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This is a phosphor bronze, shown at 200X at left and at 500X
below. Phosphor bronze is copper that has been deoxidized with
phosphorus. It is much harder and stronger than pure copper,
because of the unconsumed phosphorus.
The microstructure consists of cold-worked alpha. Notice the
sytrain markings, equiaxed structure, and annealing twins. There
are also some deformation twins, which appear needle-like and
intersecting. They are actually disk shaped in three dimensions.
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Note that the deformation twins "pinch off" where they meet
grain boundaries or other twins. That is due to the shear strain
involved in the deformation-twinning process, which can't easily occur
next to structural discontinuities such as internal boundaries.
The annealing twins are evidence of cold work prior to annealing.
Cast copper microstructures cannot contain such annealing twins.
The annealing twins seen here have been bent; therefore, the specimen
has been cold worked again after the annealing was completed.
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The stacking sequences possible in an imperfect face
centered cubic (FCC) structure explain the origin of annealing
twins. Dislocations in FCC metals are split into pairs of
imperfect "partial" dislocations separated by a band of stacking fault
(deformation fault).
Deformation introduces many stacking faults (the more, the lower the
interfacial energy of the stacking fault). These stacking faults
subsequently induce annealing twins to form during recrystallization of
the cold worked metal.
Specimen 8 shows that damage can happen
during annnealing of a cold worked structure.
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