Self-Healing Metals and Metal Matrix Composites
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- Ferguson, J.B., Schultz, B.F. & Rohatgi, P.K. JOM (2014) 66: 866. doi:10.1007/s11837-014-0912-4
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Self-healing in inorganic materials is a relatively new area in materials science and engineering that draws inspiration from biological systems that can self-repair damage. This article reviews the preliminary attempts to impart self-healing behavior to metals. Several challenges yet exist in the development of metallic alloys that can self-repair damage, including surface bonding issues, such as liquid/solid contact angle (wetting) and oxidation, and practical issues, such as capillary pressure for delivery of a liquid metal to a damaged area or crack, and the overall mechanical properties of a composite system. Although the applied research approaches reviewed have obtained marginal success, the development of self-healing metallic systems has the potential to benefit a wide range of industrial applications and thus deserves greater investment in fundamental research.
To create self-healing metallic materials is a daunting task. Due to the high melting temperatures of metals, repair processes must take place at high temperatures or in specialized environments. The reactive nature of metals with oxygen and water further complicates metallic joining as rapidly formed oxides at surfaces make bonding difficult. However, several methods are currently being developed to provide self-healing properties to metallic and metal matrix composite systems. These include solid state methods such as precipitation healing, semisolid state methods incorporating shape memory alloys, vascular networks filled with reactive low melting alloys, as well as liquid state methods such as electroplating.
The development of materials with self-repairing properties inspired by nature into inorganic systems is garnering growing interest from materials scientists. Although there have been significant advances in self-healing polymers1 and ceramics,2 progress in self-healing metals has been rather limited. Self-healing methods for metallic systems cannot be copied directly from the natural systems that inspire them, posing several particularly vexing challenges. Living systems generate healing agents, transport them to the site of damage, and then use them to repair the injury. Metallic healing, on the other hand, is often accomplished using methods similar to welding or joining, which requires the melting of the metal to be joined or the introduction of new liquid metal. In systems that make use of electroplating-like techniques, the component must be surrounded by liquid solutions of specific chemical compositions. Even solid state methods require an elevated temperature and considerable time to produce healing. Unlike ceramic or polymer-based materials, oxidation further complicates metallic joining as fresh metal oxidizes when exposed to air or water, making bonding difficult. Due to the high melting temperatures of metals, these types of repair processes must take place at significantly higher temperatures than in polymer or ceramic self-healing systems. Notwithstanding these challenges, there has been some progress in the development of self-healing metals.
He et al.5 have reported that creep damage can be healed in austenitic stainless steels containing boron and copper by dynamic precipitation of these elements from the supersaturated matrix. In their studies, creep resistance was significantly improved when precipitates partially filled nanoscale open-volume defects and thereby prevented further spread of damage. They compared the precipitation kinetics in deformed Fe-Cu and Fe-Cu-B-N alloys by positron annihilation spectroscopy, and their results suggest that open-volume defects introduced by plastic deformation in pure Fe can be recovered almost completely by self-diffusion of Fe atoms during the aging step and that this behavior is independent of the heat treatment before testing. Their tests on Fe-Cu alloys also indicate that Cu precipitation is promoted by the presence of dislocations. In related studies, the researchers concluded that due to properties of the various alloys, self-healing with Fe-Cu-B-N alloys will initially take place by the formation of BN precipitates, and may be assisted by copper precipitation when larger creep cavities are formed.
Shape Memory Alloy-Based Healing
This technique employs micron-size shape memory alloy (SMA) wires to pull together a crack and then bonds the damaged surfaces together using a partial melting technique. SMAs are materials that will revert to a “trained” shape upon the application of heat due to phase changes involving two phases known as austenite and martensite. The SMA transforms from the martensite to the austenite phase upon heating, and back to the martensite phase upon cooling. In the martensite state, the SMA can be easily deformed, responding to stress by “twinning” or changing the orientation of its crystal structure.6,7 In self-healing situations, SMA wires embedded in a metallic system revert from martensite to austenite upon heating to achieve crack closure. If the wires embedded in the sample have been prestrained, then a clamping force is exerted on the damaged material.
Manuel and Olson8 fabricated a Sn-based self-healing proof-of-concept composite using a Sn-13at.%Bi matrix and NiTi SMA wires. The SMA wire reinforcements were continuous and uniaxially oriented with a volume fraction of 1% in the matrix. Prior to casting, the wires were sputter coated with 5 nm of gold to increase the wettability of the wire surface. Tensile tests were performed to assess the mechanical behavior of the composite and matrix alloys. The composite displayed a 73% increase in uniform ductility in comparison with the unreinforced prototype alloy due to composite toughening: improving from 3.7% to 6.4%, which was attributed to either grain refinement of the alloy due to the presence of the SMA reinforcement or interfacial debonding or crack bridging along the matrix/reinforcement interface.
Manuel9 expanded their SMA work on Sn-Bi alloys with a magnesium-based cast alloy. They designed a high specific strength solution-treated Mg—5.7 at.% Zn—2.7 at.% Al proof-of-concept self-healing alloy composite reinforced with 1% volume fraction of commercial Ti—49.4 at.% Ni SMA wires. A 160% increase in uniform ductility resulted in composite toughening in the specimen. They treated their matrix to increase its strength by 40% over commercial cast magnesium AZ91 alloy and heated their SMA wires for 3 h at 500°C to increase their transformation temperatures.
The group found that as the SMA wires pulled the crack closed, the rough crack walls came into contact and prevented full closure. Unlike in the Sn-Bi matrix composite, the force applied by the SMA wires could not overcome the matrix strength of the Mg-based alloy. Poor wetting of the reinforcement and low strength of the interface was also identified as deleterious to the performance of the composite.
Composite Materials Reinforced with a Healing Agent
Self-healing in inorganic materials is a relatively new area in materials science and engineering. Inspired by nature and modern technological tools, researchers are attempting to impart the ability of engineered materials to self-repair. The approaches reviewed are preliminary attempts. Work in this area continues because self-healing can be potentially extremely beneficial for a wide range of industrial applications. The research discussed in this review points to several specific challenges facing the development of self-healing metallic systems, including improving wetting, overcoming oxidation, providing sufficient capillary pressure, and healing macroscopic damage, while not sacrificing strength or functionality.