Inquarting Gold with Aluminum if not intending to recover?/Chloroauric Acid from Hydrochloric acid and Hydrogen Peroxide?

[Ultrax]

As I already wrote in the previous post, it is possible to inquart gold with aluminum, but you will end up with a “purple plague” because of interdiffusion, and after the dissolution of the intermetallic compound in the presence of Cl(-), there will be losses of gold during precipitation process. It is almost impossible to completely remove all aluminum ions from the intermetallic compound with zero(!) losses of gold.

 

[Ultrax]

If I'm not mistaken these blue and purple Gold alloys are not workable at all.
You can cast them and grind them, but that is it.

All AuX2 gold intermetallic compounds have a crystalline structure and are therefore brittle.

 

[JimEarns]

I really enjoy watching this guy . He explains the reactions and stoichiometry along the way for those of us that need that kind of thing. Does anyone have any insight into why the annealing was necessary. What change it effected on the surface crystyline structure to homologize the color?

Gold alloys are annealed to mitigate shattering breakage or cracks after cooling and quenching. It also provides a more consitent alloy to work when sanding and polishing.

 

[Ultrax]

I really enjoy watching this guy . He explains the reactions and stoichiometry along the way for those of us that need that kind of thing. Does anyone have any insight into why the annealing was necessary. What change it effected on the surface crystyline structure to homologize the color?

This AuAl2 intermetallic effect is called "purple plague" in microelectronics. During intermetallic cooling, AuAl2 reduces its volume and creates cavities between Au and Al particles. That's why there always will be cavities on the AuAl2 surface, even with vacuum melting and a controlled cooling process. But cavities can be small enough for jewelry after polishing.

 

[Yggdrasil]

This AuAl2 intermetallic effect is called "purple plague" in microelectronics. During intermetallic cooling, AuAl2 reduces its volume and creates cavities between Au and Al particles. That's why there always will be cavities on the AuAl2 surface, even with vacuum melting and a controlled cooling process. But cavities can be small enough for jewelry after polishing.

Yes, but is it ductile enough to be worked with?
Either the blue In/Au alloy, the purple Al/Au alloy or both was considered unworkable due to brittleness.

 

[orvi]

Yes, but is it ductile enough to be worked with?
Either the blue In/Au alloy, the purple Al/Au alloy or both was considered unworkable due to brittleness.

Yup, it is brittle. Not to the touch, but I can imagine that dropping eg a ring from few meters to the concrete surface can crack it. Most of the sold jewellery has the purple gold just as decor in it. It isn't subjected to literally any forces.

 

[billgold]

Yeah, the commercial item NR bought had the purple gold set like a fragile stone. It would not surprise me if the commercial item was backed with something to pad it a bit, like a thin piece of paper or cardboard.

I am guessing that the purple gold has several crystaline phase changes, apparently one that is a least temporary after cooling down. If so, there is likely next to no research on those phases, like how broad the temperature range is - or how permanent the changes are.

 

[goldshark]

Can this stuff be annealed, to soften and give a different metallurgical property?

 

[Ultrax]

Unfortunately, no. All AuX₂ intermetallic compounds are brittle and can't be forged.

 

[Ultrax]

If so, there is likely next to no research on those phases, like how broad the temperature range is - or how permanent the changes are.

All research on purple gold or strictly speaking "purple plague" was accomplished at least 60 years ago. Specifically, in microelectronics for all possible temperature conditions because AuAl₂ is a real problem in reliable soldering processes between aluminum and gold contact surfaces inside microchips.

A study of purple plague and its role in integrated circuits (1964)