Issue 42

A. Strafella et alii, Frattura ed Integrità Strutturale, 42 (2017) 352-365; DOI: 10.3221/IGF-ESIS.42.36 364 - cracking induced by the adsorption of liquid metal atoms - accelerated failure due to the penetration of the liquid metal along the grain boundary of the solid - stress - aided dissolution of metal from a crack tip These considerations can justify the intergranular fracture mode: intergranular fracture usually occurs when the phase in the grain boundary is weak and brittle and this is probably due to the penetration of the liquid metal along the grain boundary of the steel which caused the embrittlement of phases and then the accelerated failure. It means that lead changes both mode and type of fracture: from mixed ductile/brittle to total brittle and, referring to brittle mode, from transgranular to intergranular type. That is the effect of liquid metal embrittlement [13]. C ONCLUSIONS he creep and corrosive behavior of the 15-15Ti(Si), for the nuclear systems using heavy liquid metals, has been addressed. For this purpose, the steel has been studied in a stress range of 300-575MPa at 550°C, in air and in hostile environment. From tests performed in air, the characteristic creep curve strain-time was obtained. The Norton’s law parameters n and A were calculated and it was found that the dislocation creep mechanism is the main mechanism for the 15-15Ti(Si) creep deformation. The creep curve sscr-stress (log–log plot) was plotted based on Norton's law, on experimental data and on the calculated values n and A. This curves is an important results because simulates the creep behaviour of 15-15Ti(Si) in air at all stress values. SEM micrographs showed that the specimen fracture mode in air was mixed, transgranular and cup-cone. Tests in lead were also performed and the main results can be summarized as follows: - For high stresses, the creep effect dominates the deformation in primary and secondary stages, while the effect of Pb becomes prevalent in the tertiary stage - For low stresses, the effect of Pb is more evident and LME tends to prevail on creep effect. The specimen rupture mode in lead is brittle, as confirmed by SEM micrographs that highlights the typical intergranular fracture. Moreover, the differences between mixed fracture mode, ductile-brittle/transgranular, occurring in air and fully brittle/intergranular mode, in lead were shown. It means that liquid metal embrittlement occurs: lead changes both the mode and type of fracture, from mixed ductile/brittle to total brittle and, referring to brittle mode, from transgranular to intergranular type. Since only few data on 15-15Ti(Si) characterization can be found in the literature, the obtained results provide useful information for this austenitic steel applications in extreme conditions, such as nuclear field. A CKNOWLEDGEMENT The authors are grateful to Dr. Selene Grilli for her careful revision of the manuscript. We also acknowledge the C.R. ENEA Brasimone for financial support and providing the samples to be tested. R EFERENCES [1] Latha, S., Mathewa, M.D., Parameswaran, P., Bhanu Sankara Rao, K., Mannan, S.L., Thermal creep properties of alloy D9 stainless steel and 316 stainless steel fuel clad tubes, International Journal of Pressure Vessels and Piping, 85 (2008), 866–870 Elsevier [2] Gilbon, D., Séran, T.L., Maillard, A., Touron Rivera, H.C., Lorant, H., Permet, J., Rabouille, O. Swelling microstructure of neutron irradiated Ti-stabilized austenitic steels, International conference on materials for nuclear reactor core applications, Bristol (UK), (1987) 27-29. [3] IAEA, International Atomic Energy Agency. Nuclear Energy Series Technical Reports Structural Materials for Liquid Metal Cooled Fast Reactor Fuel Assemblies — Operational Behaviour No. NF.T.4.3 Guides- Printed by the IAEA in Austria (2012). [4] Vaidya Gkss, W.V., Radiation-induced recrystallization, its cause and consequences in heavy-ion irradiated 20% cold- drawn steels of Type 1.4970, Journal of Nuclear Materials 113, North.Holland Publishing Company, (1983) 149-162. T