# Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
## 3 Synthesis of Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites  
This review will consider the following alloys analyzed as matrixes in previous studies: Sn-13 at.%Bi [39]  
Mg-5.7 at.%Zn-2.7 at.%Al [39]  
Al-Si, Al-Cu, and Al-Cu-Si [34]  
Al-3 at.%Si [68]  
Sn-20 wt.%Bi [50; 69]  
Sn-13 wt.%Bi [70]  
Bi-10 wt.%Sn [50; 69]  
Zn-0.8 Al-0.015 Cu [71] \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Self-Healing Mechanism** & \
\begin{tabular}{c} **Phase Transformation** \\ **Involved during Healing** \\ \end{tabular} &
\begin{tabular}{c} **Characteristic Length Scale** \\ **of the healed Damage** \\ \end{tabular} & **Autonomy** \\ \hline SMA clamp-and-melt-based & Liquid-assisted healing & Macroscale healing & Nonautonomous \\ self-healing & Liquid-assisted healing & Macroscale healing & Nonautonomous \\ Nano-SMA dispersoid-based & Solid-state healing & Nanoscale healing & Autonomous \\ self-healing & \hline \hline \end{tabular}
\end{table} 
Table 1: Classification of self-healing metal matrix composites. AA2014 (an aluminum alloy consisting of Al-Cu-Si-Mn-Mg-Fe) [72; 73]  
The influence of design factors such as SMA volume fraction, pre-strain of SMA wires, and healing temperature on the healing of SHMMCs has not been thoroughly investigated. [PERSON] et al. [51; 70] and [PERSON] et al. [72] conducted experimental studies using the Taguchi method [74] to investigate the impact of design factors, including volume fraction, diameter and pre-strain of SMA, specimen size, and healing temperature, on the healing process of SMA-reinforced SHMMCs. [PERSON] [38; 39] developed proof-of-concept self-healing composites by using a Tin-based metal matrix (Sn-13 at.%Bi) and equiatomic NiTi SMA wires. One percent volume fraction of continuous, uniaxially oriented NiTi SMA wires was used as reinforcement. Surface treatment techniques, including electropolishing of the SMA wires in a solution of 5% perchloric acid and 95% acetic acid, followed by a 5 nm gold sputter-coating, were employed to enhance the bonding between the NiTi SMA wires and the metal matrix. Specially designed clamps were used to maintain tension in the wires during casting, and the clamp/wire assembly was placed in a heated graphite mold coated with boron nitride (BN). BN spray treatment was applied to both the crucibles and molds to prevent carbide formation and facilitate the casting process. The specimens obtained from the casting process were subsequently cooled to room temperature through air-cooling. [PERSON] [50] fabricated SHMMC made of a Sn-20 wt.%Bi matrix with 20% volume fraction pre-strained NiTi wires reinforcement. The NiTi wires underwent etching and flux treatment to enhance the bonding between the metal matrix and NiTi reinforcement. Following the fabrication of single fiber composites through the dip coating of NiTi wire in Sn-20 wt.%Bi, the pressure infiltration technique was employed to introduce NiTi wires with a high-volume fraction into the metal matrix. The purpose of fabricating single fiber composites was to avoid wire charring and embrittlement during the pressure infiltration process. Charring can compromise wire properties and effectiveness as reinforcement, while high temperatures and pressures during infiltration can induce wire embrittlement, reducing overall strength, toughness, and durability of the composite material. [PERSON] et al. [51] fabricated SHMMCs composed of a Sn-13 wt.%Bi matrix with 0.78, 1.55, and 2.33% volume fraction NiTi strip reinforcement. Casting was used as a fabrication technique because of the equipment availability and ease of process control. A metallic mold with internal fixtures was designed and constructed to ensure proper placement of the continuous reinforcement uniaxially within the matrix. The casting process involved melting tin and bismuth in a stainless-steel crucible at 300 \({}^{\circ}\)C and pouring the molten alloy into the mold to create bending test samples.