**3. Crystallographic structures in titanium metal and alloys**

Generally, all property of materials depends directly or indirectly on the type of crystallographic phase and its constructions. The stable structure of pure Ti is the hexagonal close-packed (hcp) structure (α phase) at room temperature, which transforms to the body-centered cubic (bcc) structure (β phase) at high temperature. Apart from these stable phases, other metastable phases can emerge in a quenched alloy such as (α′) martensite with hexagonal structure, martensite with orthorhombic (α″) structure or the β phase [20] or an omega (ω) phase. There are two types of omega ω phases with hcp structure, one is athermal, which forms during quenching from β-phase at high temperature and this type is cooling rate dependent. The other is isothermal ω phases, which precipitate during aging at certain temperatures. However, the exact relationship between the two omega ω phases and the α″ martensite phase for reversible transformation is still a subject of many research studies.

For this, interests in titanium-base alloys as structural materials has inspired several studies of their phase relationships. These have provided the background essential to the development of commercial alloys, but have also revealed some unexpected, and still to some extent unexplained, aspects that are of considerable scientific interest [2]. Although the exact transus point is dependent on the composition and processing treatment for the alloy, for alloyed Ti based materials, the transformation of crystallographic phase could also be driven by alloying elements.

Titanium alloys are stabilized by solute elements that have strong effect on the transformation temperature. Alloying elements of titanium are typically grouped based on their effect on the beta-transus temperature. They are often termed as neutral, alpha stabilizers, or beta-stabilizers. According to this classification scheme the alpha stabilizing alloying elements extend the room temperature hexagonal alpha phase field to elevated temperatures, while beta-stabilizing elements shift the high temperature beta phase field to lower temperatures. Neutral elements have only minor influence on the beta-transus temperature. Among the alpha stabilizing elements aluminum is by far the most important alloying element of titanium. The interstitial elements oxygen O2, nitrogen N, and carbon C also belong to this category, which are referred to as α-stabilizers [20] (see **Figure 2**).

Some of the high temperature β phase stabilizing elements are Nb Mo; V; W; Fe; Cr; Mn; Co; Cu; Si, H and Ta at room temperature. For β-stabilizers, a minimum concentration β element is required to fully stabilize the β-phase following a quench from the high temperature. The β stabilizing elements are categorized into two groups, namely:


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of materials as well [22].

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

The β phase can be easily transformed to hexagonal (a′ and ω) or orthorhombic (α″) phases depending on the contents of the β stabilizing elements at room temperature. The (α″) phase has more slip systems than that in the hexagonal phases

Although it might be that no metallic element is purely neutral, some elements are classified as neutral because they have a minor influence on the transus temperature. They can lower the β transus slightly, but again increase it at a higher concentration. These elements include Sn, Zr and Hf, which may slightly lower the α/β transformation temperatures after certain threshold concentrations. Zr and Sn are the commonly used neutral stabilizing elements. Zr and Hf are isomorphous with titanium and therefore exhibit the same allotropic phase transformation from β to α and are completely soluble in both the α and β phases. Zr also substitutes titanium in a multicomponent alloy and thereby indirectly has a α stabilizing effect [20].

**4. Some self-healing assisting phenomena in titanium metal and alloys**

or changes its microstate, as a result of external constraints such as pressure or temperature. In effect, these materials adopt different crystal structures favorable for the minimization of their free energy. In general, the microstructural features and the order in the system changes, leading to variations in most of the important properties. By so doing, phase transformation provides an effective way to modify the microstructure of solids. If it can be activated by a mechanical or other physical force, it becomes part of the deformation process and directly affects the properties

In CP titanium and titanium alloys, the most common equilibrium phases are those of α and β, phases. The transformation of high temperature phase can occur by martensitic or by a diffusion controlled nucleation and growth process depending on cooling rate and alloy composition. Their relationship was confirmed for Zirconium by Burger [23, 24] and later for titanium by [22]. This Burgers

Phase transformation occurs whenever a materials system is not at equilibrium,

**4.1 Phase transformation in titanium metal and alloys**

*(a) Unit cell of* α *type titanium phase (b) unit cell of* β *type titanium phase [21].*

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

but fewer than in the β phase.

**Figure 2.**

*Self-Healing in Titanium Alloys: A Materials Science Perspective DOI: http://dx.doi.org/10.5772/intechopen.92348*

*Advanced Functional Materials*

areas can be found elsewhere [4].

frames, spikes in sprinters. Their low coefficient of thermal expansion is also an important factor. The ballistic properties of titanium are also excellent on a densitynormalized basis. Future applications are likely to be in the areas of steam turbine blading, flue gas desulphurization plant consumer products and many marine applications. Some of the basic characteristics of titanium and its alloys are listed in **Table 2** in [18] and compared to those of other structural metallic materials based on Fe, Ni, and Al. Detailed discussions on other applications of titanium in other

Generally, all property of materials depends directly or indirectly on the type of crystallographic phase and its constructions. The stable structure of pure Ti is the hexagonal close-packed (hcp) structure (α phase) at room temperature, which transforms to the body-centered cubic (bcc) structure (β phase) at high temperature. Apart from these stable phases, other metastable phases can emerge in a quenched alloy such as (α′) martensite with hexagonal structure, martensite with orthorhombic (α″) structure or the β phase [20] or an omega (ω) phase. There are two types of omega ω phases with hcp structure, one is athermal, which forms during quenching from β-phase at high temperature and this type is cooling rate dependent. The other is isothermal ω phases, which precipitate during aging at certain temperatures. However, the exact relationship between the two omega ω phases and the α″ martensite phase for reversible transformation is still a subject of many research studies. For this, interests in titanium-base alloys as structural materials has inspired several studies of their phase relationships. These have provided the background essential to the development of commercial alloys, but have also revealed some unexpected, and still to some extent unexplained, aspects that are of considerable scientific interest [2]. Although the exact transus point is dependent on the composition and processing treatment for the alloy, for alloyed Ti based materials, the transformation of crystallographic phase could also be driven by alloying elements. Titanium alloys are stabilized by solute elements that have strong effect on the transformation temperature. Alloying elements of titanium are typically grouped based on their effect on the beta-transus temperature. They are often termed as neutral, alpha stabilizers, or beta-stabilizers. According to this classification scheme the alpha stabilizing alloying elements extend the room temperature hexagonal alpha phase field to elevated temperatures, while beta-stabilizing elements shift the high temperature beta phase field to lower temperatures. Neutral elements have only minor influence on the beta-transus temperature. Among the alpha stabilizing elements aluminum is by far the most important alloying element of titanium. The interstitial elements oxygen O2, nitrogen N, and carbon C also belong to this

**3. Crystallographic structures in titanium metal and alloys**

category, which are referred to as α-stabilizers [20] (see **Figure 2**).

phase) such as Fe; Cr; Mn; Co; Cu; Si and H.

Mo; V; W; Nb and Ta.

Some of the high temperature β phase stabilizing elements are Nb Mo; V; W; Fe; Cr; Mn; Co; Cu; Si, H and Ta at room temperature. For β-stabilizers, a minimum concentration β element is required to fully stabilize the β-phase following a quench from the high temperature. The β stabilizing elements are categorized into two

i. β eutectoid stabilizers are elements (which lead to a partially stabilized β

ii. β isomorphous forming elements are heavy refractory BCC elements such as

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groups, namely:

**Figure 2.** *(a) Unit cell of* α *type titanium phase (b) unit cell of* β *type titanium phase [21].*

The β phase can be easily transformed to hexagonal (a′ and ω) or orthorhombic (α″) phases depending on the contents of the β stabilizing elements at room temperature. The (α″) phase has more slip systems than that in the hexagonal phases but fewer than in the β phase.

Although it might be that no metallic element is purely neutral, some elements are classified as neutral because they have a minor influence on the transus temperature. They can lower the β transus slightly, but again increase it at a higher concentration. These elements include Sn, Zr and Hf, which may slightly lower the α/β transformation temperatures after certain threshold concentrations. Zr and Sn are the commonly used neutral stabilizing elements. Zr and Hf are isomorphous with titanium and therefore exhibit the same allotropic phase transformation from β to α and are completely soluble in both the α and β phases. Zr also substitutes titanium in a multicomponent alloy and thereby indirectly has a α stabilizing effect [20].
