Solid solution
A solid crystal structure in which foreign atoms are uniformly incorporated or dissolved within the host lattice.
Mixed crystals form when foreign atoms dissolve in the lattice of the base metal during melting or at high temperature. The atoms either occupy regular lattice sites or squeeze between existing atoms. The lattice remains uniform, no separate phase forms. During welding, cooling behavior is crucial. If you cool too fast, the foreign atoms remain "frozen" in the lattice. This strains the lattice, making the material harder and more brittle. In steel with carbon, this can lead to unwanted martensite. Too many alloying elements can also increase the risk of cracking in the heat-affected zone. So adjust heat input and cooling to the material. For high-carbon steels, preheating helps; for thick sections, you must keep an eye on the interpass temperature. The goal is for the alloying elements to dissolve cleanly without excessive grain growth or hard structures forming.