Fig. 3. When this alumina microballoon is cracked in a self-healing matrix, Sn-Bi eutectic fills the balloon.
composite to the austenite transformation temperature of SMA, the wires or particles recover their original shape. This shape change provides a clamping force that shuts the cracks. This, in combination with a matrix that is partially molten at healing temperatures, can lead the cracks to weld together upon cooling. A study of the SMA method is being performed on composites with NiTi particulate and short NiTi fiber reinforcements in a tin (Sn)-20% bismuth (Bi) matrix. The cast Sn-Bi alloy with reinforcements of Nitinol short fibers is being synthesized with a typical microstructure at UWM (Figs. 1 and 2).
Interdendritic Eutectic The basis of the off-eutectic healing system is the use of
a standard microstructure found in cast metals to provide the needed components. In addition to a heat trigger, an off-eutectic system provides the other two components needed for healing—dendrites as a structural component and the liquid eutectic phase as a mobile healing agent. An off-eutectic system is a hypoeutectic or hypereutectic system in which the composition allows for the formation of liquid and solid phases in equilibrium. In this work, two off-eutectic alloys are being investi-
gated, both from the Sn-Bi system. The system has been chosen because it is a simple, purely eutectic, binary system used in solder applications. The compositions Sn-20% Bi and Bi-10% Sn were chosen because they exhibit approxi- mately the same percentage of liquid phase at the healing temperature of 338F (170C), allowing the study of the two systems simultaneously. The optimal liquid to solid ratio for self-healing was identified to be around 20-30% of the liquid phase. This percentage allows for the interdendritic liquid to flow freely while maintaining structural stability. The challenge is to ensure the interdendritic liquid prefer- entially enters cracks near the surface.
Embedding Hollow Reinforcements In a high melting point casting embedded with micro-
balloons and micro-tubes containing low melting point alloys, an advancing crack ruptures the microcapsules. The system then can be heated to above the melting point of the low melting alloy, allowing it to liquefy and flow into the crack. When the system is cooled, the low melt-
26 Metal Casting Design anD PurChasing July/august 2011
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