Materials for engineering, 3rd Edition - (Malestrom)

26
Materials for engineering
1.21 Transmission electron micrograph showing precipitation in the
solid state in a complex Al–Li alloy (8090)(× 70000).
Considering the critical nucleus size, if we apply equation [1.5] (which
applies to a phase change from liquid to solid) to the present case of nucleation
of one solid phase within another, these nuclei are in special orientations in
order to minimize the value of γ α/β , the interfacial energy between the two
phases.
Eutectoid and peritectoid processes
We have seen that eutectic and peritectic phase changes may proceed as
follows:
eutectic: liquid phase
cooling
heating
solid A + solid B
peritectic: liquid phase + solid A
cooling
heating solid B
Processes analogous to these can take place wholly in the solid state and are
then called ‘eutectoid’ and ‘peritectoid’ respectively:
eutectoid: solid A
cooling
heating
solid B + solid C
peritectoid: solid A + solid B
cooling
heating solid C
No new principles are involved in these changes and we will not discuss
them in detail here, although it must be emphasized that, being wholly in the