Spangold A shape-memory effect alloy with novel applications

Spangold, a 14-karat gold alloy containing 10% Cu and 5% Al, was invented by Wolff and Cortie [1], [2] in the Physical Metallurgy Division of MINTEK in Randburg, South Africa in the early 1990s. Spangold is a shape-memory effect (SME) alloy with a novel application. Its purpose was to use the SME to create a novel surface pattern for jewelry applications. Deon Sanders of MINTEK, and the inventors, gave me an as-cast specimen of Spangold as I had done work with other SME alloys. I decided to see if I could form some martensite by the SME when I hot compression mounted the specimen in a metallurgical mounting press. After hot mounting, I polished the surface and examined it using crossed polarized light and I could see some evidence of martensite that was created. Next, I used the traditional method on the polished specimen by heating it in boiling water, holding it for a short time and then quenching it in cold water. This created new martensite with surface “rumpling” or “spangles” at the free surface due to the volumetric expansion at the free surface due to the usual volumetric expansion associated with the austenite-to-martensite transformation. The original martensite is fainter in appearance and when the new, vivid martensite crosses the original martensite, we observe what is called “anti-spangles.” The image was photographed using Nomarski differential interference contrast (DIC) illumination to best reveal the surface topography.

The best-known shape memory alloy (SMA) is Nitinol, which is equal parts of Ni and Ti on an atomic weight basis. It was invented in 1963 at the US Naval Ordnance Laboratory when the SME was discovered in an equiatomic alloy of Ni and Ti that they called Nitinol. Nitinol has been found to be the most commercially useful of all SMAs. It has two temperature-dependent crystal structures (phases), martensite at low temperatures and austenite at higher temperatures. Austenite is the “parent” phase. Ironically, martensite in Nitinol is soft and ductile and easily deformed while austenite in Nitinol is rather strong and hard – the exact opposite of these phases in steels. Nitinol was first used in the 1970s in biomedical applications, one of the most important being for orthodontic arch wires, which worked far better than anything used before; its use as stents to open arteries did not begin until the 1990s.

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Further reading:

[1]Ira M. Wolff, Michael B. Cortie

Gold Bull., 27 (2) (1994), pp. 44-54

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[2]US Patent 5,503,691, Ira M. Wolff, Michael B. Cortie, The Aesthetic Enhancement or Modification of Articles or Components Made of Non-ferrous Metals, granted April 2, 1996. (Also, European Patent EP 0 569 239 A1, application published November 10, 1993.

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DOI: 10.1016/j.mattod.2019.06.004