Diffusion of Oxygen in Hafnium

Abstract

gain constant in the temperature range of 600~176 as a function of reciprocal absolute temperature. The activation energy was-~ 32.8 kcal/mole. X-ray examination indicated that La20~ was the only oxidation product at all temperatures. These results demonstrate that lanthanum oxidizes linearly in the temperature range of 600~176 This is in disagreement with the results of Vorres and Eyr-ing (2). No reason is advanced for this discrepancy. The weight gain of the samples used in the present investigation, if all the lanthanum were oxidized to lanthanum oxide, would be ~ 35 mg/cm 2. This agrees within experimental limits with the observed value of 32 mg/cm ~. Loriers used samples which, if completely oxidized, would gain 0.42 mg/cm 2. This is also in agreement with the observed value of 0.45 mg/cm 2. These observations and x-ray data indicate that lanthanum oxidizes completely to lanthanum oxide by either a continuous or discontinuous mechanism. The observation of a discontinuity in the weight-gain-vs.-time plots and the decrease in the initial rate as the temperature increases has been observed in 1185 columbium by Kolski (5). The discontinuity is much sharper in lanthanum. Although the phenomena observed in the two metals may be related, normal metallographic practices are unsuitable for studying lanthanum because of its instability even at room temperature. Manuscript received April 15, 1964. Any discussion of this paper will appear in a Discussion Section to be published in the June 1965 JOURNAL. REFERENCES 1. Some years ago I first reported on the determination of the diffusion coefficient of oxygen in hafnium (1). The technique was one used previously to measure the diffusion of oxygen in zirconium (2) and consisted of observations of the dissolution rate of anodically deposited interference colored oxide films. At the time of the hafnium study no data were available regarding the hafnium-oxygen phase diagram, and since calculations based on this technique required a knowledge of the solubility limit of oxygen in haf-nium, the diffusion coefficient was reported for each of three assumed solubility limits. Since this publication there have been three papers by other authors concerned with this subject. Gadd and Evans (3) report measurements of the diffusion coefficient of oxygen in hafnium in the temperature range 700 ~ 1200~ based on microhardness measurements made on oxidized samples. Wallwork and Smeltzer (4) report values for the diffusion coefficient at temperatures of 800 ~ and 950~ based on microhardness indentations on oxidized samples and calculated on the basis of a theory involving a steady-state solution for the oxygen gradient in the metal phase during linear oxidation. Rudy and Stecher (5) in a determination of the hafnium-oxygen phase diagram report a value of the solubility limit of oxygen in hafnium as 20.5 at. % at I350~ and almost independent of temperature. In recent studies I have shown that the concentration gradient of oxygen beneath the oxide/metal interface in oxidized samples of zirconium (6) and hafnium (7) can be accurately predicted by a theoretical expression involving diffusivity, time, and oxide thickness. This enables the diffusivity of oxygen in the metal to be calculated when the other parameters are accurately known. In addition, the depth of penetration of oxygen was found to increase with time during protective oxidation and, on the onset of linear oxidation, begins to decrease. Zones of constant hardness at high oxygen concentration probably due to ordered hafnium-oxygen alloys were reported. It is the purpose of this communication to review the data concerning the diffusion of oxygen in hafnium in the light of the subsequent publications and some additional studies carried out in this Laboratory. Experimental and Results In conjunction with the aforementioned study (7) the diffusion coefficient of oxygen in hafnium was re-determined by the anodization technique, this time performing measurements within a single grain of hafnium to avoid the necessity of estimating average colors, and hence average thicknesses, of interference films on a large number of grains in a polycrystalline sample. The new measurements were performed at 575 ~ 614 ~ and 655~ The diffusion coefficient D may be expressed as (x')2 D-4b2t where b satisfies the equation Co b(1 + erfb) ~e-b2 W%o "k//~ Here in time, t, there is a displacement of the oxide/ metal boundary, x', which is related to the observed decrease AL in thickness of oxide film by the following expression involving the respective molecular volumes VHf saturated x' ~ AL Vmo2 The quantity Co is the difference between the saturated concentration, C~, and initial concentration of oxygen in the metal, and mo represents the weight of oxygen removed from the HfO2 consumed in the generation of unit volume of saturated hafnium (HfO0.2~s). Values for the constants used are pHfO2 ~-10.13 g/cm~ (8), pHf saturated estimated as 13.29 g/cm ~, Cs-0.300 g/cm ~ (5), ~no ~ 2.03 g/cm 3, x' = 0.661 AL, and b ~ 0.0764. In order to determine whether extrapolation of the low-temperature diffusion data to high temperatures is justified, 3A in. diameter hafnium spheres were oxidized at 950~ for 111 hr and 1050~ for 64 hr. The sample was sectioned through the center, polished metallographically, and photographed at 400X and 800X magnifications at each of sixteen equidistant positions around the perimeter. The thickness of the oxide was determined with a planimeter and used to calculate the total quantity of oxygen contained in the oxide scale. This value when subtracted from the total weight gain gives the quantity of oxygen in solution in the metal substrate. In the case under consideration about 50% of the oxygen absorbed was contained in the metal substrate. These data were then used to calculate the diffusion coeffi-ecsdl.org/site/terms_use address. Redistribution subject to ECS license or copyright; see 202.96.46.54 Downloaded on 2013-10-25 to IP ecsdl.org/site/terms_use address. Redistribution subject to ECS license or copyright; see 202.96.46.54 Downloaded on 2013-10-25 to IP