Isothermal Annealing Conditions Research Articles

Effect of temperature range on extent of microstructural degradation during thermal cycling of unidirectionally solidified eutectic composite

Abstract A refractory carbide strengthened nickel-base alloy was directionally solidified at 5 mm h—1 to produce a regular well-aligned eutectic alloy consisting of niobium carbide fibres in a nickel–chromium matrix. The fibrous microstructure which is extremely stable under isothermal annealing conditions suffers severe degradation during thermal cycling. The upper cycling temperature and the temperature range of the cycling treatment were found to strongly affect the extent of fibre degradation, which increased as both upper cycling temperature and temperature range increased. Fibre degradation occurred by nucleation and growth of nodules at the corners of some fibres and by the dissolution of other fibres. Possible mechanisms for such microstructural breakdown are discussed.

Kinetics of the decomposition of supercooled austenite in high-speed steels R6M5, R6M5K5, and R18

1. The kinetics of the decomposition of supercooled austenite was studied and the effect of slight differences in the composition (within the nominal limits) of steels R18, R6M5K5, and R6M5 on the stability of supercooled austenite was determined. 2. Methods of mathematical statistics were used to establish the variation of austenitic and pearlitic transformation times with the chemical composition of steel R18. For the minimal concentration of carbon and carbide-forming elements the time for the austenitic and pearlitic transformations is 2–3 and 3–4 h, respectively; it is 6–7 and 11–12 h for the maximum concentration. 3. Isothermal annealing conditions were established in relation to the chemical composition of steel R18 in chamber furnaces with a given ratio of austenitic and pearlitic transformation times. 4. By taking into account the chemical composition of the heat it is possible to improve greatly the productivity of furnace equipment and the quality of annealing of high-speed steels.

The growth of fission gas bubbles in irradiated uranium dioxide

Abstract The growth of fission gas bubbles from supersaturated solution in irradiated uranium dioxide has been studied by electron microscopy under isothermal annealing conditions between 1300° and 1500°C. Measurements of the kinetics of bubble growth have enabled the diffusion coefficients of atomic xenon and krypton in irradiated uranium dioxide to be determined. The diffusion coefficients obtained may be expressed by the equation: The results obtained from the current experiments are believed to be more precise than most of those obtained using the Booth model for gas released from solid samples of irradiated uranium dioxide during a post-irradiation anneal.

Migration of gas bubbles in irradiated uranium dioxide

Migration of fission gas bubbles in irradiated UO 2 was studied, using electron microscopy, under isothermal annealing conditions at 1400, 1450 and 1500° C. Bubbles with radii greater than 37 Å had a dependence of bubble migration distance on bubble radius which was characteristic of a migration mechanism controlled by volume diffusion. Migration rates were very much smaller than would be predicted from a surface-diffusion-controlled mechanism. The activation energy for bubble migration was found to be 130 000 cal/mole. Smaller bubbles migrated at a slower rate than predicted by a volume-diffusion-controlled mechanism.