Damage evolution in metals under the gigacycle fatigue loading due to the contact with surface-active liquid

The Rehbinder effect (adsorption strength reduction effect) consists in a qualitative change of the development of multi-scale damage kinetics in a deformed sample in the presence of surfactants (in our case, liquid gallium). Its influence on the fatigue life of pure iron under gigacycle loading conditions 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 similarity of the chemical potential of the solid and 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 material. Fatigue tests of pure iron were carried out on the ultrasonic resonant fatigue machine Shimadzu USF-2000 with frequency of 20 kHz in the gigacycle fatigue loading regime with very low stress amplitude, during which the material failure occurs after realization of 109 loading cycles. Fractured surfaces were analyzed by optical and electronic 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 a surfactant are significantly reduced on the "adiabatic" surface and the region of localization of the 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 gigacycle fatigue regime, which leads to the formation of cracks in the surface layer of the sample, rather than in the volume of the material, which is characteristic of this kind of fatigue failure.