Crack-Healing in Metals By Electrochemical Method
Authors: Shi Y.N. 
Published: February 2020

Metallic materials have been widely used as structural materials for their considerable strength and toughness. During service damages will initiate and accumulate in the material which will never go down even when loads are reduced or removed. If something can be done to heal or repair these damages, the lifetime of the materials will be much extended. Crack is one of the most common form of damage. Crack healing or repairing in materials is of significant importance for sustainable development of the world. Although some progress has been made in material healing in some systems, considering the poor mobility of metal atoms at ambient temperature, it is quite challenging to realize crack healing or repairing in metallic materials. Here we present an electrochemical process named electro-healing to heal cracks in metals, where metallic ions with satisfactory mobility was employed as the healing agent, components containing cracks as the cathode.
Polycrystalline pure nickel plates with through thickness cracks were used as the experimental sample. Electro-healing of these cracked samples were carried out in solutions with high throwing power and suitable for low current density, respectively. Scanning electron microscope (SEM) and electron back scatter diffraction (EBSD) was introduced to observe the morphology of the healed cracks and the texture of the healing crystals. Transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM) were employed to characterize the microstructure of the healed cracks, boundary between healing crystals and the original crack surfaces(substrate) and boundary between the healing crystals. Micro-hardness tests and tensile tests were performed to evaluate the mechanical property of the crack-healed samples. Healing efficiency in terms of tensile strength was calculated to evaluate the effectiveness of electro-healing.
The results demonstrated that: through-thickness cracks in Ni sheets with sizes in the micrometer range or larger can be successfully healed by electro-healing. The electro-healing process starts with the vertical epitaxial growth of healing crystals from the substrates (the original crack surfaces) followed by lateral growth of healing crystals with the depletion of healing agent. The healing crystals bond with each other at atomistic level. These crystals have finer grains and higher strength compared with the substrate. Tensile tests exhibited that the healed samples have a comparable tensile strength as the original sample and some tensile ductility can be achieved for the sample of 100 μm thick. Post-fracture analysis indicated that part of the crack propagated along the substrate instead of healing crystals. The healing efficiency, ranging from 96%~33% with an increasing sample thickness, is related to the fraction of fully-healed region and the strength difference between the substrate and the healing crystals.
Further electrochemical investigation indicated that the addition of inhibitor or accelerator could change the growth mode of healing crystals with the growth velocity of healing crystals from crack tips is much faster than that from surfaces of the crack center, and therefore minimize pores or voids that left on the meeting line after electro-healing by plain solution. The morphology of the healed crack is controlled by the additive type, cathode current efficiency, convection condition and the interaction between the additives and the cathode as well.