High entropy alloy HfNbTaTiZr in as cast conditions and after high pressure torsion straining was characterized by nanoindentation. The length-scale dependent material response (indentation size effect) was characterized by indentation at various indentation depths. Hardness dependence on the characteristic length (depth of penetration) indicated decomposition of disordered high entropy alloy in the as cast sample, which probably occurred during slow cooling after casting. Subsequent severe plastic deformation by high pressure torsion led on the other hand to the short-range disorder of (originally partially decomposed as cast) structure. Further hardening was generated during high pressure torsion by the mechanisms of grain refinement and increasing dislocation density

Haušild, Petr

Kim, Hyoung Seop

Zýka, Jiří

Čech, Jaroslav

Čížek, Jakub

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

doi:10.14311/APP.2020.27.0141Acta Polytechnica 27(0):141–144, 2020 © Czech Technical University in Prague, 2020available online at http://ojs.cvut.cz/ojs/index.php/appINDENTATION SIZE EFFECT IN HIGH PRESSURE TORSIONPROCESSED HIGH ENTROPY ALLOYPetr Haušilda,∗, Jakub Čížekb, Jaroslav Čecha, Jiří Zýkac,Hyoung Seop Kimda Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Department ofMaterials, Trojanova 13, 120 00 Praha, Czech Republicb Charles University in Prague, Faculty of Mathematics and Physics, Department of Low Temperature Physics, VHolešovičkách 2, Praha 8, CZ-18000, Czech Republicc UJP PRAHA a.s., Nad Kamínkou 1345, 156 00, Praha, Czech Republicd POSTECH, Department of Materials Science and Engineering, Pohang, 790-784, South Korea∗ corresponding author: petr.hausild@fjfi.cvut.czAbstract. High entropy alloy HfNbTaTiZr in as cast conditions and after high pressure torsionstraining was characterized by nanoindentation. The length-scale dependent material response (inden-tation size effect) was characterized by indentation at various indentation depths. Hardness dependenceon the characteristic length (depth of penetration) indicated decomposition of disordered high entropyalloy in the as cast sample, which probably occurred during slow cooling after casting. Subsequentsevere plastic deformation by high pressure torsion led on the other hand to the short-range disorderof (originally partially decomposed as cast) structure. Further hardening was generated during highpressure torsion by the mechanisms of grain refinement and increasing dislocation density.Keywords: High entropy alloys, high pressure torsion, indentation size effect, nanoindentation.1. IntroductionIn high entropy alloys (HEAs), the configurationalentropy of a multicomponent solid solution phase ismaximized so that the Gibbs energy of random solu-tion may be lower than that of possible intermetallicphases. Generally, to achieve high entropy of mix-ing, the alloys contain typically five or more majorelements in equimolar concentrations [1]. The HEAsshow promising mechanical properties in applicationsranging from (high temperature) structural to biocom-patible materials [2, 3].Mechanical properties of such alloys can further beimproved by grain refinement especially by severe plas-tic deformation. However, studies of ultrafine grainedHEAs are rather scarce in the literature. An increaseof strength with decreasing grain size was achievedthrough processing by high pressure torsion (HPT) inthe probably most investigated HEA - Cantor alloyi.e., equiatomic CoCrFeMnNi with face-centered cubic(fcc) structure [4].Fewer attempts were made to process in such a wayHEAs with body-centered cubic (bcc) structure. Re-cently HfNbTaTiZr bcc HEA was successfully nanos-tructured by HPT straining [5, 6]. It was reportedthat grain refinement by HPT resulted in a significantenhancement of the strength of this bcc HEA, keepingexcellent ductility during room temperature strain-ing. Nevertheless, there is still a lack of informationabout the development of microstructure and physicalproperties of this refractory metal HEA subjected tosevere plastic deformation processing.Recent investigations [5, 6] revealed that thermo-dynamically stable system of HfNbTaTiZr alloy atroom temperature is a mechanical mixture of Zr, Hfrich hcp phase and Ta, Nb rich bcc phase. Recrystal-lization annealing after cold-rolling at 800 °C led tothe transformation of the single-phase bcc materialinto a two-phase structure composed of bcc1 and bcc2phases with lattice parameters a1 ∼ 0.3427 nm anda2 ∼ 0.3338 nm, respectively [7].Chemical analysis revealed that the matrix wasslightly enriched in Hf and Zr, while the precipitatedsecond-phase particles were on the other hand slightlyenriched in Ta and Nb. The decomposition of thesolid solution (and formation of short range order)after long-term annealing obviously leads to the de-terioration of mechanical properties (loss of ductilityand decrease of strength). The difference in hardnessof both phases is relatively small and both are softerthan the random (disordered) solid solution [6]. Onthe other hand, considerable contribution to the solidsolution strengthening can arise from atomic size mis-fit (phase separation on the nano-meter scale) which isprovoked by the high density of vacancies introducedby HPT [5].The aim of this work was clarification of the relation-ship between phase (de)composition, microstructure,lattice defects, and length-scale-dependent materialresponse of HfNbTaTiZr HEA in as cast conditions,after solution (disordering) thermal treatment andHPT straining. The length-scale-dependent materialresponse was characterized by indentation at various141P. Haušild, J. Čížek, J. Čech et al. Acta PolytechnicaAs cast Annealed 1/4 turn 1 turn 15 turnsεpl [1] 0 0 5 20 300H0 [MPa] 4049 4438 4947 5717 5953h∗ [nm] 45.7 76.3 49 33.5 29.3Table 1. List of the samples - equivalent plastic strain εpl, identified characteristic lengths h∗ and hardnesses in thelimit of infinite depth H0.indentation depths. The contributions of differentmechanisms were attributed to distance between dislo-cation pinning defects (length scale) and are reportedhere.2. Experimental details2.1. MaterialThe alloy of equimolar composition HfNbTaTiZr wasprepared by vacuum arc melting from pure Hf, Nb,Ta, Ti and Zr metals and cast into water cooled Cucrucible. In order to ensure proper mixing, the ingotwas flipped and remelted eight times. Disc of 20 mmdiameter and 1 mm thickness were HPT strained atroom temperature and pressure of 2.5 GPa. Strainingwas performed to 1/4 turn, 1 turn and 15 turns of thepiston (see Fig. 1). Corresponding equivalent plasticstrains εpl (at the characterized areas) are 5, 20 and300 (see Table 1). More details can be found inRef. [5]. One non-deformed (as cast) sample wasalso annealed for 2 mins at 1600 °C (followed by rapidcooling) for reasons described in section Results anddiscussion.    Sample Figure 1. Scheme of high pressure torsion straining.2.2. NanoindentationSurface of samples was polished by standard met-allographical procedure with final step made using0.04µm colloidal silica to avoid the surface layer af-fected by mechanical grinding and polishing. Nanoin-dentation measurements were performed using anAnton Paar NHT2 Nanoindentation Tester withBerkovich diamond indenter using instrumented in-dentation technique [8, 9].In order to obtain the mechanical response from dif-ferent depth, so called continuous multi cycle (CMC)indentations with increasing load were performed vary-ing the maximum load at each cycle from 1 mN upto 500 mN. This type of load cycle allows character-ization at different load (depth) levels at the sameposition (see e.g. Refs. [10, 11]). Loading time was 10s per each cycle for CMC, followed by 5 s hold at themaximum load and unloading time 10 s per each cy-cle. The results (indentation hardness) were evaluatedaccording to the ISO 14577 standard. The indenta-tions were made on sample surface in the positioncorresponding to the half radius, i.e. in the locationof most homogeneously distributed deformation af-ter HPT straining. At least 20 CMC indentationswere performed for each material state (deformationcondition).200030004000500060007000800090000 1000 2000 3000H(MPa)h (nm) 15  11/4As castFigure 2. Indentation size effect in as cast sampleand samples HPT strained to 1/4 turn, 1 turn and 15turns.3. Results and discussionResults of hardness measurements as a function ofpenetration depth for as cast sample and samplesHPT strained from 1/4 turn to 15 turns are shownin Fig. 2. It can be seen in this figure that hardnessincreases with increasing strain and that all samplesshow strong indentation size effect in sub-micron pen-etration depths. Using Nix nad Gao approach [12]based on the geometrically necessary dislocations, the142vol. 27/2020 Indentation Size Effect in HPT Processed High Entropy Alloydepth dependence of the hardness H can be relatedto the characteristic length h∗ through the relation:HH0=√1 + h∗h(1)where h is a given depth of penetration, and H0 isthe hardness in the limit of infinite depth (∼ macro-scopic hardness).Depth dependence of the hardness is plotted ac-cording to Eq. 1 for different levels of straining inFig. 3 and the parameters H0 and h∗ obtained byleast square fit are listed in Table 1.01020304050600 0.001 0.002 0.003 0.004 0.005 0.006 0.007H2 (GPa2)Millions1/h (nm-1)As cast15 turns1 turn1/4 turnFigure 3. Depth dependence of the hardness plottedfor as cast sample and samples HPT strained from1/4 turn to 15 turns according to Eq. 1.As can be expected, the hardness in the limit ofinfinite depthH0 is increasing with the increasing levelof HPT straining (HPT leads to the strain hardening[13]). On the other hand, the characteristic length h∗is decreasing with the increasing level of deformation.It is to note that the values of characteristic lengthsh∗ are quite low, which indicates fine microstructuralfeatures involved in the dislocation pinning process.According to Nix and Gao [12], the characteristiclength h∗ depends on the hardness in the limit ofinfinite depth H0 as follows:h∗ ∼ 1H02(2)Fig. 4 shows that characteristic depth h∗ depen-dence on the hardness in the limit of infinite depthH0 fulfils relation (2) for HPT deformed samples, butthe as-cast sample clearly does not follow this relation.As mentioned in the Introduction, the anomalousbehavior of the as cast sample is likely caused by thedecomposition of disordered (high entropy) bcc solidsolution into two bcc1 and bcc2 phases.Such short-range atomic redistribution is hard tobe characterized by conventional techniques suchX-ray diffraction or transmission electron microscopyas the variation of lattice parameter is very small0.000.010.020.030.040.050.060.070 20 40 60 80 1001/H02 (GPa-2)h* (nm)As castAnnealed1/4 turn1 turn15 turnsFigure 4. Characteristic depth dependence of thehardness in the limit of infinite depth for as cast sam-ple, annealed sample and samples HPT strained from1/4 turn to 15 turns.20003000400050006000700080000 500 1000 1500 2000 2500H(MPa)hc (nm)As castAnnealedFigure 5. Comparison of depth dependence of thehardness in the as cast and annealed samples.and the size of domains is in the order of tens of nm(see parameter h∗). Therefore, the as cast samplewas annealed at (high) temperature of 1600 °C withsubsequent rapid cooling in order to preserve therandom solid solution and subsequently characterizedby nanoindentation.The results of hardness measurements in as castand annealed state are compared in Fig. 5. As canbe seen in Table 1, the parameter h∗ significantlyincreased after annealing, which is consistent withthe hypothesis of short-range order disappearance.The tendency given by the relation (Eq. 2) is now ingood agreement for both annealed and HPT strainedsamples (Fig. 4). Thus, similarly to high temperatureannealing the HPT led to the short-range disorderof (originally partially decomposed) as-cast sample(severe plastic deformation leads to an extensionof solid solubility [14]). Subsequent hardening in143P. Haušild, J. Čížek, J. Čech et al. Acta PolytechnicaHPT strained samples is related rather to grain(cell) refinement and increasing dislocation densityas observed in Ref. [5] using transmission electronmicroscopy.4. ConclusionsThe results of the present study can be summarizedas follows:• High pressure torsion led to a significant (and pro-gressive) hardening of high entropy HfNbTaTiZralloy.• Both as cast and HPT strained samples showedlength-scale dependent material response (indenta-tion size effect).• As cast sample presented different dependence ofcharacteristic length h∗ on the hardness in the limitof infinite depth H0 than HPT deformed and an-nealed samples, which indicates short-range decom-position of disordered high entropy alloy.• It can be concluded that severe plastic deformationby HPT produced the short-range disorder of origi-nally partially decomposed as cast structure, whichwas followed by hardening due to the grain refine-ment and increasing dislocation density intorducedby HPT process.• Annealing of as cast sample at 1600 °C withsubsequent rapid cooling in order to stabilize thedisordered structure led to the same dependence ofcharacteristic length on the hardness in the limitof infinite depth as in the case of HPT strainedsamples.AcknowledgementsThis research was carried out in the frame of the projectsCZ.02.1.01/0.0/0.0/16_019/0000778 (Centre of AdvancedApplied Sciences) and 17-17016S (Czech Science Founda-tion).References[1] M.-H. 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INDENTATION SIZE EFFECT IN HIGH PRESSURE TORSION PROCESSED HIGH ENTROPY ALLOY

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INDENTATION SIZE EFFECT IN HIGH PRESSURE TORSION PROCESSED HIGH ENTROPY ALLOY

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