Meaning
Atomic diffusion along grain boundaries within a polycrystalline solid drives this deformation mechanism at elevated temperatures and low stress levels. The nabarro-herring creep process depends on the flux of vacancies from grain boundaries under tension to those under compression. It stops applying when dislocation glide or climb dominates the deformation kinetics under higher stress conditions.
Deformation Rate
A linear relationship between the strain rate and the applied stress defines this regime. The nabarro-herring creep mechanism relies heavily on temperature, as the diffusivity of atoms increases exponentially as heat rises. Grain size also dictates the efficiency of this process, since smaller grains provide shorter diffusion paths for mass transport.
Materials with coarse grains exhibit higher resistance to this type of deformation at a constant temperature.
Thermal Sensitivity
Activation energy governs the rate at which atoms migrate through the crystal lattice. This energy barrier represents the threshold that must be overcome for atoms to jump between sites during nabarro-herring creep. Laboratories measure this sensitivity by observing changes in strain rate as the sample temperature fluctuates across a controlled range.
Higher activation energies lead to a more pronounced decrease in deformation rates when the temperature drops.
Boundary Condition
Steady state deformation requires a continuous supply of vacancies at the grain boundaries. If the internal structure of the material hinders the creation or absorption of these vacancies, the rate of nabarro-herring creep declines below theoretical predictions. Metallurgists monitor these microstructural effects to assess the long-term structural integrity of components subjected to prolonged thermal loads.
The presence of specific solutes can alter vacancy concentrations and thereby influence the magnitude of the observed strain.
