The mechanical damping of iron from room temperature to 400°c at 7 megacycles/sec

Abstract

The pulse-echo method has been used to measure the Snoek damping peaks of nitrogen and carbon in α-iron at 6.65 Mc/sec. The diffusion peaks occurred at 326° ± 2° C for nitrogen and at 353° ± 4° C for carbon. The diffusion coefficient of nitrogen so determined was in good agreement with a determination by other workers at nearly the same temperature by a bulk desorption method. The carbon diffusion coefficient so determined fell on the Arrhenius line extrapolated upward from the low temperature data, but the present data accentuates the anomaly reported by both Smith and Homan. Consideration of all the available diffusion data for nitrogen in α-iron (and also δ-iron) shows that the diffusion coefficient can be represented from −50° to 1470°C by a single Arrhenius relation, namely: D = 4.88 × 10 −3 exp[ −(18,350) RT ] cm 2/ sec . The diffusion coefficient of carbon may be represented from −40° to at least 350°C by: D = 3.94 × 10 −3 exp[ −(19,160) RT ] cm 2/ sec . This new value for carbon removes the discrepancy between D 0 for carbon and nitrogen and according to the ideas of Zener, shows that the strain energy is only a part of the total activation free energy for diffusion. The work on the carbon Snoek peak indicates that supersaturated solutions of carbon in α-iron may be quite persistent around 350°C. This could possibly account for the discrepancy between the heat of solution of cementite as measured by equilibria and by internal friction methods. The orientation dependence of the nitrogen Snoek peak height was observed with longitudinal waves propagated in the [100], [110] and [111] directions in single crystals, with results confirming Polder's theory. The tem perature dependence of this relaxation in single crystals gives a possible indication that nitrogen atoms resist martensite-type ordering to a great extent in dilute iron-nitrogen alloys. Magnetic damping peaks due to carbon and nitrogen, similar to those observed by Maringer near 1 c/s, have been observed at 6.65 Mc/sec. The small nitrogen Maringer peak, however, occurs at a lower temperature than the corresponding Snoek peak.