**5.2 Thermally activated solid phase healing in titanium**

This mechanism is based on a thermoelastic displacive phase transformation design methodology. Certain strongly ordered intermetallic systems exhibit shear-dominated thermoelastic displacive transformations that involve minimal volume dilatation, a high degree of crystallographic reversibility, and a low-temperature allotrope that readily twins during plastic deformation. This combination gives rise to the well-known shape-memory effect in which plastic deformation imparted to the low-temperature martensitic phase can be reversed almost completely during transformation to the high-temperature austenitic phase.

In a recently study, by Elena et al. [62] thermally engineered self-healing was proven by observation of crack healing after annealing. Microscopic deformation and recovery of a shape-memory nickel-titanium alloy were studied. The deformation was induced by microindentation using spherical and Vickers diamond indenters. The recovery of the indents, caused by thermal annealing, was measured quantitatively using an optical surface profiler. Microindents formed by a spherical diamond in an equiatomic nickel-titanium martensite almost completely recover at low indentation load when moderately heated [63]. A smaller recovery ratio was observed for microindents formed by Vickers indenters. These observations suggest that the shape-memory effect exists at the microscopic level and under complex loading conditions. The observations were rationalized using the concept of representative strain and maximum stress under the spherical and pyramidal indenters. A representative surface profiles measured before and after heating for both spherical and Vickers indents are shown in **Figure 8(a)** and **(b)**, respectively. The degree of indent recovery was determined quantitatively from the surface profiles by defining a recovery ratio, d, as.

### **5.3 Precipitation in under-aged alloys**

This is an approach, of self "healing" is likened to an established metallurgical process of aging. In this mechanism, a defect sites (primarily microscopic voids) serve as nucleation centers for diffusion driven precipitation of oversaturated

#### **Figure 8.**

*A representative three-dimensional profile of a spherical indent at load of 15 N: (a) fresh indent and (b) after heating above the austenite finish temperature.*

**135**

**6. Summary**

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

fragile to break upon an advancing crack and not deflect.

oriented cracks being more desirable to fill.

liquefy at the healing temperature [70].

solute in the alloy and are thereby immobilized from further growth until failure. Consequently, the newly formed voids are sealed before they grow and this results in improvement of creep and fatigue properties of the alloy. This form of 'preventive' healing has been used Al-Cu alloys, known for decreasing solute solubility with decreasing temperatures. The process involves a high-temperature solution treatment, accompanied by quenching and annealing for relatively short periods of time, results in an underage microstructure that maintains substantial amounts of solute and serves as the healing agent. The processes of "secondary precipitation" in Al-Cu alloys that results into much finer precipitates from low-temperature aging [64] and investigations into dynamic precipitation in Al-Cu-Mg-Ag alloys occurring in response to moving dislocation generation under load [65], have been identified as potential healing mechanisms during fatigue and creep. This can be extended to other metallic materials. The limitation to this approach is that not all Ti alloy is heat

**5.4 Micro-encapsulated low-melting healing agent reinforced metal matrix** 

This technique derives its inspiration from polymer healing and was recently conceptualized by Rohatgi et al. [63] includes the embedding of a hollow reinforcement (micro-sphere, micro-tube) containing a low-melting alloy in a higher melting metal matrix. The encapsulation of a metallic healing agent, however, allows the microcapsule to serve as a diffusion barrier and the interface should be sufficiently

In the line of work done on hollow fibers reinforced polymers [66], attempts were also made to integrate hollow microfibers containing low-melting healing agents into metallic systems [67]. This attempted healing was implement by incorporating indium as a healing agent in carbon tubes embedded in a higher melting solder matrix. Upon heating beyond the Indium melting point, a macroscopic crack that was directed downward to gravity was repaired. Computational fluid dynamics studies were conducted by Lucci et al. [67] on this healing method and interface wettability and gravity-related crack orientation were highlighted as major factors affecting the flow of healing liquid, with more wetting systems and gravity-

**5.5 Shape memory alloy (SMA) reinforced metal matrix composites (MMCs)**

In one of the earliest trial, Manuel and co-workers [68] used this approach to heal Sn and Mg based metallic materials. The method involves reinforcing an alloy matrix with wires made of a shape-memory alloy (SMA), such as nitinol (NiTi). SMA wires have the ability to recover their original shape when heated above a critical temperature [69]. Thus, when the metal matrix composite cracks, the resulting plastic strain stretches the SMA that bridges the crack. On heating above the shape transformation temperature of the SMA, the wire shrinks back to its original shape applying compressive force to the matrix and clamping the crack. This is accompanied by welding of the crack in the matrix alloy which is so-designed as to partially

Although the production of autonomous self-healing in metallic materials has been the subject of numerous studies and will continue to be in the near future.

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

treatable like Al-Cu alloys.

**composites (MMCs)**

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

*Advanced Functional Materials*

profiles by defining a recovery ratio, d, as.

**5.3 Precipitation in under-aged alloys**

shear-dominated thermoelastic displacive transformations that involve minimal volume dilatation, a high degree of crystallographic reversibility, and a low-temperature allotrope that readily twins during plastic deformation. This combination gives rise to the well-known shape-memory effect in which plastic deformation imparted to the low-temperature martensitic phase can be reversed almost com-

In a recently study, by Elena et al. [62] thermally engineered self-healing was proven by observation of crack healing after annealing. Microscopic deformation and recovery of a shape-memory nickel-titanium alloy were studied. The deformation was induced by microindentation using spherical and Vickers diamond indenters. The recovery of the indents, caused by thermal annealing, was measured quantitatively using an optical surface profiler. Microindents formed by a spherical diamond in an equiatomic nickel-titanium martensite almost completely recover at low indentation load when moderately heated [63]. A smaller recovery ratio was observed for microindents formed by Vickers indenters. These observations suggest that the shape-memory effect exists at the microscopic level and under complex loading conditions. The observations were rationalized using the concept of representative strain and maximum stress under the spherical and pyramidal indenters. A representative surface profiles measured before and after heating for both spherical and Vickers indents are shown in **Figure 8(a)** and **(b)**, respectively. The degree of indent recovery was determined quantitatively from the surface

This is an approach, of self "healing" is likened to an established metallurgical process of aging. In this mechanism, a defect sites (primarily microscopic voids) serve as nucleation centers for diffusion driven precipitation of oversaturated

*A representative three-dimensional profile of a spherical indent at load of 15 N: (a) fresh indent and (b) after* 

pletely during transformation to the high-temperature austenitic phase.

**134**

**Figure 8.**

*heating above the austenite finish temperature.*

solute in the alloy and are thereby immobilized from further growth until failure. Consequently, the newly formed voids are sealed before they grow and this results in improvement of creep and fatigue properties of the alloy. This form of 'preventive' healing has been used Al-Cu alloys, known for decreasing solute solubility with decreasing temperatures. The process involves a high-temperature solution treatment, accompanied by quenching and annealing for relatively short periods of time, results in an underage microstructure that maintains substantial amounts of solute and serves as the healing agent. The processes of "secondary precipitation" in Al-Cu alloys that results into much finer precipitates from low-temperature aging [64] and investigations into dynamic precipitation in Al-Cu-Mg-Ag alloys occurring in response to moving dislocation generation under load [65], have been identified as potential healing mechanisms during fatigue and creep. This can be extended to other metallic materials. The limitation to this approach is that not all Ti alloy is heat treatable like Al-Cu alloys.
