3.2.4. Activation Energy for Flow

The activation energy for flow in superplastic deformation was used to assess the degree of difficulty of the material thermal deformation or for the rearrangement of atoms. The higher the dislocation density of the deformation alloy, the greater the activation energy. The activation energy, *Q*, was obtained by taking the logarithm of Equation (1) under constant strain rates. The apparent diffusion activation energy of *Q* was derived as follows:

$$Q = nR \frac{\partial \ln(\sigma - \sigma\_0)}{\partial (1/T)},\tag{3}$$

The variation in the logarithmic stress as a function of reciprocal temperatures illustrates that a low strain rate appeared to be more sensitive to temperatures. The activation energy for flow, *Q*, ranged from 135 to 139 kJ/mol and is illustrated in Figure 6. In addition, the lattice diffusion activation energy of *QL* = 143.4 kJ/mol [42] and the grain boundary diffusion activation energy of *Qgb* = 84.0 kJ/mol [43] for pure aluminum demonstrated that lattice diffusion dominated the GBS diffusion processes. The activation energy for flow, *Q*, was 139 kJ/mol, and the maximum *δ* of 437% was obtained at 550 ◦C and 5 × <sup>10</sup>−<sup>4</sup> <sup>s</sup><sup>−</sup>1.

**Figure 6.** Logarithmic stress as a function of reciprocal temperatures and the apparent activation energy *Q* of the fine-grained 5A70 alloy.
