by
Larry PHILLIPS(Thomas Jefferson Laboratory, Newport News, VA, USA)
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Europe/Rome
LNL Meeting Room (INFN LNL)
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INFN LNL
Description
Liquid metal embrittlement or LME is characterized by the reduction in the true fracture stress and/or in the strain to fracture when tested in the presence of liquid metals as compared to that obtained in air / vacuum tests. The reduction in fracture strain is generally temperature dependent and a “ductility trough” is observed as the test temperature is decreased. A ductile-to-brittle transition behaviour is also exhibited by many metal couples. The shape of the elastic region of the stress-strain curve is not altered, but the plastic region may be changed during LME. Very high crack propagation rates, varying from few centimeters per second to several meters per second are induced in solid metals by the embrittling liquid metals. An incubation period and a slow pre-critical crack propagation stage generally precede final fracture. It is believed that there is specificity in the solid-liquid metals combinations experiencing LME. There should be limited mutual solubility for the metal couple to cause embrittlement. Excess solubility makes sharp crack propagation difficult, but no solubility condition prevents wetting of the solid surfaces by liquid metal and prevents LME. Presence of an oxide layer on the solid metal surface also prevents good contact between the two metals and stops LME. The chemical compositions of the solid and liquid metals affect the severity of embrittlement. Addition of third elements to the liquid metal may increase or decrease the embrittlement and alters the temperature region over which embrittlement is seen. Metal combinations which form intermetallic compounds do not cause LME. Alloying of the solid metal alters its LME. Some alloying elements may increase the severity while others may prevent LME. The action of the alloying element is known to be segregation to grain boundaries of the solid metal and altering the grain boundary properties. Accordingly, maximum LME is seen in cases where alloy additions elements have saturated the grain boundaries of the solid metal. The hardness and deformation behaviour of the solid metal affect its susceptibility to LME. Generally harder metals are more severely embrittled. Grain size greatly influences LME. Solids with larger grains are more severely embrittled and the fracture stress varies inversely with the square root of grain diameter. Also the brittle to ductile transition temperature is increased by increasing grain size.
