Gold alloy with improved hardness

Abstract

The invention concerns a gold alloy. It comprises at least 75% of gold, 0.5% to 2.1% of aluminium able to form precipitates with gold, an additional metal able to favour a stable face centred cubic structure and able to increase the solubility of aluminium in gold, and a precipitate selected to obtain a hardness of more than 250 HV. The selected precipitate of aluminium with gold is the aluminium and gold precipitate Al 2 Au 5 . It includes 0.5% to 2.1% of aluminium and a complement of additional metal including a majority of silver. The method of obtaining this alloy regulates the controlled growth of the precipitate during a structuring tempering treatment following dilution and hardening. The invention concerns the use of the aluminium and gold precipitate Al 2 Au 5 for hardening a gold alloy. The invention concerns a timepiece or piece of jewellery including at least one component made of this type of alloy.

1 - 16 . (canceled) 
     
     
         17 . A 3N 18 carat yellow gold-based alloy, comprising in weight percent:
 at least 75% of gold;   from 0.5% to 2.1% of an aluminium metal capable of forming at least one precipitate with gold;   from 20% to 25% of at least one first additional metal having an ability to favor a stable face centered cubic (FCC) structure and to increase high temperature solubility of the aluminum metal in the gold;   0% to 0.5% of at least one component affecting fluidity and grain size refinement of the alloy; and   at least one aluminium and gold precipitate comprising Al 2 Au 5  forming an intermetallic compound which provides the alloy with more than 250 HV hardness so as to improve structural hardening of the alloy.   
     
     
         18 . The alloy according to  claim 17 , wherein the first additional metal is silver. 
     
     
         19 . The alloy according to  claim 18 , wherein the first additional metal is 10% to 12.5% weight percent silver, and the alloy further comprises 10% to 12.5% weight percent of a second additional metal of lower concentration than that of the silver. 
     
     
         20 . The alloy according to  claim 19 , wherein the second additional metal is copper. 
     
     
         21 . The alloy according to  claim 17 , wherein a content of the at least one first additional metal is between 22.4% and 24.5%. 
     
     
         22 . The alloy according to  claim 17 , wherein the at least one component affecting fluidity and grain size refinement of the alloy is selected from the group consisting of zinc, cobalt and iridium. 
     
     
         23 . The alloy according to  claim 17 , comprising, as the only precipitate of aluminium and gold, the aluminium and gold precipitate comprising Al 2 Au 5 . 
     
     
         24 . A method for obtaining a gold-based alloy with improved hardness, the method comprising:
 (i) heat treating a mixture comprising:
 0.5% to 2.1% of aluminium; 
 at least 75% of gold; 
 20% to 25% of at least one first additional metal having an ability to favor a stable face centered cubic (FCC) structure and to increase high temperature solubility of the aluminum in the gold; and 
 0% to 0.5% of at least one component affecting fluidity and grain size refinement, 
   
       to a temperature between 650° C. and 700° C. to form a heat treated mixture; then
 (ii) rapidly cooling the heat treated mixture by ambient temperature hardening to form a cooled mixture; 
 (iii) tempering the cooled mixture at a temperature of between 200° and 250° C. to form at least one aluminium and gold precipitate comprising Al 2 Au 5 , said at least one precipitate being capable of inserting into the FCC structure of the at least one first additional metal and forming an intermetallic compound capable of imparting a desired hardness of more than 250 HV to a resulting gold-based alloy; 
 (iv) maintaining the tempering for at least 60 minutes to grow the intermetallic compound in a controlled manner to obtain the resulting gold-based alloy with the desired hardness; and 
 (v) cooling the resulting gold-based alloy at ambient temperature. 
 
     
     
         25 . The method according to  claim 24 , wherein the rapid cooling (ii) is achieved by water hardening. 
     
     
         26 . The method according to  claim 24 , wherein the at least one precipitate is a single precipitate. 
     
     
         27 . The method according to  claim 24 , wherein the tempering (iii) occurs for at least 24 hours after the rapid cooling (ii). 
     
     
         28 . The method according to  claim 24 , wherein the at least one first additional metal is silver. 
     
     
         29 . The method according to  claim 24 , wherein the mixture further comprises at least one second additional metal in a lower concentration than that of the silver. 
     
     
         30 . The method according to  claim 29 , wherein the second additional metal is copper. 
     
     
         31 . The method according to  claim 24 , wherein the mixture comprises between 22.4% and 24.5% of the at least one first additional metal. 
     
     
         32 . A timepiece or a piece of jewellery, comprising at least one component made of the gold-based alloy of  claim 23 . 
     
     
         33 . A method for hardening an 18 carat gold-based alloy, the method comprising forming at least one precipitate from aluminium and gold comprising Al 2 Au 5  which forms an intermetallic compound within a gold-containing alloy and thereby imparts a resulting gold-based alloy with a hardness of more than 250 HV. 
     
     
         34 . The method of  claim 33 , wherein:
 the gold-based alloy comprises:
 at least 75% of gold; 
 0.5% to 2.1% of aluminium; 
 20% to 25% of at least one additional metal selected from the group consisting of silver and copper, said additional metal having an ability to favor a stable face centered cubic (FCC) structure and to increase solubility of aluminium in gold; and 
 0% to 0.5% of at least one component affecting fluidity and grain size refinement of the gold-based alloy; 
   the method further comprises inserting the intermetallic compound into the FCC structure during a solution heat treatment of a mixture comprising the gold, the aluminium, the at least one additional metal, and the at least one component if present; and   the at least one additional metal is selected from the group consisting of aluminium, silver, chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum, and vanadium.   
     
     
         35 . The method of  claim 34 , wherein the at least one additional metal is silver. 
     
     
         36 . The method of  claim 35 , wherein the alloy further comprises at least one second additional metal having a lower concentration than that of the silver. 
     
     
         37 . The method of  claim 36 , wherein the at least one second additional metal is copper. 
     
     
         38 . The method of  claim 34 , wherein the gold-based alloy comprises 22.4% to 24.5% of the at least one first additional metal. 
     
     
         39 . The method of  claim 34 , where the Al2Au5 is the only aluminium and gold precipitate in the gold-based alloy.