# Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
###### Abstract  
This paper reviews the synthesis, characterization, healing assessment, and mechanics of NiTi and other shape memory alloy (SMA)-reinforced self-healing metal matrix composites (SHMMCs). Challenges to synthesizing and characterizing the SMA-reinforced SHMMCs and the strategies followed to overcome those challenges are discussed. To design the SMA-reinforced SHMMCs, it is necessary to understand their microstructural evolution during melting and solidification. This requires the knowledge of the thermodynamics of phase diagrams and nonequilibrium solidification, which are presented in this paper for a model self-healing composite system. Healing assessment provides information about the autonomous and multicycle healing capability of synthesized SHMMCs, which ultimately determines their success. Different techniques to assess the degree of healing of SHMMCs are discussed in this paper. Strategies are explored to find the optimum volume fraction of SMA wires needed to yield the matrix and prevent damage to the SMA wires for the most effective healing. Finally, major challenges, knowledge gaps, and future research directions, including the need for autonomous and multicycle healing capability in SMA-reinforced SHMMCs, are outlined. self-healing metal matrix composites; shape-memory alloys; crack closure; phase transformation +
Footnote †: journal: Applied  
definition  
consumer goods. Similarly, Self-healing Metal Matrix Composites (SHMMCs) are anticipated to play a crucial role in aerospace, automotive, electronics, energy conversion, and energy storage-related sectors [28; 29; 30; 31; 32]. SHMMCs are proposed for spacecraft, extraterrestrial planet surface rovers, and deep-sea applications that are currently impractical or impossible to repair while in service [33]. SHMMCs offer significant potential for application in a wide range of aircraft structural components, providing enhanced performance and durability in fuselage skin, stingers, frames, ribs, longerons, stiffeners, doors, fuel tanks, landing gears, wheel wells, fuel lines, shock struts, empennage structures, avionics enclosures, and floor beams [34]. These advanced materials hold promise in spacecraft structures, including hulls, frames, trusses, panels, and support structures, enabling extended missions with improved reliability. Furthermore, the utilization of SHMMCs in the oil and gas industry could revolutionize structures such as oil-well casings, pipelines, offshore platforms, and drilling equipment, delivering enhanced structural integrity and maintenance capabilities. Similarly, extraterrestrial planet surface rovers could greatly benefit from SHMMCs in various critical structures, encompassing chassis, body panels, wheels, suspension components, robotic arms, and other load-bearing and protective elements, enabling reliable exploration and operation. Additionally, deep-sea applications stand to benefit from the use of SHMMCs in structures like underwater vehicles, submersibles, remotely operated vehicles (ROVs), and offshore oil and gas exploration equipment, enhancing their durability and performance in challenging marine environments. Shape-memory alloys (SMAs) are among the newest and most popular smart/ intelligent materials that can recover their original shape when heated above a critical temperature [35; 36]. This paper discusses the synthesis, characterization, healing assessment, and mechanism of NiTi and other SMA-reinforced SHMMCs. When heat is applied to fractured SMA-reinforced SHMMCs, the stretched SMA undergoes phase transformation and returns to its original shape. Compressive healing force exerted by the SMA and pre-determined matrix liquefaction at healing temperature help heal cracked SMA-reinforced SHMMCs [37; 38; 39]. Various synthesis processes were used to fabricate SMA-reinforced composites, including liquid metallurgy [40; 41; 42], powder metallurgy [43; 44; 45], ultrasonic consolidation [46; 47], hot pressing method [48], and friction stir processing [49].