ReviewA review of shape memory alloy research, applications and opportunities
Graphical abstract
Introduction
The technology push, towards ‘smart’ systems with adaptive and/or intelligent functions and features, necessitates the increased use of sensors, actuators and micro-controllers; thereby resulting in an undesirable increase in weight and volume of the associated machine components. The development of high ‘functional density’ and ‘smart’ applications must overcome technical and commercial restrictions, such as available space, operating environment, response time and allowable cost [1]. In particular, for automotive construction and design: increased mass directly results in increased fuel consumption, and automotive suppliers are highly cost-constrained. Research on the application of smart technologies must concentrate on ensuring that these ‘smart’ systems are compatible with the automotive environment and existing technologies [1]. The integration and miniaturisation of integrated micro-controllers and advanced software has enabled considerable progress in the field of automotive sensors and control electronics. However, the technical progress for automotive actuators is relatively poorly advanced [2]. Currently, there are about 200 actuation tasks are performed on vehicles with conventional electro-magnetic motors, which are potentially sub-optimal for weight, volume and reliability [3].
Shape memory alloy (SMA) or “smart alloy” was first discovered by Arne Ölander in 1932 [4], and the term “shape-memory” was first described by Vernon in 1941 [5] for his polymeric dental material. The importance of shape memory materials (SMMs) was not recognised until William Buehler and Frederick Wang revealed the shape memory effect (SME) in a nickel-titanium (NiTi) alloy in 1962 [6], [7], which is also known as nitinol (derived from the material composition and the place of discovery, i.e. a combination of NiTi and Naval Ordnance Laboratory). Since then, the demand for SMAs for engineering and technical applications has been increasing in numerous commercial fields; such as in consumer products and industrial applications [8], [9], [10], structures and composites [11], automotive [2], [12], [13], aerospace [14], [15], [16], [17], mini actuators and micro-electromechanical systems (MEMS) [16], [18], [19], [20], [21], robotics [22], [23], [24], biomedical [16], [18], [25], [26], [27], [28], [29], [30] and even in fashion [31]. Although iron-based and copper-based SMAs, such as Fe–Mn–Si, Cu–Zn–Al and Cu–Al–Ni, are low-cost and commercially available, due to their instability, impracticability (e.g. brittleness) [32], [33], [34] and poor thermo-mechanic performance [35]; NiTi-based SMAs are much more preferable for most applications. However, each material has their own advantage for particular requirements or applications.
In this work, a brief summary of SMA, its design feasibility and the variety of SMA applications are compiled and presented. SMA applications are divided into several sections based on the application domain, such as automotive, aerospace, robotics and biomedical, as well in other areas. Most of the work presented here has an emphasis on NiTi SMAs, but other forms or types of smart materials such as high temperature shape memory alloys (HTSMAs), magnetic shape memory alloys (MSMAs), SMM thin film (e.g. NiTi thin film) and shape memory polymers (SMPs) are also discussed. However, intensive topics such as metallurgy, thermodynamics and mechanics of materials will not be addressed in detail.
Section snippets
Shape memory alloy overview
SMAs are a group of metallic alloys that can return to their original form (shape or size) when subjected to a memorisation process between two transformation phases, which is temperature or magnetic field dependent. This transformation phenomenon is known as the shape memory effect (SME).
The basic application of these materials is quite simple, where the material can be readily deformed by applying an external force, and will contract or recover to its original form when heated beyond a
Designing with SMAs
To date, more than 10,000 United States patents and over 20,000 worldwide patents have been issued on SMAs and their applications, but the realisation of viable products from all this intellectual property has thus far been limited [61], [80], [81]. The reason for this lies primarily with the lack of understanding by scientists and engineers on both the technical limitations of SMAs and the methods to apply SMAs in a robust manner to achieve technical requirements of longevity and stability [69]
Other forms or types of shape memory materials
Other forms or types of SMMs have been explored due to some obvious limitations or disadvantages of SMA, such as high manufacturing cost, limited recoverable deformation, limited operating temperature and low bandwidth. Some of the SMMs can be categorised in multiple forms or types, such as Co–Ni–Ga and Ni–Mn–Ga can be categorised as HTSMA and MSMA, and Ni–Ti–Pt/Pd also can be fabricated as SMM thin film.
Shape memory alloy applications
Generally, the shape memory applications can be divided into four categories according to their primary function of their memory element as shown in Table 10 [34], [201]; where the SME can be used to generate motion and/or force, and the SE can store the deformation energy. [44].
The unique behaviour of NiTi SMAs have spawn new innovative applications in the aerospace, automotive, automation and control, appliance, energy, chemical processing, heating and ventilation, safety and security, and
Opportunities and future direction of SMA applications
The commercial and research interest in SMMs, particularly in SMAs are rapidly increasing, and many potential new applications have been proposed, such as listed in Table 17 [307]. The chance of success of a new idea can be evaluated and ranked into three different categories of applications, i.e. substitution, simplification and novel applications [218]. Applications with higher novelty and good competitive price are more interesting and have a better chance to penetrate the market. A few
Discussion
Although, more than 10,000 US SMA related patents have been proposed in many sectors [61], [81], [336] (see Fig. 4), only the four major sectors are presented in this work, due to the huge classification of sectors and applications. The most recent developments of SMMs, others than SMAs are also omitted; due to the objective of this review is to focus on SMAs. After the first commercially success SMA application as pipe coupler in 1969 [7], the demand for SMAs are increasing after 1980s,
Conclusions
In general, the important designing factors to be considered for SMA applications are as listed below:
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Operating temperature range for the actuator: Selection of SMA material and heat transfer technique to be considered.
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Force required for deforming the actuator: Selection of SMA shape, size, loading configuration and design technique to be considered.
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The required speed of the actuator: Selection of SMA material, shape, size and cooling technique.
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The stroke required: Selection of SMA material,
Future development
The identified future development for SMA applications:
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Development of more efficient and effective information platform or base for knowledge sharing within SMM communities.
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Development of new materials (including composites and hybrid SMMs), fabrication technologies and treatment processes for SMAs, which are more stable, more durable and can be utilised in a broad range of industries.
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Development of new design approaches or guidelines for creation of novel SMA applications, in existing and new
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