Binary eutectic system (example, silver-copper)
The two constituents are not completely soluble in each other
Copper (Cu)
FCC
0.128 nm atomic radii
Silver (Ag)
FCC
0.144 nm atomic radii
Mix 50% Ag + 50% Cu, gives
two solid phases (a and b)
a (saturated - mostly Cu with
some Ag)
Not this!
b (saturated - mostly Ag
with some Cu)
Eutectic reaction: One liquid phase becomes two distinct solid phases upon cooling
L cooling a + b
Liquid, L
a
a+L
b+L
b
Eutectic
temperature
a+b
Eutectic composition
Eutectic composition liquid
Eutectic reaction
Liquid, L
a
a+L
a+b
b+L
b
Eutectic Reaction
(at eutectic
temperature and
composition)
Crystal of one
phase forms
(via diffusion in
the liquid)
Local area becomes “depleted”
of one constituent.
The depleted area near the solidification pushes the local composition
into a solid forming range on the phase diagram. This causes the second
solid phase to form adjacent to the first phase.
Eutectic reaction
Liquid, L
a
a+L
b+L
b
a+b
Local area becomes
Depleted in the second
phase constituent
Second solid phase forms
Eutectic reaction
Liquid, L
a
a+L
b+L
b
a+b
The 2 phases grow
side-by-side creating a
laminated microstructure
Eutectic reaction
Liquid, L
a
a+L
b+L
b
a+b
Eutectic reaction:
Characteristic microstructure: laminates of each of the 2 phases