Microstructural Design and Analysis of Nickel Titanium Reinforced Self-Healing Metal-Metal Composites

Abstract This paper presents a recent investigation into the microstructural design and optimization of Nickel Titanium (NiTi) shape memory alloy (SMA) reinforced metal-metal composites with considerations of material strength and self-healing ability. Self-healing materials have the potential to revolutionize the design of engineering structures by changing the definition of failure, resulting in lighter weight and more efficient vehicles and structures. As a result, engineered self-healing materials and structures have been the subject of significant investigation in recent years. One of the most promising self-healing material systems is composed of NiTi SMA wires reinforcing a Bismuth-Tin (BiSn) metal matrix. After fracture, upon heating actuation, this non-autonomic self-healing system can restore bulk geometry, and then weld itself back together restoring nearly 100% of its original strength. Being entirely metallic, the entire structure is load bearing, and no consumables are expended in the healing process, meaning that healing can be repeated indefinitely. The self-healing capability of NiTi reinforced Bi-Sn has been well demonstrated, but understanding of the material structure, failure mechanisms, and self-healing activity is currently very limited. The mechanics and related properties of failure around the interface between the NiTi wires (fibers) and BiSn matrix was only recently studied. It was found that chemically etching the NiTi wires to remove native oxides drastically increased the interface strength, sufficient to initiate detwinning of the NiTi and shape memory effect (normal stress > 340 MPa) and produced alternate failure mechanisms. Composite design relating volume fractions of fiber and matrix to strength and failure mechanisms has been well studied in static materials. However, breakage in this self-healing composite system is complicated by the detwinning and accumulation of recoverable strain in the NiTi during deformation. This paper presents an investigation into the microstructural (composite) design of the NiTi-BiSn system. Design theory is developed matched to experimental results in a unidirectional NiTi-BiSn metal-metal composite for fiber diameter. Fiber diameter is related to actuation forces produced in individual NiTi fibers as well as surface area and the related stresses introduced relative to interfacial failure strengths. Overall volume fraction is also related to the potential strength of actuation (blocking stress) and potential production of residual compressive stresses after actuation through post constrained recovery residual stresses (PCRRS) in the NiTi.