If you break a piece of metal in just the right way, it can actually become harder to break instead of weaker. This surprising idea is at the center of a new material from Purdue University. Engineers there have created a cobalt-aluminium alloy that is six to ten times stronger than structural steel, making it one of the toughest metals ever tested in a lab.
This material is a cobalt-aluminium intermetallic, meaning its cobalt and aluminium atoms form a tightly ordered crystal structure instead of mixing randomly like in regular alloys. Normally, cobalt-aluminium is as brittle as chalk and breaks under pressure instead of bending. Because of this, intermetallics are rarely used in engineering, even though they are light and have high melting points. The Purdue team, led by Professor Xinghang Zhang and postdoctoral researcher Ke Xu, discovered a way to overcome this brittleness.
Their material has a yield strength of 6 gigapascals, which is the point where a metal starts to change shape permanently instead of returning to its original form. It can also stretch up to 15% before breaking at room temperature. For comparison, high-strength steels usually yield between 0.4 and 1 gigapascal. Getting six times that strength while still allowing the metal to bend instead of shatter is something researchers have struggled to achieve for years. The team describes this breakthrough in their Science Advances paper, which SciTechDaily also covered in detail.
The trick lies in how the material is built, not what it is made from. Zhang’s team used magnetron sputtering, a deposition technique that knocks atoms off a target material with charged plasma and lets them settle onto a surface one atomic layer at a time, to construct the alloy as a nanoscale laminate, alternating layers only a few nanometres thick. Critically, some of those layers are deposited in an amorphous state, meaning their atoms are arranged randomly rather than in the alloy’s usual rigid crystal lattice.
When mechanical pressure is applied, these amorphous layers turn into crystals. As this happens, they release dislocations, which are tiny defects that let layers of atoms slide past each other instead of causing the whole material to break. In a typical brittle intermetallic, there is no way for this sliding to occur, so stress builds up until the material cracks. In the Purdue laminate, the change from amorphous to crystalline acts like a controlled release valve for stress, layer by layer, throughout the material.
Engineers are thinking about tough applications for this material. Turbine blades in jet engines and power plants face extreme temperatures and spin under huge forces. Every gram saved on a blade reduces stress on the rest of the engine. A material that is much stronger than steel and flexible enough to handle real-world forces without breaking could let engineers design thinner, lighter blades that work at higher temperatures and are more efficient than current nickel-based superalloys.
This goal is still far away. Making turbine-sized parts with magnetron sputtering, which was designed for thin coatings only micrometres thick, is much harder than making small lab samples. However, the materials science behind this breakthrough is now proven. It shows that an intermetallic once thought to be unusable can be turned into one of the strongest metals ever tested.
Engineering Deep Dive: How Nanoscale Layering Defeats Brittle Fracture
To see why the Purdue cobalt-aluminium alloy matters, it helps to know why intermetallics usually fail. Most structural metals like steel, aluminium, and titanium are solid solutions, where atoms of different elements are mixed randomly in a shared crystal lattice. This disorder lets the lattice handle defects called dislocations. When a metal bends, these dislocations move through the structure, allowing the metal to change shape without breaking. This is basic materials science: ductility depends on how easily dislocations can move.
Intermetallics like cobalt-aluminium are different. Their atoms form a very ordered structure, sometimes called a superlattice, where cobalt and aluminium take specific, fixed spots instead of being mixed randomly. This order gives intermetallics useful traits like high melting points and resistance to oxidation, but it also makes the lattice very rigid. Dislocations that move easily in regular metals have to break and reform bonds in this ordered structure, which takes much more energy. Below a certain temperature, this energy is too high, so instead of bending, the material just breaks. That is why cast cobalt-aluminium acts more like ceramic than metal at room temperature.
Zhang and Xu’s solution does not change the chemistry of cobalt-aluminium. Instead, it changes the structure. Using magnetron sputtering, the team built the material as a stack of nanoscale layers. Some layers are crystalline and ordered, while others are left amorphous, meaning they have no long-range atomic order. Amorphous materials, like glass, cannot support dislocation movement as crystals do. Instead, they deform through local atomic rearrangement, where small groups of atoms shift past each other under stress. These are called shear transformation zones.
This is where the engineering gets really smart. When mechanical pressure is applied during testing, the amorphous layers do not stay that way. They crystallise under stress, a process called stress-induced crystallisation. As the atoms move into their ordered positions, this change creates dislocations at the boundaries between layers. These new dislocations can move easily because the boundary between an amorphous and a crystalline layer is more open and can handle defects better than a fully ordered lattice. The material basically creates its own ductility, right where and when it is needed.
This idea is similar to a principle from physics and product design: composite structures often work better than their individual parts because each layer does a different job. For example, plywood is strong in many directions because its thin layers are glued with grains running in different ways, so each layer makes up for the weakness of the others. The Purdue laminate uses the same idea, but at a scale about a million times smaller. By alternating amorphous and crystalline layers, the material combines strength and flexibility, so one layer’s rigidity is balanced by another’s ability to bend and prevent failure.
The next big challenge is making this material at a larger scale. Magnetron sputtering is slow, adding material just nanometres per second, which works for research samples but is too slow for making large parts like turbine blades. Researchers will need to either speed up the process or find other ways to create the same layered structure, without losing the special mix of strength and flexibility that makes this alloy unique.
Structural metals rarely improve by this much in a single step. Most gains in alloy design come from incremental changes to composition or heat treatment, shaving a few per cent off weight or adding a modest strength margin. A cobalt-aluminium alloy strength increase of six to tenfold over steel, achieved through architecture rather than chemistry, suggests nanoscale layering could become a design tool applied well beyond this one material system.
- Purdue University engineers created a cobalt-aluminium alloy six to ten times stronger than structural steel
- The material reaches a yield strength of 6 GPa while sustaining 15% plastic strain at room temperature
- Built using magnetron sputtering to create a nanoscale laminate with amorphous interfaces
- Amorphous layers crystallise under pressure, releasing dislocations that prevent brittle fracture
- Potential applications include tougher, lighter turbine blades for jet engines and power plants
