Shape Memory Alloys(SMA)

Liliana W. 2/8/2022

Imagine The Future

Memory wire

Memory wire is a strong wire that keeps its shape and is nowadays most commonly used in Jewelry making. Memory wire is a rigid and tarnish resistant material. It expands and contrasts to fit a variety of sizes. The one downside to this material is that because of all its properties, it's more expensive than other metals.

The first series of memory wire was produced by the Naval Ordnance Laboratory. When making quick and easy projects memory wire is used because its characteristics keep the design in place after completion. It was also created as a way for objects and materials to change and keep shape in different temperatures for different reasons depending on the project.

Memory wire is one of the easiest stringing materials. It's a hard temper, holds its shape and is incredibly easy to finish off as well. The company that's in the USA that produces memory wire is Beadalon and they produce two types of memory wire. There's a remembrance wire that is the highest quality available. It's a stainless steel wire wrapped into coils and is sized for jewelry making. The second type is steel memory wire. It's a carbon steel wire that can come in multiple colors most commonly used: silver, antique brass and gold. It's more ideal when you want the shimmer or shine of the plated wire.

Shape-memory alloys remember the shape that it was originally set into. So if an object made of this material is heated up into an unrecognizable shape, when it's cooled it then returns to its original shape before heating. The shape-memory effect is “programmed to remember the mold an object is placed into. This process takes place through heat/thermal energy. So memory wire can remember different shapes in very different temperatures ( hot to cold and vice versa). Memory wire demonstrates great levels of superelasticity.

When you bend an object made from shape-memory alloys and it deforms, its internal crystalline structure is what is changing. The molecules are rearranging themselves in a completely reversible way (aka solid-state phase change). When heat is applied to the crystalline structure it changes into a different form and can revert back. Materials can take a number of forms in crystalline form when it is in martensite state. In austenite form there's only one form. This form is also the most stable because it's the one with the lowest energy state. Shape-memory alloys go back and forth between austenite and martensite states. At lower temperatures, they take the form of martensite - soft, plastic, easy to shape- at a (very specific) higher temperature, they transform into austenite, which is a harder material and much more difficult to deform.

Heat is the property that determines the shape of the metal so it's the 1st property used for manipulation for formation. Transition from martensite to austenite is ONLY dependent on temperature and stress. Not the factor of time, which most phase changes require because there's no diffusion included. Martensite is in colder temperatures and when heat is added it changes to an austenite phase. The same works backwards. From an austenite phase, once the object is coles it will return to a martensite phase.

Sometimes, shape-memory alloys remember one shape when they're hot and a different one when they're cold. This is known as the two-way shape-memory effect. Pseudo-elasticity or superelasticity is when a shape-memory object can bend and twist by a large amount but can still return perfectly to its original shape (even without heating).

Gauges

Diameters are given in ranges to account for the variations during the manufacturing process. The thickness of the wire is measured using the AWG scale. Also known as the American Wire Gauge. A lower gauge is a thicker wire. For example a 16 gauge is considered too heavy for some uses so it's more for decoration. A higher gauge means thinner wire. As an example a 24 gauge is considered too thin for free forming shapes.

Works Cited

How Shape Memory Alloys Work, depts.washington.edu/matseed/mse_resources/Webpage/Memory metals/how_shape_memory_alloys_work.htm.

By: Laura Piantanida, Editor. “What Is Memory Wire?” AllFreeJewelryMaking.com, 28 Sept. 2018, www.allfreejewelrymaking.com/Hints-and-Tips/What-is-Memory-Wire.

Chris Woodford. Last updated: February 13. “How Do Shape-Memory Materials Work?” Explain That Stuff, 13 Feb. 2021, www.explainthatstuff.com/how-shape-memory-works.html.

Dunning, David. “Memory Wire Size & Gauge Information.” Sciencing, 31 Dec. 2020, sciencing.com/memory-wire-size-gauge-information-7847888.html.

“Home.” Beadalon, www.beadalon.com/products/memory_wire.asp.

“Memory Wire.” Memory Wire - BeadStuff.com, shopsite.hypermart.net/ss12.3/sc/ss_mb.cgi?storeid=*1614e3e88a8940c20751bb4fc8&ss_parm=Adc130556b88620b64377b9425166168a.

“Shape-Memory Alloy.” Wikipedia, Wikimedia Foundation, 18 Jan. 2022, en.wikipedia.org/wiki/Shape-memory_alloy.