The intermetallic equiatomic binary alloy NiTi forms the basis of the nickel-titanium shape memory alloy (SMA) family. It has a moderate solubility range enabling changes in composition and alloying with other elements to modify both shape memory and mechanical properties.
It displays common shape memory behaviour, returning to its original shape after being heated to above its phase transformation temperature. Heating to this temperature sees the alloy change from its low temperature monoclinic martensitic structure to the high temperature cubic austenitic structure.
Manufacture of Nickel-Titanium Alloys
Nickel-titanium alloys can be manufactured using a number of different techniques. These include vacuum melting techniques such as:
- Electron beam melting
- Vacuum arc melting
- Vacuum induction melting
- Plasma arc melting
Vacuum techniques are preferred due to the high reactivity of titanium, although inert atmospheres can also be used successfully.
Ingots are usually hot worked between 700 and 900°C as the next stage on the way to wire, bar or sheet formation. Processes such as press forging, rotary forging, bar rolling and extrusion are used for tis process. Alloys can be hot worked in air as they do not react readily with air.
Most cold working techniques can also be used, of which wire drawing is probably the most common, producing diameters as small as 0.05mm with excellent surfaces. However, work hardening occurs readily and constant annealing is usually required.
Deformation stress can be reduced by work hardening and suitable heat treatment. This process can also improve the strength of the austenitic phase as well as giving the alloy two way shape memory effect.
Welding of nickel-titanium alloys is possible but tends to produce brittle joints. Brittleness can be reduced by stress relieving, but degradation of the shape memory effect may occur due to the high temperatures that are required. Welding should also be carried under inert gases to prevent passive oxide buildup.
Like welding, passive oxide buildup can be problematic when soldering nickel-titanium alloys. While being difficult, it can be done using a Sn – 3.5Ag solder with an aggressive flux. Alternatively, plating with nickel or gold prior to soldering can result in good solder joints.
Nickel-titanium alloys can be joined using adhesives such as cyanoacrylates, epoxies, etc. Mechanical methods such as crimping and swaging have also been successful. The shape memory effect itself can be used and has been employed commercially in couplings.
Machining is possible using conventional methods, but high wear are not uncommon so carbide tools are recommended.
Other techniques such as electrodischarge machining and electro polishing can be used under the right conditions.
Coating and Plating
Coating and plating of nickel-titanium alloys is possible, but good adhesion is necessary to allow for the high strains encountered in some applications.
Other coating techniques can also be used, but high curing temperatures can degrade shape memory properties.