chapter
4 Pages

W

Ostwald ripening

, according to which, the

mean size

of

equiaxed

precipitates,

d

,

changes with time,

t

, at a constant temperature and a small volume fraction of

precipitates

, as follows:

d

d

=

k

(

t

t

)

The subscript 0 in the equation relates to the initial magnitudes of

d

and

t

, and the constant,

k

, is directly proportional to the energy of the

interface

between precipitates and the

matrix

,

σ

. In addition,

k

is proportional to the

diffusion coefficient

of

solutes

in the matrix, as well as to their

solubility limit

. The equation is usually referred to as

t

dependence and is characteristic of

diffusion-controlled

growth. Since

coherent

and

partially coherent interfaces

are characterized by low

σ

,

coherent precipitates

coarsen slower than do

incoherent

precipitates.