PSI - Issue 43

3

John Campbell / Procedia Structural Integrity 43 (2023) 234–239 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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However, not all bifilm defects are difficult to see. Figure 2 shows a bubble trail, a kind of open bifilm which is caused by the sloughing off the bubble’s oxide skin as it rises through the metal. This type of bifilm is clearly about 500 mm long, and probably visible to the unaided eye from 100 meters distance.

Figure 2. A bubble trail in a 20Cr 12Ni 2.5Mo duplex steel casting (Courtesy Micky Dholakia 2022).

The cracks become integrated into the metal as it solidifies, and because the faces of the crack are covered with an extremely stable ceramic (oxides such as alumina Al 2 O 3 and chromia Cr 2 O 3 are common) they tend to be resistant to closure by ‘welding’ or bonding when the metal is subjected to plastic working as in forging or rolling. It is important to note that the bifilms are not solidification defects. They are casting defects. This is the key to suggesting how they might be avoided. They are not intrinsic features of metals such as dislocations, but externally introduced phenomena. As such, we should clearly select processes which do not introduce bifilms into our metals. We shall consider this later. 3. The Micro-Cracked Structure of Metals The density of the microcracks in liquid metals is illustrated by the alumina bifilms in liquid Al alloys. Figure 3 shows metal poured in air at normal pressure (in which the bifilms have been scrambled into compact forms by the turbulence of the pour, and are seen as ghostly shapes) and the bifilms unfurled and inflated by reducing the pressure to 0.1 atm immediately after pouring. Although transgranular bifilms can be found in metals, the most common are intergranular. During solidification, transgranular bifilms arise as a result of the growth of metal grains in the liquid, which push the bifilms ahead (the grains cannot grow through the m icroscopic ‘air gap’ which separates the two films). When growing grains impinge, the bifilms are naturally trapped at the grain boundaries, and in a sense, they become the grain boundary. In the solid state, during recrystallization or grain growth, the migrating boundaries once again meet bifilms and become pinned in place, unable to cross their ‘air gap’. Once again, the bifilm commonly becomes the grain boundary. As a consequence, a large percentage of grain boundaries in our metals are not metal grain/metal grain interfaces, but are constituted by bifilms as oxide/oxide interfaces, often containing some kind of ‘air gap’ as an incipient, partly open crack. When casting in air, the air entrapped in the boundary continues to react with the matrix, consuming its oxygen and nitrogen, thickening its oxide films with more oxide and added nitrides, eventually leaving only the residual one percent of argon in the air. The argon is insoluble and becomes a permanent feature of the air gap, ensuring that it can never completely close.