Deformation and failure mechanisms of niobium and tantalum during tensile testing in uranium at 1473 K

It is well known that, when a higher-melting-point solid metal is exposed to the combination of a specific lower-melting-point liquid metal and stress, severe embrittlement can occur. This is known as liquid metal embrittlement (LME). Refractory metals with high melting points and good workability, such as the Group VB metals (V, Nb, and Ta) have been used to construct molds for casting other metals that have low melting points. One of these low-melting-point metals is U, which is used in nuclear power reactors as fuel. However, it was previously reported that severe degradation occurred when Ta was exposed to liquid U. The kinetics of the penetration of U into Ta under stress-free conditions were studied. It was found that, between 1433 K and 1623 K, despite the low solubility of U in Ta, the Ta matrix recrystallized into elongated grains, and U penetrated between the grains. The penetrated U existed in front of the transformed grains as a separate layer and along grain boundaries. For conditions in which stresses exist, it is expected that the penetration of liquid uranium could be further accelerated and thus embrittle the Ta matrix. There is no detailed study about the LME mechanisms of Group VB metals in liquid U. The authors have initiated a research activity to study the mechanisms of embrittlement of Nb and Ta by liquid uranium. Their initial results, as reported in this paper, indicate a significant difference between Nb and Ta when exposed to liquid U and subjected to tensile stress