Surface interactions in the iron-nitrogen system
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I.
INTRODUCTION
IN many respects the surface is one of the most important defects in a crystal. Not only is the surface a disruption of the atomic regularity, but many of the pretreatment processes used in preparing samples for thermodynamic equilibration experiments result in large dislocation densities and even microvoids in surface or near-surface regions. ~,2,3Such defect-rich layers have been referred to as the Beilby layer. 4 The spurious solubility data resulting from the presence of such layers can be enhanced even more by secondary defects formed on subsequent annealing or by the uptake of impurity atoms from the annealing atmosphere or "vacuum". This latter process can, of course, take place during the equilibration anneal in the gaseous second phase) Since equilibration with the second phase is a process whose kinetics are usually controlled by the diffusivity of the interstitial solute atoms in the solvent metal, the equilibration samples are invariably in the form of thin films with a large surface-to-volume ratio.6 This fact obviously leads to an increased propensity for the observation of surfacerelated spurious solubilities. Since real (bulk) solubilities usually decrease with decreasing temperature, the relative magnitude of the error related to surface effects usually increases as temperature decreases. These effects have been discussed in great detail 5 in the case of the Fe-H system. Since, however, H has such a high diffusivity in iron even at low temperature, it is difficult to obtain reproducible quantitative data connecting surface condition to solubility. Thus, in the present work the Fe-N solution, where the solute species has a much lower diffusivity than H, was chosen as the example system.
II.
liminary experiments performed on iron in the purity range 99.9 to 99.999 pct, no composition dependence of the measured N-solubility for a given surface condition was detected. Thus, all the work reported here was performed using Fe foils of a given fixed degree of purity made from MARZ-grade iron. The foil thicknesses were in the range 12 to 500/zm, arid the foils were of uniform thickness. The surface finishing and preequilibration treatment procedures used are compiled in Table I. The spectroscopic analysis of the iron is given in Table II. B. Equilibration Procedure
The prepared MARZ-grade foils were equilibrated in vycor boats in a horizontal tube furnace under 99.999 pct pure N2 gas at a constant pressure of 1.013 • 105 Pa. The details of flushing, pressure and temperature measurement, and sample quenching have been given in a previous report. 7 The N-content of the equilibrated foils was determined by fusion extraction using a system 7 calibrated regularly by NBS standards.
III.
A. Data Scattering in Prior Measurements
A number of investigations of the solubility of N in Fe at atmospheric N2-pressure as a function of temperature has been performed in the past. 7-~2 These measurements are depicted in Figure 1 in the form of plots of the atom ratio 0i of dissolved N vs reciprocal tem
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