טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentLevy Rafael
SubjectLiquid Metal Embrittlement of AISI 4340 Low Alloy Steel
by Ga-In Eutectics
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Eugen Rabkin
Full Thesis textFull thesis text - English Version


Abstract

The phenomenon of Liquid Metal Embrittlement (LME) can be defined as a brittle fracture or a catastrophic failure of solid metal when being intimately wetted by liquid metal. In most cases an external tensile stress or internal residual stresses are necessary to induce the fracture. Fracture caused by LME is characterized by a significant reduction in fracture stress, elongation, or both. Severe failures in metals and alloys that caused by LME have been reported in literature throughout the years. Therefore, understanding the phenomenon is necessary for preventing failures in various industrial fields, processes and applications.


We studied the embrittlement of AISI 4340 martensitic steel by the liquid eutectics of In-Ga (Tm = 15.3°C). We performed tensile tests in liquid metal environment at room temperature and determined the conditions for occurrence of LME (such as strain rate, stress concentrations and wetting method), as well as fracture strength of the steel. It was found that LME reduces the steel strength only in high strength notched specimens that were deformed in tension at low strain rate (0.0005 s-1).


Since there is no single accepted mechanism for the LME phenomenon we conducted further investigations for better understanding of the LME mechanisms in low alloy steels. The dependence of notch tensile strength (NTS) on strain rate was determined both for wetted and dry samples. While in dry samples the NTS was independent of deformation rate, a significant decrease of NTS with decreasing deformation rate was observed in the case of LME. The corresponding deformation activation volume was ~1.7b3, where b is the Burgers vector of steel. This low value of activation volume is in a good agreement with the results of atomistic molecular dynamics simulations of incipient plasticity reported in the literature, and is consistent with predictions of Nam-Srolovitz atomistic mechanism of LME.