12 October 2013

MIT’s self-healing metal fixes tiny flaws before they can create massive problems

When a material is damaged, you wouldn’t expect pulling it apart to suddenly make it less damaged. This counterintuitive effect is exactly what researchers at MIT observed in an experimental model recently, and it was so unexpected that the results had to be rechecked before anyone was ready to believe it. Astonishingly, it seems that under the right conditions, metal with small flaws and cracks can heal itself when tension is applied — if you pull it apart, it puts itself back together.

Researchers led by graduate student Guoqiang Xu and professor Michael Demkowicz modeled microscopic cracks in a sheet of nickel with tension applied. Instead of worsening, the cracks became smaller, then closed on their own as the edges fused together. After assuring themselves the effect was real, the next step was figuring out how it happens.

The answer has to do with the basic structure of metals, most of which are composed of microscopic crystalline grains of varying sizes and shapes. The orientation and size of these grains affects the overall mechanical strength and other characteristics of the material. Nickel is of particular interest because it is a basis for many so-called superalloys used in harsh environments like jet turbines, deep-sea oil rigs, and joints in heavy industrial equipment. It turns out that the grains making these materials so strong are not as static as scientists thought.

As the metal is pulled outward, the edge of the crystalline grains begins to migrate and can eventually fill in the crack completely. The migration of this crystalline boundary — the edge of the grain — is what heals the gaps in the material. This is different than the quest for self-healing artificial skin, but no less important.

This doesn’t affect the large-scale cracks that you are able to see with the naked eye — only damage to the microstructure. Researchers have only been able to reproduce the healing behavior with defects known as disclinations (A disclination is a small crack that extends part way through a grain.) However, it is believed that these micro-scale defects are the seeds of much larger, sometimes catastrophic cracks and metal fatigue. The geometry of disclinations can actually reverse an applied force locally, which is how the tension leads to the metal pulling itself back together.

The team believes this mechanism can be used to prevent superalloys developing structural cracks by healing the tiny flaws that can lead to real damage. Material could even be engineered to direct damage into disclination-type structures which could be easily healed with a little stress in the right direction. If the healing effect is robust enough, this could even lead to metals that don’t weaken with age.

Now read: Caltech Engineers Focus On The Nano To Create Strong, Lightweight Materials

Research paper: DOI: 10.1103/PhysRevLett.111.145501: “Healing of Nanocracks by Disclinations”


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