Self‐Healing Phenomena in Metals

Abstract

puters) would not be possible in their current form without highly developed metallic materials.Currently all metallic materials are developed along the so-called "damage prevention" paradigm, [1] i.e., their composition and microstructure are optimized such that the initiation and propagation of mechanical damage (in the form of internal damage such as pores or surface initiated cracks) leading to catastrophic failure of the product is postponed as much and as long as possible. Such a performance needs a careful tuning of the constituent atoms forming the matrix and its secondary phases, the grain size and the dislocation structure in the "pristine" state. Of course, the word "pristine" state is somewhat a misnomer as engineering metals generally have undergone multiple thermomechanical treatments once casted and are in a far-from-equilibrium state. Yet, in this context "pristine" metallic products are those in the shape and condition at the start of their life in an actual construction. All current metallic engineering materials have in common that once a local crack or defect is generated, either by the preceding plastic deformation or simply by mechanical overload of the material, this damage will remain forever and could be the initiator of the final catastrophic fracture event. In simple mathematical termsTypical examples of such an accumulation of microscopic damage leading to macroscopic failure are (low and high cycle) fatigue as well as creep failure. Generally speaking metallic systems perform very well under such conditions due to the high bond strength between the metallic atoms as well as a stable dislocation network, which tend to keep the atoms more or less in place or at least in registry, even in case of severe loading or deformation.The same positive attributes responsible for the excellent mechanical properties of metals are also the attributes for the modest success (in comparison to the field of self-healing polymers, [2][3][4] self-healing composites, [5] self-healing concrete [6] ) in developing self-healing metallic materials. For a material to become self-healing local processes leading to "local In comparison to other materials, in metals and metallic systems self-healing of cracks and crack-initiating defects is difficult to achieve due to the fact the solute atoms that act as healing agents are relatively small and generally have a relatively low mobility at the prevailing operating temperatures. In this review, the scientifically most interesting and industrially most promising approaches to self-healing metals are presented and discussed. The various approaches are separated in autonomous healing methods based on an intrinsic (solid-state diffusion) mechanism and assisted healing methods that need an external intervention. Some promising routes are identified while in other cases the approach has too many intrinsic limitations. Recently, a number of computational studies using molecular dynamics and finite element modeling have been performed to analyze the self-healing potential of...

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