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[Chihiro Tabata](https://orcid.org/0000-0001-6597-4998), [Toshio Osada](https://orcid.org/0000-0003-1539-9264), [Tadaharu Yokokawa](https://orcid.org/0000-0003-1595-6729), [Ayako Ikeda](https://orcid.org/0000-0002-1705-9004), [Kyoko Kawagishi](https://orcid.org/0000-0001-7652-9232), Shinsuke Suzuki

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[Effect of Solution Heat-Treatment on the Oxidation Resistance of Ni-Base Single-Crystal Superalloy](https://mdr.nims.go.jp/datasets/436e22ff-2718-4f7b-a97d-c5af093aa029)

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Effect of Solution Heat-Treatment on the Oxidation Resistance of Ni-Base Single-Crystal SuperalloyORIGINAL RESEARCH ARTICLEEffect of Solution Heat-Treatment on the OxidationResistance of Ni-Base Single-Crystal SuperalloyCHIHIRO TABATA, TOSHIO OSADA, TADAHARU YOKOKAWA, AYAKO IKEDA,KYOKO KAWAGISHI, and SHINSUKE SUZUKITo clarify the effect of solution heat-treatment on the oxidation resistance of Ni-basesingle-crystal superalloy TMS-238, the evaluation of dendrite/inter-dendrite segregation ofalloying elements in the as-cast and heat-treated samples, and its effect on cyclic oxidationresistance were investigated. Cyclic oxidation test results at 1100 �C for up to 150 cycles clearlyshowed that the as-cast samples with element segregations had lower oxidation resistancecompared to heat-treated samples with homogeneous structure. Further, for the as-cast sample,rapid growth and spallation of oxide consisting of NiO, Cr2O3, and Al2O3 were observedaround the dendrite core for 10 cycles of oxidation. Analysis of sub-surface on sampleisothermally oxidized at 1100 �C for 10 minutes showed that rapid oxide growth is due to theformation of discontinuous Al2O3 layer at dendrite core with lower Al concentration.Furthermore, in this study, the threshold value of Al concentration and Gibbs energy for theformation of continuous Al2O3 layer were estimated and determined to be around 5.2 wt pctand � 556.6 ± 0.5 kJ/mol, respectively. This indicated that the solution heat-treatment forTMS-238 should be conducted above 1305 �C for exhibiting oxidation resistance at 1100 �C, tomeet the threshold value within the whole region between dendrite and inter-dendrite.https://doi.org/10.1007/s11661-023-07205-7� The Author(s) 2023I. INTRODUCTIONNI-BASE single-crystal superalloys with c/c¢ two-phase structures are materials often used in the turbineblades for the hottest components in jet engines and gasturbines for power generation. To improve the thermalefficiency of the engines, the inlet gas temperatures areon the rise, and the turbine inlet temperature must beincreased. The components must also be able towithstand the highest temperature and pressure withinthe engines. Therefore, the components often usesingle-crystal (SC) alloys.[1] To improve its hightemperature creep strength and other mechanical prop-erties, optimization of chemical compositions by alloydesign,[2–5] as well as solution heat-treatment and agingheat-treatment temperatures[6–8] for Ni-base SC super-alloys are being studied.Solution heat-treatment of cast alloy is an importantprocess for achieving a desired homogeneous c/c¢two-phase structure,[9,10] since the as-cast materials havestrong segregation of elements, leading to large differ-ences in the compositions of the dendrite core andinter-dendrite[11,12] and observations of remaining eutec-tic phases. However, recent superalloys have signifi-cantly limited process windows for solutionheat-treatment, and in order to avoid formations ofeutectic phases, the heat-treatment process becomesseverely complex.[6,13] On the other hand, alloys such asTMS-238, a 6th generation alloy with excellent creepstrength, have larger process windows for heat-treat-ment.[14] But this material requires heat-treatment attemperatures as high as 1335 �C, and methods to reducethe process cost, such as decreasing the solutionheat-treatment temperature and time, are needed.[11,14]Furthermore, because Ni-base superalloys are oftenused at high temperatures, oxidation resistance becomesan important property as well. Therefore, measurementand observations of high temperature oxidation resistanceof homogenized superalloys have been conducted,[15–21]and methods to predict the oxidation resistance of theCHIHIRO TABATA, and KYOKO KAWAGISHI are with theNational Institute for Materials Science (NIMS), 1-2-1 Sengen,Tsukuba, Ibaraki 305-0047, Japan and also with the Department ofMaterials Science, Waseda University, 3-4-1 Okubo, Shinjuku-ku,Tokyo 169-8555, Japan. Contact e-mail: chihiro448@akane.waseda.jpTOSHIO OSADA, TADAHARU YOKOKAWA, and AYAKOIKEDA are with the National Institute for Materials Science(NIMS). SHINSUKE SUZUKI is with the Department of MaterialsScience, Waseda University and with the Department of AppliedMechanics and Aerospace Engineering, Waseda University, 3-4-1Okubo, Shinjuku-ku, Tokyo 169-8555, Japan and also with theKagami Memorial Institute for Materials Science and Technology,Waseda University, Tokyo 169-0051, Japan.Manuscript submitted June 21, 2023; accepted September 10, 2023.Article published online October 2, 2023METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 54A, DECEMBER 2023—4825http://crossmark.crossref.org/dialog/?doi=10.1007/s11661-023-07205-7&domain=pdfalloys at such temperatures[22] have been studied byvarious researchers. For example, Sato et al. proposed amethod using effective valence index and Gibbs energy topredict whether the homogenized alloys will likely forma-Al2O3 for oxidation at 900 �C and 1000 �C.[23,24] Manyalso consider the effect of the composition of alloys on theoxidation resistance of alloys.[17,25–29]Hagen et al. reportedon the difference in the scale width and cyclic oxidation at850 �C and 1050 �C among the as-cast, short heat-treat-ment, and long heat-treatment on Co-base SC superal-loys.[30] They reported that the as-cast sample exhibited lessresistance toward oxidation compared to the solutionheat-treated samples, and the alloy with higher Coconcentration showed lower oxidation resistance. But onlylimited studies consider the effect of the solutionheat-treat-ment and segregation of the elements on the oxidationresistance of the Ni-base SC superalloys. Therefore, it isdifficult to determine the lower limit of solution heat-treat-ment temperature necessary to maintain the level ofoxidation resistance which the designed alloy originallyhas, all while considering the reduction of the process cost.The objective of this research is to clarify the effect ofsolution heat-treatment on the oxidation resistance ofNi-base SC superalloy, by comparing the as-cast mate-rial and solution heat-treated material. The composi-tions of the dendrite core and inter-dendrite within thesuperalloy were analyzed, and the differences in thestructures of the oxide layers were evaluated.II. EXPERIMENTAL PROCEDURESSingle-crystal Ni-base superalloy TMS-238[14] wasused for this research. The alloy was cast into the SCalloy by using the directional solidification furnace.High-frequency vacuum induction heating was used tomelt the materials and kept at 1600 �C for 15 minutesunder 6� 10�2 Pa. The melt was poured into the moldand withdrawn from the heating chamber to the coolingchamber at 200 mm/h to be unidirectionally solidifiedinto single crystal cylindrical bars, with diameters of11 mm. The alloy compositions were analyzed usinginductively coupled plasma optical emission spectrom-eter (ICP-OES) for most of the elements except S, whichwas analyzed using glow discharge mass spectrometry(GD-MS). One sample was not heat-treated, which willbe referred to as the as-cast sample. The other samplewas heat-treated at 1335 �C for 20 hours, which isknown to be enough for the TMS-238 to be homoge-nized.[11] Aging heat-treatment was not conducted forthis research since we are only focusing on the dendriticsegregation of elements. Samples for observations andoxidation tests were cut normal to the growth direction,which is [001], from the cylindrical SC bars to be 5 mmin height and 9 mm in diameter. The surfaces andcross-sections of the samples were polished using U.S.Grid #600 SiC abrasive paper and mirror polished.The samples were observed using electron probemicro analyzer (EPMA, SHIMADZU EPMA1610) withthe accelerating voltage at 15 kV, the beam current at 20nA, and the beam size at 1 lm. The compositions of thedendrite core and inter-dendrite for each alloy were alsoanalyzed using EPMA. Different areas of the dendritecore and inter-dendrite were analyzed five times each,and the average concentrations were obtained. Thegradual changes in the compositions of the as-castsample were also analyzed using EPMA by pointanalysis, starting from the dendrite core and ending atthe center of inter-dendrite, with each point analysisbeing 5 lm apart.Cyclic oxidation tests were conducted at 1100 �C for150 cycles, where each cycle consists of 1 hour ofheating at 1100 �C and 1 hour of cooling in air.Separate samples oxidized at 1100 �C for 10 cycles wereobserved from the top using field emission scanningelectron microscopy (FE-SEM, ZEISS Gem-iniSEM300). Samples were also isothermally oxidizedat 1100 �C for 10 minutes, and the cross-section of theoxides scales that had formed in both the dendrite coreand inter-dendrite were observed by creating cross-sec-tions using focused ion beam (FIB) milling and observedusing scanning electron microscopy and energy-disper-sive spectroscopy (SEM-EDS, ZEISS Gemini 2 Cross-beam 550). This is sufficient amount of time for theoxide layer to grow and be intact for observation. Thecross-sectional elemental maps of the oxide scales forboth samples were obtained using SEM-EDS (ZEISSGemini 2 Crossbeam 550) as well.III. RESULTSFigure 1 shows the back-scattered electron images(BEIs) of both the as-cast and heat-treated samples. Theas-cast sample shows clear image of the dendrites, whilethe dendrites for the heat-treated sample have faded.This indicates that the diffusion of elements occurred,and some of the segregation had cleared. The compo-sitions of the alloy analyzed using ICP-OES andGD-MS, as well as the compositions at the dendritecore and inter-dendrite analyzed using EPMA areshown in Table I. Al, which tends to partition into theliquid phase during solidification, and segregates to theinter-dendrite. On the other hand, Cr, which tends topartition into the solid phase, segregates to the dendritecore. For the as-cast material, there was a cleardifference in the composition between the dendrite coreand inter-dendrite. For example, the amount of Al wassignificantly larger at the inter-dendrite. On the otherhand, the amount of Cr was larger at the dendrite core.The heat-treated sample had similar compositions at thedendrite core and inter-dendrite, suggesting that thesample had been homogenized successfully.To understand whether heat-treatment affects theoxidation resistance of the alloy, cyclic oxidation testswere conducted at 1100 �C, and the results are shown inFigure 2. The results for the cyclic oxidation ofTMS-238 samples with low S and fully heat-treated byconducting both solution heat-treatment at 1345 �C for20 hours and aging heat-treatments at 1150 �C for 2hours, and 870 �C for 20 hours are shown as Reference31. The referenced results have almost the same4826—VOLUME 54A, DECEMBER 2023 METALLURGICAL AND MATERIALS TRANSACTIONS Amicrostructure for the oxide layer, but the S segregationlevel has been suppressed, leading to the reduction inspallation of the oxides. The differences in the cyclicoxidation between the heat-treated sample and thereferenced results are most likely due to the differencesin the S segregation level. The as-cast sample hadslightly larger change in mass at the beginning of thecyclic oxidation test. By 75 cycles, there was a cleardifference between the two samples, where the heat-treated sample had less changes in mass compared to theas-cast sample.To observe the difference in the initial oxidationresistance, secondary electron images (SEIs) taken fromthe top of both samples oxidized at 1100 �C for 10 cycleswere observed, as shown in Figure 3(b) for the as-castsample, and Figure 4(b) for the heat-treated sample. Thedirections of the observations are shown in the sche-matic diagrams in Figures 3(a) and 4(a). For the as-castsample, oxide spallation can be observed around thedendrite core, shown in dotted white lines, and the shapeof the dendrites can be seen within the area where theoxides had spalled. On the other hand, the heat-treatedsample showed no signs of spallation, and the oxideswere intact. Therefore, it has been made clear thatwithout solution heat-treatment, the oxidation resis-tance of the SC superalloys decreases. To understand ifthere were any differences in the structures of the oxidelayers, samples oxidized at 1100 �C for 10 minutes werealso prepared and observed, shown in Figure 3(c) for theas-cast samples, and Figure 4(c) for heat-treated sam-ples. The cross-sections were created using FIB millingand observed using SEM. The locations where themilling and observations were conducted are shown inthe schematic diagrams in Figures 3(a) and 4(a). InFig. 1—BEIs of the as-cast and the heat-treated (1335 �C 20 h) TMS-238. The light areas are dendrite cores, while the dark areas areinter-dendrites. The analyzed areas for the chemical compositions shown in Table I are added in the BEIs as blue crosses for the dendrite cores,and orange circles for the inter-dendrites (Color figure online).Table I. Nominal[14] and Analyzed Compositions of TMS-238 (Ni bal.)Sample/LocationCompositionsAnalysis MethodWt Pct ppmCo Cr Mo W Al Ta Re Ru Hf SNominal[14] 6.5 4.6 1.1 4.0 5.9 7.6 6.5 5.0 0.1 —Analyzed 6.3 4.4 1.1 4.0 6.0 7.8 6.3 5.0 0.1 2.4 ICP-OES, (GD-MS for S)As-Cast Dendrite Core 6.6 4.6 1.1 5.7 4.8 5.4 11.5 5.3 0.2 — EPMAInter-dendrite 5.1 2.8 0.7 1.9 7.4 13.2 1.7 4.4 0.2 —Heat-Treated Dendrite Core 6.5 4.5 1.1 4.0 5.7 7.3 7.4 5.0 0.3 — EPMAInter-dendrite 6.4 4.7 1.1 4.1 5.6 7.9 5.6 5.4 0.1 —Fig. 2—Sample mass changes of the as-cast and the heat-treatedTMS-238 under 1100 �C 1 h cyclic oxidation tests. Results for fullyheat-treated (solution and aging) TMS-238 samples with low Ssegregation level are shown as a Ref. [31].METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 54A, DECEMBER 2023—4827Figure 3(c-1), which shows the BEI of the cross-sectionof as-cast sample that had been oxidized, clear differ-ences in the thickness of the oxide scales were observed,depending on the location. In Figure 3(c-2), the dendritecore had thick layers with discontinuous and spinel-likeoxides, whereas in Figure 3(c-3), for the inter-dendrite,the layers are continuous and less than half in width.For heat-treated samples, however, continuous oxidelayers had formed in both the dendrite core andinter-dendrite. As shown in Figures 4(c-1) and (c-2),the thickness of the oxide layers was roughly eventhroughout the whole sample.In order to observe the difference in the compositionsof the oxide layers, SEIs and EDS elemental maps of theoxide layers for both as-cast and heat-treated samplesoxidized at 1100 �C for 10 minutes were taken, as shownin Figure 5. The oxide scales of the samples oxidized at1100 �C usually consist of NiO on top, followed by themixture layer of mainly Cr2O3, and the Al2O3 on top ofthe substrate.[17] Therefore, we focused our analyses onthe elements known to form the oxide layers: Ni, Al, andCr. For the as-cast sample, the elemental maps of theoxide scales, taken from the area shown in blue, for thedendrite core are shown in Figure 5(a), and the resultsfor the inter-dendrite are shown in Figure 5(b). For thedendrite core, the oxide layers were mostly Cr2O3, andthe continuous Al2O3 layer could not be observed. Forthe inter-dendrite, continuous Al2O3 layer was observed,and both the Cr2O3 layer and NiO layer were half thesize of the layers observed in the dendrite core. Similarresults were obtained for the heat-treated sample shownin Figure 5(c), where continuous Al2O3 layer could beobserved in the SEI and EDS mapping for Al.IV. DISCUSSIONOur finding here clearly shows that rapid oxidegrowth at dendrite core in the as-cast alloy acceleratesthe spallation during cyclic oxidation, and the oxidegrowth rate was slower at the inter-dendrite, wherecontinuous Al2O3 layer had formed. Meanwhile, it hasbeen reported that the transition criteria for continu-ous-to-discontinuous Al2O3 could be determined byGibbs energy of the Al2O3 formation.[23,24] To furtherunderstand the effect of homogenization on the oxida-tion resistance, here, we summarized the transitiondistance of continuous-to-discontinuous Al2O3 forma-tion (Figure 6(a)), oxide scale thickness (Figure 6(b)),concentration of alloying elements on sub-surfaceFig. 3—SEM images of an oxidized as-cast sample. (a) Schematic diagrams showing the directions of observations. (b) Top view of sampleoxidized at 1100 �C for 10 cycles, with the white dotted lines showing the area of oxide spallation. (c-1) Cross-sectional BEI of isothermaloxidation test at 1100 �C for 10 min, with the blue rectangles representing where the enlarged images of (c-2) the dendrite core and (c-3)inter-dendrite were taken (Color figure online).4828—VOLUME 54A, DECEMBER 2023 METALLURGICAL AND MATERIALS TRANSACTIONS A(Figure 6(c)), and calculated Al activity and Gibbsenergy of the Al2O3 formation (Figure 6(d)), within thedendrite core to inter-dendrite for the as-cast sampleoxidized at 1100 �C for 10 minutes. The two verticaldotted lines shown in Figures 6(a) through (d) representwhere the continuous Al2O3 layer started to form. Thered circles in Figure 6(a-1) represent where the pointanalysis was conducted, and the width of the oxidescales was taken directly above these points.As SEM images shown in Figure 6(a), the transitionsfrom discontinuous-to-continuous Al2O3 formation arefound at the distance D = 21.8 lm and 59.5 lm, whichare shown in white arrows in Figures 6(a-1) and (a-2),respectively. The changes in the total thickness of theoxide scales are shown in Figure 6(b), and the thicknessof the layers decreased from the dendrite core to theinter-dendrite. In particular, the thickness of the layerswas less than 6.3 lm, shown as a horizontal dotted linein Figure 6(b), within the range of D = 21.8 to 59.5 lmwhere continuous Al2O3 layer had formed. This wasdetermined by the merging points of the vertical dottedlines and the line profile. The changes in the concentra-tion for each element measured by EPMA are shown inFigure 6(c), which was made sure that the measurementavoided the Al depletion zone.The Gibbs energy of the formation of Al2O3, DGAl2O3f ,for multicomponent alloys corresponding to each pointin Figure 6(c) can be calculated by following equation,and the results are shown in Figure 6(d):DGAl2O3f ¼ DG0 þRTlna2=3Al2O3a4=3Al�PO2� �½1�where DG0 is the standard free energy of Al2O3formation; T is the oxidation test temperature (K); Ris the gas constant (= 8.31 J/(mol K)); PO2is the partialpressure of oxygen, which is 0.021 MPa; and aAl2O3isthe activity of Al2O3 (aAl2O3= 1 for solid state); aAl isthe Al activity in multicomponent compositions. Inorder to evaluate how the changes in the compositionfrom dendrite core to inter-dendrite affect the tendencyof the formation of protective oxide scales, in this study,Al activity was calculated using Thermo-CalcTM(TCNI8: Ni-Alloys v8.0) at 1100 �C, with the compo-sitions shown in Figure 6(c). Although it is difficult toaddress the effects of each alloying element and theirsegregation on the oxidation of the alloy, the Al activitycalculated in Figure 6(d) incorporates the differences inthe compositions, implying the effect on alumina pro-tective layer formation. The absolute value of DGAl2O3fincreased at the inter-dendrite, supporting the result thatAl2O3 tends to form at the inter-dendrite, rather thanthe dendrite core for the as-cast sample. The mergingpoints of the vertical dotted lines and the line profiles ofFig. 4—SEM images of an oxidized heat-treated sample. (a) Schematic diagrams showing the directions of observations. (b) Top view of sampleoxidized at 1100 �C for 10 cycles, and (c-1) cross-sectional BEI of isothermal oxidation test at 1100 �C for 10 min, with the blue rectanglerepresenting where the (c-2) enlarged image of the interface was taken (Color figure online).METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 54A, DECEMBER 2023—4829Fig. 5—SEI and EDS elemental maps of O, Al, Cr, and Ni, taken of the cross-section of the as-cast sample at (a) dendrite core, (b)inter-dendrite, and (c) the heat-treated TMS-238. The blue rectangles represent where the EDS analyses were taken. Both samples were oxidizedat 1100 �C for 10 min (Color figure online).4830—VOLUME 54A, DECEMBER 2023 METALLURGICAL AND MATERIALS TRANSACTIONS AAl concentration and DGAl2O3f drawn represent where thethresholds most likely exist. The horizontal dotted linein Figure 6(c) and the blue area in Figure 6(d) representthe likely range where the threshold of the Al concen-tration and DGAl2O3f exists for the formation of contin-uous Al2O3 layer. The estimated threshold value of theAl concentration was around 5.2 wt pct and thethreshold for DGAl2O3f was � 556.6 ± 0.5 kJ/mol.Next, the adequacy of the borderline concentration ofAl and DGAl2O3f necessary to form the continuous Al2O3layer for oxidation at 1100 �C needs to be accessed.Several TMS-238 samples were solution heat-treated attemperatures lower than 1335 �C,[11] and the samplewith the Al concentration just above 5 wt pct at thedendrite core was selected. Figure 7 shows SEI ofanother TMS-238 sample heat-treated at 1305 �C for20 hours and oxidized at 1100 �C for 10 minutes. Theconcentration of Al at the dendrite core was around 5.2wt pct, which corresponds to DGAl2O3f =556.1 kJ/mol and is within the threshold range. Thethickness of the continuous Al2O3 layer up to 5 lm wasobserved at both the dendrite core and inter-dendrite forthis sample. This result suggests that the samples meantto be oxidized at 1100 �C should be solution heat-treated so that the Al concentration in the dendrite corebecomes higher than 5.2 wt pct. Several research alsosupport this threshold value. For Ni-Al binary alloys,the relation between the Al concentration and theparabolic growth rate of the oxides for temperaturesbetween 900 �C and 1200 �C has been previouslyreported,[32,33] and at 1100 �C, the first drop in theparabolic growth rate of oxides was around 5 wt pct Alas well. The transition to the formation of thestable a-Al2O3 layer is known to result in a decrease inscale growth, and only a-Al2O3 layers form for1100 �C.[17,34] Smialek et al. also reported that Ni-basesuperalloys containing more than 5 wt pct Al and up to6 wt pct Ta showed superior oxidation resistance.[25]Although it is important to note that elements such asMo, which is known to deteriorate the oxidationresistance of Ni-base superalloys,[25,27] have not changeddrastically for this research, elements such as these mayaffect the borderline Al concentration and DGAl2O3f . Butcan be said that for this temperature and this alloy, theborderline Al concentration and DGAl2O3f found in thisstudy are most likely reasonable.V. CONCLUSIONTo conclude, the following points have been madeclear about the effect of solution heat-treatment on theoxidation resistance of Ni-base SC superalloy,TMS-238.1. Solution heat-treatment at 1335 �C for 20 hoursimproved the oxidation resistance compared to theas-cast sample, and clear differences in the structureand composition of the oxide layers were observed.Fig. 6—(a-1, a-2) Enlarged images and (a-3) cross-sectional SEI ofthe as-cast TMS-238 sample oxidized at 1100 �C for 10 min. Lineprofiles of (b) the changes in the thickness of the oxide layers, and(c) changes in the compositions of each element taken using pointanalysis. (d) Line profile showing the changes in calculated Gibbsenergy of the formation of Al2O3, using the compositions shown in(c), and the activity of Al calculated using Thermo-CalcTM (TCNI8:Ni-Alloys v8.0).METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 54A, DECEMBER 2023—4831The as-cast sample had discontinuous Al2O3 for-mation at the dendrite core, where oxide spallationwas mainly observed during the cyclic oxidationtests. The heat-treated sample, however, had con-tinuous Al2O3 formation at both the dendrite coreand inter-dendrite, clearly suggesting the need toconsider the effect of heat-treatment on the oxida-tion resistance of alloys.2. For the oxidation of TMS-238 at 1100 �C, contin-uous Al2O3 layer had formed once the Al concen-tration was higher than 5.2 wt pct, and DGAl2O3f waslarger negative than � 556.6 ± 0.5 kJ/mol. Contin-uous Al2O3 layer was also observed in an alloy with5.2 wt pct Al andDGAl2O3f ¼� 556.1 kJ/molin the dendrite core, butwith lower solution heat-treatment temperature at1305 �C for 20 hours. For this alloy, conducting thesolution heat-treatment so that the Al concentrationand DGAl2O3f for both the dendrite core and theinter-dendrite become higher than the thresholdvalues will most likely be enough for the formationof protective Al2O3 scale, thus improving theoxidation resistance.ACKNOWLEDGMENTSThis paper is based on results obtained from a pro-ject, JPNP21007, commissioned by the New Energyand Industrial Technology Development Organization(NEDO). This research was partially supported by theCouncil of Science, Technology and Innovation(CSTI), Cross-ministerial Strategic Innovation Pro-gram (SIP), ‘‘Materials integration for revolutionarydesign system of structural materials’’ (Fundingagency: JST). This research was also supported byGrant-in-Aid for JSPS Fellows Grant Number23KJ2024. The authors would like to express our grat-itude to Dr. Makoto Osawa for the helpful discus-sions. We also would like to thank Ms. Kyoko Suzukiand Dr. Jun Uzuhashi for the support of microstruc-tural investigation, Mr. Takuma Kohata for the SEMobservations, and Mr. Yuji Takata for the preparationof the single-crystal alloys.CONFLICT OF INTERESTOn behalf of all authors, the corresponding authorstates that there is no conflict of interest.OPEN ACCESSThis article is licensed under a Creative CommonsAttribution 4.0 International License, which permitsuse, sharing, adaptation, distribution and reproductionin any medium or format, as long as you give appro-priate credit to the original author(s) and the source,provide a link to the Creative Commons licence, andindicate if changes were made. The images or otherthird party material in this article are included in thearticle’s Creative Commons licence, unless indicatedotherwise in a credit line to the material. 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Doychak: Intermetallic Compounds, eds. J. H. Westbrook, R.L.Fleischer, Wiley, New York, 1994, p. 977.Publisher’s Note Springer Nature remains neutral with regard tojurisdictional claims in published maps and institutional affiliations.METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 54A, DECEMBER 2023—4833 Effect of Solution Heat-Treatment on the Oxidation Resistance of Ni-Base Single-Crystal Superalloy Abstract Introduction Experimental Procedures Results Discussion Conclusion Open Access References