Ni-Ti Shape Memory Alloys And Their Constituents

A Preface on Shape Memory Alloys

Shape memory alloys (SMAs) are alloys that retain information about their preformed configuration. They are produced in a cold-worked state and, upon heating, revert to their pre-set or original shape. Other names by which shape memory alloys are known include Memory Alloys, Memory Metals, and Smart Metal Alloys, among others. SMAs are generally lightweight and are employed in a number of applications. The following text outlines several uses and applications of shape memory alloys.

Further information: Nitinol - A Notable Shape Memory Alloy

1. Main Applications of SMAs

Shape memory alloys are employed in several industry sectors. For example, they play a role in the aerospace industry in the manufacture of aircraft and turbine engines, thereby reducing noise. They are also used in pipework installations for water and oil conduits.
- The automotive industry utilises these alloys.
- They are applied in robotics, telecommunications, optometry, dentistry and medicine.
- Many actuators incorporate memory alloys.
- Orthopaedic surgery is another field in which these alloys are employed.

2. Ni-Ti Shape Memory Alloys

Ni-Ti Shape Memory Alloys consist of approximately equal proportions of nickel and titanium. In addition to these two elements, copper, zinc, gold and iron may be used to manufacture shape memory alloys; however, Ni-Ti is the most frequently used variant. There are several reasons for choosing these two elements when producing such alloys, the most important of which are detailed below:

- Ni-Ti shape memory alloys are preferred because they are stable, practical and exhibit improved thermomechanical performance compared with other metals.
- The yield strength of Ni-Ti-based SMAs is lower than that of steel. However, certain compositions achieve a yield strength that exceeds that of some plastics and even aluminium. Ni-Ti alloys can have a yield strength of up to 500 MPa.
- The use of Ni-Ti alloys can result in a significant degree of recoverable plastic deformation.

3. Manufacture of Ni-Ti SMAs

Nitinol, also known as Ni-Ti, is challenging to produce. This difficulty arises from the exceptionally tight control of composition required and from the high reactivity of titanium metal. The two primary melting processes used today to manufacture Nitinol are:

- Vacuum arc remelting – In this process, an electric arc is initiated between the raw material and a water-cooled copper closing plate. This melting process is performed in a high-vacuum environment. The mould consists of water-cooled copper, and no carbon is introduced during the process.
- Vacuum induction melting – In this method, the raw materials are heated in a crucible by means of alternating magnetic fields. This process is conducted in a high-vacuum environment, with the introduction of carbon into the process.

No evidence exists to indicate that one method is more effective than the other; both methods offer distinct operational benefits.