Effect of activity differences on hydrogen migration in dissimilar titanium alloy welds
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I.
INTRODUCTION
~T is well known that titanium alloys are subject to mechanical property degradation in the presence of external or internal hydrogen. Hydrogen embrittlement in a-/3 titanium alloys has been related to slow tensile strain embrittlement and sustained load cracking, tl-6] Several models explaining hydrogen embrittlement have been proposed, which include hydrogen diffusion to triaxially stressed regions, causing reduced cohesive strength; t7'81 void formation; t9,l~ surface adsorption; E1~421 and hydride precipitation and fracture, tl3~ There appears to be general agreement concerning the importance of stress-assisted hydrogen migration in contributing to overall embrittlement and cracking, tla-w] The mechanistic aspects of hydrogen embrittlement are concerned with the extent and kinetics of hydrogen movement and local enrichment brought about by gradients in solute concentration, stress, temperature, and their effects on hydrogen activity in titanium, t61 In dissimilar titanium alloy weldments, alloy compositional gradients create hydrogen activity gradients, which can cause migration of hydrogen. Previous work on a Ti6A1-4V pressure vessel welded with unalloyed titanium (Ti-CP) filler metal showed that hydrides were formed in the Ti-CP along the fusion zone/heat-affected zone interface. These hydrides ultimately caused an unexpected failure, t6'2~ 221 It was determined that the activity of hydrogen (all) in the Ti-6A1-4V alloy was higher than J.R. KENNEDY, Staff Scientist, and P.N. ADLER, Director, Materials and Structures, are with the Corporate Research Center, Grumman Corporation, Bethpage, NY 11714. H. MARGOLIN, Distinguished Research Professor, is with the Department of Materials Science and Engineering, Polytechnic University, Brooklyn, NY 11201. Manuscript submitted October 24, 1990. METALLURGICAL TRANSACTIONS A
in the Ti-CP, which led to uphill hydrogen diffusion into the Ti-CP and subsequent hydride formation, t61 Also, it has been shown that A1 increases the activity of hydrogen in alpha titanium. 123,24'251 This article presents the results of a study to determine the effect of alloying elements and temperature on hydrogen activity, as a means of predicting the tendency for hydrogen migration within dissimilar alloy welds. The relationships between hydrogen concentration and measured activity are used to estimate hydrogen redistribution among the materials studied and thereby serve as a guide for reducing the likelihood of hydride formation. II.
EXPERIMENTAL PROCEDURE
A series of seven titanium alloys containing 0 to 6 wt pct A1 and other alloying element variations was used for this study. The chemical compositions are given in Table I. The first six alloys were rolled to round bars below their respective /3 transus temperatures and had microstructures consisting of either equiaxed or elongated a. The Ti-6A1-2Nb-ITa-0.8Mo alloy (Ti-6211) was rolled to plate in the /3 field and had a Widmanstatten a morphology. The round bars were cut into specimens 12.5 mm in diameter and 9.5-ram lon
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