**1. Introduction and background**

Historically, solving material reliability issues has been an old and long term quest of material scientist and engineers, due to their implications for material safety. Considering the fact that structural materials degrade irreversibly over time owing to proliferation of damage like microscopic cracks: the growth of which eventually results in failure. And most times, these internal defects or damage are deep inside materials and difficult to perceive and repair. Recently, there has been a huge interest in materials that can self-heal, as this property can potentially extend materials lifetime, minimize replacement costs, and improve product safety and reliability [1]. Thus, having materials with intrinsic self-repair capabilities—a sort of biomimetic healing functionality, may then allow failures to be averted and the useful lives of components and structures to be extended [2, 3].

Although self-healing is an exclusive specialty of living organisms of biological origin and not easy to put in place in non-biological materials, continuous efforts are now being made to mimic natural materials and to integrate self-healing capability into polymers and polymer composites. Self-engineered healing properties, which are applied in closing and healing crack initiated in a material during its utilization, have been described in cementous [4] and polymer materials [5]. Selfhealing approaches mostly gained by surface modification [6–9] or by the creation of a composite material with some other smart material like NiTi [10, 11] are been utilized in metals and other inanimate materials. For example, damage to oxide films, which normally protect the surfaces of metals such as aluminum (Al) and titanium (Ti) from corrosion, can be repaired by reoxidation in air, which can be seen as a form of self-repair. Also identified are the self-healing properties obtained by encapsulating a solder material into a metallic matrix [11–13]. Self-healing behavior was also observed in a commercial Al alloy after suitable heat treatment [14] and some other precipitation-forming systems [15, 16]. Healing can be initiated by means of an external source of energy as was shown in the case of a bullet penetration [17] where the ballistic impact caused local heating of the material by allowing self-healing of ionomers.

There are several different strategies to impart self-healing functionality that have been developed and the number of publications dealing with various aspects of self-healing materials has increased markedly in recent years. On the whole, the vast majority of the articles deal with polymer composites and cementous materials. Research in the field of metallic systems is still in its infancy. However, the emergence of self-healing in metallic materials, such as titanium adjured to be biocompatible and explored here presents an exciting paradigm for an ideal combination of metallic and biological properties in application traditionally dominated by metallic materials. Depending on the method of healing, self-healing in metallic system can be classified into two categories: (i) intrinsic ones that are able to heal cracks or repair damage by the metals themselves and (ii) extrinsic in which healing agent has to be pre-embedded.

This chapter begins with an overview on the importance of titanium as an engineering of self-healing materials. Since all processes of self-repair, including healing in living bodies depends on rapid transportation of repair substance to the injured part and reconstruction of the tissues, Therefore, the knowledge of basic principle of solid state diffusion is essential for understanding the self-repair processes, such as phase transformation, precipitation and shape memory effects taking place titanium and other alloys, were briefly discussed. The chapter concludes by considering future research.

## **2. Titanium: A special engineering material**

Titanium has been an important development in the history of non-ferrous industry. Titanium is an attractive material with excellent corrosion resistance and high strength-to-weight ratio. It combines the strength of iron and steel with the light weight of aluminum, which accounts for its widespread use. Industrial applications of titanium materials have recently expanded widely in many areas such as the aerospace, chemical plants, automobiles, and aviation industries, and even in high performance sports equipment, and in the medical field for bone. Their biological compatibility is particularly of interest to the medical industry implants and replacement devices [17]. Currently, the chemical industry is the largest user of titanium due to its excellent corrosion resistance, particularly in the presence of oxidizing acids. The ballistic properties of titanium are also excellent on a densitynormalized basis. Some physical properties as compared with other engineering materials by Hanson are presented in **Table 1** [18]. Detailed discussions on other applications of titanium in other areas can be found elsewhere [18, 19].

Besides the areas mentioned above, building applications such as exterior walls and roofing material have emerged as a new market for titanium. Using CP titanium as building material has become especially popular in Japan [20]. One example is the Fukuoka Dome, built in 1993, which is covered with titanium roofing,

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**Table 2.**

**Table 1.**

**Figure 1.**

*Self-Healing in Titanium Alloys: A Materials Science Perspective*

*Physical properties of titanium compared with other metals [18].*

*retractable for multi-role and all-weather purposes.*

*Physical properties of titanium and some of its alloys [18].*

retractable for multi-role and all-weather purposes (**Figure 1**) [20]. Each of these building projects uses large quantities of CP titanium leading to the increased usage in the civil engineering area in Japan. Another "new area" in which titanium use is growing is the area of consumer products, such as spectacle frames, cameras, watches, jewelry, and various kinds of sporting goods. The largest application in the area of sporting goods is golf club heads. Other examples are tennis rackets, bicycle

*(a) Arial approach view of the Fukuoka dome, built in 1993, which is covered with (b) titanium roofing,* 

*DOI: http://dx.doi.org/10.5772/intechopen.92348*
