# Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
## 3 Synthesis of Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
### Negative Coefficient of Thermal Expansion (CTE) Materials as Reinforcements for SHMMCs  
The use of negative coefficient of thermal expansion (CTE) materials as reinforcement SHMMCs, as proposed by [PERSON] in a patent [86], presents an interesting concept with potential benefits for crack repair. The two-stage crack repair process involving crack closure from the contraction of the contracting constituent, followed by crack repair during partial liquefaction of the matrix material, is intriguing. However, the application of this concept is currently in the developmental stage, and experimental demonstration of self-healing ability is lacking. The authors reported that the contracting constituent can be either SMA materials (nickel-titanium-based alloys, including high-temperature modifications such as Ti(NiPt), (TiHf)Ni, Ti(NiPd), Ti(NiAu), NiTiSn, and the like, indium-titanium-based alloys, nickel-gallium-based alloys, nickel-aluminum-based alloys, copper-based alloys (e.g., copper-zinc-aluminum alloys, copper-aluminum-nickel alloys, and copper-tin and copper-gold alloys), silver-cadmium-based alloys, gold-cadmium-based alloys, manganese-copper-based alloys, indium-cadmium-based alloys, iron-based alloys, iron-palladium-based alloys, iron-platinum-based alloys, iron-chromium alloys, and iron-manganese alloys), and the like) or materials that have a negative CTE (examples of materials that exhibit negative CTE are cubic zirconium tungstate (ZrW\({}_{2}\)O\({}_{8}\)), members of the AM\({}_{2}\)O\({}_{8}\) family of materials (where A = Zr or Hf, M = Mo or W), and ZrV\({}_{2}\)O\({}_{7}\); A\({}_{2}\)(MO\({}_{4}\))\({}_{3}\) is also an example of controllable negative thermal CTE). ALLVAR alloy 30 [87] and trifluoruconscantium (ScF\({}_{3}\)) [88] also exhibit negative thermal expansion. Carbon fibers [89] show negative CTE between 20 \({}^{\circ}\)C and 500 \({}^{\circ}\)C. Quartz (SiO\({}_{2}\)) and a number of zeolites [90; 91] also show negative CTE over certain temperature ranges.  
One limitation of the concept is the availability of suitable materials with negative CTE for reinforcement. While a variety of materials are listed as examples of negative CTE materials, the practicality of using these materials as reinforcement in SHMMCs needs to be further investigated. The availability, cost, and processability of these materials may pose challenges in their practical implementation. Another limitation is the lack of experimental demonstration of self-healing ability using negative CTE materials as reinforcement. Although the concept of exerting compressive force at the cracked surfaces during healing using negative CTE materials is intriguing, there is a need for rigorous experimental testing to validate the effectiveness of this approach. The mechanism and extent of crack closure, partial liquefaction of the matrix material, and subsequent healing need to be systematically studied to understand the feasibility and limitations of this approach. Furthermore, the concept of using negative CTE materials for crack repair in SHMMCs may require careful consideration of the thermal properties and compatibility between the reinforcement and matrix material. Mismatch in thermal properties, such as CTE, between the reinforcement and matrix material may result in residual stresses, which could affect the overall performance and reliability of the composite. The potential for interface debonding, reduced mechanical properties, and premature failure because of residual stresses should be carefully evaluated.