# Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
## 4 Characterization of Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
Heat treatment at 169 \({}^{\circ}\)C for 24 h was applied to homogenize the dendritic as-cast microstructure and achieve a microstructure resembling that of healed samples, enabling direct comparison without casting effects. The micrographs revealed that the SMA wires appeared as large circles, with dark regions indicating Sn-rich phases and lighter areas representing eutectic phases. The micrographs suggested that heat treatment resulted in a more uniform structure compared to the as-cast condition. Moreover, during annealing, eutectic liquid tended to form around the SMA wires, leading to insufficient bonding between the metal matrix and the SMA wires. This finding highlights the importance of using OM to identify and understand the microstructural changes that occur during the fabrication process in SHMMCs. OM was used to evaluate the effectiveness of surface treatment methods in improving the interface bonding between the matrix and reinforcement in SHMMCs. Some studies [52; 85] observed weak bonding between the matrix material and the NiTi SMA wires. This is because NiTi wires have stable nickel and titanium oxide on the surface, which reduces the wettability of the NiTi wires in the matrix, thus weakening the interface strength. Figure 2a-c [52] illustrate the regions surrounding the SMA wires in NiTi-reinforced self-healing Al-A380 matrix composites. The micrographs reveal the SMA wires as circular structures. It is evident from these figures that there is a significant lack of bonding between the Al matrix and the SMA wires. Etching the NiTi wire with an aqueous solution of 4.8%HF-10.5%HNO\({}_{3}\) and then fluxing with phosphoric-acid-based flux (Indalloy Flux #2, Indium Corporation of America, Clinton, NY, USA) was used [50; 69; 71] as an efficient method to remove the oxide layer from the NiTi surface. An optical microscopic image of the impact of applying an etch solution on the strengthening of matrix-strip bonding is shown in Figure 3[51]. The image shows that using the etch solution to remove the oxide layer from the surface of the reinforcement improved the bonding between the matrix alloy and reinforcement. These examples highlight the capability of OM to evaluate the effectiveness of surface treatment methods in improving the interface between the matrix and reinforcement in SHMMCs. SHMMCs were investigated via optical microscope after they were tested under different loading conditions. The optical micrograph, as reported by [33], revealed debonding occurrences between an Al-3 at.%Si matrix and NiTi wires (V\({}_{t}\) = 4.43%) following tensile testing. Debonding was identified during the tensile testing through distinctive indicators such as cracking sounds and stress reduction at different strain levels. It is assumed that the decreased healing percentages observed in SHMMCs with higher volume fractions of NiTi wires are a result of this debonding phenomenon. Furthermore, debonding is expected to impede the wires from clamping the crack, particularly when the temperature rises during heat treatment for healing purposes. Figure 2: (**a–c**) Optical micrograph of the area around NiTi SMA wires. Reprinted/adapted with permission from Ref. [52]. 2014, [PERSON]. In Figure 4 [68], optical micrographs of the SHMMC samples subjected to fatigue testing are presented. As depicted in Figure 4a-d, during the initial testing phase, crack propagation primarily occurs along the intergranular region, following the eutectic zones of the composites. However, after the healing process of the specimen, the crack path becomes more convoluted. This change in crack path can be attributed to the morphological alterations and growth of the Al-3 at.%Si microstructure caused by heat treatment, which reroutes the crack propagation. As a result of these modifications, the sample exhibits improved toughness. OM has been used to evaluate the bonding between the cracked surfaces after healing. The bonding between the cracked surfaces after healing is critical in regaining the structural integrity of the composites.