Abstract
The damage evolution in a single domain aluminium/alumina freeze-cast composite has been examined using 3D X-ray computed tomography (CT). A single domain was extracted and loaded incrementally at an orientation of 45° to the lamellae, with the damage being assessed after each of eight compressive loading steps. Prior to loading, significant damage was observed at the metal-ceramic interface – this is thought to have formed during machining and can be ascribed to weak interfacial bonding associated with the Cu coating applied to the ceramic preform prior to metal infiltration. Further interfacial damage was seen to initiate after loading to 170MPa and to develop with each subsequent load step. Damage was also observed in the ceramic lamellae, with a series of parallel cracks forming across the alumina, perpendicular to the domain orientation. These sets of parallel, intra-lamellae cracks were closely spaced, but initiated independently, with coalescence only occurring at higher loads. Both the interfacial and intra-lamellae cracking initiated after loading to 170MPa, with the intra-lamellae cracks propagating into the metal matrix after loading to 240MPa. The cracks in the ceramic lamellae were found to form and develop independent of the interfacial cracks, with discrete crack paths and morphologies being observed in each case. Despite this, the underlying driving force was the same for each damage mode, with crack propagation being driven by an elastic-plastic mismatch between the metal matrix and ceramic lamellae.
Highlights
O ver the past three decades metal matrix composites (MMCs) have progressed from being a topic of academic interest to one of practical importance, with MMCs finding usage in automotive and aerospace applications [1]
There was no suggestion that the cracking was linked to damage originating in the ceramic lamellae. This is consistent with the findings presented in the previous section, in which cracks in the alumina were seen to form and progress without the assistance of cracking at the metal-ceramic interface
The Cu coating has been shown to weaken the interface between the metal and ceramic lamellae when compared to the inherently strong bond found between the alumina and aluminium [5]
Summary
O ver the past three decades metal matrix composites (MMCs) have progressed from being a topic of academic interest to one of practical importance, with MMCs finding usage in automotive and aerospace applications [1]. One of the more promising processing techniques is that of ice-templating, which is known as freeze-casting This process revolves around the formation of a second phase scaffold via segregation [3]. In the case of an MMC, this unsintered template would comprise of ceramic particles, which would be sintered and densified prior to being infiltrated with the metallic matrix. The versatility of this approach is noteworthy, with a modification of the freeze temperature and solvent supersaturation allowing for a vast array of template morphologies [3].
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