Liquid metal embrittlement of nuclear materials

Abstract

The phenomenological features of liquid metal embrittlement (LME) are reviewed and the influence of metallurgical factors and testing conditions is described. The process is shown to be similar in many respects to the elastic fracture observed at lower temperatures in some bcc and hcp. metals. An important difference in the case of LME arises from the need for the embrittler to be present at the crack-tip during fracture. This condition imposes a low temperature limit which occurs when the embrittler is no longer mobile. The relationship between susceptibility to LME and alloying characteristics is discussed. The various theories have been considered and it is concluded that a reduction in surface energy leading to lower crack-tip cohesion and hence lower plastic deformation is consistent with most of the experimental evidence.The second section of the review uses this general understanding as a basis for speculation about possible synergy between LME and radiation effects. Those which alter the plastic deformation behaviour, i.e., radiation hardening or radiation annealing, are considered likely to have the same influences on embrittlement as on elastic fracture. Radiation creep will reduce any tendency to slow crack growth under restricted embrittler availability. Radiation-induced transmutation is seen as a possible source of supply of embrittling species, particularly as fission products, but interaction with high temperature creep-cavitation fracture by helium segregation is less likely because LME tends to be more severe at lower temperatures.The review concludes with a brief catalogue of LME-susceptible couples, concentrating as far as possible on nuclear materials (i.e. ferrous and zirconium alloys) to provide an initial source of data for materials selection by designers and operators and for failure analysis.