Effect of hydrogen on creep behavior of Ti-6AI-4V alloy at room temperature
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I.
INTRODUCTION
THE effects
of hydrogen on mechanical properties of structural alloys have been investigated extensively. 1-9'14 Nevertheless, there is still a controversy as to whether hydrogen causes softening or hardening. Many investigators have reported that the yield and flow stresses of iron and steels are increased by hydrogen charging.l-4 But, under other conditions the yield and flow stresses are decreased by hydrogen. Beachem5 recognized that hydrogen decreased the torsional yield strength of medium carbon steel. Bernstein6 showed that vacuum melted iron with 0.15 pct Ti was softened by hydrogen. Kimura and co-workers, 7'8'9 investigating the effect of hydrogen on mechanical properties of pure iron, showed that cathodic hydrogen charging could cause softening at temperatures between 200 K and 300 K. They explained that this softening effect was caused by an increase in screw dislocation mobility due to the presence of hydrogen. It is well known that dissolved hydrogen causes the embrittlement of certain metals and alloys.I~ On the other hand, the data referenced above have shown that hydrogen can also cause softening of metals and alloys, decreasing the yield and flow stresses. The relation between embrittlement and softening is not yet understood. Asano 3 suggested that the "softening" noted in Kimura's works might be due to (1) unexpected internal damage caused by precipitation of hydrogen, or (2) surface effects while the thin wire samples were being charged cathodically. Oriani ~~ put forth the decohesive theory of hydrogen embrittlement in which there exists a threshold value of hydrogen fugacity. At hydrogen fugacity above the threshold value the rate of relaxation, or creep rate, is sharply increased. He deduced that this threshold effect is due to a hydrogen-induced decohesion process that promotes nucleation and growth of microvoids at internal surfaces in the steel. An interesting result was noted earlier 14 during tension testing of Ti-6A1-4V: hydrogen decreased the elastic limit of Ti-6A1-4V significantly at low strain rate (10-6/S), while the 0.2 pct yield stress was increased, as shown schematiG. Y. GAO, formerly Visiting Scholar at the University of Delaware, is at the Institute of Corrosion and Protection of Metals, Academia Sinica, Shenyang, Liaoning, People's Republic of China. S. C. DEXTER is Associate Professor, College of Marine Studies, University of Delaware, Lewes, DE 19958. Manuscript submitted March 10, 1986. METALLURGICAL TRANSACTIONS A
cally in Figure 1. This indicated that, while hydrogen softened Ti-6AI-4V at the beginning of tension at low strain rate, it also caused strong work hardening, thereby leading to embrittlement. Neither Asano's 3 suggestion nor Oriani's decohesive theory J~ can explain these results. Creep behavior has been investigated extensively by many workers, 15'16'17and the mechanism at several stages is clear. However, further work on the primary stage of room temperature creep of hydrogen-containing alloys is needed to determine the hydrogen-induced
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