Abstract
The distribution of boron in samples of Type 316 austenitic steel (containing 18 ppm by weight of B), cooled at a rate of either 50 or 500 degC/s from solution-treatment temperatures in the range 900–1350°C, has been determined using an optical microscope autoradiographic technique. Segregation of the boron to the grain-boundary regions was detected in the specimens cooled at the slower rate from the higher solution-treatment temperatures, the extent of the intergranular segregation diminishing with decreasing solution-treatment temperature; grain-boundary segregation of the boron was not detected in the specimens cooled at 500 degC/s irrespective of the solution-treatment temperature. A theoretical model is proposed to account for this non-equilibrium grain-boundary segregation of boron during cooling, based on the existence of mobile vacancy-boron complexes. The predictions of the model are in good agreement with the experimental observations if realistic values for the vacancy-boron binding energy and complex diffusion rate are assumed.
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