Materials for engineering, 3rd Edition - (Malestrom)

Determination of mechanical properties 55
fine-grained materials at elevated temperatures are low is an additional factor
leading to a large elongation. The grain-size dependence of the steady-state
strain rate at elevated temperatures may be expressed in an equation in the
same form as that equation [2.19]:
p
˙ε = ( ADGb/ kT )( b/ d) ( σ/ G o )
n
[2.22]
where A is a dimensionless constant, D is the appropriate diffusion coefficient,
G is the shear modulus, b the magnitude of the Burgers vector (see Chapter
3), k is Boltzmann’s constant, d is the grain size, and p is the inverse grain
size exponent.
In accordance with this relationship, the principal requirements for a
superplastic material are (a) a fine and equiaxed grain size (