Embrittlement by liquid U in some group VB and VIB metals and alloys during tensile loading at 1473 K
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I.
INTRODUCTION
THE Group
VB and VIB refractory metals (Ta, Mo, W, V, Nb, etc.) have higher melting point temperatures than does U. The reported equilibrium binary phase diagramst~-41 between these metals and U indicate that the thermodynamic behaviors of these systems differ significantly. At temperatures just above the melting point of U (1408 K), the mutual solubilities between liquid U and solid Ta and between liquid U and solid W are very low, less than - 2 at. pet (Figure 1). In contrast, the mutual solubilities between Nb and U and between V and U are much higher (Figure 2). Similarly, for temperatures below the melting point of U (1408 K), the mutual solubilities between W and U and between Ta and U are very low, whereas those between Nb and U and between V and U are much higher. It has been reported that Nb [5] and Mo t61dissolved uniformly, with no intergranular corrosion in liquid U as a result of their high solubilities in liquid U; because of the comparably high solubility of V in liquid U, it is expected that V will behave similarly. It has also been reported that liquid U preferentially attacked Ta, I5,7,81Ta10 wt pet W, tsl and W [9'1~ along grain boundaries. These differences could lead to significantly different failure mechanisms of these metals and alloys during tensile loading it liquid U. A previous studyt51 had shown that Ta was embrittled in liquid U under tensile loading but that Nb was not. The difference was explained as significant intergranular penetration in Ta, minimal in Nb. In this work, we extend the study to W, V, and Ta-2.5 wt pet W. On the basis that the Ta-U and W-U phase diagrams are similar, it is believed that W is likely to be embrittled by liquid U. In the field of liquid-metal embrittlement (LME) research, it is known that LME is most often associated J.S. HUANG and G.F. GALLEGOS, Metallurgists, are with the Chemistry and Materials Science Department, Lawrence Livermore National Laboratory, University of California, Livermore, CA 94550. Manuscript submitted August 30, 1989. METALLURGICAL TRANSACTIONS A
with phase diagrams in which the mutual solubility between solid and liquid elements is very small near the temperature of embrittlement, rH] The objectives of this work are to (1) investigate whether the susceptibility to embrittlement by liquid U in Group VB and VIB refractory metals can be predicted from this phase diagram Correlation and (2) establish the fundamental mechanism leading to the observed embrittlement. The approach taken to establish the mechanisms is to perform tensile tests at various strain rates and to characterize fracture surface features. II.
EXPERIMENTAL DETAILS
The materials studied are commercial grades with purity >99.9 wt pet. At least two heats for each material were obtained from different suppliers. Table I lists the typical interstitial contents, the annealing treatment given, and the average grain size after each material was annealed. The U used in this study was depleted of the 235U isotope (>99.9 wt pet 23SUisotope) with a chemical
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