BRASS
Phase diagram
Link with specific temperatures and compositions;
https://www.doitpoms.ac.uk/miclib/phase_diagrams.php?id=12
NOTES
α-phase (Solid Solution): It is a solid solution of zinc in copper. It is also known as the alpha phase. It has
a face-centered cubic (FCC) crystal structure.
β-phase (Solid Solution): It is a solid solution of copper in zinc. It is also known as the beta phase. It has a
body-centered cubic (BCC) crystal structure.
γ-phase (Solid Solution): This is a high-temperature solid solution that exists at elevated temperatures.
Alpha brass:
From the copper-zinc phase diagram we can see the solid solubility of zinc in copper, for concentrations
of zinc upto about 30 at.%, at equilibrium the alloy should be of a single phase.
Alpha brasses are often seen with a single phase, however this usually arises due to annealing. As the
alloy cools α phase copper precipitates out first, changing the composition of the remaining melt. This
may result in coring and dendritic growth as well as the formation of other phases such as the β phase
when the zinc concentration in the remaining liquid is sufficiently high. Annealing the sample to aid
diffusion means the composition becomes more uniform as zinc diffuses down the concentration
gradient and a single phase predominates.
Microstructure
This is a single phase material. Alpha brass may have up to 30% zinc in solid solution. The straight
edged twins are annealing twins. Annealing twins are stacking defects in the stacking of the
{111} close-packed planes.
Alpha-beta brass:
Another very common form of brass is α-β brass. α-β brasses have zinc concentrations of between about
30at.% and 45at.% and are two phase alloys.
The α phase precipitates out first and may form a Widmanstatten structure (see micrograph below),
solidifying in plates along preferred growth directions.
Microstructure
80μm
Click on regions of the micrograph for more information.
The alpha phase precipitates out of solid solution from the beta phase. This example was cooled quickly
in air as can be seen from the Widmanstätten microstructure.
The beta phase makes up the background matrix. This is the more zinc rich phase.
Key points;
1. α-phase (Solid Solution):

Composition: Rich in copper (Cu).

Crystal Structure: Face-Centered Cubic (FCC).

Stable at low temperatures.
2. β-phase (Solid Solution):

Composition: Rich in zinc (Zn).

Crystal Structure: Body-Centered Cubic (BCC).

Stable at low temperatures.
3. γ-phase (Solid Solution):

Composition: High-temperature solid solution.

Crystal Structure: Face-Centered Cubic (FCC).

Exists at elevated temperatures.
4. Peritectic Reaction Line:

Involves the transformation of liquid and α-phase into the β-phase.

Represents a specific composition and temperature range.
5. Eutectic Point:

Composition: Around 33% zinc (Zn) by weight.

Involves simultaneous formation of α and β phases from the liquid phase.
6. Eutectoid Point:

Composition: Around 45% zinc (Zn) by weight.

Involves transformation of γ-phase into a mixture of α and β phases.
7. Solubility Limits:

α-phase has a limited solubility for zinc.

β-phase has a limited solubility for copper.
BRONZE (copper-tin)
Phase diagram
Key points;
1. α-phase (Solid Solution):

Composition: Copper-rich phase.

Crystal Structure: Face-Centered Cubic (FCC).

Stable at low temperatures.
2. β-phase (Solid Solution):

Composition: Tin-rich phase.

Crystal Structure: Body-Centered Tetragonal (BCT) or Body-Centered Orthorhombic
(BCO).

Stable at low temperatures.
3. Peritectic Reaction Line:

Involves the transformation of liquid and α-phase into the β-phase.

Represents a specific composition and temperature range.
4. Eutectic Point:

Composition: Around 8-10% tin (Sn) by weight.

Involves simultaneous formation of α and β phases from the liquid phase.
5. Eutectoid Reaction Line:

Transformation of the β-phase into a mixture of α and γ phases.

Represents a specific composition and temperature range.
6. γ-phase (Solid Solution):

Composition: Intermediate phase in the eutectoid reaction.

Crystal Structure: Face-Centered Cubic (FCC).
The peritectic reaction (see diagram above) is an important example of a microstructural
transformation. Sn – 21wt.%Cu exhibits this transformation from a solid phase and a liquid phase to a
different, solid phase.
Microstructure;
The peritectic reaction (a solid plus liquid transforming to another, different, solid) does not go to
completion due to the new phase creating a barrier to diffusion.
The epsilon phase precipitates out first. The precipitates are visible in the micrograph as the phase
surrounded by the paler eta phase.
The background phase is actually a euctectic (as can be seen from the phase diagram) of eta and Sn. The
microstructure is too fine to be seen in this micrograph.
Calculated Invariant Equilibria
Reaction
Phase
Mass % Cu
Mass % Sn
Liquid
73.25
26.75
(Cu)
85.70
14.30
beta
78.91
21.09
Liquid
66.87
33.13
beta
74.99
25.01
gamma
72.78
27.22
gamma
57.74
43.26
Liquid
42.05
57.95
Cu3Sn
61.63
38.37
Liquid
13.07
86.93
Cu3Sn
61.63
38.37
Cu6Sn5
39.07
60.93
Liquid
0.89
99.11
Cu6Sn5
39.07
60.93
(Sn)
0.01
99.99
beta
76.12
23.88
L + (Cu) -> beta
796.0 oC
L + beta -> gamma
756.5 oC
gamma -> L + Cu3Sn
638.4 oC
L + Cu3Sn -> Cu6Sn5
415.0 oC
L -> Cu6Sn5 + (Sn)
226.8 oC
beta -> (Cu) + gamma
585.9 oC
(Cu)
84.56
15.44
gamma
73.94
26.06
gamma
72.71
27.29
(Cu)
85.15
14.85
Cu41Sn11
66.56
33.44
gamma
65.95
34.05
Cu3Sn
61.63
38.37
Cu10Sn3
64.06
35.94
gamma
68.56
31.44
Cu10Sn3
64.06
35.94
Cu41Sn11
66.56
33.44
Cu10Sn3
64.06
35.94
Cu41Sn11
66.56
33.44
Cu3Sn
61.63
38.37
Cu41Sn11
66.56
33.44
(Cu)
90.36
9.64
Cu3Sn
61.63
38.37
congruent point
61.63
38.37
gamma -> (Cu) + Cu41Sn11
518.7 oC
gamma + Cu3Sn -> Cu10Sn3
640.1 oC
gamma + Cu10Sn3 -> Cu41Sn11
590.0 oC
Cu10Sn3 -> Cu41Sn11 + Cu3Sn
582.5 oC
Cu41Sn11 -> (Cu) + Cu3Sn
348.1 oC
gamma -> Cu3Sn
677.0 oC
Cu6Sn5 -> Cu6Sn5'
allotropic
transformation
187.5 oC
39.07
60.93
Phases, Crystal Structures and Model Descriptions
Phase
Struktur- Common Names
bericht
Symbol
Prototype Spacegroup Model*
Liquid
n/a
L
n/a
n/a
(Cu,Sn)1
Fcc
A1
(Cu)
Cu
Fm-3m
(Cu,Sn)1(Va)1
Bcc
A2
(beta Cu), beta
W
Im-3m
(Cu,Sn)1(Va)3
Bct
A5
(Sn), (beta Sn)
beta Sn
I41/amd
(Cu,Sn)1
Cu3Sn.h D03
gamma
BiF3
Fm-3m
(Cu,Sn)0.75(Cu,Sn)0.25
Cu41Sn11 ...
delta
...
F-43m
Cu0.788Sn0.212
Cu10Sn3
...
zeta
...
P63
Cu0.769Sn0.231
Cu3Sn
...
epsilon
...
Cmcm
Cu0.75Sn0.25
Cu6Sn5
B81
eta, Cu6Sn5.h
NiAs
P63/mmc
Cu0.545Sn0.455
Cu6Sn5'
...
eta ', Cu6Sn5.l
...
...
Cu0.545Sn0.455