Metals
Aluminum-Zinc Solid-State Phase Change in Metals
Page 1
Aluminum-Zinc Solid-State
Phase Change in Metals
(Reference: Battelle PNNL MST Handbook, U.S. Department of Energy, Pacific
Northwest Laboratory)
Instructor Notes
Reliability
This lab works as far as making an alloy. A problem may occur when making a noticeable
phase change; this works only about 90% of the time. Increasing the annealing temperature
about 25C may make this experiment work 100% of the time (see Teacher Tip 5, for more
details).
Estimated Time for Activity
Two class periods.
Teacher Tips
1. This experiment demonstrates the concept that certain metals and alloys undergo a
structure change while in the solid state, with a consequent change in their properties.
When the structure changes, heat is discharged.
2. Experience indicates that annealed ingots may spontaneously transform back to their stable
state. It is recommended that ingots not be stored while in their metastable state; anneal the
ingots, do the phase change experiment, then store the ingots.
3. Have oven and furnace at operating temperature to save time.
4. Metals that have the same crystal structure (like silver and gold) are usually completely
miscible (they don’t separate into two or more phases* upon cooling). When alloys are
made from metals with different crystal structures, a tendency usually exists for different
phases to form in the alloy upon cooling. The aluminum (AI)/zinc (Zn) alloy studied in this
experiment is an example of a material that undergoes a phase transformation upon cooling
or heating (see Figure 5.7). Aluminum has a face-centered cubic crystal structure, and zinc
has a hexagonal-closest packed crystal structure (see Figure 5.8). [This is a martensite
reaction.]
The alloy composition used in this experiment (22 weight percent Al, 78 weight percent Zn)
is called an eutectoid composition. Eutectoid means that one solid phase transforms to two
solid phases upon cooling. In this experiment, one phase will be mostly Al and the
other phase will be mostly Zn.

A single-phase alloy is uniform throughout in chemical composition and physical state;
it is homogeneous. A two-phase alloy, if polished and examined microscopically, has
regions of different appearances that are chemically different.
U.S. Department of Energy, Pacific Northwest Laboratory
Metals
C
Aluminum-Zinc Solid-State Phase Change in Metals
Atomic Percent Zince
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F
Weight Atomic Percent Zinc
Figure 5.7. Phase Diagram for Aluminum and Zinc
When the alloy is cooled rapidly from above 275C (at which point it is single
phase) to room temperature, it is metastable (it wants to separate into two phases),
but the atoms are frozen in place. Some energy is necessary to allow the
transformation to start; this energy is provided by heating the alloy a few degrees in
your hand. This particular phase transformation is exothermic (releases heat). Any
transformation to a more stable phase releases some heat.
After the phase transformation is complete, the metal can be reheated to above
275C. At this temperature, only one phase exists (face-centered cubic, like Al), so
the transformation is reversed.
5. The temperature of 275C is used as the annealing temperature of this alloy. This
is taken directly from the phase diagram located in the student activity section. In
reality and practice, it is suggested that 300C or greater be used as the annealing
temperature for two reasons.
a. Temperature gradients exist in most furnaces, and the actual temperature of the
furnace may be quite different from the temperature readout or controller in the
furnace (± 25C is not uncommon).
b. Compositional variations of the alloy can also occur because of impurities in
purchased materials, incorrect or imprecise measurement of source materials,
and the loss of zinc because of oxidation during the melt process. As observed
on the phase diagram, these minor changes in composition cause a rapid
increase in annealing temperature.
U.S. Department of Energy, Pacific Northwest Laboratory
Metals
Aluminum-Zinc Solid-State Phase Change in Metals
Page 3
x
(a)
z
2
a1
(c)
Figure 5.8. Crystal Structure of Metals: (a) Face-centered cubic,
(b) Body-Centered Cubic, (c) Close-Packed Hexagonal
6.
Adding ice to the water used to quench the ingots is suggested. The quicker the sample
cools, the more dramatic the heat release during the phase change. More of the
metastable phase will be solidified when the alloy is rapidly cooled.
7.
Students will record weights of the crucible and materials before and after the alloying. This
is one more chance to let students observe that mass is conserved. Steps 3, 19, and 20 in
the student activity directly apply to this.
8.
This is an experiment where crucibles can be used year after year. Label the crucibles with
a number or figure using an underglazed pencil or a heat-resistant marking product as
described in item 9. Students can then choose their own labeled crucible to do the
experiment.
U.S. Department of Energy, Pacific Northwest Laboratory
Metals
9.
Aluminum-Zinc Solid-State Phase Change in Metals
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Heat-resistant marking material can be made quickly. In a small 5 to 10 ml cup or beaker,
add 1 to 3 g of iron oxide powder (Fe2O3). Mix into the powder just enough water to make a
paste or slurry. Use a fine-tipped paint brush to mark or label the crucible with the paste.
Bake on the label at 75C for about 5 mm.
Suggested Questions
10. Did crucible gain, lose, stay the same? Why?
11. Did total weight gain, lose, or stay the same? Why?
Safety
1. Do not use aluminum or zinc powder. The powders can be explosive. Use sheet or shot
material.
2.
Use extreme caution with molten metal alloy. Especially do not overheat zinc because
poisonous zinc fumes are created.
Disposal
1. Keep alloy for Caloric Output Lab.
U.S. Department of Energy, Pacific Northwest Laboratory
Metals
Aluminum-Zinc Solid-State Phase Change in Metals
Page 5
Procedure
1. Preheat furnace to 500C (9320F).
2. Obtain materials, numbered crucible, zinc, alumina, and paper cups. Label the crucibles.
3. Weigh the crucible, and record its mass in your journal.
4. Tare weight of paper cup. Add 78.0 g +/- 1.0 g of zinc. Pour zinc into crucible.
5. Using tongs, place crucible in furnace at 50000 for 15 mm.
Note: Whenever you put materials in or take them out of ovens and fumaces, always use
correct tongs and gloves, and have your partner operate the door. Door should be opened
for as little time as possible. (If underglaze pencil is used, steps 2 and 5 may be omitted.
These steps are needed to bake on heat-resistant glaze markings.)
6. Tare weight of paper cup. Add 22.0 g +/- 1.0 g of aluminum.
7.
Remove crucible from furnace (zinc should be in a liquid state, if not, return crucible to
furnace for an additional 5 mm). To keep the molten zinc from splattering, add the
aluminum VERY CAREFULLY.
8.
Return the crucible to the furnace, and increase the temperature to 725C. Allow the two
metals to become an alloy by soaking for 15 mm. [Note: Guy Whittaker uses a 1/8 in. dia.
by 12 in. long stainless steel rod to stir the mixture.]
9.
Prepare casting mold by cutting three holes, 2 in. deep in the casting sand with a 3/8-in,
hole-cutting tube. Identify your holes by writing your number in the sand with your pencil.
10. Remove crucible from furnace, and quickly pour molten metal into the mold. Place crucible
on a surface intended for hot crucibles. Allow the ingots to cool to room temperature, then
remove them from the mold.
11. Using a file, remove any rough surfaces from the ingot and make ends smooth. Remove as
little as possible. Add pieces of filed metal to the crucible.
12. Identify your ingot by stamping or engraving your number and/or initials in the end of the
ingot.
13. Anneal the ingot by using small tongs and placing the ingot in the oven at 370 - 380C for
1/2 hour.
14. Fill a No. 10 can with cold water (ice water works even better) and place near the oven.
15. Using small tongs, rapidly remove one of your ingots from the oven and quench it in water
using a figure 8 motion for 15 sec to ensure rapid cooling. Then drop the ingot to the
bottom of the can. Repeat until all ingots have been quenched.
U.S. Department of Energy, Pacific Northwest Laboratory
Metals
Aluminum-Zinc Solid-State Phase Change in Metals
Page 6
16. Remove ingots from the water and handle carefully (DO NOT BANG OR DROP). Squeeze
the cold (annealed) ingot in the palm of your hand (see Figure 5.9). Squeezing the ingot
allows better surface contact, which gives more heat transfer from your hand to the ingot.
This heat transfer is what will trigger the phase change. In less than 2 mm, a
transformation will occur accompanied by the discharge of heat. This excess energy
released by the ingot will cause its temperature to rise to as high as 60C (1400F).
(Caution—this may be hot enough to burn your hand.)
17. Annealed ingots must be stored in a cool place or the transformation may occur on its own.
This is not recommended, though, because many of the ingots will change to a more stable
state on their own.
18. Once an annealed ingot has undergone transformation, it can be reused by repeating the
heat treatment in steps 13 - 15.
19. Weigh crucible and record results in your journal. Compare weight in step 3.
20. Weigh ingots and record. Add weight to crucible weight and compare to total weight in
steps 3, 4, and 6.
Figure 5.9. Annealed Ingot
U.S. Department of Energy, Pacific Northwest Laboratory