# Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
## 4 Characterization of Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
Oxide layers that form immediately at the cracked surfaces upon exposure to air can prevent bonding of the cracked surfaces. To address this issue, [PERSON] et al. [107] proposed a thermodynamic approach to improve healing at the solid-liquid interface. The approach involved designing alloys with a reactive element alloying addition with lower oxide formation free energy compared to the base element, in order to promote effective healing. The presence of the reactive element in the base element reduces the formation of the passivating surface oxide layer on the exposed surfaces of the crack when the alloy is in its molten state, leading to the formation of a strong chemical bond. The reactive element hypothesis was tested using the model systems antimony-copper (Sb-Cu) and antimony-zinc (Sb-Zn) (Sb served as the base element here, while Zn was the more reactive element addition, and Cu was the thermodynamically less reactive element addition based on the change in Gibbs Free Energy for the respective reaction). The mechanical testing and microstructure analysis revealed that the incorporation of a highly reactive alloy element led to enhanced bonding effectiveness, as depicted in Figure 5. This improvement was attributed to the increased bond area and load-carrying capability of the  
Figure 4: Optical micrographs of an untested SHMMC specimen (**a**), a post-fatigue cracking specimen (**b**), a post-healing specimen (**c**), and a specimen after a second M(T) fatigue test (**d**). Reprinted/adapted with permission from Ref. [68]. 2018, [PERSON] et al. Figure 3: A cross-section optical micrograph of the interface between the Sn-Bi matrix and SMA strips, with (**a**) 5\(\times\) and (**b**) 20\(\times\) magnification. Reprinted/adapted with permission from Ref. [51]. 2018, [PERSON] et al. metallic system. The formation of stable oxides through the reduction of the passive oxide layer on the parent metal surface played a crucial role in establishing a strong chemical bond across the interface, thereby contributing to the enhanced bonding observed. However, the effectiveness of this approach in SMA-reinforced SHMMCs has yet to be explored. This finding demonstrates the potential of OM in evaluating the effectiveness of healing mechanisms and bonding at the interface in SHMMCs. OM plays a crucial role in characterizing SMA-reinforced SHMMCs, providing insights into microstructural changes, interface bonding, debonding, and crack propagation. However, OM has limitations that need to be considered. First, OM provides only surface-level information, relying on reflected light, which may not accurately represent the internal microstructure of SHMMCs, leading to incomplete or inaccurate characterization. Second, OM may lack sufficient resolution to clearly distinguish between phases or microstructural features in complex systems. Third, OM provides a two-dimensional view, limiting the characterization of complex three-dimensional microstructures and interface bonding in SHMMCs, potentially missing important information in the third dimension. Fourth, OM may have limitations in detecting and characterizing very small defects, such as microcracks or dislocations. Fifth, OM has limitations in studying dynamic or time-dependent behaviors of SHMMCs and may not capture in situ or real-time behavior during deformation or thermal cycling. Sixth, OM provides limited information about the chemical composition of materials, which is essential for understanding the atomic-level behavior and performance of SHMMCs. It is important to consider these limitations to obtain a comprehensive understanding of SMA-reinforced SHMMCs and complement OM with other advanced characterization techniques. Scanning Electron Microscopy (SEM):  
Scanning electron microscopy (SEM) offers superior performance compared to optical microscopy in terms of magnification, resolution, depth of focus, and observable features, making it a preferred choice for characterizing the microstructure of SHMMCs when cost is not a limiting factor.