Thermomechanical effects on hydride precipitation in zircaloy-2
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other experimental measurements of surface selfdiffusion. 923. J. A. Clum and R. D. Gretz: Fourteenth Field-Emission Symposium, National Bureau of Standards, Gaithersburg, Maryland, June 1967. 24. G. A. Odishariya: Soy. Phys. Sol. State, Engl. Transl., 1968-69, vol. 10, p. 1130. 25. J. A. Clum: Ph.D Thesis, Carnegie-Mellon University, 1968. 26. J. A. Clum: Scripta Met., 1970, vol. 4, p. 463.
J. A. C L U M
IN
his latest of a s e r i e s of important contributions to the surface diffusion literature Allen has determined surface self-diffusion p a r a m e t e r s for niobium to be: D s = 0.4,8 exp [(-46,100 + 2 6 0 0 ) / k T ]
This communication is written to provide supporting data from the literature for the activation enthalpy determined by Allen. A r e m a r k can also be made r e l a t ing to Allen's observation that long time diffusivittes for molybdenum and rhenium were affected by bulk impurities. It was determined by this author in some earlier work as that the activation enthalpy for surface selfdiffusion of niobium was Q = 48,100 cal/mol. That data was obtained by following the current-voltage c h a r a c teristics of field-emitter tips following annealing sequences which produced tip blunting by surface diffusion. The niobium was of ultra high purity (99.999 pct). A similar result was obtained by Odishariya m using another field-emission technique. In that case there was reported to be some variation in activation enthalpy if a calculated correction was made for the electric field, i . e . , Q = 49,500 c a l / m o l (field build-up) and Q = 54,700 c a l / m o l (corrected to zero electric field). Since there is some question about the actual effect of the electric field ~ the uncorrected value of Odishariya may be more appropriate for purposes of confirming Allen's result. In r e g a r d to Allen's concern about bulk impurities it has been pointed out ~8 that bulk impurity can significantly affect the diffusivities. The activation enthalpies for surface self-diffusion of niobium of two purity levels were found to be greatly reduced for the case of the p u r e r material. In that case the less pure niobium (99.7 pct) was found to have an activation enthalpy of Q = 89,000 cal/mol. Both materials were studied in the field-emission microscope and the effect of impurity could be easily recognized by changes in the fieldemission image as a function of annealing time. The less pure material showed image changes c h a r a c t e r istic of contamination after shorter times and at lower annealing temperatures. It was not possible to identify the contaminant in those field-emission experiments with as much certainty as Alien was able to do by Auger analysis of his molybdenum and rhenium specimens. In conclusion it is noted that Allen's result for niobium can be confirmed by two independent experiments using different techniques. Additionally, Allen's concern for bulk impurity effects can be reaffirmed by *B. C. ALLEN, Met. Trans., 1972, vol. 3, pp. 2544-47.
J. A. CLUMis Assistant Professor, Department of Metallu
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