A metal may look perfectly intact, but inside it a quiet separation is already underway. In iron-chromium alloys, at certain temperatures, chromium atoms start to cluster into islands, leaving behind chromium-poor regions. This is called spinodal decomposition, and it makes the material brittle and more prone to corrosion eating through it.
The trouble is that spotting the onset of this decomposition with conventional methods is difficult. You need a way to peek inside the metal without breaking it. Physicists have proposed using sound — more precisely, short laser pulses that make the alloy's surface vibrate at very high frequencies. By measuring how quickly these vibrations die out, you can calculate the material's elasticity.
It turns out that after spinodal decomposition, an iron alloy with 36% chromium becomes noticeably stiffer than it was before the separation. This is unexpected: we usually associate damage with softening. But here a subtle effect is at play: elasticity depends on chromium content nonlinearly, and at certain compositions, the decomposition results in a net stiffness higher than that of the homogeneous solution.
Calorimetry confirmed that this particular alloy releases heat during heating, characteristic of spinodal decomposition. And atomistic computer simulations showed that stiffness should indeed increase after decomposition. This method could become a simple tool for inspecting components that operate at high temperatures — for instance, in the energy sector.
Spinodal decomposition was first described back in the 1950s, but for a long time it was considered an exotic phenomenon. Today, it is found even in glasses and polymers.
In Andy Weir's novel 'The Martian', the protagonist repairs equipment relying on the strength of metal. If an alloy inside were to start separating, a routine inspection wouldn't reveal it: you'd need exactly this kind of laser test.