W. Qin

Lattice distortion and its effects on physical properties of nanostructured materials

A theoretical model is developed to understand the basic mechanism of the deviation of a crystal lattice from the perfect structure in nanostructured materials. The interface tension and the stress field induced by the excess volume in the grain boundaries are the two main reasons for lattice distortion. Based on this model, the contribution of the lattice distortion to the thermal expansion coefficient of nanostructured materials is analysed quantitatively. The results show that the unusual physical properties of nanostructured materials result not only from grain boundaries, but also from nanocrystallites.

Hydride-induced degradation of zirconium alloys: a criterion for complete ductile-to-brittle transition and its dependence on microstructure

Hydride-induced embrittlement is an important problem in hydride-forming materials. In this paper, a model is built for analysing a hydride-induced degradation process of the mechanical properties of zirconium alloys. A time-dependent criterion is proposed for determining the critical conditions of complete ductile-to-brittle transition. The dependence of this criterion on microstructure of Zr alloys is analysed in detail. The results show that many factors simultaneously govern the hydride-induced degradation. These factors include the susceptibility of hydrides to rupture, hydride size, hydride distribution, active slip systems, zirconium grain orientation, grain–boundary structure, grain–boundary density, hydrogen concentration, applied stress state and temperature. Intergranular hydrides have a more important influence on the loss in ductility than intragranular ones. The results obtained in this paper are of particular interest for zirconium alloys used in nuclear application.

A criterion for grain-size limit of polycrystalline materials

The transformation from the crystalline to the amorphous state was frequently found in some polycrystalline materials when grain refinement reaches a certain degree. Grain-size reduction induced lattice distortion and its effects on crystal structural instability were analyzed based on our recent work [Qin et al., Philos. Mag. Lett. 88, 169 (2008); Qin et al., J. Appl. Phys. 102, 124303 (2007); Qin et al., J. Phys.: Condens. Matter 19, 236217 (2007)] and Koike’s model [Phys. Rev. B 47, 7700 (1993)]. A criterion for determining the possible grain-size limit below which the amorphization occurs was proposed. Theoretical results may well account for experimental observations.