Shape Memory Alloy

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A Swedish physicist Arne Olander discovered ?the Shape Memory Effect? (SME) in goldcadmium (AuCd)? alloy in 1932. The alloy could be deformed when cool and then heated to return to original ?remembered? shape. The metal alloys with SME are called ?Shape Memory Alloys? (SMA). In 1958, SME was demonstrated at the Brussels World?s Fair, where the SME was used to cyclically lift a load mass. Researchers of U.S. Naval Ordnance Laboratory found SME in nickel-titanium (NiTi) alloy in 1961 by accident, while studying the heat and corrosion resistance of NiTi. Today, the NiTi alloys are commonly referred to as ?Nitinol?, for NiTi Naval Ordnance Laboratory.

The benefits of NiTi alloys, such as lower costs, smaller dangers (from health standpoint) and easier manufacturing and machining methods refreshed the interest in SME and its applications. In 1970?s, commercial products began to emerge. First devices were static, taking advantage of a single dimensional change, for example fasteners, couplings and electrical connectors. Then, SMA devices started to perform dynamic tasks as actuators.
Ambient temperature-controlled valves and clutches were the first applications, later actuators with resistive heating and thus electrical control were proposed to be used in micro-robotics, for example.
The main features of SMAs are:

one-way shape memory effect
superelasticity
two-ways shape memory effect
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Applications:

  • NASA,Boeing and All Nippon airways developed the Variable Geometry Chevron using shape-memory alloy that reduces aircraft's engine noise.
  • The first consumer commercial application was a shape memory coupling for piping, e.g. oil line pipes for industrial applications, water pipes 
  • Experimental solid state heat engines, operating from the relatively small temperature differences in cold and hot water reservoirs.
  • A comprehensive review of various SMA applications are particularly in automotive, aerospace, robotic and biomedical. 
  • Shape-memory alloys are applied in medicine, for example, as fixation devices for osteotomies in orthopaedic surgery, in dental braces to exert constant tooth-moving forces on the teeth.

 The term Shape Memory Alloys (SMA) is applied to that group of metallic materials that demonstrate the ability to return to some previously defined shape or size when subjected to the appropriate thermal procedure. Generally, these materials can be plastically deformed at some relatively low temperature, and upon exposure to some higher temperature will return to their shape prior to the deformation. Materials that exhibit shape memory only upon heating are referred to as having a one-way shape memory. Some materials also undergo a change in shape upon recooling. These materials have a two-way shape memory.

Although a relatively wide variety of alloys are know to exhibit the shape memory effect, only those that can recover substantial amounts of strain or that generate significant force upon changing shape are of been the nickel-titanium alloys and copper-base alloys such as CuZnAl and CuAlNi.

A shape memory alloy may be further defined as one that yields a thermoelastic martensite. In this case, the alloy undergoes a martensitic transformation of a type that allows the alloy to be deformed by a twinning mechanism below the transformation temperature. The deformation is then reversed when the twinned structure reverts upon heating to the parent phase.

History of Shape memory alloys

The first recorded observation of the shape memory transformation was by Chang and Read in 1932. They noted the reversibility of the transformation in AuCd by metallographic observations and resistivity changes, and in 1951 the shape memory effect (SME) was observed in a bent bar of AuCd. In 1938, the transformation was seen in brass (CuZn). However, it was not until 1962, when Buehler and co-workers discovered the effect in equiatomic nickel-titanium (NiTi), that research into both the metallurgy and potential practical uses began in earnest. Within 10 years, a number of commercial products were on the market, and understanding of the effect was much advanced. Study of shape memory alloys has continued at an increasing pace since then, and more products using these materials are coming to the market each year .

As shape memory effect became better understood, a number of other alloy systems that exhibited shape memory were investigated. Of all these systems, the NiTi alloys and a few of the copper-base alloys have received the most development effort and commercial exploitation.

Advantages and Disadvantages

Some of the main advantages of shape memory alloys include:

  • Bio-compatibility
  • Diverse Fields of Application
  • Good Mechanical Properties (strong, corrosion resistant)

The use of NiTi as a biomaterial has severable possible advantages.Its shape memory property and super elasticity are unique characteristics and totally new in the medical field. The possibility to make self-locking, self expanding and self- compressing thermally activated implants is fascinating. As far as special properties and good bio compatibility are concerned, it is evident that NiTi has a potential to be a clinical success in several applications in future.
There are still some difficulties with shape memory alloys that must be overcome before they can live up to their full potential. These alloys are still relatively expensive to manufacture and machine compared to other materials such as steel and aluminum. Most SMA's have poor fatigue properties; this means that while under the same loading conditions (i.e. twisting, bending, compressing) a steel component may survive for more than one hundred times more cycles than an SMA element.


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