3.2.3. Strain Rate Sensitivity

The coefficient of strain rate sensitivity, *m*, indicates the capability of an alloy to resist necking spread and symbolizes the values of the superplastic elongation-to-failure. The *m* value was derived as follows:

$$m = \frac{\partial \ln \sigma}{\partial \ln \dot{\varepsilon}},\tag{2}$$

The applied stress and strain rate data were plotted in the ln(σ)-ln(. *ε*) coordinate system. The coefficients of strain rate sensitivity at different temperatures and strain rates were obtained at *ε* = 0.6 and are shown in Figure 5a. The variation of the *m* values of the 5A70 alloy with the true strain in the superplastic deformation are shown in Figure 5b.

**Figure 5.** Coefficients of the strain rate sensitivity, *m*, at *ε* = 0.6 (**a**) and the *m* values during superplastic deformation at 400–550 ◦C (**b**).

It has been well-documented that the solute drag creep is the dominant deformation mechanism in the coarse-grained/fine-grained Al-Mg alloys at low temperatures and high strain rates [38]. The solute drag creep was previously characterized for *m* ≤ 0.33 [39,40]. However, the apparent *m* values in the present study were generally higher than 0.33 and close to 0.5. Thus, the solute drag creep might make a relatively limited contribution to the observed superplasticity. In addition, the corresponding stress exponent, *n*, values ranged from 2.13–2.50, which were close to the typical model for the superplasticity stress exponent (*n* = 2) [41]. Under the experimental temperatures and strain rates, the *m* values declined with the accumulation of the deformation strain. Therefore, at 400 and 450 ◦C, necking was prevented due to an extensive strain-hardening rate and the 5A70 alloy exhibited a moderate tensile ductility with *m* ≥ 0.33. At 500 and 550 ◦C, both strain hardening and the strain-hardening rates associated with decreased necking had low strain rates and high *δ* values.
