Coherent Phase Boundary

A coherent phase boundary forms when two crystal lattices merge seamlessly without a visible interface in between. The whole thing is stabilized by elastic strains in the lattice. For welding practice, this means: if the cooling and heat management are right, you get a clean, solid joint. But if the process goes wrong, e.g., due to too fast cooling, this boundary doesn't form properly – the part becomes brittle or warps. So you have to control the cooling. Cooling too quickly is the classic mistake that ruins coherence. Better to let it cool in a controlled way, then the weld stays stable and crack-free.