Non-Equilibrium Diffusion Zone
A microstructural transition region where concentration gradients could not be fully eliminated due to rapid cooling.
When welding dissimilar materials, a zone can form in the heat-affected zone where the alloying elements don't mix properly. This happens when cooling is too fast and the atoms don't have time to distribute evenly. The result is a structure that is harder and more brittle than the base material, and that's exactly where cracks like to start. Corrosion also sets in quickly at such spots. To get this under control, you need to slow down the cooling. You can achieve this, for example, by preheating or by not dissipating the heat too quickly. If you cool the weld too fast, you have exactly this problem. So: better work a bit slower and more controlled, then the structure stays calm and the weld holds.