|
|
|
|
|
|
|
|
|
| CSIRO | SOLVE | Issue 8 | Aug 06 | |
|
|
|
ARTICLE
 METALS: Healing Powers By Jason Major
Researchers have discovered metals that can quickly self-repair cracks and respond to fatigueStress and fatigue force a rupture. Immediately, a repair mechanism kicks in and the healing begins. ‘Heal thyself’ is a natural trait of living things, but this self-healing is also possible with metal, and has caught the attention of the world’s aerospace and transport research scientists and engineers. However, disregard thoughts of a liquid metal ‘Terminator’ appearing from a laboratory any time soon. The investigation into self-healing metals is still blue sky research that almost never began, because the self-healing mechanism seemed so obvious when it was first observed it was thought that someone must have already found and exploited it.
“Such metals could have an important application in components used in hard-to-access areas or those that are expensive to maintain,” says Dr Roger Lumley, research scientist and project leader with CSIRO Manufacturing and Materials Technology (formerly CSIRO Manufacturing and Infrastructure Technology). Testing so far has shown, under certain conditions, up to a tenfold increase in the useable life of metals showing self-healing behaviour during fatigue. Dr Lumley first observed the self-healing mechanism 10 years ago as a PhD student at the University of Queensland when he was testing the effects of different thermal processes on metal behaviour. However, it took him until this year to have a report on it published in peer-reviewed literature. “What we observed was, we thought, so obvious, that it must have been reported on before,” Dr Lumley says. “The mechanism was textbook solid-state metallurgy with much of the background theory for it appearing in the 1950s, but nobody seemed to have taken the next step.” It was not until he was working with CSIRO and using the same mechanism for improving fatigue life of metal that colleagues pointed out to him that he was onto something new and potentially significant. CSIRO is now funding Dr Lumley’s further research in this area. Metallurgical heresyMetallurgists have a philosophy that the more stable a metallic alloy, the better it is at resisting environmental and service life stresses such as fatigue and creep (changing shape at elevated temperature). What Dr Lumley proposes is almost heretical: create an unstable metal, one that takes advantage of the features a more stable metal is designed to avoid. “To achieve self-healing in metals, you need the metal to be relatively unstable,” he says. “This instability allows the metal to respond or adapt favourably to its service conditions, something a stable metal will not do.” But unlike living organisms that come with an in-built repair mechanism, a metal must have self-healing designed into it. Dr Lumley works with aluminium alloys, but says the mechanism should, in theory, apply more widely. Metals and alloys are known to degrade and lose functionality during service. Factors shortening the service life of these materials are internal defects and changed surface conditions, for example, corrosion and wear. To improve the mechanical properties of aluminium alloys, manufacturers use heat treatments to age and stabilise the metal. A self-healing metal’s secret is its combination of alloying elements, such as copper, zinc, magnesium or silver, and how these solute elements interact in the metal matrix during heat treatment. Controlled solute instability creates a metal that is capable of further change triggered by loading conditions. In a self-healing metal, areas of fatigue or stress such as tiny cracks – sometimes only tens of atoms in length – create what are termed high free-energy sites. The alloying elements are moved to these sites and, in the case of a tiny crack, will come out of solution as small crystallites to fill or repair the crack. One of the primary research tasks now is to enhance the self-healing process and properly understand, control and exploit it. CSIRO researchers have had success with some experimental alloys and are homing in on the optimum processing conditions. Although the philosophy and research into self-healing metals is still embryonic, CSIRO and its collaborators are already looking for opportunities to demonstrate the effectiveness of these metals in practical situations. APPLICATION Self-healing metal – combining alloying elements and determining how these interact in the metal matrix during heat treatment – has the ability to quickly repair cracks or respond to fatigue BENEFIT Improvements in the performance of metal components under loading stress For further information contact: |
 IN THIS ISSUE
|
| Home | About Us | eSubscribe | Links | |
|
|
||
|
|
|
CSIRO and a number of international teams – including close collaborators at the Delft University of Technology, Netherlands, and Monash University – are now working on understanding and controlling this mechanism at the atomic level. If they succeed – and they are tantalisingly close – metals that can quickly self-repair cracks and respond to fatigue will be used to improve the performance of metal components. This will expand the scope of design and applications of these metals.
Back to main