An evaluation of creep behavior in ultrafine-grained aluminum alloys processed by ECAP

Excerpt

There is now a good understanding of the creep behavior in crystalline materials. Under steady-state conditions, the creep rate, \( \dot{\varepsilon } \), varies with the applied stress, σ, the absolute temperature, T, and the grain size, d, through a relationship of the form [1, 2] where D is the appropriate diffusion coefficient [=D_o exp (−Q/RT) where D_o is a frequency factor, Q is the activation energy and R is the gas constant], G is the shear modulus, b is the Burgers vector, k is Boltzmann’s constant, p and n are the exponents of the inverse grain size and the stress, respectively, and A is a dimensionless constant. Over a wide range of intermediate stresses the creep rate is controlled by intragranular processes so that p = 0 and there is no dependence on grain size, but at low stresses intergranular creep processes may become important, such as Nabarro–Herring [3, 4] and Coble [5] diffusion creep and/or grain boundary sliding [6], and this introduces a dependence on grain size with p ≥ 1. …