technique. Discussion of ‘‘Formation of an Intermediate Layer Between Grains in Nickel-Based Superalloy Turbine Blades’’* JOHN CAMPBELL is with the Department of Metallurgy and Materials, University of Birmingham, Birmingham, UK. Contact: e-mail: jc@campbelltech.co.uk *K. Kim and P. Withey, Metall. Mater. Trans. A, 2017, vol. 48A, pp. 2932–42. Manuscript submitted May 30, 2017. Kim and Withey discuss the formation of an ‘intermediate layer’, formed between grains, which they observe in some Ni-base superalloys. This author proposes that the layer is the result of the presence of a bifilm, a double film probablyof oxide or nitride, which appears to be capable of providing a coherent explanation of all the interesting observations reported by the authors. DOI : 10.1007/s11661-017-4255-y _ The Author(s) 2017. This article is an open access publication may find itself favored as a substrate for the The curious phenomenon of the ‘‘intermediate layer’’ in the single crystal alloy CMSX-10precipitation of second phases and inter- was first observed in 2015 by Kim, Withey, and Griffiths[1] and further studied and reportedmetallics, as seems to occur commonly on byKim and Withey in Reference 2. The intermediate layer appeared to be a layer of colum bifilms.[3] The other side may be favorable nar to different second phases or intermetallics. ɣand ɣ’ sandwiched between grains. This strange feature had an additional number of puzzling features, including associated layers of Re-rich particles which appeared to beIn this case side 1 has become decorated with a Ni-based intermetallic containing a number of alloying elements. The authors examineRe-rich intermetallics (Figures 1 (d) and (e)), several potential explanations without, it seems, coming to firm conclusions. whereas side 2 remains clear. (Whether side Because the feature is sited on the boundary between the matrix grain and a stray grain, it1 is the thick or thin side cannot be known seems probable to this writer that a bifilm is highly likely to be present. For instance, bifilmsat this time, since it depends on which liquid formed in turbulently filled molds will be pushed by the advancing fronts of growing grains,flow arrived first at that location, but does not and so tend to be often sited at grain boundaries. affect our reasoning.) Asymmetric bifilms as However, in this case, the boundary appears to completely surround the stray grain, a facta result of a confluence weld problem in an strongly suggesting that the matrix metal and the stray grain metal arrived in that region ofAl alloy have been confirmed to display inter- the mold at different times and would therefore be entirely surrounded by a bifil metallic precipitates only on the thick side of the double film[4] Similarly, topologically close packed (TCP) phases in superalloys are For instance, a small jet of metal entering thataround the grain. Effectively, it is an oxideassociated with threats to performance, most lap (colloquially, and unhelpfully, known as aprobably because they appear to form on part of the mold prematurely might then be bifilms[3] and possibly starved of additional metal, the metal the-‘shut’). It is not a cold lap, since both sides ofonly on bifilms. refore simply lodged in place in such narrowthe lap, effectively constituting a bifilm, will sections by its tightly constrained and oxi-be fully hot and liquid. The sequence of fillingBecause the matrix and the stray grain are dized meniscus. Within, perhaps, a fraction ofand of the oxidation of the metal menisci isseparated by a double oxide film, containing a a second it will be overtaken and surroundedillustrated in Figures 1(a) through (c).gap between like a moat, the two orientations by the arrival of matrix metal. It is notewor- can, in principle, be completely unrelated. thy, however, that the reverse might easily beThe bifilm surrounding the stray grain will beHowever, they are not likely to be very diffe- true: the matrix metal fills most of the moldasymmetrical, having one thick and one thinrent simply because the two are probably but not the small area of the platform, whichoxide. Asymmetricbifilms are common. Thejoined far back at the point where the two only fills later as the filling pressure rises.two very different oxides usually behave instreams diverged. At this point, of course, the Either way, a double oxide film is formed quite different ways. For instance, one sideorientations will be identical. Figure 1. The stages of formation of the alternate layer. (a) arrival of maine liquid flow; (b) thickening of its oxide film with time; (c) arrival of a later flow; (d) the consequent asymmetric bifilm; (e) subsequent Re-rich precipitation onthe thick side; and (f) intermediate layer initiation and growth from the thin side. N°19 • SEPTEMBRE 2020 • 27