Essay Sample on Shape Memory Alloys: Properties, Uses & Applications

Introduction

Shape memory alloys include "a group of metallic materials that can recover a previously defined shape or length when it is subjected to a particular thermomechanical pressure" (Hodgson, Wu & Biermann, 1990). When their shape recovery is limited, these metals encourage an increased restitution force. The properties that define SMAs ensure that they are preferred for many technological uses and in different applications.

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Even though there is a large variety of metals that bring about the effect of shape memory, the ones able to recover from high strain amounts and can produce high restitution forces are preferred commercially. The alloys that are most preferred are those that are developed using "Ni-Ti and Cu, which include Cu-Al-Ni and Cu-Zn-Al" (Mantovani, 2000). SMA alloys developed using Ni-Ti are preferred in commercial settings due to their combination of shape memory and mechanical properties.

The characteristics presented by SMA have been explored from the 1930s. In 1932, it was discovered that alloy Au-Cd had reversibility properties. This was noted by observing the changes in resistivity and metallographics. The effect of shape memory from Cu-Sn and Cu-Zn alloys was discovered in 1938 (Mantovani, 2000). The technological interest in SMA was sparked in the 1960s. The US Naval Ordnance Laboratory realized the effect of shape memory on the equiatomic Ni-Ti alloy that started being known as Nitinol in lab initials. The first applications of SMA were in the aeronautic industry in the 1960s. The first medical application was in the form of a superelastic orthodontic device that was done at Iowa University in 1975 by Andreasen (Pacheco PMCL & Savi, 2000). Currently, these applications have been exploited in engineering and science.

SMA has martensite and austenite as its crystallographic phases (Smart, 2001). The martensite phases are stable at low temperatures and in the absence of additional stress. It means that its induction is determined by pressure and temperature. Martensite can be deformed easily, and it can reach larger strains (Hodgson, Wu & Biermann, 1990). Alloy transformation influences the crystalline composition of martensite, which can either be orthorhombic or monoclinic (Otsuka & Ren, 1999). Inducing martensite with temperature produces twinned martensite. There are 24 forms in the twinned martensite and 24 subtypes that have different crystalline orientations (Funakubo, 1987). Contrarily, inducting martensite by stress ensures that these 24 different variants come together to be a single variant which, develops a crystallographic orientation that is matched with the direction of stress known as detwinned martensite. Increased temperatures maintain the stability of the austenite stage.

The transformation of martensite illustrates the recovery of shape in SMA. This change only happens in a specific temperature range, and it depends on the chemical contents in the alloys (Shape Memory Applications, 2001). Generally, there are four transformational temperature definitions: "MS and MF which are the temperatures at which martensite formation begins and ends .Whereas, AS and AF are the temperatures at which austenite formation begins and ends respectively" (Shape Memory Applications, 2001).

Current studies illustrate that certain SMAs can bring about a different crystallographic stage, which is the R-phase. This phase is seen before the transformation of the martensite in the following steps: "austenite - R-phase - martensite" (Wu & Lin, 2000). It has a rhombohedric crystal structure (Wu & Lin, 2000).

The remarkable properties of SMA allow them to be also used in different non-medical applications (Van Humbeeck, 1997). SMA can solve aerospace industry problems regarding slender structure vibration control, solar panels, and non-explosive release devices (Pacheco PMCL & Savi, 2000). Robotic actuators and micromanipulators have been used in a way that mimics the movements of the human muscles, which is smooth (Webb, Wilson, Lagoudas & Rediniotis, 2000). SMA can also act as external actuators and as additional fibers in different materials. This allows them to alter the mechanical structures of these elements, thus controlling vibration and buckling (Birman, 1997).

The use of SMA in medicine has been possible due to the alloy properties, which increases surgery success (Mantovani, 2000). These alloys are biocompatible, allowing biomedical applications in surgical instruments, cardiovascular devices, orthopedic implants, and endodontic files, including orthodontic appliances (Airoldi & Riva, 1996).

References

Airoldi G & Riva G (1996). Innovative materials: The NiTi alloys in orthodontics. Bio-Medical Materials and Engineering, 6: 299-305.

Birman V (1997). Theory and comparison of the effect of composite and shape memory alloy stiffeners on the stability of composite shells and plates. International Journal of Mechanical Sciences, 39: 1139-1149.

Funakubo H (1987). Shape Memory Alloys. Gordon & Bleach, New York, NY, USA.

Hodgson DE, Wu MH & Biermann RJ (1990). Shape Memory Alloys, Metals Handbook. Vol. 2. ASM International, Ohio, 897-902.

Mantovani D (2000). Shape memory alloys: Properties and biomedical applications. Journal of the Minerals, Metals and Materials Society, 52: 36-44.

Otsuka K & Ren X (1999). Recent developments in the research of shape memory alloys. Intermetallics, 7: 511-528.

Pacheco PMCL & Savi MA (2000). Modeling and simulation of a shape memory release device for aerospace applications. Revista de Engenharia e Ciencias Aplicadas.

Shape Memory Alloy Research Team (Smart) (2001). http://smart.tamu.ed

Shape Memory Applications, Inc. (2001). http://www.sma-inc.com

Van Humbeeck J (1997). Shape memory materials: state of the art and requirements for future applications. Journal de Physique IV, 7: 3-12.

Webb G, Wilson L, Lagoudas DC & Rediniotis O (2000). Adaptive control of shape memory alloy actuators for underwater biomimetic applications. AIAA Journal, 38: 325-334.

Wu SK & Lin HC (2000). The recent development of TiNi-based shape memory alloys in Twain. Materials Chemistry and Physics, 64: 81-92.