t has been generally believed that superplastic deformation
can be observed if the so-called strain rate
sensitivity m is larger than about 0.3. Let us discuss
here the parameter from a viewpoint of dislocation
dynamics.
The strain rate sensitivity m is normally defined as
md ln s:d ln o;(ds:s):(do;:o;). (9)
Differentiation of Eq. (2) yields
do;(6b)drm(brm)d6(rm6)db. (10)
When a sudden change in o; occurs, it reflects on a
change in 6 so that it is expected drmdb0.Division
of Eq. (10) by Eq. (2) yields
do;:o;d6(brm):(b6rm)d6:6. (11)
Since 6 is given by Eq. (3), the differentiation of 6 with
respect to s* gives
Fig. 5. Changes in the mobile dislocation density rm and the true d6:6(n:s*)ds*. (12)