Lianlong He

Characterization of a new Nb–silicide (δ-Nb11Si4) in Nb–Si binary systems

A new Nb–silicide, δ-Nb11Si4, identified as a second phase precipitated from a Nb–Si solid solution, was characterized by combining crystallographic analysis with first-principles calculations. The crystal structure was determined as a body-centered orthorhombic system with space group Immm using transmission electron microscope (TEM). Based on the crystallographic relationship between the δ and Nb phases, // {200}Nb and // ⟨001⟩Nb, eight possible unit cells were proposed, which were optimized by first-principles calculations. By analyzing the experimental and calculated data, we obtained the final unit cell of the δ phase, which was confirmed by high-resolution TEM images. In addition, density of states and electron density difference of δ-Nb11Si4 revealed that the δ structure was in a metastable state at low temperature because not all the bonding states were filled.

Effect of niobium on the oxidation behavior of TiAl

Two TiO2 layers formed in TiAl oxidation for 50 h at 900 degrees C were studied using scanning transmission electron microscopy. The main efforts were placed on the investigation of the distribution of niobium. It was found that Nb enriched in TiO2 grains of mixture layer but did not exist in the outer TiO2 layer. High-resolution electron microscopy (HREM) Z-contrast image revealed that Nb substitute for Ti site leading to Nb enrichment in TiO2 grains of the mixture layer. The formation mechanism of the two TiO2 layers and the potential effect of Nb doping in the mixture layer were also discussed.

Microstructure and thermal stability of an ultrafine Al/Al2O3 composite

Studies were carried out on the microstructure and thermal stability of an ultrafine Al/Al2O3 composite with high strength and low density. Transmission electron microscopy indicated that Al2O3 shells remained undeformed below 550 degreesC, which limited grain growth of Al. Both transmission electron microscopy and x-ray diffraction analysis indicated that no obvious grain growth of Al with time occurred upon annealing at 620 degreesC. After almost all the alumina shells were destroyed following annealing at 620 degreesC, the Al2O3 fragments with various morphologies distributed in the material could still limit the migration of Al grain boundaries to increase the thermal stability of the material. After Al melted and resolidified, the grain sizes of Al were still about 200 run. The bulk composite sample showed good dimensional stability. Even if the Al grains melted, the network of Al2O3 fragments kept the sample from deforming due to the wetting of Al2O3 network with liquid Al.

Orientation relationship and interfacial structure between α-Nb5Si3 and Nb solid solution in the eutectic lamellar structure

The orientation relationship (OR) and the interfacial structure of the eutectic α-Nb5Si3 and Nb solid solution (Nbss) lamellar microstructure were investigated by transmission electron microscopy (TEM). The OR between α-Nb5Si3 and Nbss was determined as and . In spite of the significant difference in crystal structure between α-Nb5Si3 and Nbss, the interface was found to be semi-coherent with good matching. High-resolution TEM images of the α-Nb5Si3/Nbss interface are presented to describe the chemistry of the terminating plane of α-Nb5Si3. The results revealed that a pure Nb layer not Si or mixed (Nb + Si) layer of the α-Nb5Si3 phase was bonded to the Nb phase at the interface. A periodic array of edge-type misfit dislocations, with line direction and Burgers vector , was observed at the interface.

The interpretation of X-ray diffraction from the pyrocarbon in carbon/carbon composites with comparison of TEM observations

Three different carbon/carbon (C/C) composites based on needle-punched felt made of layered T700 carbon fiber cloth were fabricated by chemical vapor infiltration and were studied using transmission electron microscopy (TEM) and X-ray diffraction (XRD). The TEM observations show that one of the composites contains only low-textured pyrocarbon. The other two contain both low-textured and high-textured pyrocarbon, one with predominantly low-textured and the other with mainly high-textured pyrocarbon. High-resolution TEM images show that the high-textured pyrocarbon in the two composites has the same microstructure as local areas with the graphite structure. XRD measurements show that the interlayer spacing and crystallite size of pyrocarbon are not only affected by the poorly graphitized carbon fiber phase, but also by the amounts of the different types of pyrocarbon and the orientation of crystallites. Comparison of the TEM observations and the XRD measurements reveals that structural parameters, such as the interlayer spacing and crystallite size, of pyrocarbon in C/C composites as determined by XRD are not accurate. Therefore, XRD profiles of C/C composites should be interpreted with caution. TEM observations for detailed microstructure analysis of C/C composites are thus important.