EXPERIMENT 8
PRECIPITATION HARDENING IN 2024 ALUMINUM
Object
To study the time and temperature
variations in the hardness of Al-4%Cu alloy on isothermal aging.
Introduction
Materials can be hardened by inhibiting
the motion of crystal defects called dislocations. In pure metals, the presence of defects (as
vacancies, interstitials, dislocations, and grain boundaries) can enhance the
strength. In single phase alloys,
additional resistance to deformation may arise from the presence of foreign
atoms. In two-phase alloys, additional
stress is needed to enable the dislocation to intersect the second-phase
particles. A finely dispersed
precipitate may, therefore, strengthen the material. This phenomenon is termed precipitation
hardening.
The thermodynamics of precipitation in
2024 Al can best be understood by referring to the binary phase diagram of
Aluminum-Copper in the aluminum-rich region in Figure 8-1.
When the aluminum-copper alloy of less
than 5wt% copper is heated to temperature just above the solvus
line, only one phase (kappa, K) is
thermodynamically stable. Other solid
phases dissolve (disappear). This
process is called solid-solution heat-treatment. The only requirement is that the specimen
must be kept at this temperature for a long enough time. This means to solid-solution heat-treat a
sample of 2024 Al (4wt% Cu) the sample should be heated to 930oF (500oC) and held for 30
minutes.
When a solid-solutionized
sample is rapidly cooled to below the solvus line in
Figure 8-1, two phases are thermodynamically stable
(kappa and theta). These phases are two
different solids, physically distinct, and separated by a phase boundary. The process is similar to precipitation of
salt in supersaturated brine.
The process of precipitation is not instantaneous
as is often the case in liquid-solid precipitation. The process involves the formation of embryos
of theta through thermal fluctuations and their subsequent growth once they
achieve stability. With time, more and
more precipitates form. This process is
called aging. Once the solution achieves
an equilibrium composition given by the solvus line
for the aging temperature, precipitation stops.
For example, the precipitation of the copper-rich theta phase depletes
the kappa phase of copper to approximately 1-1/2 wt% Cu at 715oF (380oC).
The distribution of precipitates
affects the hardness and yield strength.
The hardness and yield strength are greater when the precipitates are
small and finely dispersed in the kappa matrix than when the precipitates are
large and not finely scattered. Thus, to
gain hardness in 2024 Al, the specimen should be heat treated to produce a fine
dispersion of small precipitates.
Unfortunately, there is a tendency when
thermodynamic equilibrium is reached for large precipitates to grow and small
precipitates to shrink. This will lower
the surface to volume ratio of the precipitates, the surface energy, and
therefore the energy of the system. As a result, at some point in the aging of
2024 Al, the precipitates begin to coarsen and on the average
the spacing between them gradually increases. At this point the hardness
and the yield point will begin to decrease with time of aging.
The process of aging is a function
of temperature. The higher the temperature the wider the
spacing of the precipitates. They form initially on cooling from the
solid-solution heat treatment temperature. Also, because coarsening is
dependent upon the movement of copper atoms in kappa, the maximum point is
generally reached sooner at a higher temperature than at a lower temperature,
as shown in figure 8-2.
In the present experiment, the
precipitation hardening behavior of the Al-4%Cu alloy will be studied by
measuring changes in hardness as a function of aging time.
Materials and
Equipment
Five pieces of
Al-4%Cu alloy (2024 Al)
Furnace for
heat treating specimens at 500oC (930oF)
Pail of water
Hardness
testers
Aging furnace
at 190oC (370oF)
Procedures
1. Obtain five specimens of 2024 Al.
Stamp
the specimens with an identifying mark.
Measure the hardness of all of the specimens using Rockwell B.
2. Place all five in a heat-treatment
crucible and into a furnace for solid-solution heat-treatment at 500oC (930oF) for 30
minutes.
3. Natural Aging. Remove one specimen and drop into a pail of
water. Once the specimen is cool, measure the Rockwell B hardness at intervals of
approximately 30 mins., 2 hrs., 1 day, and 1 week for this specimen. It will be necessary for one member to come
back to the lab during the week when the lab is in session to measure the
hardness.
4. Artificial Aging. Remove the remaining four specimens and drop
them into a pail of water. Once the specimens are cool remove them from the
quenching bucket. Wait 10 minutes, then measure the
Rockwell B hardness. Next, transfer the
specimens to a furnace set at 190oC (370oF). Remove one sample each after 3 mins., 10
mins., 60 mins., and 120 mins. Quench into
water and measure the Rockwell B
hardness. After one week, again measure
the Rockwell B hardness of the 3- and 10-minute artificially aged specimens.
Glossary of Terms
Understanding
the following terms will aid in understanding this experiment.
Age hardening.
A special dispersion-strengthening heat treatment. By solution treatment, quenching, and aging,
a coherent precipitate forms that provides a substantial strengthening effect
by acting as obstacles to dislocation movement.
Artificial aging.
Reheating a solution-treated and quenched alloy to a
temperature below the solvus in order to provide the
thermal energy required for a precipitate to form.
Coherent precipitate. A precipitate whose crystal structure and
atomic arrangement have a continuos relationship with
the matrix from which the precipitate formed.
The coherent precipitate provides excellent disruption of the atomic
arrangement in the matrix and provides excellent strengthening.
Dislocation.
A line imperfection in the lattice of a crystalline
material. Movement of
dislocations helps explain how materials deform. Interference with the movement of
dislocations helps explain how materials are strengthened.
Grain boundary.
A surface defect representing the boundary between two
grains. The lattice has a different orientation on either side of the
grain boundary.
Interstitialcy. Atom occupying an interstitial site not
normally occupied by an atom in the perfect crystal structure or an extra atom
inserted into the perfect crystal such that two atoms occupy positions close to
a singly occupied atomic site in the perfect structure.
Natural aging.
When a coherent precipitate forms from a
solution-treated and quenched age hardenable alloy at
room temperature, providing optimum strengthening.
Solid solution.
A solid phase that contains a mixture of more than one
element, with the elements combining to give a uniform composition everywhere.
Solid-solution treatment. The first step in the
age-hardening heat treatment. The
alloy is heated above the solvus temperature to
dissolve any second phase and to produce a homogeneous single-phase structure.
Solvus. A solubility line that
separates a single solid phase region from a two solid phase region in the
phase diagram.
Strain energy.
The energy required to permit a precipitate to fit into the surrounding
matrix during nucleation and growth of the precipitate.
Supersaturated solid solution. The solid solution formed when a material is
rapidly cooled form a high-temperature single-phase region to a low-temperature
two-phase region without the second phase precipitating. Because the quenched phase contains more
alloying element than the solubility limit, it is supersaturated in that
element.
Vacancy. A vacancy is created
when an atom is missing from a lattice point.
Write Up
1. Prepare a memo report.
2. Plot, by
hand, the hardness vs. time of aging for each aging temperature, using five cycle semi-log graph paper. (You may use
the paper provided on the next page for this plot).
3. How does aging temperature affect the time
and hardness?
4. What happens to the 190oC (370oF) 3 and 10
minute specimens after one week? Why?
5. If you were going to use 2024 Al in an
application at a temperature of 190oC (370oF), what
problems could be encountered?
6. Discuss errors in this experiment and
their sources.
References
1. ASM Handbook, Vol. 2 - Heat Treating
and Cleaning of Metals
2. Van Vlack, Elements
of Materials Science, Chapter 10
3. Smith, Science and Engineering
Materials, Chapter 8
4. Flinn &
Trojan, Engineering Materials and Their Applications, Chapters 4 and 5