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Fig. 74: Stages of Programmed Damage and

Repair strategy.

Programmed Damage and Repair a promising healing strategy for metals

Healing a material by reversing damage, such as filling small internal voids generated during loading of a ductile material, enables it to recover its performance. A new healing strategy Programmed Damage and Repair was demonstrated on an aluminium alloy. 3D X-ray nano-imaging successfully confirmed void healing.

Self-healing (SH) materials can be repaired after being damaged by previous loading. They are well-developed for polymers, cement, asphalt, and ceramics, but remain under-exploited for metallic materials due to the low mobility of atoms at room temperature in metals, often requiring an external driving force to promote mass transfer. This has to be achieved without sacrificing the strength of the material [1]. Current solid-state, precipitation-based healing metals can be divided into two categories (depending on the external trigger used to promote diffusion): Thermally activated Diffusion and Precipitation (TDP) and Pipe Diffusion and Precipitation (PDP); however, they share the same disadvantage, i.e. the size of healed damage is limited to the nanoscale.

The present work proposes a new solid-state healing strategy called Programmed Damage and Repair (Figure 74), applied to a commercial Al alloy. The strategy consists of four steps:

1. Initial Al alloy microstructure modification by incorporation of damage-localisation particles within the Al matrix. 2. During the loading of the sample, damage primarily nucleates on these particles (hence Programmed Damage), modifying the common void nucleation mechanism in commercial Al alloys (i.e., fracture of iron- rich intermetallics). 3. When loading is interrupted, a healing heat treatment (HHT) is applied, triggering fast-diffusing atoms towards the voids (hence Repair). 4. Material recovers its integrity without significant microstructural and mechanical property changes, as the selected temperature and time are moderate.

To obtain the desired microstructure, Friction Stir Processing (FSP) was applied on a commercial Al 6063 alloy of the Al-Mg-Si system. FSP leads to refinement and homogenisation of the microstructure, and the processed material typically contains many crystal defects, which provide nucleation sites for precipitation as well as increasing healing potential by the presence of diffusion shortcuts for fast-diffusing atoms [2].

The new alloy consists of an Mg-rich Al matrix containing about 0.5% of Mg2Si particles and brittle Fe-rich intermetallics (Figure 75a). Damage was initiated on Mg2Si particles by their fracture or debonding from the matrix after significant tensile deformation, while Fe- rich intermetallics remained generally intact until final fracture.

3D X-ray nano-imaging was performed at beamline ID16B using holo-tomography with a voxel size of 35 nm and the high-temperature furnace. Correlative characterisation over time was used to analyse healing ability [3]. Figure 75 shows the disappearance and progressive filling of larger voids during in-situ HHT at the selected temperature of 400°C. Figure 75b shows the results of the tracking algorithm used to follow the progressive filling of each individual void from one 3D scan to the next. A clear size effect is observed: the smallest cavities (below 200 nm) have a healing efficiency of approximately 90% after 10 minutes, while the largest cavities (larger than 400 nm) present a much lower healing efficiency (20%). Further heating shows continuous healing of voids regardless of size or the deformation level at which HHT is performed. After two hours, approximately 85% of all nucleated voids are completely filled and the biggest healed void is about 2 µm in length (Figure 75a). Further analysis showed that 10 minutes at 400°C is sufficient to significantly decrease both the number of voids close to each other and the volume fraction of voids, and thus was chosen as an optimum HHT.

The healing shown in Figure 75 can be interpreted by analogy with metal sintering, where the driving force of initial porosity closure is the decrease in interfacial energy, achieved by a reduction in free surface area per unit volume. Thermal activation, operated here at