Solidus boundary shift
The alteration of phase boundaries due to non-equilibrium conditions during rapid solidification. It leads to modified solubility limits within the crystal lattice.
Solidus boundary shift means the boundary between solid and liquid doesn't behave as you know from the equilibrium diagram. During rapid cooling or heating, diffusion in the lattice lags behind, so this boundary shifts and you get a different microstructure than expected. For you as a welder, this means: if the cooling rate isn't right, the microstructure in the heat-affected zone can become inhomogeneous. The risk is hardening cracks or zones with different toughness that can fail under load. In practice, you have to ensure uniform and controlled cooling. If the part cools too fast, it becomes hard and brittle; if it cools too slowly, you get coarse grains and lose toughness. You find the correct cooling rate from the material data and the welding procedure.