Patent Application
20130153097
The invention concerns a gold-based alloy with improved hardness.
The invention also concerns a method of obtaining a gold-based alloy with improved hardness.
The invention further concerns the use of a precipitate for hardening a gold alloy.
The invention further concerns a timepiece or piece of jewellery including at least one component made of this type of alloy.
It is an object of the invention to make a gold-based alloy, which has improved hardness qualities compared, not only to pure gold, but also to known gold-based alloys.
The main applications are horology, jewellery and dentistry.
Hardening gold is an old problem, which, since ancient times, has led to the use of alloys in order to obtain sufficient mechanical features to ensure at least the resistance of the manufactured articles. Indeed, the method of cold working material by plastic deformation, which is well suited to some metals, is ill suited to gold because gold has very little consolidation during deformation, and also recrystallises at relatively low temperatures. The grain size refinement method, which theoretically raises the elastic limit of the material, is not suitable for gold either, as gold has a face centred cubic structure, called “FCC” hereinafter, since there are enough active sliding systems for the free passage of dislocations from one grain to another.
Solution heat treating alloy elements is the method most commonly used, often empirically, and at best, it provides only mediocre hardness, of around 150 to 155 HV on the Vickers scale.
Various attempts have been made, for example to refine grain size as in EP Patent No. 0 284 699 in the name of Steinemann with a binary alloy containing gold and another metal chosen from among aluminium, gallium or silicon, or even with a similar pseudo-binary alloy also including copper, with a maximum gold concentration of 15%. This type of compound gives a centred cubic structure and a grain size of less than 50 microns, which provides some ductility, which is not the quality desired here.
The preparation of dental alloys with hardness which increases over time and at body temperature is also known, from U.S. Pat. No. 5,338,378 in the name of Kyushu University, which implements an alloy with 67% to 82% of gold, 18% to 33% of copper and 2% to 8% of at least one other metal selected from among gallium, aluminium and zinc. This alloy undergoes a hardening operation after being heated to between 650° C. and 700° C. prior to use. Likewise, EP Patent No. 0 978 572 in the name of Hafner GmbH discloses an alloy comprising 70% to 80% of gold, 15% to 25% of copper, 0% to 15% of silver and 0.1% to 5% of gallium, which, in an unexplained way, barely oxides during a second treatment at 400° C. which follows a first treatment at 800° C. and acquires a hardness which increases with time at ambient temperature.
The following documents are also known:
The document referred to as D1: JP Patent Application No 8 013060 A in the name of PILOT PEN discloses a method of obtaining a gold-based alloy with improved hardness and several possible compositions:
15% to 19% of copper and 4 to 10% of silver, and 0.3 to 1% of aluminium and/or magnesium.
or 15 to 19% of copper, 4 to 10% of silver, 0.3 to 1% of aluminium and/or magnesium, and 0.3 to 2% of zinc.
0.1% to 1% of ruthenium and/or cobalt is incorporated into this mixture. However, this composition corresponds to a rose gold, which is not claimed here.
The document referred to as D2: EP Patent Application No. 0 978 5762 A1 in the name of HAFNER discloses an alloy with 70 to 80% of gold and 15 to 25% of copper, to which 0.1 to 5% of gallium is added. According to some variants it may also contain: 0.1% to 3% of zinc, and/or 0.5 to 5% of silver, and/or 0.1 to 0.5% of silicon and/or 0.1 to 2% of iron and/or 0.1 to 0.3% of indium, and/or 0.1 to 0.5% of aluminium and/or 0.1 to 3% of tin.
The document referred to as D3: US Patent Application No. 5 38 378 A in the name of OHTA MICHIO discloses a dental gold alloy which ages slowly for 20 to 30 days during which its hardness increases further. It includes 82 to 67% of gold, 18 to 33% of copper, and 0 to 2% of a hardening accelerator chosen from between gallium and zinc. The alloy is heated to between 650 and 700° C. and water quenched. In another composition, it includes 2 to 8% of this type of accelerator which includes at least one metal chosen from among the group consisting of 1 to 4% of gallium, 0.4 to 2% of aluminium and 1 to 5% of zinc. In another composition, it includes 1 to 4% of gallium. In another composition it includes 1 to 5% of zinc.
The document referred to as D4: The article “18 carat yellow gold alloys with increased hardness” by SUSS, RAINER, published in 2004, discloses gold alloys with increased hardness and the influence of the additional metals, both as regards obtaining physical characteristics and particular colours.
The document referred to as D5: The article “Metallurgy of gold” by FISCHER-BÜHNER, published on 20 May 2010, details the metallurgy of gold and its alloys.
It has been shown that, from all the many studies made, it is known how to select gold alloys with suitable hardness, without always understanding the physico-chemical mechanism which infallibly results in the desired hardness.
In short, the known methods are often empirical, are not well grasped and create alloys which, on the one hand, only have average hardness, and on the other hand have a very particular colouring which is very different from that of pure gold.
The invention is concerned with developing an alloy which has good hardness properties, greater than 250 HV on the Vickers scale, and equivalent core hardness properties compared to hardness of around 155 HV currently obtained by solution heat treating metallic alloy elements.
It is also important for the appearance of gold and its brilliance to be preserved.
The invention therefore concerns a gold-based alloy, known as 3N 18 carat yellow gold, characterized in that it is formed of a mixture comprising in weight percent:
at least 75% of gold,
from 0.5% to 2.1% of a second metal chosen for its ability to form precipitates with gold, said second metal being aluminium, 20% to 25%, or preferably 22.4% to 24.5% of at least one additional metal chosen for its ability to favour a high temperature stable FCC structure, and for its ability to increase the solubility of said second metal in gold, and to adjust the colour of the alloy,
0% to 0.5% of one or more components selected for the fluidity and grain size refinement of said alloy, said mixture further including at least one said precipitate from the reaction between said second metal and gold, selected from among said precipitates of said second metal with gold to form an intermetallic compound providing said alloy with more than 250 HV hardness so as to improve the structural hardening of said alloy, said selected precipitate being the aluminium and gold precipitate Al2Au5.
According to another feature of the invention, said additional metal is silver.
According to yet another feature of the invention, said additional metal is silver and it is completed by another additional metal in a lower concentration than that of silver.
According to a particular feature of the invention, said other additional metal is copper.
The invention further concerns a method for obtaining a gold-based alloy with improved hardness, characterized in that:
According to a feature of the invention, the selection of said at least one selected precipitate is limited to a single precipitate.
According to another feature of the invention, said tempering structuring treatment is performed at least 24 hours after said rapid cooling.
According to another feature of the invention, silver is chosen as said additional metal.
According to another feature of the invention, silver is chosen as said additional metal and another additional metal is added in a lower concentration than the silver.
In particular, copper is chosen as said other additional metal.
The invention further concerns the use of a precipitate for hardening a gold alloy, characterized in that said precipitate is an aluminium and gold precipitate Al2Au5 for hardening a gold alloy comprising at least 75% of gold, 0.5% to 2.1% of aluminium and 20% to 25% of at least one additional metal, chosen from among silver and copper for its ability to favour a stable FCC structure on the one hand, and for its ability to increase the solubility of aluminium in gold on the other hand, and 0% to 0.5% of one or more components selected for the fluidity and grain size refinement of said gold alloy, said use resulting from the insertion of said aluminium and gold precipitate Al2Au5 into an FCC structure resulting from the solution heat treatment of said mixture comprising gold, aluminium, said at least one additional metal, said selected components if said alloy has any, and aluminium and gold precipitates, said insertion being achievable via this method.
The invention further concerns a timepiece, piece of jewellery or dental piece including at least one component made of an alloy of this type.
In the preferred embodiment of the description, concerning an 18 carat alloy, the alloy preserves the specific appearance of pure gold. Because of its increased hardness, the alloy obtained is more scratch resistant and is entirely suitable for timepiece and pieces of jewellery and in particular for the visible components thereof such as bezels and middle parts of watches, and jewellery structures, bracelets, clasps, buckles and other items.
The invention provides a method that is simple to implement and reproduce, and which reliably produces a gold alloy with the required hardness of more than 250 HV in a short treatment time. The alloy obtained can be used immediately, without requiring any additional ageing.
Other features and advantages of the invention will appear upon reading the following description, with reference to the annexed drawings, in which:
It is an object of the invention to make a gold-based alloy, which has improved hardness qualities compared, not only to pure gold, but also to known gold-based alloys.
The invention implements a structural hardening method, by the selection of particular elements, which are chosen here to form particular precipitates. Among the various precipitates which gold can form with other metals in very particular physico-chemical conditions, those precipitates whose germination and growth can be controlled, by implementing an appropriate treatment, to optimise mechanical characteristics, and in particular here to increase hardness, should be chosen.
In particular, the mechanical feature which the present invention improves, by creating a particular method, is hardness which concerns both the core hardness of the alloy, and surface hardness which is very important in horology and jewellery to ensure scratch resistance or at least to minimise the effects of scratches.
This is therefore a very different case to most gold alloys used in jewellery, which are usually developed to include the minimum gold content required to ensure an appearance close to that of gold, and with the requirement for a high level of formability, so as to allow lamination, or stretching, in hollow bodies or sheets, which are easy to shape and weld.
The inventive step of the invention has consisted in researching the possibility of inserting precipitates into a face centred cubic structure or FCC, and letting them grow in a controlled manner, so as to obtain a greater hardness than the usual hardness
The invention more particularly concerns the field of gold alloys with a high gold content, and more specifically, 18 carat alloys comprising at least 75% of gold content by weight.
The selection of aluminium is peculiar to the invention because of the ability of this metal to form different precipitates with gold: Al2Au5, AlAu2, Al Au. These three precipitates can produce alloys with improved hardness.
These alloys Au/Ag/Al/ Al2Au5 or Au/Cu/Al/ Al2Au5 or Au/Ag/Cu/Al/ Al2Au5 do not exist in the natural state and they have to be made in order to be used. The invention proposes a preferred manufacturing method below.
It is preferable to carry out the method so as to obtain the Al2Au5 precipitate, which when incorporated in an alloy provides normal resistance during machining or transformation operations. Thus, this Al2Au5 precipitate is created and preferably only this Al2Au5 precipitate is created since it yields better properties than the other two precipitates AlAu2 and Al Au.
The Al2Au5 precipitate must be obtained at the core of the FCC structure. A binary alloy formed solely of gold and aluminium is difficult to work and is very fragile, which makes it unsuitable for most jewellery uses. It is therefore necessary to stabilise the FCC phase by incorporating at least one other alloy element to ensure the solubility of the intermetallic compound at a high temperature, and also to ensure the longest possible FCC phase, i.e. for the broadest possible range of aluminium content. High temperature stability means that only the FCC phase is present in the temperature range concerned, which can be seen along the elongated single domain of the equilibrium diagram of
Different trial pseudo-binary alloys have been tested.
The second metal can be chosen from among aluminium, silver, chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum and vanadium although this list is not exhaustive.
An additional metal may preferably be chosen from among silver, aluminium, chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum and vanadium.
Experiments demonstrate that the choice of silver as the additional metal is the most particularly favourable for the solubility of the intermetallic compound at a high temperature, and for obtaining a long FCC phase, since the silver-gold solubility is complete, and since the silver can also dissolve the aluminium.
Innovatively, the invention has attempted to create the pseudo-binary alloy phases Au—Ag—Al as seen in
This is a partial solubility diagram showing the limits of solubility, which are substantially vertical in the diagram, and which separate the phases. Each phase has a defined composition which is different to the next phase. In each of these phases, the atoms are reorganised locally to form precipitates, which are defined compounds of fixed composition.
In order to obtain the desired Al2Au5 precipitate, and only that precipitate, it is advisable to remain within a first domain called A in
A second domain called B in
The third domain called C in
The diagram in
The alloy is then made metastable. The rise in temperature, achieved for example between around 400° C. and 700° C. for the extreme part of domain A and ideally around 650° C., is followed by rapid cooling, such as water hardening or similar. Thus, the aluminium atoms do not have time to reorganise. After a variable period time, but preferably close to 24 hours, the alloy undergoes a structuring tempering treatment, within the temperature range defined by the solvus of domain A for the aluminium concentration concerned. In any event, this structuring tempering treatment does not exceed a temperature of 400° C. During tempering, the Al2Au5 precipitates develop and grow. Preferably, the structuring tempering temperature is higher than 200° C. to facilitate the precipitate growth, and also to limit the duration of the heat treatment.
If the structuring tempering treatment is performed at a lower temperature, for example 100° C., a hardness of more than 200 HV will only be obtained after 10 to 15 hours, and the treatment must be extended further to reach a level of around 250 HV.
The Al2Au5 precipitate obtained is harder than gold.
It is essential, according to the invention, to favour the presence of the Al2Au5 precipitate and preferably to restrict the formation of precipitates comprising only gold and aluminium to this one Al2Au5 precipitate, which has the best features, in order to resolve the technical problem of hardening the alloy.
Preferably, in order for the Al2Au5 precipitates to develop optimally, the alloy does not include any other metal apart from gold, aluminium and an additional metal, preferably silver, selected to increase intermetallic compound solubility and to lengthen phase D as much as possible in terms of the amplitude of the range of aluminium concentrations.
Returning to some of the aforecited prior art teachings, it is evident that the role of aluminium is not demonstrated.
Document D1 discloses a pink gold.alloy. It cannot be affirmed that aluminium is significant as the alloy hardening agent by analysing the variance of the results of document D1. Copper, like silver, is a hardening element, α=0.05. The effect of copper is to create an order/disorder reaction with gold, with the formation of an AuCu type compound, with increased hardness. The effect of the silver/copper mixture is similar to the effect of copper by itself.
The same is true of document D2, which also discloses a pink gold. The addition of aluminium does not seem to play any particular part therein, which is understandable if the processing method is not carried out for the purpose of forming a single gold and aluminium precipitate which is advantageous for increasing the hardness of the alloy.
Document D3 is also dedicated to fabricating pink gold. This document claims that the alloy remains single phase, which is physically impossible: the teaching of this document cannot be accepted.
Document D4 discloses a pinkish yellow gold. It is impossible to evaluate the specific role of aluminium since all the other elements present in the alloy already contribute to the hardening process. This document comments that with more than 0.4% aluminium, the alloy blackens considerably. This drawback is not visible in the case of the alloy developed according to the invention, even with 2% aluminium content. It should be noted that the experimenter who follows the instructions of document D4 will not obtain Al2Au5 precipitate not only because of the added elements such as zinc, which modify the solubility of aluminium, but also because of the low aluminium concentration >0.4%.
Document D5 explains the hardening of the gold (75%)-silver-copper alloy by the well known order/disorder transformation with copper. The composition of 75% gold-12.5% silver-12.5% copper of
In short, the fabrication of the aluminium and gold precipitate Al2Au5 increases the HV hardness by around 50 HV.
In short, the invention differs from the prior art in that it creates the conditions for developing Al2Au5 precipitate within an alloy of suitable composition comprising gold, aluminium and at least one additional metal selected for its ability both to favour a stable FCC structure and to increase the solubility of aluminium in gold, this additional metal preferably being silver.
The optimum weight percent composition is from 0.1% to 2.1% of aluminium, and preferably from 0.5% to 2.1% and at least 75% of gold so as to respect the legal standard for jewellery, and the complement formed by at least one additional metal, which may be completed by a small proportion of at least one other component selected for fluidity and grain size refinement.
The additional metal may also be copper. It is also possible to combine several metals each having the properties that this additional metal is required to have, namely the ability to favour a stable FCC structure on the one hand and the ability to increase the solubility of aluminium in gold on the other hand.
Silver is the best element, and the other metal elements in the list set out above may be added to adjust the colour of the alloy. This list of elements was drawn up so that the elements comprised therein satisfy the condition of increasing the solubility of aluminium in the FCC structure at a high temperature.
In particular, copper is less favourable than silver in satisfying these particular conditions in the presence of gold and aluminium. The use of copper remains possible for reasons of cost, but is considerably less advantageous than silver, and if used should always be combined with silver, while ensuring that the concentration of silver is always higher than the concentration of copper in the alloy.
With the use of additional metals other than silver or copper, for example chosen from among chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum and vanadium (this list is not exhaustive), it should be borne in mind that aluminium could form precipitates with certain of these additional metals, but that preferably it is desired to form Al2Au5 precipitates. Thus, in addition to silver and aluminium, preferably only these elements should be used: chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum and vanadium.
Moreover, each new composition with different additional metals requires thorough testing to define the corresponding phase diagrams, which do not exist in the literature, to analyse the precipitates and other intermetallic compound components created within each of the phases, and to check that these compounds do not adversely affect the mechanical properties of the gold alloy. These studies and experiments take a long time and are expensive and cannot be conducted at random. The object thereof is also to determine, on a case by case basis, the aluminium concentration range to be observed to obtain Al2Au5 precipitates and preferably only this precipitate.
In short, the invention provides a 3N 18 carat yellow gold-based alloy, characterized in that it is formed of a mixture comprising in weight percent:
at least 75% of gold,
from 0.5% to 2.1% of a second metal chosen for its ability to form precipitates with gold, said second metal being aluminium,
20% to 25%, or preferably 22.4% to 24.5% of at least one additional metal chosen for its ability to favour a high temperature stable FCC structure, and for its ability to increase the solubility of said second metal in gold, and to adjust the colour of the alloy,
0% to 0.5% of one or more components selected for the fluidity and grain size refinement of said alloy,
said mixture further including at least one said precipitate from said second metal and gold, selected from among said precipitates of said second metal with gold to form an intermetallic compound providing said alloy with more than 250 HV hardness so as to improve the structural hardening of said alloy, said selected precipitate being the aluminium and gold precipitate Al2Au5.
Advantageously, the second metal is aluminium and the precipitate chosen is the aluminium and gold precipitate Al2Au5 which provides an alloy with very good hardness characteristics, i.e. a hardness of more than 250 HV and close to 280 HV. This Al2Au5 precipitate also gives the alloy very good resistance during transformation or when machined, since it does not make the alloy brittle.
Preferably, the additional metal is silver, which ensures that the whole of the mixture is properly soluble.
In a particular embodiment, the additional metal is silver, preferably in a weight percent content of 10% to 12.5% of the total, and it is completed by another additional metal, preferably in a weight percent content of 10% to 12.5% of the total, to adjust the colour of the alloy.
Advantageously, at least one said component selected for the fluidity and grain size refinement of said alloy is chosen from among zinc, cobalt or iridium.
The method for obtaining a gold-based alloy, with improved hardness, preferably consists in:
Preferably, the choice of selected precipitates is limited to one precipitate, in this case the aluminium and gold precipitate Al2Au5.
Advantageously, the structuring tempering treatment is performed at least 24 hours after the rapid cooling.
Preferably, aluminium is chosen as the second metal, and the aluminium and gold precipitate Al2Au5 is chosen as the selected precipitate.
Advantageously, silver is selected as the additional metal.
In a variant, silver is selected as the additional metal and another additional metal having similar characteristics to silver is added, to adjust the colour of the alloy.
Other methods such as mechanosynthethis or PVD may also involve the creation of the aluminium and gold precipitate Al2Au5. However, as in the operating modes listed above, the conditions for inserting this very particular precipitate into the face centred cubic structure of the alloy and the conditions for developing this precipitate in order to give the alloy the desired hardness characteristics should be created.
The invention further concerns the use of a precipitate for hardening a gold alloy. According to the invention, this precipitate is an aluminium and gold precipitate Al2Au5 for hardening a gold alloy comprising at least 75% of gold, form 0.5% to 2.1% of aluminium and from 20% to 25% or preferably from 22.4% to 24.5% of at least one additional metal selected from among silver and copper and chosen for its ability to favour a stable FCC structure on the one hand, and for its ability to increase the solubility of aluminium in gold on the other hand, and from 0% to 0.5% of one or more components selected for the fluidity and grain size refinement of said gold alloy, said use resulting from the insertion of said aluminium and gold precipitate Al2Au5 into an FCC structure resulting from the solution heat treatment of said mixture formed of gold, aluminium, said at least one additional metal, said selected components if said alloy includes any, and aluminium and gold precipitates, said insertion being preferably achievable by the method described above.
The invention further concerns a timepiece, piece of jewellery or dental piece including at least one component made of an alloy of this type.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10167859.7 | Jun 2010 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/060041 | 6/16/2011 | WO | 00 | 3/8/2013 |
