# Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
## 4 Characterization of Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites  
SMA-reinforced SHMMCs were characterized by different characterization techniques such as optical microscopy (OM) [33; 34; 39; 50; 51; 52; 58; 59; 68; 70; 86; 107], scanning electron microscopy (SEM) [33; 34; 39; 50; 69; 72; 73; 73; 85; 108], energy dispersive X-ray spectroscopy (EDS) [33; 50; 69], X-ray photoelectron spectroscopy (XPS) [50], inductively coupled plasma-atomic emission spectroscopy (ICP-AES) [33; 39], X-ray diffraction (XRD) [50], microindentation testing [39; 50; 73], dynamic mechanical analysis (DMA) [50], and differential scanning calorimetry (DSC) [33; 39; 50]. Among these techniques, OM and SEM have been the most commonly used techniques for characterizing SMA-reinforced SHMMCs, indicating their significance in understanding the composite's structure and properties. Evaluating the advantages and limitations of the different characterization techniques used to characterize SMA-reinforced SHMMCs is critical in providing comprehensive insights into these composites' complex behavior and performance. This section will provide a critical analysis of the existing characterization techniques used in the field, shedding light on their strengths and weaknesses, and providing valuable guidance for a better understanding of the structure and properties of these composites. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Year, Author** & **Matrix Alloy** & **Reinforcement (Pre-Strain)** & **Bonding and Special Distribution of SMA** \\ \hline
2009, [PERSON] [85] & Sn-20\%Bi, Bi-10\%Sn & NiTi SMA short fibers with a diameter of 100 microns (no pre-strain) & Poor wettability of Copper-coated NiTi; NiTi fibers were randomly oriented. \\ \hline
2013, [PERSON] et al. [50; 69] & Sn-20 wt.\%Bi, Bi-10 wt.\%Sn & Sn-20 wt.\%Bi, (4.5\% pre-strain as provided by the manufacturer) & Bonding between matrix and NiTi wires was improved; NiTi wires were uniaxially oriented, but the pattern seems to be random; No proof of whether pre-strain was retained in the fabricated composites. \\ \hline
2015, [PERSON] et al. [71] & Zn-0.8 Al-0.015 Cu & 0.92, 1.54, 2.01 vol\% NiTi SMA wires (wires were etched and flux treated) of 0.38 mm diameter (no pre-strain) & The preform frame consisting of 100 mm \(\times\) 20 mm \(\times\) 2 mm steel bars and 5 mm diameter threaded steel rod was used to train the wires. \\ \hline
2018, [PERSON] et al. [70] & Sn-13 wt.\% Bi & NiTi SMA strips (strips were etched and flux treated) with different reinforcement vol.\% (0.78, 1.55, and 2.33); dimension of the strip is 0.762 mm \(\times\) 0.254 mm. Different pre-strains (0\%, 2\%, or 6\%) of the wires & Good bonding; a custom-made metallic mold was designed and constructed with internal fixtures to keep the SMA strips in desired position and direction. \\ \hline
2018, [PERSON] et al. [68] & Al-34.\%Si & Al-34.\%Si &
\begin{tabular}{c} A custom-designed graphite mold coated with BN and equipped with an end-wire holder containing pre-aligned NiTi wires (Ni-49.3 at.%Ti, \(\mathcal{O}\) = 0.87 mm). \\ \end{tabular} \\ \hline
2020, [PERSON] et al. [72] & AA2014 matrix & NiTi wires vol\% (0.5\%, 1.3\%) and solder alloy; wire diameters (0.47 mm, 0.96 mm) & A steel frame was wound with pre-strained NiTi wires annealed at 500 \({}^{\circ}\)C for 10 min. After that, the NiTi wires were quenched in cold water at 20 \({}^{\circ}\)C so that they memorize their cold state shape. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: \(Cont.\)Optical Microscopy (OM):  
OM is the most commonly used characterization technique to characterize SMA-reinforced SHMMCs. It is performed on the specimens to confirm the expected microstructures, detect porosity, and inspect the interface between the matrix material and SMA. OM [39] was employed to investigate the microstructures of Sn-based proof-of-concept SHMMCs in both as-cast and heat-treated conditions.