G.E. Fuchs

J. Campbell

John Campbell

K. Kim

Crossref

Metallurgical and Materials Transactions A

Discussion of “Formation of an Intermediate Layer Between Grains in Nickel-Based Superalloy Turbine Blades”*

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 probably of oxide or nitride, which appears to be capable of providing a coherent explanation of all the interesting observations reported by the authors

Campbell, John

University of Birmingham Research Portal

  Discussion of “Formation of an Intermediate LayerBetween Grains in Nickel-Based Superalloy TurbineBlades”*Campbell, JohnDOI:10.1007/s11661-017-4255-yLicense:Creative Commons: Attribution (CC BY)Document VersionPublisher's PDF, also known as Version of recordCitation for published version (Harvard):Campbell, J 2017, 'Discussion of “Formation of an Intermediate Layer Between Grains in Nickel-BasedSuperalloy Turbine Blades”*', Metallurgical and Materials Transactions A, vol. 48, no. 10, pp. 5157-5159.https://doi.org/10.1007/s11661-017-4255-yLink to publication on Research at Birmingham portalPublisher Rights Statement:The final publication is available at Springer via http://doi.org/10.1007/s11661-017-4255-yGeneral rightsUnless a licence is specified above, all rights (including copyright and moral rights) in this document are retained by the authors and/or thecopyright holders. The express permission of the copyright holder must be obtained for any use of this material other than for purposespermitted by law.•	Users may freely distribute the URL that is used to identify this publication.•	Users may download and/or print one copy of the publication from the University of Birmingham research portal for the purpose of privatestudy or non-commercial research.•	User may use extracts from the document in line with the concept of ‘fair dealing’ under the Copyright, Designs and Patents Act 1988 (?)•	Users may not further distribute the material nor use it for the purposes of commercial gain.Where a licence is displayed above, please note the terms and conditions of the licence govern your use of this document.When citing, please reference the published version.Take down policyWhile the University of Birmingham exercises care and attention in making items available there are rare occasions when an item has beenuploaded in error or has been deemed to be commercially or otherwise sensitive.If you believe that this is the case for this document, please contact UBIRA@lists.bham.ac.uk providing details and we will remove access tothe work immediately and investigate.Download date: 01. Feb. 2019Discussion of ‘‘Formation of anIntermediate Layer Between Grainsin Nickel-Based Superalloy TurbineBlades’’*JOHN CAMPBELLKim and Withey discuss the formation of an ‘interme-diate layer’, formed between grains, which they observein some Ni-base superalloys. This author proposes thatthe layer is the result of the presence of a biﬁlm, a doubleﬁlm probably of oxide or nitride, which appears to becapable of providing a coherent explanation of all theinteresting observations reported by the authors.DOI: 10.1007/s11661-017-4255-y The Author(s) 2017. This article is an open accesspublicationThe curious phenomenon of the ‘‘intermediate layer’’ inthe single crystal alloyCMSX-10was ﬁrst observed in 2015by Kim, Withey, and Griﬃths[1] and further studied andreported by Kim andWithey in Reference 2. The interme-diate layer appeared to be a layer of columnar c and c¢sandwiched between grains. This strange feature had anadditional number of puzzling features, including associ-ated layers of Re-rich particles which appeared to be aNi-based intermetallic containing a number of alloyingelements. The authors examine several potential explana-tions without, it seems, coming to ﬁrm conclusions.Because the feature is sited on the boundary betweenthe matrix grain and a stray grain, it seems probable tothis writer that a biﬁlm is highly likely to be present. Forinstance, biﬁlms formed in turbulently ﬁlled molds willbe pushed by the advancing fronts of growing grains,and so tend to be often sited at grain boundaries.However, in this case, the boundary appears to com-pletely surround the stray grain, a fact strongly sug-gesting that the matrix metal and the stray grain metalarrived in that region of the mold at diﬀerent times andwould therefore be entirely surrounded by a biﬁlm.For instance, a small jet of metal entering that part ofthe mold prematurely might then be starved of addi-tional metal, the metal therefore simply lodged in placein such narrow sections by its tightly constrained andoxidized meniscus. Within, perhaps, a fraction of asecond it will be overtaken and surrounded by thearrival of matrix metal. It is noteworthy, however, thatthe reverse might easily be true: the matrix metal ﬁllsmost of the mold but not the small area of the platform,which only ﬁlls later as the ﬁlling pressure rises.Either way, a double oxide ﬁlm is formed around thegrain. Eﬀectively, it is an oxide lap (colloquially, andunhelpfully, known as a ‘shut’). It is not a cold lap, sinceboth sides of the lap, eﬀectively constituting a biﬁlm,will be fully hot and liquid. The sequence of ﬁlling andof the oxidation of the metal menisci is illustrated inFigures 1(a) through (c).The biﬁlm surrounding the stray grain will beasymmetrical, having one thick and one thin oxide.Asymmetric biﬁlms are common. The two very diﬀerentoxides usually behave in quite diﬀerent ways. Forinstance, one side may ﬁnd itself favored as a substratefor the precipitation of second phases and intermetallics,as seems to occur commonly on biﬁlms.[3] The other sidemay be favorable to diﬀerent second phases or inter-metallics. In this case side 1 has become decorated withRe-rich intermetallics (Figures 1(d) and (e)), whereasside 2 remains clear. (Whether side 1 is the thick or thinside cannot be known at this time, since it depends onwhich liquid ﬂow arrived ﬁrst at that location, but doesnot aﬀect our reasoning.) Asymmetric biﬁlms as a resultof a conﬂuence weld problem in an Al alloy have beenconﬁrmed to display intermetallic precipitates only onthe thick side of the double ﬁlm.[4] Similarly, topolog-ically close packed (TCP) phases in superalloys areassociated with threats to performance, most probablybecause they appear to form on biﬁlms[3] and possiblyonly on biﬁlms.Because the matrix and the stray grain are separatedby a double oxide ﬁlm, containing a gap between like amoat, the two orientations can, in principle, be com-pletely unrelated. However, they are not likely to be verydiﬀerent simply because the two are probably joined farback at the point where the two streams diverged. Atthis point, of course, the orientations will be identical.In the case of a stray grain which has formed by late(or early!) arrival in this way, the diﬀerence in orienta-tion may be relatively insigniﬁcant from the point ofview of the performance of the casting. The major defectis, of course, the extensive biﬁlm acting as a crack(traditionally described as a ‘weak grain boundary’,which in a way, it is).The formation of the columnar arrays of c and c¢occurs during the cooling down from the solution heatJOHN CAMPBELL is with the Department of Metallurgy andMaterials, University of Birmingham, Birmingham, UK. Contacte-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.Article published online August 1, 2017METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 48A, OCTOBER 2017—5157treatment temperature (Figure 1(f)). The biﬁlm is afavored substrate for the formation of any new phasewhich involves a volume or shape change in the solidstate, because this can occur so much more easilyadjacent to the empty volume in the lattice provided bythe biﬁlm. (This eﬀect is widely observed with thenucleation of ferrite during the cooling of austenite. Thewhite-etching cracks (WECs) seen in the failures of windturbine bearings are a clear example delineating biﬁlmswhich have survived from the poorly controlled castingprocess[5]).It follows that c and c¢ will be nucleated earlybecause of the presence of a favored biﬁlm surface.This writer is inclined to think that the nucleation willbe most favored at the thin oxide which can deformeasily, in contrast to the thick side of the biﬁlm whichmay be too rigid to allow any accommodation ofstrain. It is also noteworthy that initiation of thephases will almost certainly not be inﬂuenced partic-ularly by surface energy eﬀects, which is rathercommon in metallurgy, especially in the liquid state.In contrast, in the solid state, the reduction of strainenergy by accommodating the volume and shapechanges will be dominant.The columnar region will start to form early, at itsfavored higher temperature. As the temperature falls,the remainder of the stray metal ahead of the interme-diate layer will transform en mass, bringing the steadyprogress of the layer to a stop, limiting the width of thecolumnar grain region. This general growth ahead ofthe layer will, of course, be initiated from the tips ofthe grains in the layer, ensuring that the layer and thegrain have a coherent interface as observed by theauthors.In summary, the defect described by the authors is anasymmetrical biﬁlm resulting from some kind of per-turbation during ﬁlling, resulting in an oxide lap defect.A schematic summary is shown in Figure 2.As an aside, the widespread nature of biﬁlms inNi-base superalloys is lent interesting support in theirﬁgure 8b of a grain boundary in alloy SRR99. It ispossible to discern a very ﬁne linear feature betweenthe two grains for which it is diﬃcult to imagine beinganything other than a simple biﬁlm, uncomplicated byprecipitates or intermediate layers. It appears to beonly about 20 nm thick, typical of many biﬁlms. Theauthors are to be congratulated on a signiﬁcant imagewhich would be well worth further investigation. Thepresence of a biﬁlm is to be expected in SRR99 alloy asa result of the high Al content in this alloy, and, itseems, zero B content. It is important to note that thealloy of our current discussion, CMSX-10, also doesnot appear to contain B, which is why the biﬁlmexplanation proposed in this communication is possi-ble, as clariﬁed below.These biﬁlm appearances contrast with investigationsinto IN100 alloy in which biﬁlms were searched-for,but not observed.[6] Only later was it realized thatbiﬁlms would not be expected in this alloy.[7] This wasa result of the eﬀect of boron greatly reducing themelting temperature of the oxide ﬁlm, composed ofFig. 1—The stages of formation of the intermediate layer. (a) Arrival of main liquid ﬂow; (b) thickening of its oxide ﬁlm with time; (c) arrival ofa later ﬂow; (d) the consequent asymmetric biﬁlm; (e) subsequent Re-rich precipitation on the thick side; and (f) intermediate layer initiation andgrowth from the thin side.Fig. 2—The biﬁlm crack originating in the liquid, creating favoredsites for Re-rich intermetallic precipitates from the liquid and growthof c and c¢ intermediate layer during cooling from heat treatment.5158—VOLUME 48A, OCTOBER 2017 METALLURGICAL AND MATERIALS TRANSACTIONS Aborates with melting points in the region of only1273 K (1000 C), on its surface. Analogously, boronsteels are similarly well known for their strength andtoughness, explained by this author to be a result oftheir low melting point oxide on the liquid steel,avoiding biﬁlms during the casting of these steels.[8](Unfortunately, the turbulence of the pouring actionsduring steelmaking results in huge quantities of otherentrainment defects, such as inclusions together withargon bubbles—the 1 pct residue in air—which canhelp initiate failure by cracking. Thus, unfortunately,these steels cannot be completely free from crack-ini-tiating defects[8]).For some steels and Ni-base alloys therefore, biﬁlmscan be avoided by taking advantage of the high metaltemperatures, arranging for the oxide on the liquidmetal surface to be liquid. Quite separately, biﬁlms canbe avoided by avoiding surface turbulence duringmelting and casting. The beneﬁts of good melting andcasting practice extend to the suppression of otherentrainment defects (bubbles and external inclusions) inaddition to biﬁlms, and so are to be recommended as abeneﬁt to supplement the beneﬁt of appropriatechemistry.In conclusion, biﬁlms are to be expected in most castmetals, including many steels and Ni-base alloys cast invacuum. Their near-universal presence in metalsrequires a rethinking of much of our traditional metal-lurgy. Ultimately, the elimination by improved technol-ogy of all entrainment defects, including biﬁlms,promises beneﬁts not only to vacuum-cast turbineblades, but across the whole of engineering.OPEN ACCESSThis article is distributed under the terms of theCreative Commons Attribution 4.0 InternationalLicense (http://creativecommons.org/licenses/by/4.0/),which permits unrestricted use, distribution, and re-production in any medium, provided you give appro-priate credit to the original author(s) and the source,provide a link to the Creative Commons license, andindicate if changes were made.REFERENCES1. K. Kim, P. Withey, and W.D. Griﬃths: Metal. Metall. Trans. A,2015, vol. 46A (March), pp. 1024–29.2. K. Kim and P. Withey: Metal. Metall. Trans. A, 2017, vol. 48A(June), pp. 2932–42.3. J. Campbell: Complete Casting Handbook, 2nd ed., Elsevier,Oxford, 2015.4. O. Garcia-Garcia, M. Sanches-Araiza, M. Castro-Roman, andB.J.C. Escobedo: In shape casting: 2nd International SymposiumTMS; Editors P.N. Crepeau, M. Tiryakioglu, J. Campbell.5. J. Campbell: Mater. Sci. Technol., 2012, vol. 28 (11), pp. 1358–67.6. G.E. Fuchs and M.A. Kaplan: Metal. Metall. Trans. A, 2016, vol.47A (May), pp. 2346–75.7. J. Campbell: Communication submitted to MMTA April 2017.8. J. Campbell: Steel Res. Int., 2016, vol. 87 (9999), p. 1.METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 48A, OCTOBER 2017—5159

  University of BirminghamDiscussion of “Formation of an Intermediate LayerBetween Grains in Nickel-Based Superalloy TurbineBlades”*Campbell, JohnDOI:10.1007/s11661-017-4255-yLicense:Creative Commons: Attribution (CC BY)Document VersionPublisher's PDF, also known as Version of recordCitation for published version (Harvard):Campbell, J 2017, 'Discussion of “Formation of an Intermediate Layer Between Grains in Nickel-BasedSuperalloy Turbine Blades”*', Metallurgical and Materials Transactions A, vol. 48, no. 10, pp. 5157-5159.https://doi.org/10.1007/s11661-017-4255-yLink to publication on Research at Birmingham portalPublisher Rights Statement:The final publication is available at Springer via http://doi.org/10.1007/s11661-017-4255-yGeneral rightsUnless a licence is specified above, all rights (including copyright and moral rights) in this document are retained by the authors and/or thecopyright holders. The express permission of the copyright holder must be obtained for any use of this material other than for purposespermitted by law.•	Users may freely distribute the URL that is used to identify this publication.•	Users may download and/or print one copy of the publication from the University of Birmingham research portal for the purpose of privatestudy or non-commercial research.•	User may use extracts from the document in line with the concept of ‘fair dealing’ under the Copyright, Designs and Patents Act 1988 (?)•	Users may not further distribute the material nor use it for the purposes of commercial gain.Where a licence is displayed above, please note the terms and conditions of the licence govern your use of this document.When citing, please reference the published version.Take down policyWhile the University of Birmingham exercises care and attention in making items available there are rare occasions when an item has beenuploaded in error or has been deemed to be commercially or otherwise sensitive.If you believe that this is the case for this document, please contact UBIRA@lists.bham.ac.uk providing details and we will remove access tothe work immediately and investigate.Download date: 01. Mar. 2020Discussion of ‘‘Formation of anIntermediate Layer Between Grainsin Nickel-Based Superalloy TurbineBlades’’*JOHN CAMPBELLKim and Withey discuss the formation of an ‘interme-diate layer’, formed between grains, which they observein some Ni-base superalloys. This author proposes thatthe layer is the result of the presence of a biﬁlm, a doubleﬁlm probably of oxide or nitride, which appears to becapable of providing a coherent explanation of all theinteresting observations reported by the authors.DOI: 10.1007/s11661-017-4255-y The Author(s) 2017. This article is an open accesspublicationThe curious phenomenon of the ‘‘intermediate layer’’ inthe single crystal alloyCMSX-10was ﬁrst observed in 2015by Kim, Withey, and Griﬃths[1] and further studied andreported by Kim andWithey in Reference 2. The interme-diate layer appeared to be a layer of columnar c and c¢sandwiched between grains. This strange feature had anadditional number of puzzling features, including associ-ated layers of Re-rich particles which appeared to be aNi-based intermetallic containing a number of alloyingelements. The authors examine several potential explana-tions without, it seems, coming to ﬁrm conclusions.Because the feature is sited on the boundary betweenthe matrix grain and a stray grain, it seems probable tothis writer that a biﬁlm is highly likely to be present. Forinstance, biﬁlms formed in turbulently ﬁlled molds willbe pushed by the advancing fronts of growing grains,and so tend to be often sited at grain boundaries.However, in this case, the boundary appears to com-pletely surround the stray grain, a fact strongly sug-gesting that the matrix metal and the stray grain metalarrived in that region of the mold at diﬀerent times andwould therefore be entirely surrounded by a biﬁlm.For instance, a small jet of metal entering that part ofthe mold prematurely might then be starved of addi-tional metal, the metal therefore simply lodged in placein such narrow sections by its tightly constrained andoxidized meniscus. Within, perhaps, a fraction of asecond it will be overtaken and surrounded by thearrival of matrix metal. It is noteworthy, however, thatthe reverse might easily be true: the matrix metal ﬁllsmost of the mold but not the small area of the platform,which only ﬁlls later as the ﬁlling pressure rises.Either way, a double oxide ﬁlm is formed around thegrain. Eﬀectively, it is an oxide lap (colloquially, andunhelpfully, known as a ‘shut’). It is not a cold lap, sinceboth sides of the lap, eﬀectively constituting a biﬁlm,will be fully hot and liquid. The sequence of ﬁlling andof the oxidation of the metal menisci is illustrated inFigures 1(a) through (c).The biﬁlm surrounding the stray grain will beasymmetrical, having one thick and one thin oxide.Asymmetric biﬁlms are common. The two very diﬀerentoxides usually behave in quite diﬀerent ways. Forinstance, one side may ﬁnd itself favored as a substratefor the precipitation of second phases and intermetallics,as seems to occur commonly on biﬁlms.[3] The other sidemay be favorable to diﬀerent second phases or inter-metallics. In this case side 1 has become decorated withRe-rich intermetallics (Figures 1(d) and (e)), whereasside 2 remains clear. (Whether side 1 is the thick or thinside cannot be known at this time, since it depends onwhich liquid ﬂow arrived ﬁrst at that location, but doesnot aﬀect our reasoning.) Asymmetric biﬁlms as a resultof a conﬂuence weld problem in an Al alloy have beenconﬁrmed to display intermetallic precipitates only onthe thick side of the double ﬁlm.[4] Similarly, topolog-ically close packed (TCP) phases in superalloys areassociated with threats to performance, most probablybecause they appear to form on biﬁlms[3] and possiblyonly on biﬁlms.Because the matrix and the stray grain are separatedby a double oxide ﬁlm, containing a gap between like amoat, the two orientations can, in principle, be com-pletely unrelated. However, they are not likely to be verydiﬀerent simply because the two are probably joined farback at the point where the two streams diverged. Atthis point, of course, the orientations will be identical.In the case of a stray grain which has formed by late(or early!) arrival in this way, the diﬀerence in orienta-tion may be relatively insigniﬁcant from the point ofview of the performance of the casting. The major defectis, of course, the extensive biﬁlm acting as a crack(traditionally described as a ‘weak grain boundary’,which in a way, it is).The formation of the columnar arrays of c and c¢occurs during the cooling down from the solution heatJOHN CAMPBELL is with the Department of Metallurgy andMaterials, University of Birmingham, Birmingham, UK. Contacte-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.Article published online August 1, 2017METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 48A, OCTOBER 2017—5157treatment temperature (Figure 1(f)). The biﬁlm is afavored substrate for the formation of any new phasewhich involves a volume or shape change in the solidstate, because this can occur so much more easilyadjacent to the empty volume in the lattice provided bythe biﬁlm. (This eﬀect is widely observed with thenucleation of ferrite during the cooling of austenite. Thewhite-etching cracks (WECs) seen in the failures of windturbine bearings are a clear example delineating biﬁlmswhich have survived from the poorly controlled castingprocess[5]).It follows that c and c¢ will be nucleated earlybecause of the presence of a favored biﬁlm surface.This writer is inclined to think that the nucleation willbe most favored at the thin oxide which can deformeasily, in contrast to the thick side of the biﬁlm whichmay be too rigid to allow any accommodation ofstrain. It is also noteworthy that initiation of thephases will almost certainly not be inﬂuenced partic-ularly by surface energy eﬀects, which is rathercommon in metallurgy, especially in the liquid state.In contrast, in the solid state, the reduction of strainenergy by accommodating the volume and shapechanges will be dominant.The columnar region will start to form early, at itsfavored higher temperature. As the temperature falls,the remainder of the stray metal ahead of the interme-diate layer will transform en mass, bringing the steadyprogress of the layer to a stop, limiting the width of thecolumnar grain region. This general growth ahead ofthe layer will, of course, be initiated from the tips ofthe grains in the layer, ensuring that the layer and thegrain have a coherent interface as observed by theauthors.In summary, the defect described by the authors is anasymmetrical biﬁlm resulting from some kind of per-turbation during ﬁlling, resulting in an oxide lap defect.A schematic summary is shown in Figure 2.As an aside, the widespread nature of biﬁlms inNi-base superalloys is lent interesting support in theirﬁgure 8b of a grain boundary in alloy SRR99. It ispossible to discern a very ﬁne linear feature betweenthe two grains for which it is diﬃcult to imagine beinganything other than a simple biﬁlm, uncomplicated byprecipitates or intermediate layers. It appears to beonly about 20 nm thick, typical of many biﬁlms. Theauthors are to be congratulated on a signiﬁcant imagewhich would be well worth further investigation. Thepresence of a biﬁlm is to be expected in SRR99 alloy asa result of the high Al content in this alloy, and, itseems, zero B content. It is important to note that thealloy of our current discussion, CMSX-10, also doesnot appear to contain B, which is why the biﬁlmexplanation proposed in this communication is possi-ble, as clariﬁed below.These biﬁlm appearances contrast with investigationsinto IN100 alloy in which biﬁlms were searched-for,but not observed.[6] Only later was it realized thatbiﬁlms would not be expected in this alloy.[7] This wasa result of the eﬀect of boron greatly reducing themelting temperature of the oxide ﬁlm, composed ofFig. 1—The stages of formation of the intermediate layer. (a) Arrival of main liquid ﬂow; (b) thickening of its oxide ﬁlm with time; (c) arrival ofa later ﬂow; (d) the consequent asymmetric biﬁlm; (e) subsequent Re-rich precipitation on the thick side; and (f) intermediate layer initiation andgrowth from the thin side.Fig. 2—The biﬁlm crack originating in the liquid, creating favoredsites for Re-rich intermetallic precipitates from the liquid and growthof c and c¢ intermediate layer during cooling from heat treatment.5158—VOLUME 48A, OCTOBER 2017 METALLURGICAL AND MATERIALS TRANSACTIONS Aborates with melting points in the region of only1273 K (1000 C), on its surface. Analogously, boronsteels are similarly well known for their strength andtoughness, explained by this author to be a result oftheir low melting point oxide on the liquid steel,avoiding biﬁlms during the casting of these steels.[8](Unfortunately, the turbulence of the pouring actionsduring steelmaking results in huge quantities of otherentrainment defects, such as inclusions together withargon bubbles—the 1 pct residue in air—which canhelp initiate failure by cracking. Thus, unfortunately,these steels cannot be completely free from crack-ini-tiating defects[8]).For some steels and Ni-base alloys therefore, biﬁlmscan be avoided by taking advantage of the high metaltemperatures, arranging for the oxide on the liquidmetal surface to be liquid. Quite separately, biﬁlms canbe avoided by avoiding surface turbulence duringmelting and casting. The beneﬁts of good melting andcasting practice extend to the suppression of otherentrainment defects (bubbles and external inclusions) inaddition to biﬁlms, and so are to be recommended as abeneﬁt to supplement the beneﬁt of appropriatechemistry.In conclusion, biﬁlms are to be expected in most castmetals, including many steels and Ni-base alloys cast invacuum. Their near-universal presence in metalsrequires a rethinking of much of our traditional metal-lurgy. Ultimately, the elimination by improved technol-ogy of all entrainment defects, including biﬁlms,promises beneﬁts not only to vacuum-cast turbineblades, but across the whole of engineering.OPEN ACCESSThis article is distributed under the terms of theCreative Commons Attribution 4.0 InternationalLicense (http://creativecommons.org/licenses/by/4.0/),which permits unrestricted use, distribution, and re-production in any medium, provided you give appro-priate credit to the original author(s) and the source,provide a link to the Creative Commons license, andindicate if changes were made.REFERENCES1. K. Kim, P. Withey, and W.D. Griﬃths: Metal. Metall. Trans. A,2015, vol. 46A (March), pp. 1024–29.2. K. Kim and P. Withey: Metal. Metall. Trans. A, 2017, vol. 48A(June), pp. 2932–42.3. J. Campbell: Complete Casting Handbook, 2nd ed., Elsevier,Oxford, 2015.4. O. Garcia-Garcia, M. Sanches-Araiza, M. Castro-Roman, andB.J.C. Escobedo: In shape casting: 2nd International SymposiumTMS; Editors P.N. Crepeau, M. Tiryakioglu, J. Campbell.5. J. Campbell: Mater. Sci. Technol., 2012, vol. 28 (11), pp. 1358–67.6. G.E. Fuchs and M.A. Kaplan: Metal. Metall. Trans. A, 2016, vol.47A (May), pp. 2346–75.7. J. Campbell: Communication submitted to MMTA April 2017.8. J. Campbell: Steel Res. Int., 2016, vol. 87 (9999), p. 1.METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 48A, OCTOBER 2017—5159

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Discussion of “Formation of an Intermediate Layer Between Grains in Nickel-Based Superalloy Turbine Blades”*

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