# Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
## 4 Characterization of Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
Figure 6a [69] shows the SEM microstructure of Sn-20 wt.%Bi reinforced with 20 vol% of NiTi wires at a magnification of 300\(\times\). Figure 6b provides a closer view at a magnification of 1000\(\times\), highlighting a specific area within the microstructure marked in Figure 6a. The figures clearly depict that NiTi wires are well-bonded with the metal matrix, and the presence of a light phase in the matrix is enriched in bismuth. Figure 7 [69] shows the elemental mapping of Sn-20 wt.%Bi-NiTi SHMMC close to the interface. The magnified area under investigation captured a eutectic region in the Sn-20 wt.%Bi matrix. A distinct interface was observed at the boundary of NiTi and the Sn-20 wt.%Bi matrix. The presence of an inter-metallic zone consisting of Sn-Ti-Ni was found by mapping elemental pairs, which proves that good wetting had occurred at the interface after surface treatment of NiTi Wires. A good interface can transfer the load better from the fiber to the matrix during healing. Figure 5: Representative micrographs for the reactive element hypothesis (**a**) Sb-4 at.%Cu and (**b**) Sb-4 at.%Zn demonstrating fracture line after bonding. In Sb-Zn alloys, cracks occurred away from the bonded interface, which became more robust than the matrix. (**c**) Sb-4 at.%Zn bonded microstructure in which ZnO is highlighted along the bonded interface formed by the reduction of Sb\({}_{2}\)O\({}_{3}\) by Zn. Reprinted/adapted with permission from Ref. [107]. 2019, [PERSON] et al. SEM offers significant advantages in terms of magnification, resolution, and elemental mapping for microstructure characterization of SHMMCs, making it a preferred choice when cost is not a limitation. The SEM images presented in Figures 6 and 7 provide clear evidence of the capabilities of SEM in revealing the microstructural details of Sn-20 wt.%Bi-NiTi SHMMC, supporting the claim that SEM is a valuable tool for characterizing the microstructure of such materials. X-ray Diffraction (XRD):  
X-ray diffraction (XRD) was employed for phase characterization, as reported in [50]. Qualitative analysis of XRD patterns revealed a gradual decrease in the intensity of peaks associated with tin, eventually disappearing with an increase in bismuth content. Moreover, certain compositions exhibited pronounced texturing effects. The unique crystal structures, lattice parameters, and XRD patterns of each phase enabled accurate identification of the composition of the phases through XRD characterization. Though XRD has limitations, including the need for careful sample preparation, and the potential cost of equipment and maintenance, XRD remains a powerful tool for phase analysis in material-characterization studies. Differential Scanning Calorimetry (DSC):  
Differential scanning calorimetry (DSC) was utilized to determine the transformation temperatures of the SMAs. The DSC test involved maintaining a constant heating and cooling rate during the measurement. The martensite finish temperature (M\({}_{\text{f}}\)), martensite start temperature (M\({}_{\text{s}}\)), austenite start temperature (A\({}_{\text{s}}\)), and austenite finish temperature (A\({}_{\text{f}}\)) of the SMA reinforcement were determined using the ASTM Standard F 2004-05 (2010) [39; 50; 109] approach, which involves identifying the intersection of tangent lines on the steepest slope of the DSC curve. As reported in [39], DSC test results for the TiNi SMA (Ti-49.4 at.%Ni) annealed at 500 \({}^{\circ}\)C for 3 h revealed that the M\({}_{\text{f}}\), M\({}_{\text{s}}\), A\({}_{\text{s}}\), and A\({}_{\text{f}}\) temperatures of the SMA reinforcement were measured as 58, 71, 88, and 105 \({}^{\circ}\)C, respectively. DSC, while a valuable technique for determining transformation temperatures and enthalpy  
Figure 6: SEM image at (**a**) 300\(\times\) and (**b**) 1000\(\times\) magnification of Sn-20 wt.%Bi-NiTi (d = 500 \(\upmu\)m) SHMMC. Reprinted/adapted with permission from Ref. [69]. 2022, [PERSON] et al. Figure 7: Elemental mapping of Sn-20 wt.%Bi-NiTi SHMMC close to fiber–matrix interface.