Ultrasonic Determination of the Diffusion Coefficient of Carbon in α-Iron at 408° and 430°C

Abstract

The ultrasonic attenuation at 31 MHz and 60 MHz has been measured in an iron single crystal from room temperature to 625°C, using zinc sulfide vacuum-evaporated, thin-film transducers. Longitudinal waves propagating along the [110] direction were used, and the attenuation was measured in both the “carbon-free” condition (wet-hydrogen-treated) and the carburized condition. We have apparently observed the carbon Snoek peaks in the carburized specimen. Assuming that the Snoek peak occurs only via octahedral site to octahedral site jumps, we obtain a diffusion coefficient of carbon in a α iron, at 408 ±5°C, of Dc = 4.52 × 10−9 cm2/sec, and at 430°±5°C, of Dc = 8.85 × 10−9 cm2/sec. These values fall in a region previously devoid of diffusion data. The new values connect, in a relatively smooth manner, the low temperature Snoek-effect-determined diffusion coefficients with the high temperature mass-flow-determined coefficients. It must be mentioned, however, that in the case of the diffusion of carbon in α-iron, the extraction of the diffusion coefficient from the Snoek damping data is by no means obvious. This point is discussed.