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RUZIC, Jovana, [WATANABE, Ikumu](https://orcid.org/0000-0002-7693-1675), GOTO, Kenta, OHMURA, Takahito

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[Nano-Indentation Measurement for Heat Resistant Alloys at Elevated Temperatures in Inert Atmosphere](https://mdr.nims.go.jp/datasets/c73c7279-36c6-42f9-9f26-fab5e209ec54)

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Nano-Indentation Measurement for Heat Resistant Alloys at Elevated Temperatures in Inert AtmosphereNano-Indentation Measurement for Heat Resistant Alloys at Elevated Temperaturesin Inert AtmosphereJovana Ruzic, Ikumu Watanabe+, Kenta Goto and Takahito OhmuraResearch Center for Structural Materials, National Institute for Materials Science, Tsukuba 305-0047, JapanA new approach of nano-indentation measurements at elevated temperatures has been developed to evaluate a temperature-dependentmechanical behavior of heat resistant alloys on a submicron scale, where the measurement is conducted in an inert atmosphere to prevent thesurface oxidation of a sample and heat an indenter passively. The developed approach enables us to perform nano-indentation testing with adiamond indenter in the temperature range of 23³800°C. The developed approach has been applied to gamma single-phase single-crystal of anickel-based superalloy. Then the degradations of the sample surface and the indenter tip have been discussed in this case.[doi:10.2320/matertrans.MD201909](Received February 28, 2019; Accepted April 19, 2019; Published July 25, 2019)Keywords: high-temperature nano-indentation, hardness, elastic modulus, nickel-based superalloy1. IntroductionNano-indentation measurements have been used exten-sively in materials research owing to its non-destructivenature, relatively simple specimen preparation, the ability toextract mechanical properties directly from load-displace-ment curves, etc.1,2) Furthermore, it enables the observationand analysis of small-scale deformation, dislocation nuclea-tion, size-dependent plasticity, phase transformation, andsurface degradation at elevated temperatures.3­7) Recently,high-temperature nano-indentation measurements have beenwidely employed in industries such as aerospace, automotive,nuclear, cutting tool, and biomaterials. The need for materialcharacterization at elevated temperatures has increasedtoward the development of novel structural materials whichcan withstand challenging operating conditions, such as hightemperatures.Pioneering works on nano-indentation tests at elevatedtemperature was done by Poisl, et al.8) and Suzuki andOhmura.9) Poisl, et al.8) performed the indentation measure-ments on amorphous selenium samples up to the maximumtemperature of 34°C by controlling the room temperature.Suzuki and Ohmura9) reported the first high-temperatureindentation measurements up to 600°C on silicon samples.Since 2000, the concern on measurement systems of high-temperature nano-indentation testing including systems ofheating and atmosphere control has been growing. Thedevelopment of commercial high-temperature nano-inden-tation testing systems is outlined in Table 1; products whichcan be mounted in scanning electron microscope andtransmission electron microscope chambers are excludedfrom this list.The feature and possible applications of these commercialsystems were summarized in the previous studies.10­12) Therestill remain several operational issues in these high-temper-ature nano-indentation systems: thermal stability (control ofthermal equilibrium and drift), appropriate indenter selection,preventing oxidation (using vacuum or inert environment),and proper experimental design. Especially the thermalmanagement is important to reduce the thermal drift andobtain reliable and reproducible results. There are twoheating system options: active and passive. The activeheating is a system to heat an indenter and a sampleindependently.3,4,11) On the other hand, in the passive heatingsystem, an indenter gains heat from a sample and atmosphere.Past studies13,14) reported that although the passive heatingprovides favorable results, the passively heated indenter actsas a local heat sink, whence it is difficult to reach theequilibrium temperature between an indenter and a sample.The material of an indenter is another concern in high-temperature nano-indentation measurements. With theobjective of high elastic stiffness and high hardness atelevated temperatures, diamond, sapphire, cubic boron nitride(cBN), tungsten carbide (WC), and boron carbide (B4C) havebeen widely used. Despite diamond possesses the highesthardness and better thermal conductivity in comparison to theother candidates of the indenter material, it cannot be used inair at temperatures over 400°C due to the oxidation and forsteel samples at temperatures over 500°C due to formationof iron carbides.12,13) Therefore, sapphire and cBN are chosenas good substitutes because of the better chemical stability.The present work aims to develop an approach of nano-indentation measurement to evaluate mechanical behaviors ofheat resistant alloys at elevated temperatures. For this aim,the nano-indentation equipment using a high-temperaturestage has been set up in a vacuum chamber, where theindenter is passively heated in an inert environment. Thenwe have investigated and discussed issues of the thermalmanagement and the durability of a diamond indenter inapplication on a sample of a nickel-based superalloy.2. Thermal and Atmosphere Control System in High-Temperature Nano-Indentation MeasurementFor measurements at elevated temperatures, nano-inden-tation testing equipment using high-temperature stage(Bruker Co.) placed in a vacuum chamber on a vibrationisolation stage (Minus K Technology Inc.) is used. Thevacuum chamber is equipped with gas inlets allowing innerenvironment control and gas introduction, and with an+Corresponding author, E-mail: WATANABE.Ikumu@nims.go.jpMaterials Transactions, Vol. 60, No. 8 (2019) pp. 1411 to 1415Special Issue on Recent Advances in Indentation Technique©2019 The Japan Institute of Metals and Materialshttps://doi.org/10.2320/matertrans.MD201909external cooling system. The vacuum chamber is cyclicallyevacuated up to 1.33mPa (10¹5 Torr) and backfilled withargon gas (99.9999% purity) to reduce the oxygen gas levelbefore heating.A sample is placed between two independently-controlledheaters and heated simultaneously from the top and thebottom as illustrated in Fig. 1. The dual heating system isemployed to generate a uniform temperature within thesample, in which the heating rate sets up slow (³10°C/min).Here the indenter is heated passively. At room temperature(before heating), the indenter tip is brought to 100 µm heightabove the sample surface and heated together with thesample, where it can be assumed that the tip is heated withalmost the same heating rate as the sample. The tip and thesample are held at the predetermined temperature for 1­2hours to achieve the thermal equilibrium between them forthe thermal stabilization before the nano-indentation meas-urements. Heat radiation and convection are the primarysources of heat transfer during heating and thermalstabilization period. It is noted that the thermal stabilizationprocess contributes to reduce the thermal drift in high-temperature measurements. Without this process, themeasurements are failed due to the thermal drift caused bythe high temperature gradient between the indenter tip andthe sample. Additionally, in this study, the indenter tip is kepton the sample surface with a small force (³1µN) for a periodof 180 sec to ensure the better thermal equilibrium beforeeach indentation.The thermal and atmosphere control system enables us touse a diamond indenter for the following high-temperaturenano-indentation measurements.3. Application to Nickel-Based Superalloy3.1 Sample and experimental conditionsIn this study, a sample made of gamma single-phasesingle-crystal of a nickel-based superalloy was prepared forthe measurements, which was 2mm in height and 8mmTable 1 Historical development of commercially available high-temperature nano-indentation devices.J. Ruzic, I. Watanabe, K. Goto and T. Ohmura1412in diameter. The sample was mechanically polished, inparticular, with 0.05 µm colloidal silica suspension (Master-Met, Buehler) as a final step to minimize the surface damage.The nano-indentation measurements at elevated temper-atures up to 800°C were performed using a 3,000 µN appliedload with a diamond Berkovich indenter. The loading/unloading rate and the dwell time were changed in the rangeof 50­300 µN/sec and 10­60 sec, respectively, to investigatethe creep effect in Section 3.3. The loading/unloading rate of300 µN/sec and the dwell time of 10 sec was employed in theother sub-sections.The thermal drift rate which is monitored by the nano-indentation system was on the order of «0.06 nm/sec inalmost all experiments and did not vary significantly withincreasing temperature (Table 2). The thermal drift can beestimated from the holding segment at the maximum appliedload.11,15,16)3.2 Geometry effect of indenter tipIn the present approach, values of elastic modulus (Er) andhardness (H ) are calculated from the load-displacement curveon the basis of Oliver-Pharr method,17) same as a standardnano-indentation measurement at room temperature. Forthe sake of simplicity, the effect of pile-ups and sink-ins isnot taken into account in the calculation of elastic modulusand hardness in the conventional approach. According toprevious studies,11,18) if the thermal drift is measured to below, it has a negligible effect on elastic modulus and hardnessvalues which are obtained from nano-indentation measure-ments performed within seconds. Consequently, in thepresent approach, the measured elastic modulus and hardnessvalues can be used without any additional correction.A calibration procedure of the indenter tip is required toconsider the current non-perfect geometry of the indentertip, which is characterized by area function (AF), with astandard sample for which fused silica is used usually. Thecalibration can be done at room temperature before and afterhigh-temperature measurements. In the present approach,the indenter tip is calibrated at the same temperature asthe corresponding nano-indentation measurements, in whichfused silica is available as the standard sample because thetemperature dependency of elastic modulus and hardness isknown.18,19) These two approaches are compared in Fig. 2, inwhich the values of elastic modulus and hardness with the AFcalibrated at room temperature exhibit higher (about 20%higher at 800°C) than those with the AF calibrated at thesame temperature as the nano-indentation measurements. Itis noted that the values with the AF calibrated at the sametemperature as the nano-indentation measurements showbetter agreement with the values in the literature.20)Therefore, this approach was adopted for the further dataanalysis in this study.Fig. 1 Schematic view of high-temperature nano-indentation system insidevacuum chamber.Table 2 Thermal drift rates measured by nano-indentation system.Fig. 2 Differences in calculated elastic modulus and hardness values ofnickel-based superalloy sample using area function calibrated at roomtemperature and at the same temperatures with nano-indentation measure-ments.Nano-Indentation Measurement for Heat Resistant Alloys at Elevated Temperatures in Inert Atmosphere 14133.3 Creep effectIn high-temperature measurements, the effect of creep isinevitable and one of critical issues. In general, the creepbecomes dominant beyond 400°C, when the negativestiffness can be observed in the unloading segment of theload-displacement curve.21) The effect can be minimizedby high loading/unloading rate and short dwell period at themaximum applied load because it is definitely a time-dependent behavior. Figure 3 shows the dependency of theloading/unloading rate and the dwell period at the maximumapplied load on the load-displacement curve at 800°C. In thecase of very slow loading/unloading rate (50 µN/sec), thenegative stiffness can be observed at the unloading segment.Following the results in Fig. 3, we here employ themeasurement condition, 300 µN/sec loading/unloading rateand dwell time of 10 sec.3.4 Measurements at elevated temperaturesUsing the above-mentioned approach, load-displacementcurves, the elastic modulus and hardness of the sample wereevaluated at elevated temperatures. The results are shownin Fig. 4. In both elastic modulus and hardness, a continuousdecreasing trend appeared and became obvious above400°C.3.5 Degradation of sample surface and indenter tipIn high-temperature measurements, surface degradation byoxidation is inevitable even if the measurement is conductedin an inert or vacuum atmosphere.7,22) Figure 5 shows thesurface roughness of the sample after 3 hours of exposureto the corresponding temperature in a high purity argonatmosphere, which was measured using scanning probemicroscope. The surface roughness increased above 600°Cand became drastically high at 800°C. In high-temperaturemeasurements, particular attention should be given to thesurface state and an indentation should be performed at anappropriate area.The degradation occurs not only on the sample surface butalso the indenter tip. Although a tip is worn by a long-termusage even at room temperature, the degradation of the tipbecomes significant in high-temperature measurements.Figure 6 shows the comparison between an unused indentertip and one after the usage of more than 3,000 indentations atvarious temperatures up to 800°C. Blunting of the indentertip can be observed after the long-term usage. In high-temperature measurements, a chemical reaction between theindenter tip and the sample can happen, which is one ofthe causes for the degradation of the tip in addition toFig. 3 Effect of loading/unloading rate and dwell period at maximumapplied load at 800°C.Fig. 4 Temperature-dependency of nickel-based superalloy sample atelevated temperatures up to 800°C: a) load-displacement curves andb) elastic modulus and hardness.Fig. 5 Degradation of sample surface after 3 hours exposure to elevatedtemperature.J. Ruzic, I. Watanabe, K. Goto and T. Ohmura1414the mechanical wearing. Therefore the chemical stabilitybetween materials of an indenter and a sample at elevatedtemperatures should be considered. Although the degradationof the indenter tip is inevitable, it can be monitored andcorrected by calibrating AF at each temperature before andafter a measurement.4. ConclusionA new approach of nano-indentation measurement atelevated temperatures up to 800°C in an inert atmospherewas developed in this study and the temperature-dependentmechanical behavior of gamma single-phase single-crystal ofa nickel-based superalloy was evaluated. By passive heatingand calibration of area function at each temperature, reliablemeasurements could be performed with a diamond indenter.The developed approach is effectual to evaluate temperature-dependent mechanical properties at a submicron scale.The creep and the degradations of a sample and an indentertip are essential and characteristic phenomena in the high-temperature measurements. These are key issues for furtherstudies.AcknowledgmentsThis research was financially supported by the Cross-Ministerial Strategic Innovation Promotion Program onStructural Materials for Innovation (directed by the Cabinetof Japan) and the Amada Foundation for Metal WorkTechnology (No. AF-2018035-C2).REFERENCES1) J. Ruzic, S. Emura, X. Ji and I. Watanabe: Mater. Sci. Eng. A 718(2018) 48­55.2) K. Goto, I. Watanabe and T. Ohmura: Int. J. Plast. 116 (2019) 81­90.3) J.C. Trenkle, C.E. Packard and C.A. Schuh: Rev. Sci. Instrum. 81(2010) 073901.4) S. Korte, R.J. Stearn, J.M. Wheeler and W.J. Clegg: J. Mater. Res. 27(2012) 167­176.5) J.S.K.-L. Gibson, S.G. Roberts and D.E.J. Armstrong: Mater. Sci. Eng.A 625 (2015) 380­384.6) P.S. Phani and W.C. Oliver: Acta Mater. 111 (2016) 31­38.7) Y. Li, X. Fang, B. Xia and X. Feng: Scr. Mater. 103 (2015) 61­64.8) W.H. Poisl, W.C. Oliver and B.D. Fabes: J. Mater. Res. 10 (1995)2024­2032.9) T. Suzuki and T. Ohmura: Philos. Mag. A 74 (1996) 1073­1084.10) Z. 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