Effect of temperature on transitions in creep mechanisms in class-A alloys

Alloy class materials exhibit viscous glide controlled creep, where dislocations are locked by solute atoms thereby decreasing their glide velocity while dislocation annihilation by climb still occurs. The creep-rate here varies as cube of the applied stress (stress exponent, n = 3) while the activation energy for creep becomes equal to that for solute atom diffusion. At high enough stress, the dislocations get freed from solute atom locking and transition to climb controlled creep is noted with n ∼ 5 while at still higher stresses, power-law breakdown is observed. For relatively small grain-sizes, viscous creep mechanisms such as Nabarro–Herring or Coble creep usually occur at low stresses. A close examination of the model equations reveals that at temperatures below a critical temperature, a transition from diffusional viscous creep to dislocation climb occurs without the intermediate viscous glide creep. Recent experimental results on a Zr–Nb sheet clearly follow these formulations exhibiting narrower intermediate region with decreasing temperature.