Shape Memory Alloy Reinforced Self-healing Metal Matrix Composites

Abstract

A metal matrix composite system incorporating Nickel Titanium (NiTi) Shape Memory Alloy (SMA) as smart reinforcement with self-healing potential for crack closure is investigated. Lead free proof of concept solder matrix-alloys of Sn-Bi with off eutectic compositions were designed, synthesized and characterized. This was aimed at enabling partial melting of the matrix at healing temperature - the temperature at which the reinforced SMA reverts to its original shape, whereby welding the cracks shut. In this composite system, NiTi long fibers (wires) have been incorporated as reinforcements. The reinforcements were etched and flux treated to improve wettability with the solder matrix and enhance the fiber matrix interface strength. This was followed by dip coating the fiber with matrix alloy and pressure infiltration of the Single Fiber Composites (SFCs) thus obtained to get a high volume fraction of the reinforcement in the resulting composite. Microscopic characterization of the matrix and interface via optical and scanning electron microscopy followed by electrical and mechanical testing of the composite was undertaken. Etching and fluxing with subsequent Pressure infiltration of SFCs to get self-healing composites was demonstrated as a viable method of synthesizing self-healing composites. Finally, macroscopic shape recovery and healing of the microstructure were demonstrated in the obtained samples. Complete crack closure evaluated by metallography of cross-section for Bi-10%Sn/NiTi with 23% liquid (eutectic) at a healing temperature of 145˚C was achieved, with the eutectic melting and sealing the crack. Complete recovery of flexural strain was observed in all bent samples. 92% of flexural strength was recovered for Sn-20% Bi /NiTi composite and 88% of the strength was recovered for Bi-10% Sn/NiTi composite within an hour of healing.