Baifeng Luan

Optimization of N18 Zirconium Alloy for Fuel Cladding of Water Reactors

In order to optimize the microstructure and composition of N18 zirconium alloy (Zr-1Sn-0.35Nb-0.35Fe-0.1Cr, in mass fraction, %), which was developed in China in 1990s, the effect of microstructure and composition variation on the corrosion resistance of the N18 alloy has been investigated. The autoclave corrosion tests were carried out in super heated steam at 400 C/10.3 MPa, in deionized water or lithiated water with 0.01 mol/L LiOH at 360 C/18.6 MPa. The exposure time lasted for 300-550 days according to the test temperature. The results show that the microstructure with a fine and uniform distribution of second phase particles (SPPs), and the decrease of Sn content from 1% (in mass fraction, the same as follows) to 0.8% are of benefit to improving the corrosion resistance; It is detrimental to the corrosion resistance if no Cr addition. The addition of Nb content with upper limit (0.35%) is beneficial to improving the corrosion resistance. The addition of Cu less than 0.1% shows no remarkable influence upon the corrosion resistance for N18 alloy. Comparing the corrosion resistance of the optimized N18 with other commercial zirconium alloys, such as Zircaloy-4, ZIRLO, E635 and E110, the former shows superior corrosion resistance in all autoclave testing conditions mentioned above. Although the data of the corrosion resistance as fuel cladding for high burn-up has not been obtained yet, it is believed that the optimized N18 alloy is promising for the candidate of fuel cladding materials as high burn-up fuel assemblies. Based on the theory that the microstructural evolution of oxide layer during corrosion process will affect the corrosion resistance of zirconium alloys, the improvement of corrosion resistance of the N18 alloy by obtaining the microstructure with nano-size and uniform distribution of SPPs, and by decreasing the content of Sn and maintaining the content of Cr is discussed.

Hot deformation and processing maps of Al2O3/Al composites fabricated by flake powder metallurgy

Abstract The deformation behaviors of Al 2 O 3 /Al composites were investigated by compressive tests conducted at temperature of 300–450 °C and strain rates of 0.001–1.0 s −1 with Gleeble–1500D thermal simulator system. The results show that the flow stress increases with increasing strain rate and decreasing temperature. The hyperbolic sine constitutive equation can describe the flow stress behavior of Al 2 O 3 /Al composites, and the deformation activation energy and constitutive equations were calculated. The processing maps of Al 2 O 3 /Al-2 μm and Al 2 O 3 /Al-1 μm composites at strain of 0.6 were obtained and the optimum processing domains are in ranges of 300–330 °C, 0.007–0.03 s −1 and 335–360 °C, 0.015–0.06 s −1 for hot working, respectively. The instability zones of flow behavior can also be recognized by the maps.

Deformation behaviors and processing maps of CNTs/Al alloy composite fabricated by flake powder metallurgy

Abstract Deformation behaviors of CNTs/Al alloy composite fabricated by the method of flake powder metallurgy were investigated by hot compression tests, which were performed in the temperature range of 300–550 °C and strain rate range of 0.001– 10 s −1 with Gleeble–3500 thermal simulator system. Processing maps of the CNTs/Al alloy at different strains were calculated to study the optimum processing domain. Microstructures before and after hot compressions were characterized by electron backscattered diffraction (EBSD) method. Stress–strain curves indicate that the flow stress increases with the increase of strain rate and the decrease of temperature. The processing maps of the CNTs/Al alloy at different strains show that the optimum processing domain is 500–550 °C, 10 s −1 for hot working. EBSD analysis demonstrates that fully dynamic recrystallization occurs in the optimum processing domain (high strainrate 10 s −1 ), whereas the main soften mechanism is dynamic recovery at low strain rate (0.001 s −1 ).

Dislocation density and configuration in fully pearlitic steel during wire drawing

Abstract This study explores the dislocation density and configuration in fully pearlitic steel wires during the cold wire drawing process by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD results show that the dislocation density increases from 1·1×1014 m−2 to 1·4×1015 m−2, with the drawing strain increasing from 0 to 1·65. Measurement methods by XRD and TEM for the dislocation density are compared. Dislocation cells, walls and tangled dislocations are observed as the main dislocation substructures in ferrite lamellae. The dislocation cell size is related to the width of the ferrite lamellae, which is gradually reduced with increasing drawing strain.

Evolution of microstructure and second-phase particles in Zr–Sn–Nb–Fe alloy tube during Pilger processing

ABSTRACT Evolution of microstructure and second-phase particles (SPPs) in Zr–Sn–Nb–Fe alloy tube were investigated during Pilger process using electron backscatter diffraction, secondary electron and transmission electron microscopy imaging techniques. Results show that the Pilger rolled tubes present heterogeneous structures with the C axes of less deformed grains mostly concentrated in the axial direction. During the Pilger rolling, the increase of deformation caused weakening of linear distribution of second-phase particles. The mean diameters of the precipitates are in the range of 70–100 nm in all specimens, and the growth mechanism of SPPs follows second-order kinetics. The grain growth is controlled by Zener pinning in the Pilger rolling–annealing specimens. Clusters containing the Zr(Nb,Fe)2 and βNb precipitates formed in the Zr–1.0Sn–1.0Nb–0.12Fe alloy. Most of the particles located in grain boundaries are the Zr(Nb,Fe)2 Laves phase with hexagonal structure, and stacking faults have been found in the Zr(Nb,Fe)2 precipitates. The types, morphology and distribution of precipitates depend on the constituent and structural fluctuations of the nucleation area.

Deformation mechanisms and dynamic recrystallisation of Zr alloy with different initial textures compressed at 700°C

Abstract The hot deformation behaviour depends on the initial texture. In this study a hot rolled Zr alloy plate with a bimodal basal texture has been compressed at 700°C along two directions to study the effect of initial texture on deformation mechanisms and dynamic recrystallisation (DRX). Four strain levels (10, 20, 30 and 45%) of uniaxial compression were analysed. Microstructures and textures of deformed specimens were characterized and compared by electron backscatter diffraction (EBSD). At 10% strain, twins were observed in the two textures, but {10–12} tensile twins predominate in the sample with the bimodal basal texture normal to the compression direction. As the compression strain accumulates to 20%, DRX is also affected by the initial texture. When the c-axis is close to the compression axis (CA), extensive DRX process was observed. If the c-axis is perpendicular to the CA, DRX was obviously retarded. Due to the initiation of twinning, the stored energy accumulated by dislocations was relatively low. After 45% compression, the two specimens developed the same bimodal basal texture with the c-axis of most grains rotated toward CA.

Effect of Mo and Bi Additions on the Microstructure of Zr–Cr–Fe Alloy After β-Quenching

This work investigated the solid-states phase transformation behavior of Zr–Cr–Fe alloys during rapid cooling from β-phase region. Scanning electron microscopy (SEM) was used to characterize the microstructure evolution of Zr–Cr–Fe alloys containing different Mo and Bi contents. The results show that two different phase transformation modes were involved during β to α transformation for different domains within prior β grains: (i) Martensitic transformation resulting in lath-shaped grains occured within prior β grain interiors. (ii) Massive transformation generating massive-shaped grains initiated along two adjacent prior β parent grain boundaries. Alpha (α) lath width reduced with increasing Mo concentration while Mo strongly retarded massive phase transformation. Interestingly microstructures exhibited no significant variation in the case of specimens containing different Bi contents irrespective of the phase transformation modes.

Anisotropic Biaxial Creep of Textured Nb-Modified Zircaloy-4 Tubing

Biaxial creep of Nb-modified Zircaloy-4 (HANA-4) tubing is investigated at varied ratios of hoop and axial stresses at a constant temperature of 500 °C using internal pressurization superimposed with axial load while monitoring the hoop and axial strains using non-contact laser telemetric extensometer and linear variable differential transducer, respectively. Steady-state creep rates along the hoop and axial directions were evaluated in the power-law creep regime from which the creep locus was derived at a constant energy of dissipation. The resulting creep locus was compared with that predicted by the anisotropy parameters, R and P in the Hill’s formulation for generalized stress for anisotropic materials. Crystallographic texture of the tubing was characterized using electron backscatter diffraction technique. Research is supported by NSF grant #DMR0968825.

Phase dependent restoration mechanisms of TC8 titanium alloy during hot compression in the two phase region

The TC8 titanium alloy was isothermally compressed at 1133 K and 1213 K in the (α+β) two phase region. The microstructural evolution and restoration mechanism in the α and β phases were characterized by optical microscopy and transmission electron microscopy. The results show a significant effect of phase content on the microstructural evolution and restoration mechanism. The grain refinement occurs in the α phase at both temperatures, but in the β phase only at the higher temperature of 1213 K. This difference in microstructural evolution is attributed to the different temperature dependence of restoration mechanisms in the two phases. A significant increase in the volume fraction of β phase makes the restoration mechanism in the β phase change from dynamic recovery (DRV) to dynamic recrystallization (DRX), which subsequently induces the β grain refinement.