Meaning
High temperature deformation where adjacent crystalline grains displace relative to each other along their common interfaces constitutes a primary mechanism of creep. This sliding action, known as grain boundary sliding, becomes a dominant contributor to total strain when the operating temperature rises above half the absolute melting point of the metal. It occurs because the disordered atomic structure at the grain boundaries behaves like a viscous fluid under low strain rates and sustained loads.
The process requires accommodation by diffusions or dislocation movement to prevent immediate cavity formation.
Atomic Migration
Diffusion of atoms along the boundary plane accommodates the relative shift of the crystal grains to prevent the opening of voids. When the diffusion rate cannot match the mechanical displacement rate, microscopic wedges open at the triple points where three grains meet. These wedges act as stress concentration sites that initiate cracks.
Alloy Design
Adding solid solution elements or fine precipitates along the interfaces blocks the movement of adjacent crystals to inhibit the sliding action. This microstructural pinning increases the creep resistance of the alloy significantly. Large grain materials naturally resist this deformation mode due to their lower total boundary surface area.
Failure Pattern
Continuous shear displacement eventually produces microvoids that join together into a macroscopic fracture path. The resulting rupture exhibits an intergranular appearance, showing that the failure occurred along the boundaries rather than through the grain interiors. This behavior represents the final limit of the material under constant mechanical stress.
