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IN the last decade, there has been considerable interest in the embrittlement of group VA metals, noibium, ~ 9 tantalum, ~~and vanadium 4,1H4 in the presence of hydrogen. When pure, this group of metals has appreciable ductility over a wide range of temperature down to liquid nitrogen) 5 When hydrogen is dissolved, however, the mechanical properties of these metals are greatly affected. The ductility decreases sharply with decreasing temperature, resulting in the raising of the ductile-brittle transition temperature and the embrittlement of these materials. As the temperature is lowered further, the ductility of the metal containing hydrogen reaches a minimum and stays brittle for period of temperature, while the hydrogen-free specimens show considerable ductility. At very low temperatures, the ductility begins to exhibit an apparent rise, which indicates that the effect of hydrogen is less pronounced at these temperatures. The extent of this ductility minimum depends upon strain rate, where the embrittlement becomes less pronounced as the strain rate is increased. 1.4.8:~For higher hydrogen concentrations, two ductility minima are observed in niobium) '3'4 As the temperature is lowered, an earlier transition from ductile-to-brittle behavior occurs, and upon further cooling a ductility minimum followed by a limited recovery is exhibited. The recovery of ductility is curtailed by a second ductile-brittle transition from which there is no recovery upon further cooling. While the general features of the embrittlement of group VA by hydrogen are in relative agreement, no general understanding of the mechanisms involved has been established. Two mechanisms have been considM. M. FARAHANI, formerly graduate student at the University of Houston, is now research metallurgical engineer with Mostek Co., Carrollton, TX 75006. F. ATTIA, formerly graduate student at the University of Houston, is now design engineer with Brown and Root Inc., Houston, TX 77001. K. SALAMA, Professor in the Department of Mechanical Engineering, University of Houston, Houston, TX 77004. Manuscript submitted December 19, 1979. METALLURGICAL TRANSACTIONS A

ered to be the cause of the hydrogen embrittlement in this group of metals. The first mechanism, namely hydrogen decohesion, was first proposed by Troiano ~6 for the embrittlement of high-strength steels. The mechanism deals with the presence of hydrogen as a point defect which has been shown to be favored by the application of macroscopic stresses) 7,18Hydrogen diffuses along a stress-induced activity gradient to regions of triaxial stressing and produces the loss of lattice cohesion which facilitates crack nucleation and subsequent growth. This process continues until the growth is terminated due to the exhaustion of the hydrogen in the volume within ready diffusio,~ distance of the crack tip. 4 The second mechanism deals with the other extreme, where a phase change at the crack tip occurs, and the metal hydride is formed. Group VA metals form metal hydrides at relatively low hydrogen concentrations