EVALUATION OF INTERFACE STRENGTH AND FAILURE BETWEEN NICKEL-TITANIUM SHAPE-MEMORY-ALLOY WIRE AND BISMUTH-TIN MATRIX FOR THE DESIGN OF SELF-HEALING COMPOSITES

Self-healing materials can lead to a paradigm shift in engineering design by enabling lighter weight & more efficient structures, reducing maintenance requirements, and changing the definition of failure. Among self-healing materials, metal-metal composites have the potential for some of the most advanced capabilities, being entirely structural and healing without consumable components. Despite potential advantages, metal-metal self-healing composites are challenging to synthesize, as a result most research has focused on polymeric or ceramic self-healing materials. NiTi fiber reinforced off-eutectic BiSn matrix composite structures have demonstrated some of the most advanced self-healing abilities. After one of these non-autonomic self-healing structures incurs damage, the selfhealing capability can be activated through the application of heat. Increase in temperature activates the shape memory effect in the NiTi, restoring bulk geometry and closing fractures. Continued heating results in melting of eutectic portions of the BiSn matrix which solders the matrix back together. Combined these abilities result in restoration of macro-scale geometry and near 100% restoration of strength without expending any internal consumable reagents. Therefore, the process can be repeated indefinitely. One of the primary challenges in designing this composite structure is understanding the interface between the NiTi and BiSn. The strength and mechanisms of failure around this interface are critical to designing the composite (i.e. sizing the fibers) and understanding the resulting effects on strength and self-healing ability. A primary obstacle is that NiTi forms an inert titanium oxide (TiO2) surface layer almost instantly upon exposure to air. The TiO2 layer severely inhibits bonding between NiTi and BiSn (or most any other materials). This paper presents an experimental and theoretical investigation into the properties of the interface between NiTi and BiSn in both a control state with the native TiO2 present and an experimental state where chemical etching processes were performed in an inert environment to remove the TiO2 and prevent is re-formation. Experimental specimens were synthesized, mechanically tested, and microscopically inspected. Modeling was performed to understand internal states of the structure and theory was developed consistent with and explaining observed results. The results of this work quantified the improvement in interface strength between NiTi and BiSn achieved through the etching process. Quantification of this overall strength provides vital information for composite design optimization and sizing of wires. However, these analyses will differ from inactive composites to account for internal loads generated by the healing process and potential development of TiO2 upon incurring damage.