Bai Guanghai

Evolution of Oxidation in Ni-Cr-W Alloy at 1100 °C

Abstract The oxidation process of a newly developed Ni-Cr-W alloy in air at 1100 °C has been investigated by varying the holding time. The microstructures and compositions of the surface scale as well as the oxide in cross-section were characterized by Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectrum (EDS). The phase identification of the surface scale was carried out using X-ray diffraction (XRD). Results show that in the initial stage of oxidation ( 7 h), a NiCr2O4 layer with spinel structure grows out gradually in the outer layer of Cr2O3. Once the outer layer of the compact spinel is formed, the double multi-components layer NiCr2O4·Cr2O3 will further decelerate the oxidation rate of the alloy. The degradation of the alloy subsurface includes the inner oxidation of grain boundaries and the formation voids and carbides-free area. The high concentration of W in the alloy does not impair the corrosion resistance of the alloy obviously.

Effects of Solution Heat Treatment on Carbide of Ni-Cr-W Superalloy

The microstructure features of a kind of Ni-Cr-W superalloy after solution heat treatment at different temperatures (1230–1300 °C) for various time (10–120 min) were investigated by means of scanning electron microscope (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results show that the precipitated phases of the alloy after solution heat treatment are identified as W-rich fcc M6C and Cr-rich fcc M23C6 carbides. The volume fraction of carbides decreases with increasing of the solution heat treatment temperature and time. At the temperature above 1270 °C for 120 min, most of carbides are dissolved into the Ni-matrix, and the carbides distributing at twin boundaries may be dissolved into the matrix first. M6C carbide has no any fixed orientation relationship with the Ni-matrix, but the angle of M23C6 (220) to the matrix (11) is 30°.

Burst Test Research on Zirconium Alloy for Nuclear Fuel Cladding Tubes

Abstract To evaluate the mechanical properties and promote a better understanding of failure behavior of the newly developed Zr-Nb cladding tubes during reactivity initiated accidents (RIAs), burst tests were carried out with the biaxial stress state under different pressurization rates at the temperature ranging from 293 to 623 K. The influence of pressurization rate and test temperature was characterized in this test. The pressurization rates were 0.97 GPa/s at 293 K, 0.62 GPa/s at 473 K, and 0.49 GPa/s at 623 K for the fast pressurization test, and 0.23 MPs/s for the ASTM burst test. Maximum hoop stress was increased by 10.61%, 9.32% and 8.26% compared to the ASTM burst test at 293, 473 and 623 K, respectively. The results show that the burst ultimate hoop stresses (UHS) at rupture decrease while the total circumferential elongation (TCE) increased monotonically with the rise of temperature, the uniform circumferential elongation (UCE) is not significantly affect by the test temperature. All the burst tests result in ductile ballooning failure, and cracks initiate and propagate along the axial direction above the maximum hoop stress. A higher pressurization rate leads to shorter but wider cracks. As the test temperature is raised, the ductility increases and the propagation of the axial crack is suppressed.

Effect of Processing Techniques on Precipitation Behavior of Secondary Phase Particles in Zr-1Nb-0.01Cu Alloy

Abstract The precipitation behavior of secondary phase particles (SPPs) was investigated in Zr-1Nb-0.01Cu alloy prepared by various processing techniques. The results show that with the decrease of cold rolling and annealing steps, intermediate annealing temperatures, final annealing temperatures and time, the average sizes of SPPs decrease accordingly. The β -Zr phase formed at the intermediate annealing temperature (higher than 640 °C) is hard to dissolve in the matrix after final annealing treatment. During the long-time final annealing process, the SPPs with smaller size coalesce into larger particles by diffusion through the matrix gradually because of Ostwald ripening mechanism. It shows that the decrease of intermediate annealing temperature is more effective than other processing techniques for minimizing the size of SPPs. The average particle size smaller than 50 nm can be achieved by low intermediate/final annealing temperature (≤520 °C) or short annealing time (≤2 h) treatment. The investigations in the present paper are very meaningful to control the SPPs in Zr-Nb based alloys.