# Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
## 3 Synthesis of Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
Finally, the cast SHMMC sample was allowed to cool, and subsequent heat treatment was conducted at 592 \({}^{\circ}\)C for 24 h to achieve the desired post-healing microstructure. [PERSON] et al. [72] fabricated a hybrid SHMMCs consisting of an AA2014 matrix reinforced with NiTi wires (Ni55 Ti45) and solder alloy (Sn60 Pb40). To fabricate the composite, a customized steel split die pattern was utilized. First, a steel frame was wound with pre-strained NiTi wires and annealed at 500 \({}^{\circ}\)C for 10 min. After that, quenching of the NiTi wires was conducted in cold water at 20 \({}^{\circ}\)C so that the NiTi wires memorized their cold state shape. Next, an AA2014 ingot was melted in the melting furnace at 800 \({}^{\circ}\)C. The melt was poured directly into the mold cavity, containing a steel frame and wound pre-aligned NiTi wires in the die. The die was designed to allow space for longitudinally filling the solder alloy reinforcement throughout the length of the sample. The solder alloy was loaded externally into the mold, and the opposite end of the hole was sealed with refractory cement to prevent solder alloy drainage during the subsequent healing heat treatment of the composite. [PERSON] et al. [75] successfully fabricated a NiTi-reinforced SHMMC using the semi-solid metal process via the rapid slurry formation (RSF) technique, ensuring careful handling to preserve the self-healing effect. The synthesis involved several steps, starting with the preparation of A356 alloy using the RSF process, where a graphite crucible was filled with the alloy billet and subjected to an electrical resistance furnace. Equiatomic NiTi wires were etched, pickled, and coated with molten A356 alloy to enhance their integration with the matrix. These wires were randomly placed in the semi-solid slurry of A356 alloy using a scope, and the slurry-wire mixture was compressed inside a hydraulic press mold to form the composite material. Rapid quenching in water preserved the microstructure and improved the mechanical properties of the alloy. Testing and characterization were performed on the prepared self-healing material. The quenched samples were reheated below the recrystallization temperature to relieve internal stresses, rectify microvoids, and enhance the integration of the reinforcement with the matrix. Controlled heating at a specific temperature induced a phase transformation in the NiTi wire, effectively training it in a new orientation. Additionally, surface finishing processes were applied to enhance the alloy's surface. The sample was then subjected to controlled loading, resulting in the creation of the first crack. Maintaining the structural integrity allowed for subsequent recoverability analysis. By placing the cracked and bent sample in a muffle furnace at 363.15 K for 5 min, the activation temperature of the shape memory alloy was reached, causing austenite to transform into martensite and providing the necessary restoring force for partial closure of the crack. The synthesis process aimed to achieve an SHMMC with improved mechanical properties and crack closure capabilities through the incorporation of NiTi wires and the design of the A356 alloy matrix to undergo partial liquefaction at the healing temperature. [PERSON] was granted three patents [76; 77; 78] that introduced the concept of incorporating macro, micro, and nano-sized SMA particles and fibers into metal matrices to induce self-healing characteristics in fabricated composites. The healing mechanism of SHMMCs involves the phase transformation of SMAs, which is associated with shape change that generates compressive forces on the matrix, effectively closing and healing cracks in the material. The patents proposed various methods for triggering self-healing in localized areas, such as electromagnetic induction, microwave, ultrasonic, ballistic, and laser energy.