Phase boundary diffusion
The accelerated mass transport along transition zones between different phases, which facilitates diffusion processes at elevated temperatures.
Phase boundary diffusion is basically the movement of atoms across the boundary between two different materials or microstructural states. During welding, this happens constantly when the filler material meets the base material and alloys with it. The problem is: if diffusion is too vigorous or you combine the wrong materials, the structure in the heat-affected zone can become unfavorable. Then you get brittle spots, cracks, or the weld doesn't hold properly. In practice, this means: make sure the materials are compatible and don't unnecessarily put too much heat into the part. If you cool the weld properly after welding, you have better control over diffusion. And if you're unsure whether materials are compatible, ask beforehand rather than repair later.