You’re slicing up veggies for your dinner when the knife slips and cuts your finger. Immediately, your body initiates a series of physiological events, developed over millions of years of evolution. Before long, the wound is gone, and you’ve learned to be a little more careful with sharp objects.
Non-living things, of course, lack this inherent ability to heal: snap a pencil or shatter a window, and they will never be as good as new. In our imperfect universe, all materials, inorganic or organic, inevitably succumb to damage and deterioration. That’s why those who build the materials that comprise our lives have always concentrated on making things stronger, tougher, long-lasting. But many materials scientists and engineers are now taking a different approach, creating materials that are either easier to repair or can even “heal” themselves like living tissue.
Most such work has concentrated on soft polymeric substances— plastics, films, rubbers, and paints—because the molecular structure of such materials is easier to break down and alter with readily available techniques. “People have been trying for a long time to come up with mechanisms for physically closing cracks that are nucleating in materials,” explains Cem Tasan, the Thomas B. King Career Development Professor of Metallurgy at MIT. “But it’s almost impossible in the case of metals, because the required transformations in metals take place very slowly at room temperature.”
The same incredibly strong atomic bonds that make metals so durable also rule out the usefulness of self-healing concepts effective for polymers. “The field of self-healing in bulk metallic materials is in a rather early childhood stage,” notes a recent textbook co-authored by Tasan.
Uncovering the new knowledge and developing the new techniques necessary to bring that field to maturity is the quest of Tasan and his research group in MIT’s Department of Materials Science and Engineering.
Fulfilling that quest, Tasan notes, requires a departure from metallurgists’ time-honored practice of crafting metals and alloys “in terms of strength, ductility, hardness, and other engineering properties.” For millennia, that made perfect sense, but in a 21st century facing catastrophic environmental change from carbon-polluting industrial processes such as steel manufacturing, Tasan argues that reusability and repairability must also be considered. “Recycling is beneficial from a resource efficiency point of view, but not always from a CO2 emissions point of view,” he notes. “Instead of full recycling processes, feasible metals repair and reuse options need to be developed.”
Showing the cracks
Metals rarely fail for a single reason. They may undergo constant or repeated physical strains, temperature changes, pressure variations, hydrogen embrittlement, and many other effects that are still only partially understood. All these stresses accumulate over time, resulting in the formation of microscopic cracks and voids that weaken the metal’s structure. As the weakened metal loses its ability to withstand the stresses, the microscopic cracks coalesce to form macroscopic cracks. Ultimately, the metal fails completely, with consequences that can range from inconvenient (a broken door hinge) to catastrophic (an airplane crash).
So Tasan’s first major objective, he explains, “is to develop novel techniques that give us a clearer physical understanding of how metals fail under different conditions, so that we can interrupt these processes with certain treatments and either remove the damage or reverse the process of damage accumulation.”
“To be able to apply a repair treatment, one first has to understand how the microstructure, the atoms and the bonds and the crystals, evolve as a result of the external conditions,” Tasan says. So he and his team subject metal specimens to various stresses under tightly controlled laboratory conditions. They have designed machines that deform or put hydrogen in metals, for example, inserting those machines into electron microscopes, then observing failure processes in situ and in real time.
Armed with such nanoscale insights, Tasan is devising better ways to repair existing metals. Repair strategies and treatments vary depending on the damage mode, and can involve local or bulk heat treatments and deformation processes, among other approaches. As Tasan explains, there are two consequences of damage evolution during metal failure. “One is a structural change that is also causing a change in properties, and the second is a change that is related to the form of the component, the shape of it. There are failure mechanisms that don’t change the shape of a component at all but cause an abrupt failure, and there are failure modes that change the shape and also the structure.”
Failure modes that don’t involve shape change are generally more conducive to repair treatments, but there’s a complication. “Metals were never designed with the idea of reusability,” Tasan points out, “so many of our alloys have either unknown or limited repairability.”
Microstructures inspired by nature
That leads to the second aspect of Tasan’s work, developing new metallic materials. “We have to make sure that we put sufficient effort in following a parallel route in updating our material design, so that the material can respond to even simpler repair treatments. Ideally you just want to repair the component without replacing it, by applying a short, feasible treatment for a second, not putting it in a furnace at a thousand degrees for a week.”
One of Tasan’s recent projects addresses the fatigue process in metals, which can lead to failure due to cyclic stresses. “Typically, the structural materials we use today are tough,” he notes. “Even when a micro-crack is nucleated, the materials’ microstructure can arrest the micro-crack to avoid its growth. The problem is that the mechanisms involved with the arresting of the crack, such as local plasticity and phase transformation, are one-time mechanisms. So if there’s another stress cycle, the crack will propagate.” Tasan took some hints from the structure of bone tissue to design a form of steel with a layered microstructure capable of both resisting the formation and restraining the propagation of microscopic cracks. The material, still in the testing stages, is suitable for scaling up for commercial applications. It’s a highly promising example of Tasan’s approach, which he prefers to call “structural resetting” rather than “self-healing.” (“We’re not trying to create alloys that close cracks themselves,” he clarifies—“we’re trying to create alloys that have tough microstructures that, once damaged, can be repaired easily to retain their original crack arresting capability.”)
Even with the practical economic and environmental benefits that easily repairable metals can offer, Tasan acknowledges that “there are certain applications where it just doesn’t make sense.” For example, his group abandoned its interest in designing reusable bolts after realizing the construction industry would be unlikely to go to the trouble of repurposing such a small, inexpensive component after it had sat in a building for decades.
Larger and more expensive metal components such as airplane or automobile parts are a different story. When such parts are failing, weakened by one or more fatigue cracks, 99.999% of the metal is still in good shape. “Imagine you could apply a treatment that would play with the atomic structure to avoid or to delay this process,” he says. “Then it makes a lot of sense, because by treating only 0.001% of the material, you save the rest.”
And it’s more than costly equipment that is being saved: Tasan sees his work as a worthy strategy to reduce carbon emissions and fight climate change. “If we do not come up with ways of reusing and repairing materials, there’s no way that we can cut down CO2 emissions to reasonable levels,” he says.