Zhaoke Chen

Crystal lattice expansion of nanocrystalline materials

Nanocrystalline materials are characterized structurally by ultra-fine crystallites separated by grain boundaries. The structure of nanocrystallites shows significant differences in comparison to perfect crystals. A model is proposed to account for the effects of the defects in the grain boundaries of nanocrystalline materials on the crystal lattice, based on the fact that the grain boundaries of nanocrystalline materials contain a large number of vacancies and vacancy clusters. A 1/x3 law for the stress field caused by the vacancies and the vacancy clusters is considered, and the crystal lattice expansion of nanocrystalline materials is estimated. The theoretical results can interpret experimental observations well.

Stability of the austenitic phase in ultra-fine particles of Fe-based alloys

The effect of the ultra-fine particle size of Fe-based alloys on the martensitic transformation was analyzed qualitatively. It is shown that the stabilities of the austenitic phase and the martensitic phase are reversed as the particle size becomes smaller than a given critical value. The austenitic phase may become more stable than the martensitic phase and the martensitic transformation cannot take place even when cooled to temperatures approaching 0 K. This may be the main reason why nanocrystalline morphologies may prevent martensitic transformation.

Dislocation stability and configuration in the crystallites of nanocrystalline materials

Abstract Nanocrystalline materials have been characterized by ultra-fine crystallites separated by grain boundaries. The dislocations in the crystallites of nanocrystalline materials form some special configurations and the dislocation density drastically decreases. A model is proposed to account for the effects of the defects in the grain boundaries of nanocrystalline materials on the dislocation stability and configuration in the crystallites. Based on the fact that the grain boundaries of nanocrystalline materials contain a large number of vacancies and vacancy clusters, a 1/ x 3 law for the stress field induced by the vacancies and the vacancy clusters is considered. The region of the dislocation stability in the crystallites is estimated and the dislocation configurations in the crystallites are discussed. The theoretical results are shown to interpret the experimental observations well.

Preparation and Microstructure of Carbon/Carbon Composites Modified with Zr-Ti-C Fabricated by Reacted Melt Infiltration

A novel anti-ablation material of carbon/carbon (C/C) composites modified with the carbides of zirconium-titanium was prepared by reacted melt infiltration (RMI) and their synthetic mechanism and microstructure were investigated. Carbon fiber felts were firstly densified by carbon using chemical vapor infiltration to obtain a porous C/C skeleton. The zirconium-titanium melt was then infiltrated into the porous C/C skeleton at high temperatures to obtain the final composites. Models based on these results were built up to describe the diffusion behavior of carbon atoms in the carbides and the kinetic process of RMI. The results also show that the synthesized composites have good interface cohesion between carbon fibers, pyrocarbon and carbide. The carbide with high content of Zr atom in the final composites is composed of the main phase of Zr0.83Ti0.17C0.92, dispersively distributed with the phase of Ti0.82Zr0.18C0.92, while the carbide with high content of Ti composed of Zr0.57Ti0.43C1.01.