Meaning
High-temperature plastic flow in coarse-grained materials is driven by the movement of vacancies through the bulk crystal lattice rather than along boundary lines. This process of nabarro-herring diffusion dictates how structural alloys behave when subjected to stress at temperatures near their melting points. The rate of deformation is inversely proportional to the square of the grain size, making it less significant in very fine-grained materials.
It defines the operational limit for turbine blades and high-temperature reactor components.
Atomic Migration
Applied tensile stress creates a chemical potential gradient across each crystal grain. Under these conditions, vacancies migrate from grain boundaries experiencing tension toward those experiencing compression. This movement of empty lattice sites corresponds to a net flux of atoms in the opposite direction, which causes the grain to elongate.
Temperature Dependency
Because atomic migration occurs directly through the grain interior, the activation energy for this process is high. Consequently, nabarro-herring diffusion requires elevated temperatures to activate the bulk-diffusion mechanism. At lower temperatures, grain boundary diffusion takes over as the dominant creep mechanism.
Structural Integrity
Preventing creep deformation in high-stress thermal environments requires careful control of the material’s grain structure. Engineers specify single-crystal or coarse-grained superalloys to minimize the boundaries that act as vacancy sources and sinks.
