Comments on an electrochemical model for hot-salt stress-corrosion of titanium alloys
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Comments on An Electrochemical Model for Hot-Salt Stress-Corrosion of Titanium Alloys
called geometrically necessary dislocations. This interaction should necessarily be between dislocations and not directly between dislocations and the particles. The stress for the latter interaction is high and should not, therefore, be thermally activated. Experimentally determined values of the activation volume also lend support to this hypothesis. ~ has been calculated for a fully flexible dislocation assuming ~ = Lb z, where L is the mean interparticle spacing between the geometrically necessarily dislocation arrays) and b the Burgers vector. These values (shown in Table II) agree well with the experimentally measured values, considering all the approximations involved. The authors are grateful to Professor J. C. M. Li for a stimulating discussion on multlprocess activation and to Dr. B. A. Wilcox for materials and for useful discussions. This research was carried out when one of the authors, V. S. Arunachalam, was a visiting scientist at the Aerospace Research Laboratories at Wrlgfit-Patterson Air Force Base. I. C. Y. Cheng and U. F. Kooks: Proc. of the Secondlnt. Conf. on the Strength of Metals and Alloys, Asflomar, 1970, p. 199. 2. H. A. Lipsitt and V. S. Arunachalam: unpublished research, i972. 3. F. Guiu and P. L Pratt: Phys. Status So/M/, 1964, vol. 6, p. 111. 4. P. Guyot: Oxide Dispersion Strengthening, p. 405, Gordon and Breach, 1968. 5. A. Lasalmonie and M. Sindzingrr Acts Met., 1971, vol. 19, p. 57. 6. V. S. Arunachalam and H. A. lJpsitt: unpublished research, 1972. 7. B. A. Wilcox and A. H. Clauet: 1968, NASA CR-72367. 8. Von Eckard Macherauch: Z. Metallk., 1964, vol. 55, p. 60. 9. M. E. Ashby: Ox/de Dispersion Strengthening, p. 143, Gordon and Breach, 1968. 10. A. H. Clauer and B. A. Wilcox: Metal Sci. J., 1967, vol. 1, p. 86. 11. U. F. Kooks: Trans. Japan Inst..Metals, 1968, vol. 9, p. 1.
9 excellent cbntributlon to our understanding of a complex phenom.en0n was made by M. Garfinkle. I The comments ~r fo~l~ow relate to the difference between pure metals and alloys in stress-corrosion behavior. R is generally recognized that pure metals do not stress-corrosion crack in any environment, z Apparently the reason for this is that they have such low flow stress that relaxation at stress risers or at a
crack tip readily occurs. Titanium alloys are corroded by hot-salt at temperatures of 900~ (755 K) and higher, but do not stress corrode. Again stress relaxation appears to be the reason. Creep occurs so readily at these temperatures that relaxation at the base of a corrosion pit occurs. The unalloyed titanium d~ Garfinkle's study Is not pure, and it has been cold worked to a relatively high strength. Therefore, it stress corrodes at 170~ (350K); but at 650~ (616K) it creeps at such a high rate that stress corrosion9 is not possibie. NOte that the strength drops 57,000 psl (390 M N / m z) up,on raising the temperature from 170 ~ to 650~ (350 to 616K). Such a precipitous drop in short-time strength portends an even gre
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