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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In an alloy where ductility has been limited by the strengthening of the matrix, the propagation of the crack is described by the well-known rules of fracture mechanics. Conversely, in a ductile metal the crack will not propagate, but the crack tip will blunt as the crack opens up. The plastic shearing process is the mechanism by which the first crack can link to the next, as the material between bifilms necks down to 100% reduction in area. The final macroscopic crack is a series of bifilms at different levels close to the general propagation direction, which are joined by plastic steps. The relatively smooth, fairly planar bifilm regions between the plastic ledges are often designated quasi-cleavage regions. They are bifilms hiding in plain sight. The effect of bifilms on tensile elongation is dramatic, increasing elongation values for heat treated Al-7Si-Mg alloys from a typical 3% to 20% or more for well-cast alloys (Campbell 2015). Turning to the effect of bifilms on creep; grain boundaries which are effectively weakened by lack of bonding would be expected to slide with greater ease. Creep resistance would therefore be expected to be impaired. There seems good evidence for this in the contrasting behavior of single crystals and polycrystals. The Ni-based superalloys solidified conventionally nucleate and grow their grains randomly, so that the bifilms in suspension are pushed ahead of the grains, and finally finish in the boundaries. Most of the bifilm population becomes entrapped in this way. In contrast, the single crystal is grown slowly, in an upward direction, and will push the bifilms ahead and out of the casting. The conventional explanation that weak grain boundaries are eliminated is true (Reed 2006), because of the near-universal presence of bifilms in cast metals. It is interesting to speculate that if the bifilms could be avoided the polycrystal might have its creep resistance raised to equal or exceed those of the single crystal. Benefits to creep resistance in steels for heat exchangers used in power generation could be valuable for the next generation of nuclear stations. The benefits of resistance to fatigue has been well documented for aluminium alloys; there seems to be a linear relation between fatigue life and bifilm content as assessed by the tensile elongation (Tiryakioglu 2014). In addition, the author has drawn attention to the difference in the fatigue lives of flake and spheroidal graphite cast irons which have been shown to differ by a factor of about 10 5 . This amazingly high number (which he thinks is an underestimate!) arises because the flake graphite appears to form on silica (SiO 2 ) bifilms, and so is a clear representation of the bifilm population. Because the addition of magnesium immediately reduces the SiO 2 to Si + MgO, the bifilms are eliminated so that graphite can now only form by wrapping completely around nuclei in suspension, forming spheroidal graphite iron. This brief explanation is significantly simplified. The reader is directed to full descriptions elsewhere (Campbell 2015, 2020). These illustrations from non-ferrous and ferrous metals both relate to the situation where the bifilms are relatively small, but numerous, as a dense population. In other circumstances, bifilms of large size (measured typically in cm or even fractions of meters) are present, segregated into the centres of large ingot castings. The failure of larger components such as wind turbine main bearings appear to be the direct result of the presence of major bifilm inclusions, measured in cm or larger, and which are linked by highly stressed ligaments of matrix which is subject to failure by classical step-wise advance of cracks (Campbell 2020). Centimetre-sized bifilms are predicted in vacuum arc remelted (VAR) which have cause aircraft, particularly helicopters, to fail with loss of life (Campbell 2022). In the opinion of this author, VAR steels and Ni alloys cannot be recommended for safety-critical applications because of the large bifilms they can contain, and which are not easily detected. Invasive corrosion leading to corrosion pits and perhaps even stress corrosion cracking (SCC) are to be expected if bifilms are present. Certain heat treatments lead to precipitation of new phases such as carbides. These will precipitate against bifilms to reduce the strain energy of formation, which in turn forces open the bifilm. In this way corrodents are expected to be sucked long distances into the alloy by capillary attraction. The bifilm-free ligaments of matrix holding the alloy together against a tensile force are now subjected to raised stress, which, together with the chemical potential differences between the matrix and carbides, will enhance the dissolution of the ligaments leading to failure. Hydrogen embrittlement is another failure mode which seems consistent with the concept of bifilms. When chemically charging metals with hydrogen, researchers have complained about the formation of blisters, which upset their experiment. Furthermore, the complaint is that the nucleation of the blisters “cannot be understood”. The bifilm concept removes any nucleation problem because of the pre-existing crack population: the blisters are merely the inflation of bifilms close to the surface. The subject is described in more detail elsewhere (Campbell 2020).