Patent Application
20230366064
The present invention relates to a platinum alloy composition, in particular a platinum alloy composition for use in jewellery and an alloy composition for jewellery with improved castability and to jewellery.
For jewellery applications pure platinum does not have sufficient hardness to provide wear resistance. To improve the wear resistance of platinum alloys, additional elements are added. These elements increase alloy hardness providing increased resistance to wear.
While it is desirable to add elements to platinum to increase hardness, the addition of alloying elements can adversely affect the castability of the material. For example, additions of certain elements to Pt will increase the overall melting temperature of the alloy. A higher melting temperature will increase the reaction of molten metal with the mould material during the casting process and reduce surface quality. A higher melting temperature may also result in reduced alloy fluidity during casting as it may be difficult to achieve sufficient ‘superheat’ prior to casting given the already high melting point of pure platinum.
It is an aim of the present invention to provide a platinum alloy with suitable hardness for jewellery which has reduced melting temperature, and superior casting properties.
The present invention provides a platinum alloy composition consisting, in weight percent, of: 0.0 to 10.0 gold, 0.0 to 5.0 cobalt, 0.0 to 10.0 copper, 0.0 to 7.0 iron, 0.0 to 4.0 gallium, 0.0 to 3.0 indium, 0.0 to 5.0 iridium, 0.0 to 10.0 manganese, 0.0 to 7.0 nickel, 0.0 to 15.0 palladium, 0.0 to 5.0 rhenium, 0.0 to 5.0 rhodium, 0.0 to 10.0 ruthenium, 0.0 to 3.0 tin, 85.0 or more platinum and incidental impurities, wherein two or more of gallium, indium and tin are present in an amount of 0.1 or more, wherein the following equation is satisfied in which WCo, WCu, WFe, WGa, WIn, WNi, WPd, WSn, WRh, WIr, WAu, WRu, WRe, and WMn are the weight percent of cobalt, copper, iron, gallium, indium, nickel, palladium, tin, rhodium, iridium, gold, ruthenium, rhenium, and manganese in the alloy respectively
60+WPd*2.5+WRh*3.4+WIr*6.455+WAu*11.93+WRu*13.241+WCu*14.328+WRe*16.6+WNi*16.9+WMn*18.48+WCo*18.69+WFe*21.879+WIn*29+WSn*28.207+WGa*42.379≥150
−0.1028*WCo−0.1201*WCu−0.2113*WFe−0.3368*WGa−0.1125*WIn−0.1639*WNi−0.015*WPd−0.1959*WSn+17.276261−0.20*WMn+0.0678*WRu+0.035*WIr+0.045*WRh−0.059*WAu+0.066*WRe≤16.0 when WPt<95.0
−0.1028*WCo−0.1201*WCu−0.2113*WFe−0.3368*WGa−0.1125*WIn−0.1639*WNi−0.015*WPd−0.1959*WSn+17.276261−0.20*WMn+0.0678*WRu+0.035*WIr+0.045*WRh−0.059*WAu+0.066*WRe≤16.6 when WPt≥95.0
0.35WAu+0.6WSn+0.6WIn+WGa≤3.75
W
Co
+W
Pd
+W
Fe
+W
Ni
+W
Cu≥1.0
W
Sn
+W
In
+W
Ga≥0.25.
Such an alloy is suitable for use in jewellery, has a melting point significantly below that of elemental platinum and lower than that of prior art alloys and has increased hardness compared to elemental platinum and several commonly used platinum alloys. The alloys according to the present invention are well suited for the fabrication of jewellery and other ornamental articles because they exhibit superior castability and mechanical properties relative to a large number of benchmarks. In particular, the alloys have a low solidification range and lower prevalence of intermetallics making them suitable for casting and subsequent forming without the need for heat treatment.
In an embodiment the platinum alloy composition consists, in weight percent, of 90.0 or more platinum or sum of platinum and iridium, preferably 95.0 or more platinum or sum of platinum and iridium. Such an alloy meets the 900Pt or 950Pt standard for platinum jewellery respectively.
In an embodiment the platinum alloy composition satisfies the following equation in which WCo, WCu, WFe, WGa, WIn, WNi, WPd, WSn, WRh, WIr, WAu, WRu, WRe, and WMn are the weight percent of cobalt, copper, iron, gallium, indium, nickel, palladium, tin, rhodium, iridium, gold, ruthenium, rhenium, and manganese in the alloy respectively
60+WPd*2.5+WRh*3.4+WIr*6.455+WAu*11.93+WRu*13.241+WCu*14.328+WRe*16.6+WNi*16.9+WMn*18.48+WCo*18.69+WFe*21.879+WIn*29+WSn*28.207+WGa*42.379≥200
60+WPd*2.5+WRh*3.4+WIr*6.455+WAu*11.93+WRu*13.241+WCu*14.328+WRe*16.6+WNi*16.9+WMn*18.48+WCo*18.69+WFe*21.879+WIn*29+WSn*28.207+WGa*42.379≥225
60+WPd*2.5+WRh*3.4+WIr*6.455+WAu*11.93+WRu*13.241+WCu*14.328+WRe*16.6+WNi*16.9+WMn*18.48+WCo*18.69+WFe*21.879+WIn*29+WSn*28.207+WGa*42.379≥250
60+WPd*2.5+WRh*3.4+WIr*6.455+WAu*11.93+WRu*13.241+WCu*14.328+WRe*16.6+WNi*16.9+WMn*18.48+WCo*18.69+WFe*21.879+WIn*29+WSn*28.207+WGa*42.379≥275
60+WPd*2.5+WRh*3.4+WIr*6.455+WAu*11.93+WRu*13.241+WCu*14.328+WRe*16.6+WNi*16.9+WMn*18.48+WCo*18.69+WFe*21.879+WIn*29+WSn*28.207+WGa*42.379≥300
Such an alloy has superior hardness making it suitable for jewellery.
In an embodiment the platinum alloy composition satisfies the following equation in which WCo, WCu, WFe, WGa, WIn, WNi, WPd, WSn, WRh, WIr, WAu, WRu, WRe, and WMn are the weight percent of cobalt, copper, iron, gallium, indium, nickel, palladium, tin, rhodium, iridium, gold, ruthenium, rhenium, and manganese in the alloy respectively
60+WPd*2.5+WRh*3.4+WIr*6.455+WAu*11.93+WRu*13.241+WCu*14.328+WRe*16.6+WNi*16.9+WMn*18.48+WCo*18.69+WFe*21.879+WIn*29+WSn*28.207+WGa*42.379≤280
60+WPd*2.5+WRh*3.4+WIr*6.455+WAu*11.93+WRu*13.241+WCu*14.328+WRe*16.6+WNi*16.9+WMn*18.48+WCo*18.69+WFe*21.879+WIn*29+WSn*28.207+WGa*42.379≤260
60+WPd*2.5+WRh*3.4+WIr*6.455+WAu*11.93+WRu*13.241+WCu*14.328+WRe*16.6+WNi*16.9+WMn*18.48+WCo*18.69+WFe*21.879+WIn*29+WSn*28.207+WGa*42.379≤240.
Such an alloy may be more suitable for jewellery applications where gem setting is necessary.
In an embodiment the platinum alloy composition satisfies the following equation in which WCo, WCu, WFe, WGa, WIn, WNi, WPd, WSn, WRh, WIr, WAu, WRu, WRe, and WMn are the weight percent of cobalt, copper, iron, gallium, indium, nickel, palladium, tin, rhodium, iridium, gold, ruthenium, rhenium, and manganese in the alloy respectively
−0.1*WCo−0.4933*WCu−0.32*WFe−1.16377*WGa−0.54278*WIn−0.08612*WNi+0.06915*WPd−0.69928*WSn+4.169+0.04*WMn+0.133*WAu≤3.941
−0.1*WCo−0.4933*WCu−0.32*WFe−1.16377*WGa−0.54278*WIn−0.08612*WNi+0.06915*WPd−0.69928*WSn+4.169+0.04*WMn+0.133*WAu≤3.5
−0.1*WCo−0.4933*WCu−0.32*WFe−1.16377*WGa−0.54278*WIn−0.08612*WNi+0.06915*WPd−0.69928*WSn+4.169+0.04*WMn+0.133*WAu≤3.0
−0.1*WCo−0.4933*WCu−0.32*WFe−1.16377*WGa−0.54278*WIn−0.08612*WNi+0.06915*WPd−0.69928*WSn+4.169+0.04*WMn+0.133*WAu≤2.5
Such an alloy has superior resistance to hot cracking.
In an embodiment the platinum alloy composition satisfies the following equation in which WCo, WCu, WFe, WGa, WIn, WNi, WPd, WSn, WMn, WRu, WIr, WRh, WAu and WRe are the weight percent of cobalt, copper, iron, gallium, indium, nickel, palladium, tin, manganese, ruthenium, iridium, rhodium, gold and rhenium in the alloy respectively
−0.1028*WCo−0.1201*WCu−0.2113*WFe−0.3368*WGa−0.1125*WIn−0.1639*WNi−0.015*WPd−0.1959*WSn+17.276261−0.20*WMn+0.0678*WRu+0.035*WIr+0.045*WRh+0.059*WAu+0.066*WRe≤16.0
−0.1028*WCo−0.1201*WCu−0.2113*WFe−0.3368*WGa−0.1125*WIn−0.1639*WNi−0.015*WPd−0.1959*WSn+17.276261−0.20*WMn+0.0678*WRu+0.035*WIr+0.045*WRh+0.059*WAu+0.066*WRe≤15.5
−0.1028*WCo−0.1201*WCu−0.2113*WFe−0.3368*WGa−0.1125*WIn−0.1639*WNi−0.015*WPd−0.1959*WSn+17.276261−0.20*WMn+0.0678*WRu+0.035*WIr+0.045*WRh+0.059*WAu+0.066*WRe≤15.0
−0.1028*WCo−0.1201*WCu−0.2113*WFe−0.3368*WGa−0.1125*WIn−0.1639*WNi−0.015*WPd−0.1959*WSn+17.276261−0.20*WMn+0.0678*WRu+0.035*WIr+0.045*WRh+0.059*WAu+0.066*WRe≤14.5
Such an alloy has a lower melting temperature and therefore reacts less with mould walls during casting and so has superior surface quality.
In an embodiment the platinum alloy composition consists, in weight percent, of 5.0 or less nickel. Such an alloy will be unlikely to cause a reaction on human skin contact.
In an embodiment the platinum alloy composition consists, in weight percent, of 3.0 or less iridium. Such an alloy has reduced cost.
In an embodiment the platinum alloy composition consists, in weight percent, of 3.0 or less rhodium. Such an alloy has reduced cost.
In an embodiment the platinum alloy composition consists, in weight percent, of 5.0 or less ruthenium, preferably 3.0 or less ruthenium. Such an alloy will have superior casting properties.
In an embodiment the platinum alloy composition consists, in weight percent, of 3.0 or less rhenium. Such an alloy will have a lower melting temperature leading to superior casting properties.
In an embodiment the platinum alloy satisfies the following equation in which WGa, WIn, and WSn are the weight percent of gallium, indium, and tin in the alloy WSn+WIn+WGa≥0.40. Such an alloy will have a higher hardness
In an embodiment the platinum alloy composition consists, in weight percent, of 2.5 or less indium, preferably 2.0 or less indium. Such an alloy will produce fewer intermetallic phases on cooling and has a lower solidification range.
In an embodiment the platinum alloy composition of any of the preceding claims, consisting, in weight percent, of 2.5 or less tin, preferably 2.0 or less tin. Such an alloy will produce fewer intermetallic phases on cooling and has a lower solidification range.
In an embodiment at 1000° C. the alloy composition comprises 0.55 or more volume fraction solid solution FCC gamma phase, preferably 0.6 or more volume fraction gamma phase, more preferably 0.7 or more volume fraction gamma phase, even more preferably 0.8 or more volume fraction gamma phase, most preferably 0.9 or more volume fraction gamma phase. Such an alloy is desirable as the risk of reduced ductility is lowered.
In an embodiment the alloy composition has a solidification range of 200° C. or less, preferably wherein the alloy composition has a solidification range of 150° C. or less, more preferably wherein the alloy composition has a solidification range of 125° C. or less, more preferably wherein the alloy composition has a solidification range of 100° C. or less, even more preferably wherein the alloy composition has a solidification range of 75° C. or less, most preferably wherein the alloy composition has a solidification range of 50° C. or less. Such an alloy will have superior casting properties with lower propensity for pore formation.
In an embodiment the platinum alloy composition of any preceding claim, wherein at least two, preferably at least three, elements selected from the following list are present: gold, cobalt, copper, iron, gallium, indium, iridium, manganese, nickel, palladium, rhenium, rhodium, ruthenium, tin. Such alloys have been shown to exhibit the best combination of properties sought in this application.
In an embodiment indium and tin are present in an amount of 0.1 or more in the platinum alloy composition. Such an alloy has reduced intermetallic precipitation.
In an embodiment the platinum alloy composition of any of the preceding claims, wherein the following equation is satisfied in which WGa, WSn, and WAu are the weight percent of gallium, tin and gold in the alloy respectively
WAu*0.35+WSn*0.6+WIn*0.6+WGa≤3.0
Such an alloy is likely to have a lower solidification range.
In an embodiment the platinum alloy composition satisfies the following equation in which WCo, WFe, WNi, and WPd are the weight percent of cobalt, iron, nickel, and palladium in the alloy respectively
(WCo+WPd)/11+(WFe+WNi)/2.2≥1.0
Such an alloy is likely to be easily castable without the addition of other alloying elements as it likely to have a low melting point while retaining a narrow solidification range.
In an embodiment the platinum alloy composition of any of the preceding claims, consisting, in weight percent, of 5.0 or less gold, preferably 3.0 or less gold. Such an alloy has reduced solidification range.
In an embodiment the platinum alloy composition of any of the preceding claims, consisting, in weight percent, of 9.0 or less copper, preferably 8.0 or less copper. Such an alloy has improved castability as formation of slag during melting for casting is less likely and the alloy has a lower solidification range.
In an embodiment the platinum alloy composition consists, in weight percent, of 2.0 or less gallium, preferably 1.5 or less gallium. Such an alloy has a lower solidification range and lower chance of precipitation of intermetallic phases.
In an embodiment the platinum alloy composition the following equation is satisfied in which WIr and WRu are the weight percent of iridium and ruthenium in the alloy respectively
2.5WIr+3.0WRu≤7.5
Such an alloy has reduced melting temperature.
In an embodiment the platinum alloy composition of any of the proceedings claims, wherein on cooling from liquid, gamma phase is first to form in which all alloying elements are in solid solution. Such an alloy will have superior ductility as the formation of large brittle grains made of intermetallic phases on solidification will be avoided.
In an embodiment the platinum alloy composition consists, in weight percent, 0.0 to 1.0 total sum weight percent of gold, iridium, manganese, rhenium, rhodium, and ruthenium. Such an alloy is preferred because it is possible to achieve favourable properties in terms of harness and castability (e.g. low melting point, low solidification range and/or low hot cracking propensity) whilst maintaining a high platinum content.
In an embodiment the platinum alloy composition satisfies the following equation in which WCo, WCu, WFe, WGa, WIn, WNi, WPd, WSn, WMn, WRu, WIr, WRh, WAu and WRe are the weight percent of cobalt, copper, iron, gallium, indium, nickel, palladium, tin, manganese, ruthenium, iridium, rhodium, gold and rhenium in the alloy respectively
0.041WAuWCo+0.122WAuWIn+1.96WAuWNi+1.87WAuWSn+0.903WAu2+1.74WCoWGa+13.4WCoWIn+1.24WCoWMn+5.04WCoWSn+1.02WCo2+8.97WCuWFe+1.74WCuWGa+4.38WCuWIn+1.16WCuWMn+0.491WCuWNi+3.69WCuWSn+0.22WCu2+1.68WFeWGa+3.31WFeWIn−1.26WFeWMn+5.07WFeWSn+0.199WFe2+5.35WGaWIn+0.086WGaWMn+3.27WGaWRe+33.3WGaWRh+4.56WGaWRu+2.21WGaWSn+29.0WGa+8.49WGa2+9.09WInWMn−0.28WInWNi+15.4WInWRh+0.992WInWRu+11.7WIn+6.68WIn2+0.863WIr2+5.73WMnWNi−0.02WMnWRu+5.68WMnWSn+18.4WMn−0.89WMn2+0.49WNiWRu+0.186WNi2+4.48WReWSn+1.15WRe2−5.11WRhWSn+2.04WRh2+0.885WRuWSn+1.28WRu2+1.58WSn+9.49WSn211.6≥140
preferably
0.041WAuWCo+0.122WAuWIn+1.96WAuWNi+1.87WAuWSn+0.903WAu2+1.74WCoWGa+13.4WCoWIn+1.24WCoWMn+5.04WCoWSn+1.02WCo2+8.97WCuWFe+1.74WCuWGa+4.38WCuWIn+1.16WCuWMn+0.491WCuWNi+3.69WCu WSn+0.22WCu2+1.68WFeWGa+3.31WFeWIn−1.26WFeWMn+5.07WFeWSn+0.199WFe2+5.35WGaWIn+0.086WGaWMn+3.27WGaWRe+33.3WGaWRh+4.56WGaWRu+2.21WGaWSn+29.0WGa+8.49WGa2+9.09WInWMn−0.28WInWNi+15.4WInWRh+0.992WInWRu+11.7WIn+6.68WIn2+0.863WIr2+5.73WMnWNi−0.02WMnWRu+5.68WMnWSn+18.4WMn−0.89WMn2+0.49WNiWRu+0.186WNi2+4.48WReWSn+1.15WRe2−5.11WRhWSn+2.04WRh2+0.885WRuSn+1.28WRu2+1.58WSn+9.49WSn211.6≥120
more preferably
0.041WAuWCo+0.122WAuWIn+1.96WAuWNi+1.87WAuWSn+0.903WAu2+1.74WCoWGa+13.4WCoWIn+1.24WCoWMn+5.04WCoWSn+1.02WCo2+8.97WCuWFe+1.74WCuWGa+4.38WCuWIn+1.16WCuWMn+0.491WCuWNi+3.69WCuWSn+0.22WCu2+1.68WFeWGa+3.31WFeWIn−1.26WFeWMn+5.07WFeWSn+0.199WFe2+5.35WGaWIn+0.086WGaWMn+3.27WGaWRe+33.3WGaWRh+4.56WGaWRu+2.21WGaWSn+29.0WGa+8.49WGa2+9.09WInWMn−0.28WInWNi+15.4WInWRh+0.992WInWRu+11.7WIn+6.68WIn2+0.863WIr2+5.73WMnWNi−0.02WMnWRu+5.68WMnWSn+18.4WMn−0.89WMn2+0.49WNiWRu+0.186WNi2+4.48WReWSn+1.15WRe2−5.11WRhWSn+2.04WRh2+0.885WRuWSn+1.28WRu2+1.58WSn+9.49WSn211.6≥100
even more preferably
0.041WAuWCo+0.122WAuWIn+1.96WAuWNi+1.87WAuWSn+0.903WAu2+1.74WCoWGa+13.4WCoWIn+1.24WCoWMn+5.04WCoWSn+1.02WCo2+8.97WCuWFe+1.74WCuWGa+4.38WCuWIn+1.16WCuWMn+0.491WCuWNi+3.69WCuWSn+0.22WCu2+1.68WFeWGa+3.31WFeWIn−1.26WFeWMn+5.07WFe WSn+0.199WFe2+5.35WGaWIn+0.086WGaWMn+3.27WGaWRe+33.3WGaWRh+4.56WGaWRu+2.21WGaWSn+29.0WGa+8.49WGa2+9.09WInWMn−0.28WInWNi+15.4WInWRh+0.992WInWRu+11.7WIn+6.68WIn2+0.863WIr2+5.73WMnWNi−0.02WMnWRu+5.68WMnWSn+18.4WMn−0.89WMn2+0.49WNiWRu+0.186WNi2+4.48WReWSn+1.15WRe2−5.11WRhWSn+2.04WRh2+0.885WRuWSn+1.28WRu2+1.58WSn+9.49WSn211.6≥80
most preferably
0.041WAuWCo+0.122WAuWIn+1.96WAuWNi+1.87WAuWSn+0.903WAu2+1.74WCoWGa+13.4WCoWIn+1.24WCoWMn+5.04WCoWSn+1.02WCo2+8.97WCuWFe+1.74WCuWGa+4.38WCuWIn+1.16WCuWMn+0.491WCuWNi+3.69WCuWSn+0.22WCu2+1.68WFeWGa+3.31WFeWIn−1.26WFeWMn+5.07WFeWSn+0.199WFe2+5.35WGaWIn+0.086WGaWMn+3.27WGaWRe+33.3WGaWRh+4.56WGaWRu+2.21WGaWSn+29.0WGa+8.49WGa2+9.09WInWMn−0.28WInWNi+15.4WInWRh+0.992WInWRu+11.7WIn+6.68WIn2+0.863WIr2+5.73WMnWNi−0.02WMnWRu+5.68WMnWSn+18.4WMn−0.89WMn2+0.49WNiWRu+0.186WNi2+4.48WReWSn+1.15WRe2−5.11WRhWSn+2.04WRh2+0.885WRuWSn+1.28WRu2+1.58WSn+9.49WSn211.6≥60
Such an alloy has lowered solidification range leading to better castability, particularly less porosity on casting.
In an embodiment the platinum alloy composition satisfies the following equation in which WCo, WCu, WFe, WNi, and WPd are the weight percent of cobalt, copper, iron, nickel and palladium in the alloy WCo+WPd+WFe+WNi+WCu≥2.0. Such an alloy has a lower melting point without dramatic increase in solidification range.
In an embodiment the two or more of gallium, indium and tin are present in an amount of 0.25 or more, preferably wherein the two or more of gallium, indium and tin are present in an amount of 0.5 or more. Such an alloy has increased hardness.
In an embodiment the platinum alloy composition satisfies the following equation in which WCo, WFe, and WNi, are the weight percent of cobalt, iron, and nickel in the alloy 3.0≤WCo+WFe+WNi≤4.5. Such an alloy, particularly in the absence of copper and palladium has reduced melting point without a corresponding increase in solidification range.
In an embodiment the platinum alloy composition satisfies the following equation in which WGa, WIn, and WSn, are the weight percent of gallium, indium and tin in the alloy 0.4≤WGa+WIn+WSn≤2.2. This gives an alloy with superior hardness without a corresponding increase in solidification range.
In an embodiment the platinum alloy composition is made up of 95 weight percent or more of platinum and satisfies the following equation in which WCo, WFe, and WNi, are the weight percent of cobalt, iron, and nickel in the alloy 3.0≤WCo+WFeWNi≤4.5 as well as the following equation in which WGa, WIn, and WSn, are the weight percent of gallium, indium and tin in the alloy 0.4≤WGa+WIn+WSn≤2.2. Such an alloy has a good balance of castability and hardness while still meeting the 950 platinum hallmarking requirements.
In an embodiment of the platinum alloy composition the two or more of gallium, indium and tin are indium and gallium and the following equation is satisfied in which WGa and WIn, are the weight percent of gallium and indium in the alloy 0.5 WIn≤WGa≤1.5 WIn. Such an alloy has reduced chance of intermetallic precipitation.
In an embodiment of the platinum alloy composition the two or more of gallium, indium and tin are tin and gallium and the following equation is satisfied in which WGa and WSn, are the weight percent of gallium and tin in the alloy 0.5 WSn≤WGa≤1.5 WSn. Such an alloy has reduced chance of intermetallic precipitation.
In an embodiment of the platinum alloy composition the two or more of gallium, indium and tin are indium and tin and the following equation is satisfied in which WIn, and WSn, are the weight percent of indium and tin in the alloy 0.5 WIn≤WSn≤1.5 WIn. Such an alloy has reduced chance of intermetallic precipitation.
In an embodiment of the platinum alloy composition the two or more of gallium, indium and tin are indium, tin and gallium and the following equations are satisfied in which WGa, WIn, and WSn, are the weight percent of gallium, indium and tin in the alloy 0.3 WIn≤WGa≤1.3 WIn; 0.3 WSn≤WGa≤1.3 WSn; 0.3 WIn≤WSn≤1.3 WIn. Such an alloy has reduced chance of intermetallic precipitation.
The term “consisting of” is used herein to indicate that 100% of the composition is being referred to and the presence of additional components is excluded so that percentages add up to 100%.
The invention will be more fully described, by way of example only, with reference to the accompanying drawings in which:
The hardness of platinum alloys is derived from two chemically determined mechanisms:
While it is desirable to add elements to platinum to increase the hardness, the additions of alloying elements can adversely affect the castability of the material in the following ways:
Superior castability is achieved in the present invention by optimising several material properties, reflected in merit indices. These include the melting point index and optionally the hot cracking index. Their values are correlated to the risk of typical casting defects: shrinkage porosity, gas porosity, formation of inclusions, poor surface finish and poor form filling.
Superior mechanical properties are achieved by improving wear resistance and gem setting ability by increasing hardness. The alloys in the invention have a tunable hardness and a hardness of 150 HV or more can be achieved, offering the potential to trade off the ease of formability for improved wear resistance, depending on the application.
In addition, the as-cast microstructures of the alloys can have sufficient ductility which facilitates further processing steps such as gem setting. The inventors have determined that this can be achieved if the composition has at least 0.3 volume fraction of ductile gamma Pt matrix at 1000° C. A higher level of gamma phase is desirable as this further increases the ductility. The as-cast microstructure may not be the same as the equilibrium microstructure at 1000° C.
A modelling-based approach used for the isolation of new grades of platinum alloys addressing at least some of the above issues is described here. This approach utilises a framework of computational materials models combined with machine learning to estimate design-relevant properties across a very broad compositional space. In principle, this alloy design tool allows the so-called inverse problem to be solved; identifying optimum alloy compositions that best satisfy a specified set of design constraints.
The first step in the design process is the definition of an elemental list along with the associated upper and lower compositional limits. The compositional limits for each of the elemental additions considered in this invention—referred to as the “alloy design space”—are detailed in Table 1. These limits were selected by the inventors on the basis of the explanations given below. Some of the insights are from metallurgical experience whilst others, such as the effects on melting point, castability of the platinum alloys and presence of intermetallic phases have been established by the inventors based on thermodynamic calculations described below on a wider range of compositions than set out in Table 1.
It is relatively easy to increase the hardness of pure platinum by adding alloying elements. However, doing this at the same time as maintaining good castability is not so easy. Among other things, it requires limiting any rise in solidification range of the alloy caused by introduction of alloying elements, avoiding harmful precipitation of intermetallic phases in the final stages of solidification, and avoiding the formation of slag caused by reactive elements, all of which cause casting defects. Some elements which would otherwise be suitable to meeting at least some of these conflicting requirements must be limited for other reasons. Bearing this in mind, the elements and their ranges in Table 1 were selected for the following reasons:
The minimum amount of platinum in the alloys in weight percent was set to 85.0 as this is a minimum acceptable amount of platinum for jewellery applications. In some cultures, any iridium content in a platinum alloy is considered equivalent to platinum meaning that the platinum content of an alloy is considered as being equivalent the sum of platinum and iridium. Preferably the minimum amount of platinum (or sum of platinum and iridium) in weight percent is 90.0 to adhere to internationally recognised standards for platinum jewellery for example 900 Pt (90 wt % platinum). Desirably a minimum amount of platinum (or sum of platinum and iridium) in weight percent is 95.0 to adhere to 950 Pt (95 wt % platinum).
Nickel, cobalt, copper, iron and manganese: all lower the melting point of pure Pt and increase hardness by a combination of solid solution and precipitation strengthening. In addition, they are relatively unreactive, meaning their alloys with Pt can be repeatedly remelted without appreciably changing the composition of the alloy due to reactions with the atmosphere, crucible walls or mould walls. Cobalt, manganese and iron amounts are limited in Table 1 because case additions beyond the ranges specified in Table 1 are unlikely to bring additional benefits as they may not appreciably reduce the melting point further, may result in an excessive fraction of intermetallic phases upon cooling of the casting or may increase the solidification range. Additionally high cobalt can render an alloy ferromagnetic which can cause fabrication issues, iron may cause undesirable ferromagnetism and can also form an intermetallic phase at high temperatures which is suspected to harm ductility and high manganese can evaporate from melt thereby causing processing issues. Each of these elements may independently of one another be limited to 10.0 wt % or less. For nickel, there is a concern about its use in jewellery applications because of allergy concerns. Therefore the amount of nickel is kept at 7.0 wt % or less, preferably 5.0 wt % or less or even 4.0 wt % or less. On the other hand nickel is particularly useful for the purposes given above for this group of elements and so nickel is preferably present in an amount of 3.0 wt % or more. Copper is limited to 10.0 wt % or less because it is relatively easily oxidised compared to the other elements in this group meaning that formation of slag during melting for casting is more likely. Slag formation is undesirable. Additionally, copper has been found to increase solidification range at higher concentrations, which is undesirable. Therefore, copper is preferably limited to 9.0 wt % or less, more desirably 8.0 wt % or less.
Gold has a small effect of reducing the melting temperature of platinum alloys at low concentrations and increases hardness. But gold increases solidification range and so is limited to 10.0 wt % or lower. Preferably gold is limited to 5.0 wt % or 3.0 wt % or less due to its adverse effect on solidification range. Preferably gold is absent in the alloy as its presence can hinder recyclability due to difficulty in separating it from platinum.
Palladium slightly increases hardness by a combination of solid solution and precipitation strengthening. Palladium has only a slightly reducing effect on the melting temperature and so must be supplemented by other alloying elements to reduce the melting point sufficiently (equation (1)). However, palladium is unreactive and so is limited in Table 1 only by the minimum required amount of platinum. Thus, in an embodiment the alloy contains palladium and at least two further alloying elements (two or more selected from tin, indium and gallium, as described below). In an embodiment, palladium is absent in the alloy as its presence can hinder recyclability due to difficulty in separating it from platinum (in which case one or more of cobalt, iron, nickel and copper is present, as described below). In addition, palladium is currently more than twice the cost of platinum and is therefore undesirable as it increases the cost of the alloy. Furthermore, platinum-palladium alloys are known to be poorly workable—the palladium content is therefore preferably reduced to 5.0wt % or less when good workability is to be achieved. In an embodiment the alloy is essentially palladium free (i.e. consists of 0.0 wt % or less).
Rhodium, iridium, ruthenium and rhenium: These elements are very noble meaning their alloys with Pt can be repeatedly remelted without appreciably changing the composition of the alloy due to reactions with the atmosphere, crucible walls or mould walls. In addition, they increase hardness by solid solution strengthening. However, excessive additions of iridium and/or rhodium and/or rhenium may appreciably increase the cost of the alloy and raise its melting point which adversely affects its castability. Therefore, the amounts of rhodium, iridium and rhenium are limited to 5.0 wt % or less each, preferably 3.0 wt % or less each. Ruthenium may result in excessively poor castability and so is limited to 10.0 wt % or less, preferably 5.0 wt % or less or even 3.0 wt % or less. In an embodiment one or more of rhodium, iridium, rhenium and ruthenium are absent in the alloy as their presence can hinder recyclability due to difficulty in separating them from platinum.
Tin, indium, gallium: all lower the melting point of pure Pt significantly and strongly increase hardness by precipitation strengthening. However, excessive additions may result in an excessive fraction of intermetallic phases upon cooling of the casting or may excessively increase the solidification range. Thus the amount of indium is limited to 3.0 wt % or less (preferably 2.0 wt % or less or even 1.0 wt % or less), and the amount of tin is limited to 3.0 wt % or less (preferably 2.0 wt % or less or even 1.0 wt % or less). The amount of gallium is limited to 4.0 wt % or less (preferably 3.0 wt % or 2.0 wt % or less). Indium and tin have been found particularly effective and so preferably indium is present in an amount of 0.5 wt % or more and/or tin is present in an amount of 0.5 wt % or more.
In addition to the ranges in Table 1 the alloys in the invention may also contain small amounts of other elements, as incidental impurities. These include titanium, aluminium, chromium, zinc, yttrium, hafnium, zirconium, vanadium, niobium, tantalum, molybdenum, tungsten, silver, scandium, any lanthanide, germanium. Total incidental impurities make up 1.0 wt % or less of the alloy, preferably 0.5 wt % or less of the alloy. Any single impurity element is present at a level of 0.5 wt % or less, preferably 0.25 wt % or less or even 0.1 wt % or less. Many of these elements are highly reactive and may reduce castability and/or lead to the formation of intermetallic precipitates which in large quantities can lead to brittleness and cracking at grain boundaries which reduces ductility. In an embodiment aluminium and/or chromium and/or titanium may be effectively absent.
The second step relies upon thermodynamic calculations used to calculate the phase diagram and thermodynamic properties for a specific alloy composition. Often this is referred to as the CALPHAD method (CALculation of PHAse Diagrams).
A third stage involves isolating alloy compositions which have the desired properties as calculated in the second step. The candidate alloys in the investigated composition space were selected based on the various merit indices indicative of the two targets: good castability and good mechanical properties.
The merit indices for castability are:
The merit indices for mechanical properties are:
Using the above described methods in merit indices were calculated for a range of common jewellery alloys shown in Table 2. The relevant property values predicted by the method for these alloys are shown in Table 3 and were used as guides for alloy selection.
Table 3 shows that all benchmark alloys except Pt1.5In3Ga and Pt5Ni have a melting point index above 16.6 indicating a high melting point. High melt temperatures during casting are known to exacerbate metal-mould reactions, leading to increased shrinkage and gas porosity, increased risk of inclusions and investment cracking and contamination with alloy-mould reaction products. An alloy with a lower melting point also allows for a higher superheat relative to one with a higher melting point. A higher superheat increases fluidity and improves the form filling characteristics of the alloy. A platinum alloy with a low melting point is therefore desirable from the point of view of castability. Pt1.5In3.0Ga has a large solidification range (due to the high level of gallium) as well as a gamma phase fraction of below 1.0, meaning that the possibility of intermetallic phase precipitation is increased.
No single alloy is the best at everything. For example, the popular Pt5Co alloy is known to produce high-quality castings but can be easily magnetised, which can cause manufacturing problems. Another popular example is the Pt5Ru alloy which is non-magnetisable but suffers from casting porosity and poor form filling. Manufacturers also prefer hardness values of at least 175 HV. Apart from Pt1.5In3Ga, none of the other benchmark alloys meeting the 950 Pt hallmark fit this requirement in an as-cast condition. Even for benchmark alloys meeting the 900 Pt hallmark, only Pt10Ru and Pt10Au meet the hardness requirement without subsequent post-processing steps (aging or work hardening).
Based on calculations made on a wider range of composition than shown in Table 1, the effect of each of the alloying elements on the various merit indices was determined. With this knowledge, the bounds in Table 1 were created. For alloys within the composition range in Table 1, the thermodynamic calculations of step 2 were used to calculate the melting points, the solidification range, the hot cracking index, the porosity index and the phase constitution of thousands of alloy compositions within the range. The results were also used in calculating hot cracking index, porosity and shrinkage.
The alloy selection procedure was based on several guidelines: first, the main metric for improved castability is a low melting point index. A melting point index of below 16.0 results in an alloy with a significantly lower melting point than any of the prior art alloys in Table 3. Such a low melting point index is harder to achieve for the high platinum containing alloys and for such alloys (i.e. with a Pt content of 95.0 wt % or more), the melting point index requirement is relaxed to 16.6 or less. This is acceptable because for high value jewellery (i.e. high Pt content), some machining (e.g. polishing) after casting is acceptable and there may be more limitations on the alloying elements which can be used considering potential requirements for high hardness and low solidification range. Second, for improved mechanical properties, hardness values should exceed 150 HV but should be tuneable as hardness requirements may vary depending on the application.
On the basis of these results, the following equation was derived which expresses the melting point index as a function of composition. The equation is derived from results of alloys falling within the range of Table 1. Therefore, for a platinum alloy falling within the compositional range of Table 1 and in which equation (1) below is 16.0 or less, such an alloy will have superior castability compared to the prior art alloys of Table 2 and will also likely have higher hardness. For an alloy with a platinum content of 95.0 wt % or more and for which equation (1) below gives a value of 16.6 or less (or even 16.4 or less or 16.2 or less (all of the example alloys of the invention in Table 4 meet this harshest criterion)), the castability will be improved compared to all of the prior art alloys of Table 2 with a similar high platinum composition except Pt1.5In3.0Ga and Pt5Ni. However, these alloys suffer from very high solidification range (due to excessive Ga) and low hardness, respectively.
−0.1028*WCo−0.1201*WCu−0.2113*WFe−0.3368*WGa−0.1125*WIn−0.1639*WNi−0.015*WPd−0.1959*WSn+17.276261−0.20*WMn+0.0678*WRu+0.035*WIr+0.045*WRh−0.059*WAu+0.066*WRe (1)
in which WCo, WCu, WFe, WGa, WIn, WNi, WPd, WSn, WMn, WRu, WIr, WRh, WAu and WRe are the weight percent of cobalt, copper, iron, gallium, indium, nickel, palladium, tin, manganese, ruthenium, iridium, rhodium, gold and rhenium in the alloy respectively.
Alloy compositions exist within the range of Table 1 with an even higher melting point index. Preferably irrespective of the platinum content of the alloy, equation (1) is 16.0 or less. Alloys with lower values of equation (1) are preferred, particularly those in which equation (1) is 15.5 or less, more preferably 15.0 or less and most preferably 14.5 or less.
The hardness of a platinum alloy can be approximated according to the following equation (2) based on experimental results:
60+WPd*2.5+WRh*3.4+WIr*6.455+WAu*11.93+WRu*13.241+WCu*14.328+WRe*16.6+WIn*16.9+WMn*18.48+WCo*18.69+WFe*21.879+WIn*29+WSn*28.207+WGa*42.379 (2)
in which WCo, WCu, WFe, WGa, WIn, WNi, WPd, WSn, WRh, WIr, WAu, WRu, WRe, and WMn are the weight percent of cobalt, copper, iron, gallium, indium, nickel, palladium, tin, rhodium, iridium, gold, ruthenium, rhenium, and manganese in the alloy respectively.
If equation (2) is greater than or equal to 150 (as is achievable by alloys of the present invention as shown in Table 5 below), the hardness of the platinum alloy will be acceptable. Preferably, equation (2) is greater than or equal to 200, preferably 225, more preferably 250, even more preferably 275 or most preferably 300, in which case an alloy with a correspondingly increased hardness results and this may be preferred for certain applications. Those skilled in the art of jewellery know hardness should be high enough to improve wear resistance, and facilitate gem setting as well as polishing, but if hardness is too high, softer gemstones risk getting damaged during setting. The composition space allows a wide hardness range and alloys in the invention have a hardness of at least 150 HV. When setting hard gemstones such as diamonds or making gemstone-free jewellery, harder alloys may be preferred. In some embodiments lower hardness is preferred (for example to facilitate gem setting), so that preferably equation (2) is less than or equal to 280, more preferably less than or equal to 260, and most preferably less than or equal to 240.
Solidification range: excessive range often results in solidification defects. The alloy preferably has a solidification range of less than 200° C., preferably the alloy composition has a solidification range of 150° C. or less. More preferably wherein the alloy composition has a solidification range of 125° C. or less, more preferably wherein the alloy composition has a solidification range of 100° C. or less, even more preferably wherein the alloy composition has a solidification range of 75° C. or less, most preferably wherein the alloy composition has a solidification range of 50° C. or less. The inventors have found ensuring that the following equation is satisfied: 0.35 WAu+0.6 WSn+0.6 WIn+WGa≤3.75 helps to produce an alloy with a lower solidification range. Preferably 0.35 WAu+0.6 WSn+0.6 WIn+WGa≤3.0. As described below, equation (4) must be fulfilled so that the desired melting point can be achieved without increasing the solidification range excessively. The complicated interaction between elements means that it has not been possible for the inventors to derive an accurate equation using simple ratios of all the elements in Table 1 defining the solidification range from the thermodynamic data. However, solidification range can be measured experimentally by differential scanning calorimetry. The difference (in Kelvin) between the onset and end of the exothermic peak associated with the phase transition from liquid to solid upon slow cooling (at the rate of 10 K/min or less) is defined as the solidification range. The below Table gives the solidification range of various binary Pt alloys at 95 wt % Pt and shows the relative effects of a selection of alloying elements from which the skilled person can see which elements are most likely to increase the solidification range. Note, however, that the values in the Table were derived from binary phase diagrams of those elements with platinum—they represent solidification ranges when the system is near thermodynamic equilibrium throughout solidification. Because of kinetic effects, however, solidification ranges measured experimentally are always larger than equilibrium ranges.
In an embodiment, the alloy consists, in weight percent, of: 0.0 to 5.0 cobalt, 0.0 to 10.0 copper, 0.0 to 7.0 iron, 0.0 to 4.0 gallium, 0.0 to 3.0 indium, 0.0 to 7.0 nickel, 0.0 to 15.0 palladium, 0.0 to 3.0 tin, 85.0 or more platinum and incidental impurities. Such alloys are preferred because gold and manganese have low effect on melting point and hardness, but increase the solidification range and iridium, rhenium, rhodium and ruthenium tend to increase the melting point. Thus additions of those elements are not as powerful as additions of the other elements and so their use is less preferred, given the low amounts of alloying element which can be used (in order to keep the platinum content high to meet hallmarking requirements). However, a small amount (up to 1.0 weight percent in sum) of the elements gold, iridium, manganese, rhenium, rhodium, and ruthenium can be contained in such an alloy.
The following equation (3) was derived which expresses the solidification range index as a function of composition and is valid for alloys falling within the range of Table 1.
0.041WAuWCo+0.122WAuWIn−1.96WAuWNi+1.87WAuWSn+0.903WAu2+1.74WCoWGa+13.4WCoWIn+1.24WCoWMn+5.04WCoWSn+1.02WCo2+8.97WCuWFe+1.74WCuWGa+4.38WCuWIn+1.16WCuWMn+0.491WCuWNi+3.69WCuWSn+0.22WCu2+1.68WFeWGa+3.31WFeWIn−1.26WFeWMn+5.07WFeWSn+0.199WFe2+5.35WGaWIn+0.086WGaWMn+3.27WGaWRe+33.3WGaWRh+4.56WGaWRu+2.21WGaWSn+29.0WGa+8.49WGa2+9.09WInWMn−0.28WInWNi+15.4WInWRh+0.992WInWRu+11.7WIn+6.68WIn2+0.863WIr2+5.73WMnWNi−0.02WMnWRu+5.68WMnWSn+18.4WMn−0.89WMn2+0.49WNiWRu+0.186WNi2+4.48WReWSn+1.15WRe2−5.11WRhWSn+2.04WRh2+0.885WRuWSn+1.28WRu2+1.58WSn+9.49WSn2+11.6 (3)
in which WCo, WCu, WFe, WGa, WIn, WNi, WPd, WSn, WMn, WRu, WIr, WRh, WAu and WRe are the weight percent of cobalt, copper, iron, gallium, indium, nickel, palladium, tin, manganese, ruthenium, iridium, rhodium, gold and rhenium in the alloy respectively. Preferably the value of equation (3) is equal to or less than 140. Such an alloy has improved castability due to lower solidification range. Preferably the value of equation (3) is equal to or less than 120 or even equal to or less than 100, or even equal to or less than 80 and most preferably equal to or less than 60.
The alloys in the invention are characterised by a combination of sufficient hardness as well as a narrow solidification range and a low melting point, whose combination leads to good castability. Two distinct groups of alloying elements can be identified: those which improve castability by lowering the melting point and not excessively increasing the solidification range (Ni, Co, Fe, Pd and Cu) and those which increase hardness and decrease the melting point but decrease castability by increasing the solidification range (Sn, In, Ga). The alloys in the invention need at least one of the elements from each group in order to satisfy both criteria according to equations
W
Co
+W
Pd
+W
Fe
+W
Ni
+W
Cu≥1.0 (4)
W
Sn
+W
In
+W
Ga≥0.25 (5)
Preferably WCo+WPd+WFe+WNi+WCu≥2.0 as such an alloy has lower melting point without a large increase in solidification range.
It was also found that having two or more elements from the group of Sn, In and Ga is beneficial in reducing the amount of intermetallic phases formed in interdendritic regions in the last stages of solidification (‘terminal intermetallics’). Because of their brittleness and coarse morphology, terminal intermetallics reduce the ductility of castings and adversely affect subsequent forming and machining. Their fraction should therefore be reduced.
As can be seen from
It is expected that using more than one of In, Ga or Sn to reduce the fraction of terminal intermetallics holds more generally for the alloys in this invention. The underlying physical reason is likely that for a given weight percent of alloying elements (e.g. 10%), solid solution is thermodynamically more favourable when the number of alloying elements increases.
As can be seen, alloys with indium and tin, of the group of gallium, indium and tin, two of which are compulsory, have lower levels of terminal intermetallics than alloys without those two elements. Therefore preferably indium and tin are the two elements of gallium, indium and tin which are present, where reduced intermetallic precipitation is preferred over other properties.
By mixing the elements of gallium, indium and tin present in amounts close to being equal, the amount of intermetallic can be reduced compared to the case where only small amounts of the second or third of those three elements are present. Thus in the case that the two or more of gallium, indium and tin are indium and gallium, preferably the following equation is satisfied in which WGa and WIn, are the weight percent of gallium and indium in the alloy
0.5WIn≤WGa≤1.5WIn.
In the case that the two or more of gallium, indium and tin are tin and gallium preferably the following equation is satisfied in which WGa and WSn, are the weight percent of gallium and tin in the alloy
0.5WSn≤WGa≤1.5WSn.
In the case that the two or more of gallium, indium and tin are indium and tin preferably the following equation is satisfied in which WIn, and WSn, are the weight percent of indium and tin in the alloy
0.5WIn≤WSn≤1.5WIn.
In the case that the two or more of gallium, indium and tin are indium, tin and gallium preferably the following equations are satisfied in which WGa, WIn, and WSn, are the weight percent of gallium, indium and tin in the alloy
0.3WIn≤WGa≤1.3WIn
0.3WSn≤WGa≤1.3WSn
0.3WIn≤WSn≤1.3WIn.
Hot cracking: excessive values often result in solidification defects, particularly cracking. The Pt10Au benchmark alloy has the largest index value yet it is not excessively prone to cracking during solidification. Hot cracking index was therefore limited to somewhat less than the hot cracking value of Pt10Au. The following equation (6) was derived which expresses the hot cracking index as a function of composition. The equation is derived from results of alloys falling within the range of Table 1. Therefore, for a platinum alloy falling within the compositional range of Table 1 and in which equation (6) below is 4.0 or less, such an alloy will have superior castability compared to Pt10Au. Preferably the hot cracking index of equation 3 is 3.941 or less. All examples of the invention in Table 4 fall within that range.
−0.1*WCo−0.4933*WCu−0.32*WFe−1.16377*WGa−0.54278*WIn−0.08612*WNi+0.06915*WPd−0.69928*WSn+4.169+0.04*WMn+0.133*WAu (6)
wherein WCo, WCu, WFe, WGa, WIn, WNi, WPd, WSn, WMn and WAu are the weight percent of cobalt, copper, iron, gallium, indium, nickel, palladium, tin, manganese and gold in the alloy respectively.
Even more preferably, the hot cracking index is 3.5 or less, or even 3.0 or less, or even 2.5 or less. Thus equation (6) is preferably 3.5 or less, or even 3.0 or less or most preferably 2.5 or less.
Many alloying elements which were investigated, particularly some first-row transition metals (e.g. Fe) and some p-block elements (e.g. Ga, Sn), may form large quantities of intermetallic precipitates which can reduce ductility. In nickel superalloys which are metallurgically similar, the maximum allowable precipitate volume fraction is some 0.7, ensuring there is enough ductile matrix phase to prevent brittleness. Based on this insight, the minimum equilibrium volume fraction of the ductile gamma matrix phase at 1000° C. is desirably 0.3, which is achieved by many of the examples. Preferably the alloy composition comprises 0.55 or more volume fraction gamma phase at 1000° C., preferably 0.6 or more volume fraction gamma phase, more preferably 0.7 or more volume fraction gamma phase, even more preferably 0.8 or more volume fraction gamma phase, most preferably 0.9 or more volume fraction gamma phase. Results of thermodynamic calculations have not allowed a simple equation to be derived which predicts the volume fraction gamma prime based on the composition. However, composition can be determined experimentally by the following method. After a substantially long thermal exposure (for example 200 h) at 1000° C. the specimen is quenched in water, a section is taken through the material and polished using conventional/standard metallurgical preparation techniques for scanning electron microscopy. Once prepared, the microstructure should be observed in a scanning electron microscope. A minimum of 10 images should be taken which provide a statistically representative dataset. The images should cover an area of at least 1 mm2. The 2-dimensional images which reveal the microstructure, should be processed to identify all the precipitate particles larger than 30 nm, and their area fraction should be measured. Their combined area fraction is complementary to the fraction of the gamma matrix phase with the relationship M=1−P, where P is the volume fraction of precipitate phases and M is the volume fraction of the gamma matrix phase. The measured area fractions correspond directly to volume fractions.
The formation of intermetallic ordered phases directly from the melt produces large brittle grains which severely reduce ductility. To further ensure sufficient ductility, the alloys preferably are those which first form gamma phase on cooling from liquid. In the gamma phase all alloying elements are in solid solution. This can be determined experimentally by following the procedure similar to the one in the paragraph above. The procedure is modified by instead measuring the area fraction of any simple polygonal precipitates whose orientation is random and whose largest diameter exceeds 0.2 mm. If their area fraction exceeds 0.025, this is indicative of excessive intermetallic precipitation directly from the melt.
In an embodiment the condition 2.5 WIr+3 WRu≤7.5 is preferably met in order to achieve a low melting point.
The results show that if the following Equation 5 is satisfied, that a lower melting temperature and a narrow solidification range of can be achieved in which WCo, WFe, WNi, and WPd are the weight percent of cobalt, iron, nickel, and palladium in the alloy respectively. This is particularly the case when other alloying elements are present in small amounts or are absent.
(WCo+WPd)/11+(WFe+WNi)/2.2≥1.0 (7)
In an embodiment the alloy consists of platinum, cobalt, iron, nickel and palladium, with any other elements being present in a sum amount of 1.0 wt % or less (preferably 0.5 wt % or less) and any individual other element present in an amount of 0.5 wt % or less (preferably 0.25 wt % or less). This is advantageous because Fe and Ni lower the melting point more effectively than Co and Pd so more of the latter is needed to achieve the same effect.
The most favourable combination of properties have been found when the following equation in which WCo, WFe, and WNi, are the weight percent of cobalt, iron, and nickel in the alloy is satisfied 3.0≤WCo+WFe+WNi≤4.5, particularly in the absence of copper and palladium. Such an alloy has reduced melting point without a corresponding increase in solidification range. Additionally or alternatively a good combination of properties is achieved when the following equation in which WGa, WIn, and WSn, are the weight percent of gallium, indium and tin in the alloy is fulfilled 0.4≤WGa+WIn+WSn≤2.2. This gives an alloy with superior hardness with a limited corresponding increase in solidification range. In an embodiment the platinum alloy composition is made up of 95 weight percent or more of platinum and satisfies the following equation in which WCo, WFe, and WNi, are the weight percent of cobalt, iron, and nickel in the alloy 3.0≤WCo+WFe+WNi≤4.5 as well as the following equation in which WGa, and WSn, are the weight percent of gallium, indium and tin in the alloy 0.4≤WGa+WIn+WSn≤2.2. Such an alloy has a good balance of castability and hardness while still meeting the 950 platinum hallmarking requirements.
Table 4 gives several example alloy compositions which fall within the present invention. That is, all alloy compositions fall within the range of compositions of Table 1. All values are in weight percent.
Table 5 shows the main properties achieved by these examples, namely gamma fraction as predicted by the thermodynamic model, hardness as predicted by the above-mentioned equation, hot cracking index as indicated by the abovementioned fit, melting point index as obtained by the above-mentioned fit, and solidification range as measured by the thermodynamic model. As can be seen, the hot cracking index of the range of alloys compares favourably to the hot cracking index of many of the prior art alloys. Although the thermodynamic calculations indicate that some alloys have high gamma fraction and the hardness predication indicates that such alloys have high hardness, it is thought that this is due to the formation of precipitates below 1000° C.
As can be seen from Table 3, achieving low melting temperature in a platinum alloy is difficult with the presence of only one of the commonly used alloying elements. As shown in Tables 4 and 5, the examples of the present invention achieve the desired low melting point and high hardness by incorporating two or more, preferably three or more of the gallium, indium and tin and one or more of cobalt, palladium, iron, nickel and copper.
Casting trials demonstrated that examples from this invention (Example 0) indeed have a better combination of castability and hardness than Comparative examples 1 to 3. The alloys were investment cast into preheated ceramic moulds. The shape of the castings is shown in
Machinability trials on showed that examples from this invention (Example 0) have superior machinability compared to the commonly used Comparative example 2. A CNC rig using tungsten carbide tooling was used to machine a groove for gem setting in a simple ring. The comparison is shown in
Results from additional casting trials on complex ring design are shown in
The alloy of example 0 is particularly promising and has excellent properties. A particularly preferred alloy consists, in weight percent, of 0.0 to 2.0 indium, 0.0 to 5.0 nickel and 0.0 to 2.0 tin, with the balance being platinum and incidental impurities, preferably 0.5 to 1.0 indium, 3.0 to 4.0 nickel and 0.5 to 1.0 tin.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015742.6 | Oct 2020 | GB | national |
| 2103613.2 | Mar 2021 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2021/052542 | 10/1/2021 | WO |
