Intermetallic compounds

Intermetallic compounds are disclosed in which a platinum/aluminium compound includes copper in proportions chosen to provide desirable color, from yellow through orange to copper/red, to the compound. The compound contains from 50 to 81, preferably 57 to 80% by weight platinum; from 5 to 30, preferably 12.5 to 30% by weight aluminium; and from 1 to 47.5 and preferably 5 to 30% by weight copper.

FIELD OF THE INVENTION 
This invention relates to intermetallic compounds and, more specifically, 
to intermetallic compounds of platinum and aluminium. 
Still more particularly the invention is concerned with the modification of 
the colour of platinum/aluminium intermetallic compounds to provide 
aesthetically appealing colours to such compounds to render them appealing 
for use in the jewellery trade. 
BACKGROUND TO THE INVENTION 
Platinum itself has a rather plain silver colour and, in consequence, is 
not considered to be particularly appealing for use in the jewellery 
trade. On the other hand, certain platinum intermetallic compounds, 
particularly those with aluminium, do have colours different from that of 
the constituent metals and, in particular, the intermetallic compound 
PtAl.sub.2 has a bright yellow colour. 
However, such a colour does not necessarily render platinum, in this form, 
attractive for use in the jewellery trade as the yellow colour is not 
particularly distinctive over and above that of various gold alloys which 
are substantially more easy to work and form into jewellery whereas the 
platinum/aluminium intermetallic compounds are hard and brittle and not 
easy to form into attractive parts of articles of jewellery. 
Some attention has been given to intermatallic compounds, that provide 
colour. Attention has thus been given in a number of publications to 
intermetallic compounds of gold and aluminium. 
Regarding the physical properties of intermetallic compounds, European 
Patent Application No. 87810140 claims to provide an expedient for 
obtaining more workable intermetallic compounds from a physical point of 
view. This patent specification embraces an enormous range of possible 
compounds, both with and without precious metals. It fails, however, to 
teach any particularly useful platinum based compounds from a colour point 
of view. 
It is the object of this invention to provide intermetallic compounds of 
platinum and aluminium which have the colour thereof modified to render 
them more attractive and aesthetically appealing for use as component 
parts of articles of jewellery. 
SUMMARY OF THE INVENTION 
In accordance with this invention there is provided an intermetallic 
compound of platinum and aluminium comprising: 
(i) from 50 to 81 weight per cent platinum; 
(ii) from 5 to 30 weight per cent of aluminium; and, 
(iii) from 1 to 47 5 weight per cent copper. 
Further features of the invention provide for the intermetallic compound to 
comprise: 
(i) from 57 to 80 per cent by weight platinum; 
(ii) from 12.5 to 30 per cent by weight aluminium; and, 
(iii) from 5 to 30 per cent by weight copper; 
and for the intermetallic compound to be made either by adding copper in 
the appropriate quantity to the preformed intermetallic compound 
PtAl.sub.2, or, by simply melting together the required quantities of the 
three pure metal constituents. 
It has been found that various different colours of the intermetallic 
compounds result from differing additions of copper. Thus, for example, an 
addition of 10 weight per cent of copper to a PtA1.sub.2 intermetallic 
compound results in the colour being changed to an orange colour. 
Additions of 20% and 25% cause the intermetallic compound to assume a 
pinkish/mauve shade. 
In general it has been found, and is a feature of the invention, that the 
following ranges of compositions have the general colour stated: 
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Yellow compounds: 
Platinum 70 to 77 weight % 
Aluminium 20 to 23 weight % 
Copper 1 to 8 weight % 
Orange compounds: 
Platinum 63 to 70 weight % 
Aluminium 18 to 21 weight % 
Copper 8 to 15 weight % 
Copper-red compounds: 
Platimum 54 to 62 weight % 
Aluminium 15 to 20 weight % 
Copper 20 to 30 weight % 
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The invention still further provides that the preferred compositions of the 
intermetallic compound be chosen such that the chromaticity (Yxy) when 
measured using a standard CIE source C illuminent, and a standard observer 
angle of 2.degree. has an "x" value and a "y" value in respect of 
intermetallic compound samples polished to a lum mirror finish that 
provide a percentage colour of at least 9,8. Most preferably, the "x" 
value is at least 0,34 and the "y" value is at least 0,33. 
The modified intermetallic compounds provided by this invention can be made 
in any suitable manner such as, conveniently, by heating the constituents 
under an inert atmosphere, in particular argon, in a suitable ar furnace. 
In order that the invention may be more fully understood, various 
experimental results and a discussion thereof are set out below with 
reference to the accompanying drawings.

DETAILED DESCRIPTION OF EXPERIMENTAL COMPOUNDS PRODUCED 
In the experimental intermetallic compounds produced either various amounts 
of copper were added to PtAl.sub.2 (in respect of compound numbers 5 to 14 
in Table 1) or the required amounts of the three constituent metals were 
simply weighed out separately (in respect of compound numbers 15 onwards 
in Table 1); the mixture was melted in an arc furnace under an argon 
atmosphere and the resultant intermetallic compound allowed to solidify. 
The following compositions were made, in amongst others, the compositions 
being given in Table 1 together with their colour measurement results on a 
Spectrogard Colour Spectro-photometer, which are further described below: 
TABLE 1 
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COMPOUND COMPOSITION BY WEIGHT 
No. % Pt 
% Al 
% Cu 
x y L* a* b* % Colour 
Colour 
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1 
Platinum 100 0.32 
0.32 
89.12 
-0.13 
-0.54 
4.12 
2 
PtAl 87.8 
12.2 0.32 
0.32 
82.93 
1.86 
2.28 
3.58 
3 
Pt2A13 83 17 0.31 
0.32 
78.34 
-1.41 
1.93 
2.57 
4 
PtA12 77 23 0.35 
0.35 
83.91 
0.67 
17.71 
20.14 Yellow 
5 
PtA12 + 2% Cu 
75.5 
22.5 
2 0.34 
0.35 
80.45 
0.43 
14.45 
17.13 Yellow 
6 
PtA12 + 5% Cu 
73.2 
21.8 
5 0.37 
0.37 
79.05 
0.56 
26.85 
31.45 Yellow 
7 
PtA12 + 6% Cu 
72.4 
21.6 
6 0.37 
0.37 
78.71 
2.26 
24.08 
28.72 Yellow 
8 
PtA12 + 7% Cu 
71.6 
21.4 
7 0.36 
0.36 
79.50 
1.93 
22.17 
26.32 Yellow 
9 
PtA12 + 8% Cu 
70.8 
21.2 
8 0.36 
0.36 
79.70 
2.42 
20.98 
25.04 Yellow 
10 
PtA12 + 9% Cu 
70.1 
20.9 
9 0.36 
0.36 
79.90 
2.61 
22.12 
26.30 orange/yellow 
11 
PtA12 + 10% Cu 
69.3 
20.7 
10 0.36 
0.36 
79.06 
4.07 
22.02 
26.69 orange 
12 
PtA12 + 15% Cu 
65.5 
19.5 
15 0.36 
0.34 
75.33 
10.07 
13.62 
19.08 orange/red 
13 
PtA12 + 20% Cu 
61.6 
18.4 
20 0.34 
0.33 
75.70 
7.89 
9.25 
13.41 copper/red 
14 
PtA12 + 25% Cu 
57.8 
17.2 
25 0.34 
0.33 
77.44 
7.63 
7.76 
11.38 copper/red 
19 54 16 30 0.34 
0.33 
78.87 
5.37 
8.20 
11.25 orange 
20 50 15 35 0.34 
0.34 
73.75 
5.01 
9.94 
13.89 orange 
22 77 8 15 0.32 
0.33 
76.32 
0.35 
4.22 
5.74 pale yellow/orange 
24 81 9 10 0.32 
0.32 
66.88 
0.66 
3.62 
5.61 pale yellow/orange 
26 78 12 10 0.31 
0.32 
75.17 
0.04 
2.07 
3.15 pale yellow 
27 74 11 15 0.32 
0.32 
77.45 
-0.90 
3.37 
4.41 pale yellow 
28 65 10 25 0.32 
0.33 
78.91 
-0.38 
4.23 
5.42 pale yellow/orange 
29 56.5 
8.5 35 0.32 
0.33 
76.37 
-0.10 
4.42 
5.85 fairly pale yellow/orange 
30 50.5 
7.5 42 0.32 
0.33 
81.81 
0.44 
4.63 
5.93 pale pink/orange 
31 78 17 5 0.32 
0.33 
75.07 
- 0.15 
4.34 
6.09 pale yellow 
32 74 16 10 0.33 
0.33 
76.50 
0.77 
6.40 
8.38 pale yellow/orange 
33 69.5 
15.5 
15 0.33 
0.33 
76.09 
2.81 
7.22 
9.87 orange/yellow 
34 65.5 
14.5 
20 0.32 
0.32 
78.75 
0.30 
3.72 
4.99 orange/yellow 
36 61 29 10 0.33 
0.33 
73.81 
3.46 
5.46 
7.98 fairly pale orange/pink 
37 57.5 
27.5 
15 0.33 
0.33 
71.85 
4.02 
6.65 
9.87 orange/pink 
38 54.5 
25.5 
20 0.33 
0.33 
73.13 
3.75 
7.12 
10.24 orange/pink 
39 69 26 5 0.33 
0.33 
74.14 
2.90 
6.38 
9.04 orange/yellow 
45 70 2.5 27.5 
0.32 
0.32 
76.68 
-0.22 
2.55 
3.61 pale yellow 
46 50 8 42 0.33 
0.33 
81.52 
0.47 
7.70 
9.37 pale yellow 
47 50 4 46 0.33 
0.33 
82.29 
1.67 
7.92 
9.82 pale yellow/orange 
48 50 8 42 0.32 
0.33 
81.43 
0.81 
6.54 
8.17 pale yellow/orange 
49 50 4 46 0.33 
0.33 
80.10 
0.86 
7.37 
9.21 pale yellow/orange 
50 58 8 34 0.32 
0.33 
81.18 
0.05 
4.87 
6.14 pale yellow 
51 50 4 46 0.33 
0.33 
79.11 
1.72 
8.95 
11.31 pale yellow/orange 
52 58 8 34 0.32 
0.33 
79.81 
-0.19 
4.51 
5.77 pale yellow 
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The above intermetallic compounds had the colours stated which proved to be 
aesthetically pleasing and suitable for providing a novel appearance to 
components of articles of jewellery. Compounds numbers 1 to 4, which fall 
outside of the scope of this invention, and which form part of the prior 
art, were made for comparison purposes. 
The intermetallic compound samples were prepared in a button arc furnace 
under an argon atmosphere. 
Samples were mounted and polished to a lum mirror finish for colour 
measurements. FIG. 1 shows the sample compositions used for further 
measurements. 
Colour measurements were made using Spectrogard Colour Spectro-photometer. 
A standard CIE source C illuminent was used (average daylight). A CIE 
observer angle of 2.degree. was used for all calculations. Both the Yxy 
(chromaticity) and CIElab colour scales were calculated from the measured 
data. The chromaticity data is plotted on a colour locus in FIG. 3. FIG. 4 
shows an enlargement of the colour locus showing the points of relatively 
high colour saturation relative to white, pure gold and copper. The % 
colour saturation values are given in Table 1. 
The CIElab data is plotted in FIGS. 5 and 6. This data defines composition 
areas having high colour co-ordinates. The a* values plotted in FIG. 5 
give a measure of the red and green colour component of a sample. 
Increasing positive a* values indicate an increasing red component and a 
negative a* value indicates an increasing green component. The b* values 
plotted in FIG. 6 give a measure of the yellow and blue colour components. 
Increasing positive b* indicate an increasing yellow component and 
negative b* values indicate the blue component. By mapping out the a* and 
b* values as a function of composition, it enables one to exactly match a 
desired colour by choosing the corresponding composition. 
From these colour-composition triangles, it is clear that the composition 
range having the highest colour saturation is: 
Pt 81 wt. % to 50 wt. % 
Al 30 wt. % to 5 wt. % 
Cu 47.5 wt. % to 1 wt. % 
Compositions outside of these limits do have colour but of low saturation 
and it is therefore difficult to observe the difference in colour, 
relative to platinum, with the human eye. 
Intermatallic compounds are known to be hard and brittle as is found with 
the platinum-aluminium intermetallic compounds. The addition of copper to 
the intermetallic compounds has no notable effect on the hardness of 
platinum-aluminium intermetallics. There is, however, a large decrease in 
the Vickers Micro-hardness values when no, or very little, aluminium is 
present in platinum-copper alloys. Vickers hardness values as low as 124 
Hv were measured in the as-cast state of platinum-copper alloys. The 
hardness values measured are given on a hardness-composition triangle in 
FIG. 7. 
The melting point of the intermetallic compounds having a high colour 
saturation, as determined from Table 1, have been determined. Dual thermal 
analysis was carried out on all of these samples and the melting point was 
calculated from the onset temperature of the endothermic peak. Table 2 
gives the melting point measured for 10 intermetallic compounds. It is 
evident that the addition of copper to the PtAl.sub.2 intermetallic 
compounds causes a large decease in melting point. This is very 
advantageous to manufacturing jewellers who will be able to work with the 
material using standard jewellery equipment. 
TABLE 2 
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COMPOUND NO MELTING POINT .degree.C. 
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4 1413.5 
5 1324.3 
6 1406.2 
11 1380.0 
12 1352.4 
13 1335.3 
14 1287.7 
20 1210.2 
51 1121.3 
19 1179.4 
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It is envisaged that the compounds of the invention could be used for 
making, amongst other articles, cabochons and facetted pieces. It is also 
envisaged that the intermetallic compounds may be cast to form rings or 
other articles which can be made by a casting process as the compounds are 
not ductile and therefore not particularly workable. However facets can 
easily be formed on bodies of the intermetallic compounds. 
Based on the above results, it is envisaged that various interesting 
colours can be produced with the colour compositions in the range 
indicated above. 
The actual change in light reflectivity has been measured and the results 
are illustrated in FIG. 2. 
Accordingly the invention provides intermetallic compounds of platinum and 
aluminium with modified colours brought about by th addition of various 
quantities of copper to the compound and which, it is envisaged, will be 
highly useful in the jewellery trade.