The liquid metal embrittlement effect in armko-iron during high and gigacycle fatigue loading

Abstract

The liquid metal embrittlement effect (Rehbinder effect) consists in a qualitative change in the development of multi-scale damage kinetics in a deformed specimen in the presence of a surfactant (in our case, a liquid galliumeutectic alloy). Its influence on the fatigue life of pure iron is associated with a qualitative change of the role of the surface. The property of the surface to serve as a high power “sink” for defects is significantly reduced due to the similarity of the chemical potential of the solid and the surfactant as a result of “filling” the incomplete atomic planes,which provides the “adiabatic” character of the process of damage accumulation in the volume of the material. Fatigue tests of pure iron were carried out on a servohydraulic machine with a frequency of 20 Hz in the high-cycle fatigue regime anda Shimadzu USF-2000 ultrasonic resonant machine with a frequency of 20 kHz in the gigacycle fatigue loading regime with very low stress amplitude (the material failure occurs after 109 loading cycles). The fractured surfaces were analyzed by optical and electron microscopes to identify the depth of penetration of liquid metal into the fracture area. It has been shown that the durability and strength of the material coming into contact with the surfactant are significantly reduced on the “adiabatic” surface and the region of localization of defect density is shifted to the surface layer, which is characteristic of “embrittlement” materials being in contact with a surfactant. The Rehbinder effect radically changes the mechanism of crack initiation in the gigacycle fatigue regime, which leads to the formation of cracks in the surface layer of the specimen, rather than in the volume of the material, which is characteristic of this kind of fatigue failure.