microsegregation

Microsegregation is the uneven distribution of alloying elements within a single grain of a metal or alloy, occurring at the scale of tens to hundreds of micrometers. During solidification, solute atoms move more slowly than the advancing liquid-solid interface, causing them to concentrate in the last material to freeze. This creates compositional gradients between the core and the outer edge of each dendrite (the branching crystal structure that forms as metal cools), even though the grain itself is chemically homogeneous when viewed at lower magnification.

The severity of microsegregation depends on the cooling rate and the degree to which each alloying element favors the solid or liquid phase. Fast cooling rates reduce segregation because atoms have less time to diffuse, while slow cooling allows the composition to homogenize. Binary alloys show the simplest behavior, but commercial steels and superalloys contain multiple elements, each with its own segregation coefficient, making the dendrite structure chemically complex in the radial direction.

Consequences and remedies

Microsegregation creates mechanical and corrosion vulnerabilities. Regions enriched in manganese or chromium may be brittle; regions depleted in carbon may be soft. In stainless steels, chromium-depleted zones can precipitate chromium carbides, reducing corrosion resistance in those bands. Heat treatment can reduce microsegregation through diffusion, but complete homogenization requires prolonged exposure at high temperature and is often impractical for large ingots or castings.

The term distinguishes this phenomenon from macrosegregation, which occurs at scales of millimeters to centimeters and involves bulk movement of liquid during solidification. Microsegregation is inevitable in cast metals and welds; metallurgists accept it and compensate through composition control, processing speed, and post-solidification heat treatment. In powder metallurgy and rapidly solidified alloys, microsegregation is suppressed because the cooling rate exceeds 103 K/s, far faster than conventional casting.