- Analysis of Metal Sample
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- Mr. White, a Missouri resident, provided the National
Institute for Discovery Science (NIDS) a piece of material that he purportedly
obtained under unusual circumstances.
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- Since the material had not been previously tested, NIDS
decided to have a battery of tests conducted. The material was delivered
to New Mexico Tech, and the tests were conducted under the direction of
Dr. Paul Fuierer, an Assistant Professor in the Materials Engineering Department.
On August 23, 1996, Dr. Fuierer submitted his analysis, Sample Analysis
Report: Sample #2.
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- The analysis were double blinded. The following is the
entire text of the report. No conclusions are made by NIDS.
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- Sample Description
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- Sample #2 in its as-received condition can be described
as a finger-shaped piece of metal approximately 30 mm long, 7 mm thick,
18 mm wide at its larger end and 10 mm wide at the smaller end. The interior
of the specimen is silver-white in color, and highly reflecting. The outside
surface has a tarnished, darker gray appearance with overlapping scale-like
features. The sample mass was 5.11524 g.
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- Chemical Analysis
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- A semi-quantitative elemental analysis was obtained using
a Philips 2400 X-ray Fluorescence (wavelength dispersive) Spectrometer.
A scan of the entire bulk sample identified as major constituents: 85 wt
% A1 and 9 wt % Si, and minor constituents: 2 wt% Fe, 0.9 wt% Ca, 0.7 wt%
S, 0.6 wt% C1, and 0.6 wt% Na along with several other elements (see Table
I). This was in agreement with the qualitative analysis done via Energy
Dispersive Spectroscopy (EDS) during the electron microscopy investigation
(next section), which detected aluminum, silicon, and iron. In terms of
its chemical composition, sample #2 appears to be similar to what is known
as a "360 aluminum casting alloy".
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- Phase Identification
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- An X-ray diffraction scan was performed on a slice of
the sample with an area of about 1 cm^2. The diffraction pattern is shown
in Fig. 1. A13.21Si0.47 is seen to be an excellent match. This composition
calculates to 86.8 wt% A1 and 13.2 wt% Si, very close to that of sample
#2. The four largest peaks are attributed to the aluminum metal, while
the three small peaks (d = 3.1349, d = 1.9221, and d = 1.6375) are due
to the presence of silicon. The aluminum peaks are shifted slightly to
lower angles and larger d-spacings, as a result of the incorporation of
a small amount of other larger metallic impurities into the lattice. The
sample can therefore be described as a two-phase mixture of an aluminum
solid solution as the majority phase, and silicon as a minor phase. This
is in agreement with the equilibrium phase diagram for the Al-Si binary
system, shown in Fig. 2. The composition of sample #2 is close to the eutectic
composition, which is the composition have the lowest melting point (12.6
% Si according to the published phase diagram).
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- Microstructural Analysis
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- Scanning Electron Micropcopy (SEM)
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- The same slice of sample used for XRD was prepared by
grinding and polishing to a mirror finish, followed by etching with 0.5%
HF acid for 40 sec. Examination under the electron microscope revealed
the microstructure to be quite uniform throughout the sample. Fig. 3 shows
low and high magnification shots of the prepared surface. The sample contained
a large amount of porosity (darker areas are pits and voids). Also apparent
are the small particles (tenths of microns in size) surrounded by the continuous
matrix material. EDS revealed the small, light particles to be silicon
rich, while the darker gray matrix is Al-rich, as expected (see Fig. 4).
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- Optical Microscopy
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- Two samples were cut for metallographic examination;
one sliced perpendicular to the length of the sample, and one parallel.
These sections were also ground, polished and etched with 0.5% HF. Fig.
5 shows a couple of low mag shots of the parallel sample. The high porosity
is very apparent, along with the very fine microstructure. Flow lines are
also apparent in these two shots. The coarse ones are easy to see, outlined
by the porosity. A more subtle flow line can be seen in the 250X shot upon
close examination, defined only by a slight difference in the density of
the darker particles on either side. These kinds of flow lines are commonly
observed in poor sand or die castings. These are caused by a failure of
molten streams of metal to merge due to poor filling of a mold, incorrect
die lubrication or incorrect injection pressures.
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- Only under 1000X magnification can one start to pick
out individual black particles (Fig. 6). These tiny black particles are
the same Si particles seen under the SEM as light particles. Unfortunately,
sub-micron sized features approach the theoretical limit of resolution
for a light microscope, and therefore do not reveal themselves very clearly.
However, they seem to have grouped together in certain locations to form
longer bands, either straight or curved. These are probably dislocations
(highly strained defects in the Al lattice) where precipitation of a second
phase often occurs.
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- The fine microstructure observed in sample #2 is exactly
what one might expect from a near eutectic composition of the Al and Si
with a significant amount of impurities. Since the eutectic is rich in
aluminum, one would predict an Al-rich solid solution matrix with isolated
particles of silicon.....just what we have. The fact that the Si particles
are spheroidized (essentially equiaxed spherical particles) as opposed
to the classic eutectic lamellar shape (elongated plates or needles) can
be explained by the significant quantity of impurities like Na, Mg, etc.
These are regarded as modifiers in metal alloy technology, which alter
surface energies and affect the morphology of the second phase silicon.
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- Putting all of the above microstructural observations
together, one can surmise that the metal was being deformed (pushed or
blown) as it cooled from the molten state through the eutectic temperature
(~ 577 degrees C) and solidified. It probably cooled fairly rapidly since
the Si particles are rather small. Also the alloy may have been undercooled
such that excess Si was left in solid solution, and then later precipitated
out at the dislocations during either normal or artificial aging.
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- Physical Properties
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- Density
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- Bulk density of the sample was measured based on the
Archimedes Principle by immersion in toluene. The mass, while immersed,
increased over a long period of time, finally stabilizing after 3 hours,
indicative of a highly porous sample. The density of sample #2 was calculated
to be 2.47 g/cm^3. This is 91% of that of the theoretical density of pure
A1 (2.71 g/cm^3). This is not surprising, considering the presence of undisolved
silicon (2.33 g/cm^3) and significant amount of void space observed under
the microscope.
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- Hardness
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- The same samples (both perpendicular and parallel cuts)
used for optical microscopy were used to measure hardness. A Vickers Hardness
number was obtained from a Leco Tester using a diamond tip micro-indenter.
Five indentations were made for each sample. The size of the resulting
indentations were measured under the light microscope, and averaged. This
average value, along with the known applied load were used to come up with
the Vicker's Hardness number. For the perpendicular cut, VH = 60. For the
parallel cut, VH = 62. The difference is probably within experimental error.
These values are slightly higher than pure aluminum, and typical for aluminum
alloys.
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- Stiffness
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- An attempt was made to prepare a specimen for measuring
the strength and stiffness (elastic modulus) of sample #2 using an Instron
machine, however; the sample proved to be to small.
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- Electrical Properties
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- A four-point probe must be used for accurate measurement
for highly conducting materials like metals. For this, a rectangular slab
sample was cut and polished to dimensions: length = 11.16 mm, width = 3.36
mm, and thickness = 0.38 mm. A digital multimeter was used to measure the
current (provided by a constant current source) through the length of the
sample, while an electrometer was used to measure the voltage drop (to
the nearest 0.00001 V) across a length of the slab. The resistivity of
sample #2 was measured to be 2.90 x 10^-5 Ohm-cm. This is about an order
of magnitude higher than that of pure aluminum (3 x 10^-6 Ohm-cm) and five
times higher than that of 360 Al alloy (6x10^-6 Ohm-cm). This is completely
reasonable, considering the large number of Si particles, dislocation lines,
and amount of porosity.
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- Summary and Conclusions
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- Results from the analysis of sample #2 are quite conclusive.
The specimen is an aluminum-silicon alloy, with a substantial amount of
variety of impurities, including iron, calcium, sulfur, chlorine, sodium,
magnesium and others. The composition is one that could be used as an aluminum
casting alloy. The closest commercial material has the trade name "360
alloy" [Lyman, 1961]. This is a die casting alloy used in applications
where excellent castability and resistance to corrosion are required. It
is used for miscellaneous thin-walled and intricate castings. Since this
type of alloy is very close to the eutectic (lowest melting) composition,
it has excellent fluidity at relatively low temperatures.
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- The microstructure of the sample is one to be expected
from the composition: second phase eutectic silicon particles in a matrix
of aluminum solid solution. However, due to a few undesirable structural
characteristics, it would be regarded as a poorly cast aluminum alloy when
compared with published micrographs of commercial materials [Lyman, 1972].
The large amount of porosity would certainly lead to a decreased strength
and decreased corrosion resistance. The presence of porosity together with
the apparent flow lines suggests that uncontrolled cooling took place.
The significant amount of impurities like sodium accounts for the fineness
and rounded nature of the silicon particles, rather than the larger, longer,
more angular particules usually observed. Dislocations (planes of slip
caused by plastic deformation) appear to be decorated by silicon particles.
In many cases, these dislocations follow the flow lines. This suggests
some forced flow during solidification of the melt (in the range of temperatures
600 degrees C to 575 degrees C).
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- There are no anomalies in the results of this analysis.
The detected phases are accounted for, and the microstructure lends itself
to standard metallurgical interpretation. The physical properties that
were measured (density, hardness, and electrical resistivity) all fall
within the expected range.
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- References
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- Lyman, T. (editor), "Properties and Selection of
Metals", Vol. 1 in Metals Handbook, American Society for Metals (1961).
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- Lyman, T. (editor), "Atlas of Microstructures of
Industrial Alloys", Vol. 7 in Metals Handbook, American Society for
Metals (1972).
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- Lyman, T. (editor), "Metallography, Structures and
Phase Diagrams", Vol. 8 in Metals Handbook, American Society for Metals
(1973).
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- Mondolfo, L.R., Aluminum Alloys: Structure and Properties,
Butterworth Inc. (1976).
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