technique. In the case of a stray grain which has formed good melting and casting practice extend to by late (or early!) arrival in this way, the the suppression of other entrainment defects difference in orientation may be relatively (bubbles and external inclusions) in addition insignificant from the point of view of the to bifilms, and so are to be recommended as performance of the casting. The major defect a benefit to supplement the benefit of appro- is, of course, the extensive bifilm acting as a priate chemistry. crack (traditionally described as a ‘weak grain boundary’, which in a way, it is). In conclusion, bifilms are to be expected Theformation of the columnar arrays of ɣ in most cast metals, including many steels andɣ’ occurs during the cooling down from and Ni-base alloys cast in vacuum. Their the solution heat treatment temperature near-universal presence in metals requires a (Figure 1(f)). rethinking of much of our traditional metallurgy. Ultimately, the elimination by improved The bifilm is a favored substrate for the for- technology of all entrainment defects, inclu- mation of any new phase which involves a ding bifilms, promises benefits not only to volume or shape change in the solid state, Figure 2. The bifilm crack originating in the liquid, vacuum-cast turbine blades, but across the because this can occur so much more easily creating favored sites for Re-rich intremetallic whole of engineering. adjacent to the empty volume in the lattice precipitatesfrom the liquid and growth of and ‘ɣɣ intremediate layer during cooling from heat provided by the bifilm. (This effect is widely treatment. >>> OPEN ACCESS observed with the nucleation of ferrite during the cooling of austenite. The white-etching other than a simple bifilm, uncomplicated by This article is distributed under the terms of cracks (WECs) seen in the failures of wind precipitates or intermediate layers. It appears the Creative Commons Attribution 4.0 Inter- turbine bearings are a clear example deli- to be only about 20 nm thick, typical of many national License (http://creativecommons. neating bifilms which have survivedfrom the bifilms. The authors are to be congratulated org/licenses/by/4.0/), which permits unres- poorly controlled casting process[5]). on a significant image which would be well tricted use, distribution, and reproduction in worth further investigation. The presence of any medium, provided you give appropriate Itfollows thatɣ ɣand ’ will be nucleated early a bifilm is to be expected in SRR99 alloy as a credit to the original author(s) and the source, because of the presence of a favored bifilm result of the high Al content in this alloy, and, provide a link to the Creative Commons surface. This writer is inclined to think that it seems, zero B content. It is important to license, and indicate if changes were made. the nucleation will be most favored at the thin note that the alloy of our current discussion, oxide which can deform easily, in contrast to CMSX-10, also does not appear to contain B, >>> REFERENCES the thick side of the bifilm which may be too which is why the bifilm explanation proposed rigid to allow any accommodation of strain. in this communication is possible, as clarified 1. K. Kim, P. Withey, and W.D. Griffiths: It is also noteworthy that initiation of the below. Metal. Metall. Trans. A, 2015, vol. 46A phases will almost certainly not be influenced (March), pp. 1024–29. particularly by surface energy effects, which These bifilm appearances contrast with is rather common in metallurgy, especially in investigations into IN100 alloy in which bifilms 2. K. Kim and P. Withey: Metal. Metall. the liquid state. In contrast, in the solid state, were searched-for, but not observed.[6] Only Trans. A, 2017, vol. 48A (June), pp. the reduction of strain energy by accommo- later was it realized that bifilms would not be 2932–42. dating the volume and shape changes will be expected in this alloy.[7] This was a result of dominant. the effect of boron greatly reducing the mel- 3. J. Campbell: Complete Casting The columnar region will start to form early, ting temperature of the oxide film, composed Handbook, 2nd ed., Elsevier, Oxford, at its favored higher temperature. As the of borates with melting points in the region of 2015. temperature falls, the remainder of the stray only 1273 K (1000 °C), on its surface. Analo- metal ahead of the intermediate layer will gously, boron steels are similarly well known 4. O. Garcia-Garcia, M. Sanches-Araiza, transform en mass, bringing the steady pro- for their strength and toughness, explained M. Castro-Roman, and B.J.C. Escobedo: gress of the layer to a stop, limiting the width by this author to be a result of their low mel- In shape casting: 2nd International of the columnar grain region. This general ting point oxide on the liquid steel, avoiding Symposium TMS; Editors P.N. Crepeau, growth ahead of the layer will, of course, be bifilms during the casting of these steels.[8] M. Tiryakioglu, J. Campbell. initiated from the tips of the grains in the (Unfortunately, the turbulence of the pouring layer, ensuring that the layer and the grain actions during steelmaking results in huge 5. J. Campbell: Mater. Sci. Technol., 2012, have a coherent interface as observed by the quantities of other entrainment defects, such vol. 28 (11), pp. 1358–67. authors. as inclusions together with argon bubbles— In summary, the defect described by the the 1 pct residue in air—which can help ini- 6. G.E. Fuchs and M.A. Kaplan: Metal. authors is an asymmetrical bifilm resulting tiate failure by cracking. Thus, unfortunately, Metal. Trans. A, 2016, vol. 47A (May)pl, p. from some kind of perturbation during filling, these steels cannot be completely free from 2346–75. resulting in an oxide lap defect. A schematic crack-initiating defects[8]). summary is shown in Figure 2. 7. J. Campbell: Communication sub- For some steels and Ni-base alloys therefore, mitted to MMTA April 2017. As an aside, the widespread nature of bifilms bifilms can be avoided by taking advantage in Ni-base superalloys is lent interesting sup- of the high metal temperatures, arranging 8. J. Campbell: Steel Res. Int., 2016, vol. port in their figure 8b of a grain boundary in for the oxide on the liquid metal surface 87 (9999), p. 1. alloy SRR99. It is possible to discern a very to be liquid. Quite separately, bifilms can fine linear feature between the two grains for be avoided by avoiding surface turbulence which it is difficult to imagine being anything during melting and casting. The benefits of 28 • N°19 • SEPTEMBRE 2020