Metals are fated, like all things, to eventually decline and fail as various forms of damage are inflicted upon them over time. The lifetime of a metal is a feature of extreme importance; how long can you trust the girders of a bridge to support the weight of a car? Common sense tells us that eventually it must fail, but when? Being able to repair or “heal” metals would extend their lifetime considerably. These methods of healing could be external or integrated into the metal from inception. In biological systems it is the damage that triggers the healing, although recreating such an affect in metals is exceptionally difficult.
One process of self healing is to embed microcapsules into the metal lattice which will break open if a local crack forms. The chosen healing agent within the capsules will be released and repair the damage. Although the conductivity of the embedded metals could be returned the mechanical properties of the metal, could not be. Another process can occur if the metal is being weakened by external heat. This heat can trigger the precipitation of a healing agent at the crack boundaries as these are the highest stress areas. This restricts further crack growth and so slows down the degradation. The self healing is only really required as to avoid the need to detect the location of defects. Simply causing the metal to undergo plastic flow under high heat and pressure also eliminates this need as the metal flows into any crack present; but the mechanical proprieties do suffer under this process.
The brilliant insight that was made was regarding the electrical properties of a metal with defects. The cracks in the metal lattice can be seen as simple air gaps, which themselves have an electrical property. They are tiny areas with infinite resistance within a large conductor. This means that current must be denser around these areas and so the effect of resistive heating must be greater. By pulsing electric current through the damaged area a self healing (sort of) process can be induced which was shown to be successful in this paper in a titanium alloy. This process is environmentally friendly and doesn’t require targeting or a healing agent. It is also widely applicable to most metals and their alloys.