intergranular

Intergranular describes cracking, corrosion, or fracture that propagates along the boundaries between adjacent crystals (grains) in a metal, rather than cutting across them. These grain boundaries are regions of atomic misalignment where atoms are loosely packed compared to the ordered interior of each grain. When a metal fails intergranularly, the crack path takes the line of least resistance, moving grain-to-grain like a fault line through rock.

The most common intergranular problem in industrial practice is intergranular corrosion in stainless steel. When austenitic stainless steel is heated to 500, 900°C during welding or heat treatment, chromium carbides precipitate at grain boundaries. These carbide-rich zones strip chromium from the adjacent material, creating passivation-starved regions vulnerable to attack. Exposure to acidic or chloride-bearing media then selectively dissolves along these pathways, eventually disintegrating the metal structurally. This is why stabilized grades (with niobium or titanium) and low-carbon variants were developed for welded service.

Intergranular stress corrosion cracking (IGSCC) occurs when tensile stress combines with a corrosive environment, often in nickle-base superalloys or titanium alloys in service. The crack grows along grain boundaries with minimal plastic deformation, making it insidious because the metal may show little external sign of distress until sudden failure. Susceptibility depends on grain size, residual stress, grain boundary composition, and electrolyte chemistry; finer grains and lower stress typically improve resistance.

Intergranular fracture can also result from thermal or mechanical cycling in coarse-grained material. When grains are large, fewer grain boundaries exist per unit volume, and the boundaries themselves become relative weak points. Temper embrittlement in low-alloy steels, for example, involves phosphorus and tin segregating to grain boundaries during slow cooling, making them brittle and fracture-prone at service temperature.

Detection requires microscopy. Optical examination after etching reveals the grain structure; scanning electron microscopy of fractured surfaces shows the telltale faceted appearance of intergranular fracture. Industrial mitigation includes grain refinement, stress relief, controlled cooling rates, and in aggressive environments, material selection or surface protection. The term distinguishes itself from transgranular failure, which cuts across grains and typically signals different root causes like inclusion-induced fatigue or localized plastic instability.