B.X. Zhou

Effect of Water Chemistry and Composition on Microstructural Evolution of Oxide on Zr Alloys

The microstructure of oxide films formed on Zircaloy-4 and Alloy No. 3, which has a composition similar to ZIRLO™, was investigated by high resolution transmission and scanning electron microscopy, and by scanning probe microscopy after corrosion tests performed at 360°C/18.6 MPa in deionized water or lithiated water with 0.01 M LiOH. The microstructural evolution of the oxide films was analyzed by comparing the microstructure at different depths in the oxide layer. The defects, consisting of vacancies and interstitials, such as points, lines, planes, and volumes, were produced during the oxide growth. Monoclinic, tetragonal, cubic, and amorphous phases were detected and their coherent relationships were identified. The characteristic of oxide with such microstructure had an internal cause, and the temperature and time were the external causes that induced the microstructural evolution during the corrosion process. The diffusion, annihilation, and condensation of vacancies and interstitials under the action of stress, temperature, and time caused stress relaxation and phase transformation. It was observed, in the middle of the oxide layer, that the vacancies absorbed by grain boundaries formed pores to weaken the bonding strength between grains. Pores formed under compressive stress lined up along the direction parallel to the compressive stress. Thus, cracks developed from the pores were parallel to the oxide/metal interface. Li+ and OH− incorporated in oxide films were adsorbed on the wall of pores or entered into vacancies to reduce the surface free energy of the zirconium oxide during exposure in lithiated water. As a result, the diffusion of vacancies and the formation of pores were enhanced, inducing the degradation of the corrosion resistance. The relationship between the corrosion resistance of zirconium alloys and the microstructural evolution of oxide films affected by water chemistry and composition is also discussed.

Formation and Control of Sharp {100} Texture in Electrical Steel

The formation and control of sharp {100} <021> texture in electrical steels were experimentally investigated and the main influencing factors were discussed. The steels of a base composition of Fe-3. 2%Si with a certain amount of carbon and manganese were employed to obtain {100} <21> texture through one stage cold rolling and annealing. The results indicated that by careful control of the amount of alloying elements and the annealing temperature, a relatively sharp {100} <021> texture in fairly thick sheets is obtained. After primary recrystallization, a weak component of {100} <021> and strong component of (447) <18 11 4> texture, which constitute Σ9 coincidence site lattice boundaries with a common <011> axis for 39° difference, are presented. This special grain boundary promotes the growth of {100} <021> grains in addition to the lower surface energy of (100) grains during secondary recrystallization.

Study of the Initial Stage and Anisotropic Growth of Oxide Layers Formed on Zircaloy-4

An in situ investigation of the epitaxial oxide layer formed on a thin specimen heated in a transmission electron microscope was carried out. Some dot-like grains about 10 nm in size were formed on the surface of a relatively thin area. The dot-like grains are monoclinic zirconium oxide and have an orientation relationship of ( 001 ) m / / ( 0 1 ¯ 11 ) α -Zr , ( 111 ) m / / ( 1 1 ¯ 01 ) α -Zr with the α-Zr matrix. Some long strip-like grains, probably a new kind of zirconium suboxide, were formed on the surface of a relatively thick area. The strip-like grains have a bcc structure with a lattice parameter a=0.66 nm and have an orientation relationship of ( 110 ) bcc / / ( 10 1 ¯ 0 ) α -Zr , [ 1 1 ¯ 0 ] bcc / / [ 0001 ] α -Zr with the α-Zr matrix. The relationship between the thickness of oxide layers and the grain orientations of the α-Zr matrix was studied with coarse-grained Zircaloy-4 specimens through autoclave corrosion tests at 500 and 400°C in superheated steam, and at 360°C in both lithiated and deionized water for long time exposure. The results show that the anisotropic growth of oxide layers on the grain surface with different orientations is considerable. However, the relationship between the thickness of oxide layers and the grain orientations of the α-Zr matrix varies with corrosion temperature and water chemistry. The largest variation of oxide thickness developed during corrosion tests at 500°C. The thickest oxide layers were formed on those grains whose surface orientations were distributed around the planes from ( 01 1 ¯ 0 ) to ( 1 ¯ 2 1 ¯ 0 ) . The thicker oxide layers on these grains were further developed into nodular corrosion. When the specimens were corroded at 360°C in lithiated water, the thickest oxide layers formed on those grains, whose surface orientations tilted from 15 to 30° away from the (0001) plane. When the specimens were corroded at 400°C in superheated steam and at 360°C in deionized water, the difference between the thickness of oxide layers on different grain surfaces was less prominent.

Microstructural Evolution in Nd2Fe14B/α-Fe Bearing Zr-Nb Nanocomposite Magnet

Abstract Microstructural evolution in Nd 2 Fe 14 B/α-Fe bearing Zr-Nb nanocomposite magnet has been investigated. The magnetic properties, especially the coercivity were enhanced with the addition of Zr and Nb elements for the fine and uniform microstructure. The results of three-dimensional atom probe (3DAP) revealed that (Zr,Nb)Fe 2 intergranular phase existed at the boundary of grains, suppressing the grain growth during crystallization process. The interaction coupling between grains was enhanced and the magnetic properties were improved for the refinement of grains.