chapter
12 Pages

P

Ostwald ripening

) or the lamellae thickness in

colonies

(see

discontinuous coarsening

).

particle drag

Drag force

exerted by

dispersed particles

or

pores

. If the particles of the volume fraction

f

and the mean diameter

d

are arranged at the

grain boundaries

as, e.g., pores in

crystalline

ceramics

, the drag force is:

g

=

a

γ

f

/

d

where

a

is a coefficient, and

γ

is the

grain-boundary energy

(it is supposed equal to the energy of the

phase boundary

between the particles

and matrix). Pores inhibit grain boundary migration if they are relatively large and immobile; small pores can migrate together with the boundary and affect its motion to a lesser extent than immobile pores. In the case of uniformly distributed particles, the drag force is referred to as

Zener drag

and is proportional to

f/d

.