and improved the mechanical properties of the composites.

Abstract: By incorporating the Taylor-based nonlocal theory of plasticity, the finite element method (FEM) is applied to investigate the effect of particle size on the deformation behavior of the metal matrix composites. In the simulation, the two-dimensional plane strain and random distribution multi-particles model are used. It is shown that, at a fixed particle volume fraction, there is a close relationship between the particle size and the deformation behavior of the composites. The yield strength and plastic work hardening rate of the composites increase with decreasing particle size. The predicted stress-strain behaviors of the composites are qualitative agreement with the experimental results.

Abstract: In this work, Ni-Al-Ti-B composite powders with Ni: Al: Ti: B atom ratio of 5.7: 1.9:1:1 were prepared by using mechanical activation (MA) method. The MA processes were performed by vibration ball mill in a water-cooled chamber for 30 h. The ball-to-powder weight ratio was 10:1. In order to reduce the oxidation of the powders during milling process, ball milling was performed under an Argon atmosphere. The evolution of morphology and microstructure of Ni-Al-Ti-B composite powders were examined at different milling stages using scanning electron microscope and X-ray diffraction. Distribution of powders size was analyzed and the results showed that there were three stages for the change of powders size: (1) at the first stage, powders size increased due to more welding than fracture of the powders within 2h of MA, (2) at the second stage, particle size continuously decreased until 16h of MA due to more fracturing than welding, and (3) after 16h of MA, the particle size remained constant when welding and fracture reached an equilibrium. Calculation results according to Scherrer equation from X-ray pattern showed that the effective grain size continuously decreased with MA time. X-ray diffraction pattern confirmed that the formation of Ni and Ti solid solution during MA process of the Ni-Al-Ti-B composite powders.

Abstract: In this paper, a BaPb0.6Ce0.4O3 (BPCO)/Al matrix composite with a good anti-radiation property was fabricated by hot pressing in air for the first time. Differential scanning calorimeter (DSC) and Thermal Gravimetric (TG) analysis of the compacted blended powders were performed in air to study the oxidation behavior, by which the hot-pressing parameters were determined. Transmission electron microscope (TEM) images and X-ray diffraction pattern showed that there was an incomplete reaction between aluminum alloy matrix and BPCO particles during fabrication process. Compared with the aluminum alloy matrix, the X-ray and γ-ray absorption abilities of the composite were increased by 68.05% and 46.63%, respectively.

Abstract: In this paper, pure aluminium and Al-Cu (1%, 3% and 5% in mass fraction) alloy matrix composites reinforced with 35vol% 3Al2O32SiO2 short fibers were fabricated by a squeeze-casting technique. Interfacial structure and distribution of element of the composites were investigated by means of SEM, EPMA and TEM. The experimental results indicated that when pure aluminium was used as matrix, the interface was slick and chemical reaction occurred at the interface between amorphous SiO2 and matrix Al. When Cu element was added into the Al matrix alloy, the amorphous SiO2 on the fiber surface remained. Further more, with the increase of Cu element, the amount of the retained amorphous SiO2 increased. The interfacial reaction was inhibited due to the Cu element diffusion and enrichment towards the short fiber surface. SEM observation showed that large amount of fiber was drawn out from the matrix and some of them have been ruptured. Tensile test showed that the tensile strength of the 35vol% 3Al2O32SiO2sf/Al-Cu composite increased with the increasing of Cu content as compared with the Al2O3f/pure Al composite, the increment of the fracture strength of 3Al2O32SiO2sf/Al-Cu composite were 102%, 146% and 171%, respectively.

Abstract: In order to improve wear resistance of titanium alloy, with pre-placed B4C and NiCrBSi powders on Ti-6Al-4V substrate, a process of laser melting-solidifying metal matrix composite coating was studied. The coating was examined using XRD, SEM and EDS. A good metal matrix composite coating was obtained in a proper laser process. There is a metallurgical interface bonding between the coating and the substrate. During laser melting-solidifying process, high energy of laser melted the pre-placed powders and a part of Ti-6Al-4V substrate, which made Ti extend into a melting pool. A reaction between Ti and B4C took place in the melting pool, which in-situ synthesized TiB2 and TiC reinforcements in the coating. The composite coating mainly consists of γ-Ni matrix, TiB2, TiC and CrB reinforcements. Microstructure of the reinforcements obtained using the laser melting-solidifying is not as same as that of reinforcements obtained using general producing methods. Due to high cooling rate of the melting pool, TiC nucleated primarily and grew up in dendrite morphology from undercooled liquid. Encircling TiC, TiB2 precipitated later and grew up in hexagonal prism morphology. TiC and TiB2 formed an inlaid microstructure.