# Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
## 8 Conclusions  
Research on SMA-reinforced SHMMCs is imperative because of their immense potential in the automotive, spacecraft, aerospace structures, and defense industries. This paper provides an in-depth analysis of the synthesis, characterization, and healing assessment techniques used by various researchers in the field of SMA-reinforced SHMMCs. The findings indicate that casting is the most commonly used synthesis process for fabricating SMA-reinforced SHMMCs, while SEM and optical microscopy are the preferred characterization techniques. Solidification processing of SMA-reinforced SHMMCs is presented in this paper; the scope of study of this topic is not fully explored. Future research is needed to understand the solidification processing of SMA-reinforced SHMMCs.  
One of the major concerns in SMA-reinforced SHMMCs is the reinforcement-metal wettability. It is found that etching the SMA wire with an aqueous solution of 4.8%HF-10.5%HNO\({}_{3}\) and then fluxing with phosphoric-acid-based flux is the most efficient method to remove the oxide layer from SMA wires and improve the bonding between SMA wires and the matrix material. Mechanical tests such as tensile, bending, and fatigue tests have been used to assess the performance of SMA-reinforced SHMMCs, but impact properties are yet to be investigated. The impact properties of SMA-reinforced SHMMCs need to be investigated, as these composites are envisioned for use in spacecraft that may be impacted by meteoroids or space debris during their missions in space.  
The autonomous and multicycle healing capability of SMA-reinforced SHMMCs is an area that requires further research, as no comprehensive studies have been conducted in this field. The paper presents an analytical model to determine the optimum volume fraction of SMA wires required for effective healing without damaging the SMA wires. However, the model is found to be insufficient for determining minimum SMA volume fractions in Al-based composites, and more precise analytical models need to be developed for this purpose. Additionally, a systematic study is needed to quantify the required pre-strain, optimum orientation and distribution of SMA fibers in the metal matrix to achieve the most effective healing in SMA-reinforced SHMMCs. The importance of bonding and load transfer between SMA fibers and the matrix, as well as the effect of pre-strain in SMA fibers on load transfer, should also be assessed.  
It is imperative to accurately predict the rate of crack propagation, rate of crack closure, and shape recovery in SMA-reinforced SHMMCs to design them effectively. Further research should be conducted to explore new self-healing mechanisms for high-temperature and high-mechanical-property alloys, develop autonomous healing techniques, and design novel testing and characterization techniques for evaluating healing. Additionally, improving wetting between the reinforcement and matrix during synthesis, overcoming oxidation, providing adequate capillary pressure, and preserving strength and functionality during healing are recommended areas for further investigation.  
Conceptualization, M.B., M.N. and P.R.; methodology, M.B., M.N. and P.R.; software, M.B., M.N. and P.R.; validation, M.B., M.N. and P.R.; formal analysis, M.B., M.N. and P.R.; investigation, M.B., M.N. and P.R.; resources, M.B., M.N. and P.R.; data curation, M.B., M.N. and P.R.; writing--original draft preparation, M.B., M.N. and P.R.; writing--review and editing, M.B., M.N. and P.R.; visualization, M.B., M.N. and P.R.; supervision, M.B., M.N. and P.R.; project administration, M.B., M.N. and P.R.; funding acquisition, M.B., M.N. and P.R. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.