Metallography of Platinum and Platinum Alloys

Transcription

Metallography of Platinum
and Platinum Alloys
Introduction
Optical metallography is an investigation technique typical of material science
and aimed at describing the microstructure of a metal alloy both qualitatively and
quantitatively. Here the term ‘microstructure’ refers to the internal structure of
the alloy as a result of its composing atomic elements and their three-dimensional
arrangement over distances ranging from one micron, that is one thousandth of
a millimeter, to one millimeter.
Many alloy properties depend on the microstructure (e.g., mechanical strength,
hardness, corrosion resistance), not to mention aesthetical aspects such as
color and surface finishing. The surface of an alloy containing some kinds of
microstructural defects will never be particularly bright nor will it appear
polished, even after the proper final polishing steps. Metallography is then a
fundamental tool supporting research and failure analysis.1,2 This holds in all
industrial fields where alloys are dealt with and particularly in the goldsmith’s
world, where special procedures for specimen preparation are needed due to
the peculiarity of the materials involved. A great deal of literature is
available to whoever wants to study in detail the typical methods of optical
metallography.3,4,5
Optical metallography is both craft and science. It is beyond doubt that an
essential prerequisite for a correct analysis is to have specimens free of artificial
features created by the preparation procedures. As a consequence, good
experience is required to be able to recognize any alterations that may lead to an
incorrect interpretation of results and, therefore, wrong conclusions. As regards
gold and silver-based precious metals, a lot of useful information is available,6,7,8,9
whereas for platinum and its alloys the related scientific literature is not so
broad.
This work aims to give some basic information for a successful metallographic
analysis of platinum alloys, especially micro-sections obtained from jewels
or semi-finished goldsmith objects. Some examples of platinum alloy
microstructures will be provided. In dealing with this topic, it is taken for granted
that the reader is familiar with the concept of crystal structure of metal alloys.10
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Paolo Battaini
8853 SpA
Pero (MI), Italy
Battaini
Materials and Methods
It is well known that many kinds of platinum alloys are used in the goldsmith’s
field11,12,13,14,15,16 and in the industry.9,17,18,19 Since different goldsmiths’ markets
acknowledge different standards, some specific alloys are used in some countries
only. However, the industrial platinum alloys turn out to be a valuable tool for
the setting up of specimen preparation procedures and data interpretation of
goldsmith alloys.
In this work the procedures for metallographic analysis, which apply to most
platinum alloys belonging to the goldsmith’s art, are described. As one of the
most critical steps is the metallographic etching, which is intended to reveal the
microstructure, the simplest and most general methods are suggested. In so
doing, preference is given to those chemical agents that turn out to be the least
dangerous from the point of view of safety. The alloys whose microstructures
are discussed here are listed in Table 1. These do not represent all the alloys
available on the market but can exemplify the obtainable results. Some of the
alloys in Table 1 are not used in the jewelry field. The related Vickers micro
hardness of the material, measured on the metallographic specimen with a load
of 200gf, is given for each microstructure.
The preparation of the metallographic specimens consists of the following steps:
1. Sectioning
2. Sample embedding in resin
3. Polishing of the metallographic section
4. Sample etching for microstructure detection
The detailed description of these steps will not be given here, as they are well
known and already discussed in other works.3,4,5,6,7,8,9 Particularly recommended
is the reading of the paper by Dieter Ott7 because, despite being specific for
gold-based alloys, it gives instructions regarding specimen preparation, which
can be applied to platinum alloys as well. Here additional advice is provided and
only the most important aspects relevant to platinum alloys are highlighted.
If the metallographic analysis is aimed at comparing the microstructure of
different alloys in their as-cast condition, the initial samples must have the same
size and shape. In fact, it is widely recognized that the investment casting process
can produce various microstructures with different grain size and shape just by
changing the size and shape of the wax patterns. Therefore, when possible, the
comparison was performed between specimens obtained under conditions that
were as similar as possible, the casting process included. These specimens were
obtained by arc melting and pressure-cast in argon atmosphere with the shape
as in Figure 1. A ‘Yasui Platinum Investment’ was adopted with a flask final
pre-heating temperature of 650°C (1202ºF). The captions of the micrographies
specify whether the original specimen is of the type described above.
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Figure 1 General aspect of specimens prepared by investment casting. The
microstructures of different alloys obtained by investment casting can be
compared provided that the specimens have the same size and shape. The
dashed line shows the position of the metallographic sections examined here.
Table 1 Chemical composition of some platinum alloys. Alloys 2-6 are among the most
common in the goldsmith’s field. Some melting ranges are not yet known. The
microhardness value refers to the microstructure in the corresponding figure.
Micro-hardness
Alloy
No.
Composition
Melting range ºC (ºF)
1
100Pt
1769 (3216.2)
58 (Figure 45)
HV100
2
95Pt5Cu
1725 - 1745 (3137 - 3173)
130 (Figure 6)
3
95Pt5Co
1750 - 1765 (3182 - 3209)
130 (Figure 8)
4
95Pt5Au
1705 - 1740 (3101 - 3164)
127 (Figure 12)
5
95Pt5Ir
1780 - 1790 (3236 - 3254)
95 (Figure 10)
6
95Pt4CuCo
(non magnetic)
-
169 (Figure 14)
7
95Pt5InGaZr
-
350 (Figure 29)
8
95Pt3Au2Rh
-
120 (Figure 28)
9
90Pt10Rh
Solidus T. 1840 (3344)
95 (Figure 27)
10
70Pt29.8Ir
1870 -1910 (3398 - 3470)
330 (Figure 25)
11
70Pt30Rh
Solidus T. 1910 (3470)
127 (Figure 26)
12
73Au27Pt
1210 - 1415 (2210 - 2579)
95 (Figure 49)
156 (Figure 50)
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1. Sectioning
In general, the available samples have rather small sizes so they must be handled
carefully. If the material is deformed during sampling, what is observed
afterwards is not representative of its original condition and, as a consequence,
the conclusions may be biased. At the same time, the sample must be
representative of both the material and the problem to be studied.
The orientation of the metallographic section is crucial as well. Different
orientations can correspond to different microstructures. An example is offered
by the longitudinal and transversal sections of a drawn wire. In this case it is
necessary to analyze both sections in order to have more complete information
about the material.
2. Sample Embedding in Resin
The embedding must be made with cold-curing resin, and special precaution
must be taken when embedding specific kinds of specimens. Furthermore, as
shown at section 4 below, it will be necessary to establish an electrical contact with
the specimen as its microstructure is revealed by electrolytic etching. To do this,
two solutions are possible. The first involves using a platinum wire contacting the
specimen surface (Figure 2).
Figure 2 The electrode consisting of a platinum wire is put on the
surface of the specimen dipped in the electrochemical solution in order to
have electrical contact. The sample and the solution are dipped into a
platinum crucible. In this way the platinum crucible acts as a counter-electrode.
This is the typical set-up for a DC electrolytic etching.
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Figure 3 The electrical contact can be obtained by soldering the sample to a copper
wire. If the specimen is too small, it is better to solder the sample to a copper support
first – see the photo above, left. Then the specimen is put into the rubber mold
with the copper wire appropriately bent as in the photo above, right.
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However, this is not the optimal solution because the specimen surface is
damaged at the contact point. Furthermore, the electrolytic etching is not
homogenous on the exposed surface as the electrical field changes significantly
with the distance from the platinum wire. Finally, with small specimens, the
platinum wire electrode and the specimen may have comparable sizes, making
the whole process unfeasible. A better alternative is to solder the specimen to a
copper holder and continue as shown in Figure 3.
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Regarding wires, it is possible to proceed as in Figure 4. In this way the electrolytic
etching will be homogeneous over all the exposed section of the material.
Figure 4 The wire to be examined (diameter 0.5mm) is wrapped around a U-shaped
copper support to which the electrical cable is soldered (photo 1). The cross section
to be obtained is indicated by the dashed line A – A. The frame is put into the rubber
mold where the cold-curing resin is cast (photo 2). After grinding and polishing
down to the plane A-A, the emerging copper support is covered with nail varnish
(photo 3). When the varnish has dried the specimen is ready for electrolytic etching
(photo 4), providing different transversal sections to be examined (see the arrows).
3. Polishing of the Metallographic Sections
The preparation of metallographic sections of platinum alloys requires the same
care as gold alloys. If the metallographic specimen shows porosity along the
section, it will be difficult to obtain a perfectly polished surface due to residues,
which can accumulate in pores and come out during subsequent steps. In this
case, ultrasonic cleaning in alcohol between one polishing step and the
following one is required. For a better finishing condition it is advisable to
polish the specimen with finer and finer diamond pastes down to 1µm and then
with a non-crystallizing colloidal silica polishing suspension.
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4. Sample Etching for Microstructure Detection
Table 2 Electrolytic and chemical etchings of general use for platinum alloys.
The list contains the etchant whose handling is the simplest. Only the Etchant No. 3 is
chemical so the specimen is just dipped into the solution and no power supply
is required. However, it is dangerous for both the operator and the environment
as it contains hexavalent chromium. It must be handled with care.
Etchant
No.
1
100cm3 HCl (37% conc.) + 10g NaCl , Electrolytic 3-6 V AC
2
10cm3 HCl + 90 cm3 H2O + 1g FeCl3 Electrolytic 3-6 V AC
3
100cm3 HCl (37% conc.) + 3-6g CrO3
The electrolytic etching can be considered as a “forced corrosion” where the
previously polished specimen surface corrodes inhomogeneously, revealing the
different microstructural features that vary from area to area. The surface corrodes
selectively as a consequence of different grain orientations, crystal defects such as
dislocations and grain boundaries, cold-worked regions, and second phases.
The electrolytic etching is usually carried out by means of a DC power supply
where the specimen works as the anode. However, for platinum alloys and the
NaCl/HCl saturated solution, the best results are obtained with an AC power
supply. The degree of electrolytic etching depends on the applied voltage and the
etching time. The counter-electrode can be graphite and the platinum crucible can
be replaced by one made of quartz.
One of the advantages of electrolytic etching is that the process can be stopped
whenever necessary to check the results and taken up again to reach the optimal
result gradually, a practice that is not always feasible with chemical reagents.
Figure 5 shows the equipment needed for this phase.
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The most convenient way to reveal the microstructure of platinum alloys is
electrolytic etching.3,4,5,6,8,9 The most widely used electrolytic solution is a
saturated solution of sodium chloride in 37% concentrated hydrochloric acid.
Other solutions are possible as well (see Table 2).
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Figure 5 The photo shows the AC power supply connected to the metallographic
specimen and to the graphite counter-electrode by its electrodes. The power supply must
allow the regulation of voltage in alternate current. Better results are obtained if the
scale can be varied from 0.1 to 10 volts and the power supply can provide a current of 10
ampere at least. The specimen dipped in the solution must be kept under a fume hood.
In order to carry out metallographic analyses properly, a laboratory must
necessarily be equipped with a grinding and polishing machine that allows
the use of grinding papers and polishing cloths, an AC power supply, and a
metallographic optical microscope complete with a digital image acquisition
system. A precision saw with diamond wafering blade can be very useful to
prepare small specimens. Of course, there exist many possibilities of equipping a
suitable laboratory, even coming to automatic systems for specimen preparation
and sophisticated optical microscopes. The costs can then vary. However, the
minimum cost is about $20,000 US.
Microstructures of Different Alloys
In this section of the paper the microstructures of some platinum alloys in
different metallurgical conditions are presented. As already stated, this is just an
exemplifying and not complete set of the alloys that are currently on the market.
If not otherwise specified, the microstructure was revealed by the electrolytic
etching (No.1 in Table 2).
1. As-cast Microstructures - Metallography of Crystallization
Some examples of as-cast microstructures are shown in Figures 6-29. It can be seen
that the size and shape of the crystal grains vary from alloy to alloy. However, a
noticeable dendritic grain structure is quite common. The largest grain size was
found in Pt-Cu (Figures 6 and 7) and Pt-Au (Figures 12 and 13) alloys. The core of
the dendritic grains showed a higher concentration of the element whose melting
temperature is the highest. In both alloys the electrolytic etching tended to
operate a preferential dissolution of the interdendritic copper- or gold-rich
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The melting range of the alloy and the flask pre-heating temperature affect the
size and shape of crystal grains significantly. In order to decrease the dendritic
size and obtain a more homogeneous microstructure, the temperature of the
material containing the solidifying alloy is lowered as much as possible; however,
the effectiveness of such an operation is limited by the width of the melting range
and by the chemical composition of the alloy. An example of this is given in
Figures 18 and 19, which refer to Pt-Au and Pt-Ir alloys poured into a flask with
a final pre-heating temperature of 890°C (1634ºF). The two microstructures are to
be compared with those in Figures 12 and 10 respectively, whose final pre-heating
temperature was 650°C (1202ºF). While the Pt-Ir alloy grains are significantly
smaller when using a pre-heating temperature of 650°C, in the Pt-Au alloy the
difference is not nearly as clear. Furthermore, increasing the flask pre-heating
temperature for the Pt-Au alloy can give rise to a higher surface roughness
(Figures 20 and 21). However, if the Pt-Au alloy is poured into a copper mold,
where heat removal from the solidifying metal is much faster, the crystal grain
size decreases considerably and an equi-axic grain structure is established
(compare Figures 22 and 13).
The effect of homogenizing thermal treatments results in a microstructural
change. The comparison between Figures 23 and 24 highlights a reduction in
microsegregation in the Pt-Cu alloy as a consequence of a homogenization
treatment performed at 1000°C (1832ºF) for 21 hours. Figures 25 to 29 show how
the alloy microstructure changes as a function of the alloy chemical composition
and the casting methods.
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regions, respectively. This behavior, known as ‘microsegregation,’ has been
widely described.10,20,21 In the Pt-Ir alloy (Figures 10 and 11), since the iridium
melting temperature is equal to 2454°C (4449°F), i.e., higher than that of
platinum, which is 1769°C (3216°F), the dendritic crystals are enriched in iridium
in the first cooling stage. The Pt-Cu-Co alloy (Figures 14 and 15) is characterized
by a smaller grain than Alloys 2, 3, 4 and 5 and it is biphasic. Here, ‘phase’ stands
for a portion of the alloy whose properties and composition are homogeneous
and that is physically distinct from other parts of the alloy. It is important to point
out that the higher or lower visibility of microsegregation within the dendrites
is not directly related to the chemical inhomogeneity but to the effectiveness of
the electrolytical etching in revealing it. For example, the microsegregation in the
Pt-Co alloy No. 3 in Table 1 is hardly visible in Figures 8 and 9, despite being
easily measurable by other techniques.21 On the other hand, when observing the
same specimen by Differential Interference Contrast Illumination (DIC), the effect
of microsegregation appeared clearly (compare Figures 8-9 with Figure 17). It
is worthwhile remembering that metallographic preparation reveals only a few
microstructural features. By changing the preparation or the observation
technique, some microstructural details may appear or become more clearly
defined, whereas others may not be visible.
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Figure 6 Etchant No. 1. Sample shape in
Figure 1, 95Pt5Cu. Dendritic grains
with copper microsegregation, flask
temperature 650°C (1202 °F). 130 ± 4HV200
Figure 7 Detail of Figure 6 showing
the copper microsegregation
alongthe dendritic arms
Figure 1, 95Pt5Co. Flask temperature
650°C (1202°F). 130 ± 6HV200
Figure 9 Detail of Figure 8. The cobalt
microsegregation is not clearly visible in this
image but is present (see Figure 17).
Figure 1, 95Pt5Ir. Flask temperature
650°C (1202°F). 95 ± 2HV200
Figure 11 Detail of Figure 10
showing the iridium microsegregation
along the dendritic arms
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Figure 1, 95Pt5Au. Flask temperature
650°C (1202°F). 127 ± 9HV200
Figure 13 Detail of Figure 12 showing the
gold microsegregation along the dendritic arms
Figure 1, 95Pt4CuCo. Flask temperature
650°C (1202°F). 169 ± 10HV200
a secondary phase along the dendritic
arms (see a detail in Figure 16)
a secondary phase along the dendritic arms
Figure 17 Etchant No. 1. Sample shape
in Figure 1, 95Pt5Co. Detail of the sample
seen in Figure 8, obtained by Differential
Interference Contrast Illumination (DIC),
better showing the cobalt micro-segregaion
along the dendritic arms.
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Figure 1, 95Pt5Au. Flask temperature
890°C (1634 °F). 150 ± 9HV200
Figure 1, 95Pt5Ir. Flask temperature
890°C (1634°F). 105 ± 2HV200
Figure 20 95Pt5Au. Detail of Figure 18.
Flask temperature 890°C (1634°F). The
sample surface is rough because
of the high flask temperature.
Figure 21 95Pt5Au. Detail of Figure 12. Flask
temperature 650°C (1202°F). The sample
surface is smooth. Compare with Figure 20.
Figure 22 Etchant No. 1, 95Pt5Au.
Alloy poured into a copper mold at 25°C
(77°F). Compare with Figure 13. The crystal
grains are smaller and equi-axed though their
dendritic nature and the microsegregation
are still visible. 100 ± 3HV200
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Figure 23 Etchant No. 1, 95Pt5Cu. Dendritic
grains with copper micro-segregation.
Compare with Figure 24. 130 ± 4HV200
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Figure 24 Etchant No. 1, 95Pt5Cu.
Microstructure after thermal treatment
at 1000°C (1832°F) for 21 hours. The
micro-segregation of copper is reduced.
Compare with Figure 23. 120 ± 4HV200
Figure 25 Etchant No. 1, 70Pt29.8Ir.
From an ingot transversal section. A
high iridium content contributes
to grain refinement. 330 ± 4HV200
Figure 26 Etchant No. 1, 70Pt30Rh.
From the transversal section of an ingot.
A high rhodium content enhances the
grain refinement. 127 ± 9HV200
Figure 27 Etchant No. 1, 90Pt10Rh. From
the transversal section of an ingot.
Sample with gas porosity. 95 ± 5HV200
Figure 28 Etchant No. 1, 95Pt3Au2Rh.
From the transversal section of an ingot.
120 ± 10HV200
Figure 29 Etchant No. 1, 95Pt5InGaZr.
Flask temperature 650°C (1202°F).
Large dendritic grains with second phases
along the dendritic arms. 350 ± 8HV200
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2. Work-Hardened and Annealed Microstructures - Metallography of
Deformation and Recrystallization
Various microstructures obtained after work hardening and recrystallization
thermal treatments are discussed here (see Figures 33-52). In some cases the
corresponding alloy underwent several work hardening and annealing cycles.
Optical metallography allows the drawing of recrystallization diagrams like the
one in Figure 30, therefore being a valuable aid in setting up the working cycles.
Microstructure changes significantly with respect to the as-cast condition.
Particularly, dendrites are no longer visible. See, for example, Figures 34 and 7,
which refer to the same Pt-Au alloy in the two conditions. The recrystallization
structure resulting from the thermal treatments carried out after work hardening
is well revealed in most Pt alloys by means of the electrolytic etching No.1. This
observation is fundamental in setting up the working cycles in order to control
the grain size.
Figure 30 Recrystallization diagram of a platinum-rhodium alloy annealed
at tan (°C) for a given time after a deformation of e% (adapted from Reference 9).
By increasing the annealing temperature, the grain size increases. After
annealing, the grain size also increases if the deformation is reduced.
Even though the wire is very thin, as in Figures 44-47, the analyses can be
easily performed by using the precautions described in Figure 4. Alloy 12 is
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It is worthwhile pointing out that some binary Pt alloys have a miscibility gap at
low temperatures, as shown by their phase diagrams.17,18,22 Examples of this are
given in Figures 31 and 32. Similar behavior is observed for Pt-Co, Pt-Cu, and
Pt-Rh alloys.
Figure 31 Pt-Ir phase diagram showing a
miscibility gap at low temperatures.18
Figure 32 Pt-Au phase diagram showing a
miscibility gap at low temperature.22
As a consequence, a biphasic structure is expected of each of them. However,
this may not occur for different reasons. The phase diagrams refer to equilibrium
conditions, which hardly ever correspond to the as-cast conditions. One of the two
phases is sometimes present but in low volumetric fraction due to the chemical
composition of the alloy, in which one of the two elements has a low
concentration. Furthermore, the thermal treatments may have homogenized the
alloy. Finally, the metallographic preparation may not be able to reveal such
biphasic structures. Therefore, it is necessary to use other analytical techniques to
detect the kind and the concentration of the alloy phases. The biphasic structure
can be revealed only in specific cases. Figures 48 and 50 are an example of this.
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actually a gold alloy as the platinum concentration is only 27%. Its microstructure
is typically biphasic, as revealed by the electrochemical etchant No. 2. In this case
the electrolytic etching with etchant No. 1 is not appropriate since it is too strong
for such an alloy whose Pt content is low. A homogenizing thermal treatment
makes Alloy 12 monophasic (Figure 49). This treatment gave rise to cavities,
which probably grew by atomic diffusion, nucleating on small bi-dimensional
defects. Such cavities are likely to correspond to pre-existent gas porosity pressed
by cold rolling and not visible in Figure 48, even if they are present. A further
thermal treatment returned the alloy microstructure into biphasic (Figure 50),
very different from the original one, though.
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The metallographic etchant No. 3 can be used only with some of the Pt alloys
considered here and is not effective with pure Pt. Like all the chemical etchings,
it requires skill when using it. In addition to this, the quality of the revealed
microstructure is slightly lower than that attained with electrochemical etchant
No. 1 (compare Figures 51 with 52). Moreover, given its toxicity, it is advisable to
replace it with electrochemical etching.
Figure 33 Etchant No. 1, 9Pt5Cu. From
the transversal section of a work-annealed
ingot, then reduced 40% in thickness.
211 ± 10 HV200
showing the crystal grains deformed
along the rolling direction.
Transversal section of a drawn
cold worked wire. 190 ± 4 HV200
Figure 36 Etchant No. 1. Detail of Figure 35
showing the deformation of the crystal grains.
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Longitudinal section of the wire
seen in Figure 35. 190 ± 4 HV200
showing the crystal grains deformed
along the rolling direction.
Transversal section of the wire seen in
Figure 35, after oxygen-propane
flame annealing. 104 ± 6 HV200
showing the recrystallized grain
Figure 41 Etchant No. 1 70Pt30Rh.
Transversal section of a work hardened
wire after various stages of oxygenpropane flame annealing and
drawing. 255 ± 6 HV200
showing the deformed crystal grain.
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Figure 43 Etchant No. 1. Longitudinal
section of the wire seen in Figure 42. The
crystal grains are deformed along the rolling
direction. Compare with Figure 42.
Figure 44 Etchant No. 1, pure Pt
wire. Transversal section of the wire
after various stages of drawing and
annealing. 65 ± 3 HV100
Figure 45 Etchant No. 1, pure Pt wire.
Transversal section of the wire after
various stages of drawing and annealing.
The recrystallization is more advanced than
in the sample of Figure 44. 58 ± 2 HV100
Figure 46 Etchant No. 1, 90Pt10Rh
after various stages of drawing
and annealing. 92 ± 4 HV100
Figure 47 Etchant No. 1, 70Pt30Rh
after various stages of drawing
and annealing. 146 ± 2 HV100
Figure 48 As polished sample, 73Au27Pt.
Work-hardened alloy seen on the rolling plane
(reduction 78%). Biphasic alloy with a phase
richer in gold. 172 ± 6 HV200
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Figure 49 Etchant No. 2, 73Au27Pt. Alloy of
Figure 48 after annealing at 1070°C (1958°F)
for 75 minutes. Recrystallization and
appearance of round cavities. 95 ± 5 HV200
Figure 50 Etchant No. 2, 73Au27Pt.
Alloy of Figure 49 after thermal treatment
at 550°C (1022°F) for 18 hours. Growth
of a new phase along the grain boundaries.
156 ± 50 HV200 (large S.D.)
Figure 51 Etchant No. 3, 95Pt5Cu. Same
sample as Figure 52. Transversal section of
a work-hardened ring after various stages of
annealing and rolling. 131 ± 13 HV200
Figure 52 Etchant No. 1, 95Pt5Cu. Same
sample as Figure 51. The electrolytic
etching No. 1 is more efficient in
revealing grain boundaries. 131 ± 13 HV200
3. Further Investigation
Optical metallography is only the first indispensable step towards the study of
the microstructure of an alloy. The present work is not meant to enter the details
of the wide variety of analytical techniques that allow a far more complete
knowledge of the microstructure.1 It is only worth remembering that one of the
most widely used techniques, among many, is the scanning electron microscopy
(SEM). By means of SEM, important aspects otherwise left unsolved can be
clarified. For example, it is possible to observe the grain boundary phase of the
alloy in Figure 50 at a higher magnification (Figure 53). In addition to this, X-ray
energy dispersive microanalysis allows to find out the relative concentration
of the contained chemical elements. Further studies can be performed by X-ray
diffraction, which reveals the different crystal phases present in the alloy.
Regarding Pt alloys, only very small specimens are usually available so it might
be necessary to turn to more recent techniques to study them. One of these is
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the FIB (Focused Ion Beam), which can produce micro-sections of a specimen
(Figure 54). The micro-sections are then analyzed by other techniques. It is also
possible to obtain thin films from the material (Figure 55) to be analyzed by
transmission electron microscopy (TEM). In this case the details of microstructure
can be detected due to the high spatial resolution and the crystal structure of
phases, and its second phases can be studied by electron diffraction.
Figure 53 Etchant No. 2, 73Au27Pt.
Alloy of Figure 50. Detail of the phase formed
along the grain boundaries. 156 ± 50 HV200
Figure 54 95Pt5Cu. FIB section of the sample
seen in Figure 33. The section allows the study
of the internal microstructure without etching
and alterations due to the sample preparation.
Another interesting technique is the nano-indentation, which uses very small
indenters (Figure 56) to perform hardness measurements along sections
previously obtained by FIB, even very close to the material surface. In this
case, the spatial resolution is far better than that obtained with the ordinary
micro-indenters. Also, a proper elaboration of these data leads to the
measurement of fundamental mechanical properties of the alloy such as the
elastic modulus (Young modulus).
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Figure 55 95Pt5Cu. A thin plate of the alloy
(arrow) is obtained by FIB and removed
from the sample to be analyzed by
transmission electron microscopy (TEM).
Figure 56 95Pt 5Cu. Micro-indentation - a
method to study the material hardness
with a high spatial resolution and to
measure the Young modulus of the alloy.
Conclusions
The metallographic analysis of platinum alloys can be profitably carried out by
using a specimen preparation methodology based on the techniques used for
gold-based alloys. However, electrochemical etching is required in order to reveal
the alloy microstructure, and this implies the adoption of some precautions in
the preparation and embedding of the specimen. The saturated solution of NaCl
in HCl can be successfully applied to a great many platinum alloys that are of
interest to the goldsmith’s field. In this case the electrolytic etching is performed
with alternating current and at voltages between 4 and 6V. Optical metallography
is the first indispensable step to take in the study of the alloy microstructure, but it
does not complete the task. Other techniques must be applied in order to achieve
a more complete knowledge of the material, the effects of the working cycles on
it, and to interpret and solve the possible problems.
Aknowledgements
I thank Eddie Bell and the Santa Fe Symposium® for inviting me to present this
paper and my wife Angela for the translation of the paper from Italian into
English. I am also grateful to my friend Attilio Bartolucci for manufacturing
the AC power supply used for the metallographic etchings and Professor
Edoardo Bemporad (University of Roma 3) for the FIB and nano-indentationrelated pictures.
References
1. Stewart Grice, “Know Your Defects: The Benefits of Understanding Jewelry
Manufacturing Problems,” The Santa Fe Symposium on Jewelry Manufacturing
Technology 2007, ed. Eddie Bell (Albuquerque: Met-Chem Research, 2007).
May 2010
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2. Paolo Battaini, “Metallography in Jewelry Fabrication: How to Avoid
Problems and Improve Quality,” The Santa Fe Symposium on Jewelry
Manufacturing Technology 2007, ed. Eddie Bell (Albuquerque: Met-Chem
Research, 2007).
3. George F. Vander Voort, Metallography Principles and Practice, 1st ed., 3rd
printing (October 2004).
4. ASM Handbook Vol. 9, Metallography and Microstructures, ed. George F.
VanderVoort (Metals Park, Ohio: American Society for Metals International,
December 2004).
5. Günter Petzow, Metallographic Etching, 2nd ed., 3rd printing (ASM
International, July 2008).
6. Tony Piotrowski and Dante J. Accinno, “Metallography of the Precious
Metals,” Metallography 10 (1977): 243-289.
7. Dieter Ott and Ulrike Schindler, “Metallography of Gold and Gold Alloys,”
Gold Technology 33 (2001): 6 – 11.
8. ASTM E407-07, “Standard Practice for Microetching Metals and Alloys,”
ASTM International Standard (2007).
9. E. Savitsky, V. Polyakova, N. Gorina and N. Roshan, Physical Metallurgy of
Platinum Metals, 1st ed. (Moscow: MIR Publishers, 1978).
10. Mark Grimwade, Introduction to Precious Metals – Metallurgy for Jewelers and
Silversmiths (Bryanmorgen Press, 2009).
11. Jurgen J. Maerz, “Platinum Alloy Applications for Jewelry,” The Santa Fe
Symposium on Jewelry Manufacturing Technology 1999, ed. Dave Schneller
(Lafayette, CO: Met-Chem Research, 1999).
12. James Huckle, “The Development of Platinum Alloys to Overcome Production
Problems,” The Santa Fe Symposium on Jewelry Manufacturing Technology 1996,
ed. Dave Schneller (Lafayette, CO: Met-Chem Research, 1996).
13. D.P. Agarwal and Greg Raykhtsaum, “Manufacturing of Lightweight
Platinum Jewelry and Findings,” The Santa Fe Symposium on Jewelry
Manufacturing Technology 1996, ed. Dave Schneller (Lafayette, CO: Met-Chem
Research, 1996).
14. Jurgen J. Maerz, “Platinum Alloys: Features and Benefits,” The Santa Fe
Symposium on Jewelry Manufacturing Technology 2005, ed. Eddie Bell
(Albuquerque: Met-Chem Research, 2005).
15. R. Lanam, F. Pozarnik and C. Volpe, “Platinum Alloy Characteristics: A
Comparison of Existing Platinum Casting Alloys with Pt-Cu-Co,” Platinum
Alloy Characteristics, Platinum Day 1997 (Platinum Guild International
U.S.A.). http://www.platinumguild.com/files/pdf/V3N1W_platinum_
alloy.pdf.
48
16.
G. Normandeau and D. Ueno, “Platinum Alloy Design for the Investment
Casting Process,” Platinum Manufacturing Process 7 (Platinum Guild
International USA, 2002). http://www.platinumguild.com/files/pdf/
V8N7W_platinum_alloy.pdf.
18. E. Savitsky, Handbook of Precious Metals, (New York: Hemisphere Publishing
Corporation, 1989).
19. K. Vaithinathan and R. Lanam, “Features and Benefits of Different Platinum
Alloys,” (2005). http://www.platinumguild.com/files/pdf/V13N3_features_
benefits.pdf.
20. D. Miller, T. Keraan, P. Park-Ross, V. Husemeyer and C.Lang, “Casting
Platinum Jewellery Alloys,” Platinum Metals Rev. 49, no. 3 (2005): 110 – 117.
21. John C. McCloskey, “Microsegregation in Pt-Co and Pt-Ru Alloys,” The
Santa Fe Symposium on Jewelry Manufacturing Technology 2006, ed. Eddie Bell
Albuquerque: Met-Chem Research, 2006).
22. Smithells Metals Reference Book, 7th ed. (Oxford: Butterworth-Heinemann Ltd.,
1992).
May 2010
49
Battaini
17. R.F. Vines, The Platinum Metals and Their Alloys, ed. E.M.Wise (The International
Nickel Company, Inc., 1941).
Battaini
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Metallography of Platinum and Platinum Alloys

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