Zhang Xi-yan

EFFECTS OF UNTWINNING AND {1012} TWINLAMELLAR STRUCTURE ON THEMECHANICAL PROPERTIESOF Mg ALLOY

Strengthening mechanism related to {1012} twinning plays an important role in improving mechanical properties of Mg alloy.In this work,a hot-rolled AZ31 Mg alloy sheets were subjected to dynamic plastic deformation(DPD) along the rolling direction and tests were interrupted at strains of 1%,3%and 5%with the aim of introducing {1012} twins,and then were annealed at 200℃for 3 h to eliminate the dislocations.The tensile deformation behavior and the microstructural evolution of these pre-deformed samples containing {1012} twin lamellar structure with the aim of analyzing effects of untwinning and {1012} twin lamellar structure on the mechanical properties of materials were investigated.When subsequent tensile deformation is carried out along the DPD direction,untwinning causes a significant increase in the maximum flow stress. And the tensile yield stress and the maximum flow stress increases significantly with the volume fraction of twins.The texture hardening which is caused by the texture change attributable to untwinning plays an important role in improving mechanical properties of materials.When tension is carried out along the initial transverse direction,slip dominated plastic deformation and {1012} twinning activity is restrained.The tensile yield stress increases slightly with pre-strain, suggesting that the hardening contribution of initial grain refinement by {1012} twin lamellae is not very significant during deformation.

Deformation Induced Internal Friction Peaks in Nanocrystalline Nickel

We report the mechanical spectroscopy study of the cold-rolling induced dynamical behavior of crystalline defects in nanocrystalline (NC) nickel. The results show that internal friction (IF) peaks in NC nickel can be induced by cold-rolling. An IF peak, originating from dislocation activity, occurs when the strain is in the range of 9.7–32.8%. Two Bordoni peaks occur when the strain is 39.0% and an IF peak associated with deformation twinning appears when the strain is 42.6%. These results mean that deformation of NC nickel is mediated by different kinds of defects as the strain increases.

Quantitative Characterization of Partial Dislocations in Nanocrystalline Metals

Partial dislocations in nanocrystalline metals are introduced and a modified dislocation density formula for partial dislocations is established by x-ray line profile analysis theories. Effects of factors on the determination of partial dislocation density are discussed. From the correlation between the partial and perfect dislocations, partial dislocation density is simply quantitative characterized by drawing on the evaluation methodology of perfect dislocations. Dislocation densities of nanocrystalline nickel calculated from two different equations are compared additionally.

Curie Transition of NC Nickel by Mechanical Spectroscopy and Magnetization Study

Mechanical spectroscopy measurement is performed to study the internal friction of nanocrystalline ( NC) nickel with an average grain size of 23 nm from room temperature to 610 K. An internal friction peak is observed at about 550 K, which corresponds to the Curie transition process of the NC nickel according to the result of magnetization test. Moreover, the fact that the Curie temperature of NC nickel is lower than that of coarse-grained nickel is explained by an analytical model based on the weakening of cohesive energy.

A Study of the Lattice Constant of ZrO2 during the Oxidation of Nanocrystalline Zircaloy-4

Abstract A nanocrystalline zircaloy-4 was corroded under 673 K/10.3 MPa, and the change of the lattice constant of ZrO 2 during the phase transition was studied. The results show that the change of the lattice constant of ZrO 2 formed on the nanocrystalline base is very different from that of lattice constant of ZrO 2 formed on the coarse grain base. The unit cell volume of the tetragonal ZrO 2 and monoclinic ZrO 2 formed on the nanocrystalline base is constricted during the oxidation process. However, for the coarse grain base, the unit cell volume of the tetragonal ZrO 2 is constricted, and the unit cell volume of the monoclinic ZrO 2 is inflated. This phenomenon was discussed.

Internal Friction Of Bend-Deformed Nanocrystalline Nickel By Mechanical Spectroscopy

Internal friction of nanocrystalline nickel is investigated by mechanical spectroscopy from 360 K to 120 K. Two relaxation peaks are found when nanocrystalline nickel is bent up to 10% strain at room temperature and fast cooling. However, these two peaks disappear when the sample is annealed at room temperature in vacuum for ten days. The occurrence and disappearance of the two relaxation peaks can be explained by the interactions of partial dislocations and point defects in nanocrystalline materials.