PSI - Issue 43

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

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Figure 3. Typical bifilm crack population in cast aluminium alloy; the scrambled compact bifilms in (a) are straightened and inflated by applying a vacuum of approximately 0.1 atm to (b).

The presence of the bifilm crack, as an air gap, at many boundaries may be of great importance for the precipitation of inclusions and second phases, because their volume and shape changes during their formation would be predicted to plastically deform the surrounding matrix, whereas at a bifilm boundary, these changes can be far more easily accommodated by deforming into the air gap, elastically prizing open the bifilm instead of plastically deforming the matrix. This process can be shown to be of the order of 100 to 1000 times less energic than plastic deformation of the matrix. It follows that, contrary to appearances, inclusions and second phases probably never form on grain boundaries, but only on bifilms (Campbell 2020). This may explain the observation that in many micrographs showing grain boundaries, it is often observed that some boundaries are decorated with inclusions or pores, whereas others remain completely clear. The ease of nucleating inclusions on a bifilm is interestingly analogous to the ease of initiating pores or cavities on a bifilm. The diffusion of gas to a bifilm, particularly but not exclusively in the liquid metal state, easily diffuses into the air gap and starts to expand the gap. The bifilm therefore starts to grow in volume, possibly becoming a spherical pore which can greatly exceed the size of the originating bifilm. This pore initiation mechanism avoids the usual considerations of the problems of nucleating an atomic-sized embryo in a liquid metal; suddenly, pore initiation becomes easy; the nucleation difficulty is avoided by the simple expansion between macroscopic skins. This fact corroborates much research on pore initiation during solidification, in which the pores appear to nucleate at trivial pressures, close to atmospheric, of gases in solution rather than the hundreds or thousands of atmospheres pressure required for the classical nucleation of an embryo. The positive pressure of gas inside the pore as discussed above, is mechanically analogous to the tensile stress outside the pore. Both internal gas, or external stress, both grow pores in the liquid state, and tend to grow cracks in the solid state. The population of cracks pre-exists, so that no nucleation of pores or cracks is necessary, only growth . 4. The Bifilm Population and Properties In the absence of gas in solution in the matrix, the bifilm can also expand as a result of the application of tensile stress, mechanically pulling the bifilm open. This is, of course, merely a crack opening process in which the bifilm is the original (Griffith) crack, and commences tensile failure by its subsequent propagation.