# Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
## 7 Future Research Directions  
Over the last decade, the study of SMA-reinforced SHMMCs has emerged as a significant new research field in materials science and engineering. Self-healing properties offer numerous benefits, such as improved resilience, longer lifespan, enhanced safety, and reduced maintenance costs, making them desirable for various applications including building structures, biomedical implants, defense sectors, and transportation systems. Different matrix materials, such as low-melting alloys like tin-based alloys/solders, aluminum-, magnesium-, and zinc-based alloys, have been used to demonstrate self-healing in SHMMCs. However, there is a need to develop new SHMMCs using high-temperature alloys and alloys with high mechanical properties and explore new types of SMAs or materials with negative CTE as reinforcements. Incorporating reactive elements into the matrix of SHMMCs can also improve the bond at the interface and enhance healing efficiency. Achieving optimal wetting between the reinforcement and matrix during synthesis is crucial for ensuring effective load transfer and bonding between the two materials. This can be challenging, as inadequate wetting can result in weak interfaces, reduced mechanical properties, and compromised healing performance. Therefore, more research is needed to develop effective techniques to enhance wetting and promote strong bonding between SMA fibers and the matrix. The oxidation of SMA fibers and metal matrices during synthesis and healing can significantly impact their functional properties. Oxidation can decrease the recovery stress and strain of SMA fibers and prevent bonding of the cracked surfaces of metal matrices, thereby limiting their healing capabilities. Investigating strategies to overcome oxidation and preserve the integrity of SMA fibers and metal matrices in SHMMCs is essential to ensure their optimal performance. Providing adequate capillary pressure during the healing process is critical for promoting crack closure and enhancing the healing efficiency of SMA-reinforced SHMMCs. Capillary pressure plays a vital role in driving the healing agents into the cracks and promoting their closure, which is essential for effective healing. Therefore, further research is needed to understand the factors that influence capillary pressure in SHMMCs and develop strategies to optimize it for efficient healing. Preserving the strength and functionality of SHMMCs during the healing process is crucial for maintaining their structural integrity and performance. Healing mechanisms such as annealing or thermal treatment can affect the mechanical properties of the composite, including its strength, toughness, and fatigue resistance. Therefore, investigating methods to ensure that the healing process does not compromise the overall strength and functionality of the composite is critical for its successful application in practical scenarios. Autonomous and multicycle healing techniques need to be developed for SHMMCs, as most current methods require external fields such as thermal heat or electric current. Remote structural health monitoring should also be integrated into SHMMCs and components for improved performance. Traditional methods for healing assessment, such as a hot plate, may need to be replaced with electric heating or a hot bath to achieve uniform heating. Characterization techniques such as strain imaging, X-ray, CT scan, and ultrasonic tests can be used to evaluate damaged samples and healing of micro voids and strains. Additionally, creep tests, low cycle fatigue tests, and compression-after-impact tests can provide valuable insights into the healing behavior of SHMMCs. Developing more complex and accurate models for the strain field, considering the shape-memory effect of SMAs, will also aid in experimentation. Further research is needed to investigate the effect of random SMA fibers on healing and the potential of incorporating SMA nanoparticles into the metal matrix to induce autonomous self-healing.