Metastable Solid Solution
A state in which an alloying element remains dissolved in the base lattice beyond its equilibrium solubility. It serves as the starting basis for targeted precipitation hardening.
A metastable mixed crystal forms when you 'freeze' alloying elements in the lattice during welding that actually don't fit anymore. That happens when the material cools too quickly after welding and the atoms don't have time to sort themselves out cleanly.
That can make the weld hard and brittle. If you want to bring the structure back to normal later, only targeted heat treatment helps, i.e. tempering or stress-relief annealing. Without that, you risk the weld breaking under load.
In practice, that means: Don't let the weld cool down sharply after welding, but let it cool slowly or insulate it. If you notice hardness or cracks after welding, that's a sign of such an unstable structure – then off to the oven with the part.