High entropy alloy HfNbTaTiZr was successfully processed by severe plastic deformation using high pressure torsion (HPT) and ultrafine grained microstructure was achieved. The microstructure of HPT-deformed HfNbTaTiZr alloy was characterized by X-ray diffraction and compared with conventionally cast ingots. The lattice defects introduced by HPT processing were characterized by positron annihilation spectroscopy. The X-ray diffraction profiles of HTP-deformed samples were extremely broadened due to small sizes of coherently diffracting domains and a high microstrain introduced by severe plastic deformation.11Ysciescopu

Cizek, J.

Dudr, M.

Harcuba, P.

Janecek, M.

Kim, H. S.

Kuriplach, J.

Lukac, F.

Malek, J.

Moon, J.

Vlasak, T.

Zyka, J.

Acta Physica Polonica A

포항공과대학교

Vol. 134 (2018) ACTA PHYSICA POLONICA A No. 3Proceedings of the International Symposium on Physics of Materials (ISPMA 14), September 10–15, 2017, PragueDefects in High Entropy Alloy HfNbTaTiZrPrepared by High Pressure TorsionF. Lukáča,b,∗, M. Dudra, J. Čížekb, P. Harcubab, T. Vlasákb, M. Janečekb,J. Kuriplachb, J. Moonc, H.S. Kimc, J. Zýkad and J. MálekdaInstitute of Plasma Physics CAS, Za Slovankou 3, 182 00, Praha, Czech RepublicbFaculty of Mathematics and Physics, Charles University, V Holesovickach 2, 180 00 Praha 8, Czech RepubliccDepartment of Materials Science and Engineering, POSTECH, Pohang 790-784dUJP PRAHA a.s., Nad Kamínkou 1345, 156 00, Prague, Czech RepublicHigh entropy alloy HfNbTaTiZr was successfully processed by severe plastic deformation using high pressuretorsion (HPT) and ultrafine grained microstructure was achieved. The microstructure of HPT-deformed HfNbTa-TiZr alloy was characterized by X-ray diffraction and compared with conventionally cast ingots. The lattice defectsintroduced by HPT processing were characterized by positron annihilation spectroscopy. The X-ray diffraction pro-files of HTP-deformed samples were extremely broadened due to small sizes of coherently diffracting domains anda high microstrain introduced by severe plastic deformation.DOI: 10.12693/APhysPolA.134.891PACS/topics: 78.70.Bj, 61.72.Ff1. IntroductionHigh entropy alloys exhibit various combinations of in-teresting physical properties due to the formation of solidsolution stabilized by high configuration entropy. Refrac-tory metal high entropy alloy HfNbTaTiZr exhibits sin-gle phase solid solution with bcc structure [1, 2]. Thealloy has high yield strength (σ0.2 = 929 MPa in com-pression [1] and σ0.2 = 1155 MPa in tension [2]). Struc-ture refinement may improve the strength and ductility ofHfNbTaTiZr alloy as was already demonstrated by con-ventional cold rolling [3]. Methods of severe plastic de-formation (SPD) enable a strong grain refinement downto nanoscale [4–7] and may, therefore, further improvemechanical properties of HfNbTaTiZr alloy. HPT is themost efficient technique for structure refinement by se-vere plastic deformation [5,6]. In the present work HPTwas employed for preparation of ultrafine grained (UFG)HfNbTaTiZr alloy. Severe plastic deformation does notonly cause grain refinement but also introduces a highdensity of lattice defects (dislocations and vacancies) [8].HPT straining introduces the lateral inhomogeneity ofstrain with the minimum in the centre of the disk andcontinuous increase towards its periphery and results inthe inhomogeneous microstructure distribution of coarsegrains in the centre and fine grains near the disk periph-ery. The microstructure inhomogeneity is continuouslysmeared out with increasing strain (number of HPT rev-olutions) [5, 6]. Hence the structure refinement startsat the margin of the sample disk and gradually extendstowards the centre and relatively homogeneous ultrafine∗corresponding author; e-mail: lukac@ipp.cas.czgrained structure across the sample is usually achievedafter sufficient number of HPT revolutions [5]. In thepresent work the development of microstructure duringHPT processing of HfNbTaTiZr alloy was investigatedusing electron microscopy combined with X-ray line pro-file analysis and positron annihilation spectroscopy.2. ExperimentalIngots of HfNbTaTiZr alloy were prepared by vacuumarc melting from pure metals of 99.9% purity placed inwater cooled copper crucible. Casting was performedeight times and the ingot was flipped between each twomelts to mix the elements properly and suppress anychemical heterogeneity [2]. Thin discs of diameter 20 mmand thickness of 1 mm were cut from cast ingot and pro-cessed by high pressure torsion (HPT) under the pres-sure of 2.5 GPa for different numbers of HPT revolutionsN = 1/4, 1/2, 1, 5, 15. The sample only pressed by thehigh pressure without any torsion straining (N = 0) hasbeen prepared as well.Microstructure of the material before and after de-formation was investigated using electron backscatterdiffraction (EBSD) technique. The sample surfaces werepolished with argon ion polisher. Zeiss Auriga Compactscanning electron microscope (SEM) operated at 10 keVequipped with EDAX EBSD camera was used.Phase composition was measured by X-ray diffractionon a D8 Discover (Bruker, DE) diffractometer equippedwith 1D detector LynxEye and Cu Kα line beam anode.The X-ray diffraction studies have been performed in theBragg-Brentano symmetrical geometry with 20 mm widedivergent beam. The whole diffraction profile was fit-ted by means of Rietveld refinement of XRD peaks inTOPAS #v5 software [9]. Correction of peak intensitiesof powder diffraction patterns for preferred orientationwas treated by the March-Dollase approach [10]. The(891)892 F. Lukáč, et al.size of coherently diffracting domains (CDD) and the mi-crostrain were determined from broadening of Lorentzianand Gaussian components of pseudo-Voigt profile func-tion, respectively.Positron annihilation spectroscopy (PAS) was per-formed using a 22Na positron source with activity of1.0 MBq deposited on a 2 µm thick mylar foil. Adigital spectrometer described in [11] was employed forpositron lifetime (LT) measurements. The spectrometerwas equipped with BaF2 scintillators and fast photomul-tipliers Hamamatsu H3378. Detector pulses were sam-pled using a couple of 8-bit ultrafast digitizers Acqiris DC211 (Agilent Technologies) with the sampling frequencyof 4 GHz. The spectrometer exhibits an excellent timeresolution of 145 ps (full width at half maximum of theresolution function). At least 107 positron annihilationevents were collected in each LT spectrum. The sourcecontribution to the LT spectra consisted of two compo-nents with lifetimes of 368 ps and 1.5 ns and correspond-ing relative intensities 11 and 1% representing contribu-tions of positrons annihilating in the 22Na source spotand the covering mylar foil, respectively.3. Results and discussionThe EBSD map of as-cast HfNbTaTiZr alloy is shownin Fig. 1a. The as-cast sample exhibits the mean grainsize around 300 µm. The EBSD map of the only pressedsample (N = 0) is shown in Fig. 1b. One can see in thefigure that compression by high pressure without torsionstraining did not lead to any significant grain refinement.The EBSD map of the sample subjected to HPT strainingapplying N = 1/2 HPT revolution is shown in Fig. 1c.The EBSD map was measured in the centre of the sam-ple disc corresponding to the rotation axis. Substantialstructure refinement caused by HPT processing is evidentin Fig. 1c. However, the structure of the HPT-deformedsample is not homogeneous after 1/2 HPT rotation. Onecan see in Fig. 1c that the sample N = 1/2 consists of re-gions with UFG structure co-existing with coarse grainscontaining deformation twins. Hence, plastic deforma-tion of HfNbTaTiZr alloy during HPT processing occursby the slip of dislocations and also by twining. FurtherHPT straining led to so high degree of imposed strainthat EBSD indexing was impossible.Figure 2 shows X-ray diffraction patterns for the as-cast sample and HPT-deformed samples subjected tovarious numbers N of HPT revolutions. The as-castsample contains single bcc phase with the lattice pa-rameter of a = 3.4059(1) Å which is consistent withthe value reported by Senkov et al. [1]. Samples sub-jected to HPT deformation exhibit extremely broadeneddiffraction peaks. Moreover, the texture change has beenobserved in the samples subjected to HPT straining.Fig. 1. EBSD maps of HfNbTaTiZr alloy (a) as-cast sample (b) sample compresed by high pressure of 2.5 GPa withoutany torsion deformation (number of HPT revolutions N = 0) (c) HPT-defomed sample N = 1/2. The EBSD map wasmeasured in the centre of the HPT-deformed sample.Defects in High Entropy Alloy HfNbTaTiZr Prepared by High Pressure Torsion 893March-Dollase [10] treatment of texture in powderdiffraction method includes a correction of peak intensi-ties since the angle between the preferred orientation vec-tor and the diffraction vector may change. Therefore, twopreferred orientations of grains are determined indepen-dently, which alter the intensity of all model diffractionpeaks. In the as-cast and only pressed sample (N = 0)the texture components (110) and (200) dominate, whilein samples deformed by HPT straining the dominatingtexture components are (110) and (211) already after 1/4of rotation.Fig. 2. The development of X-ray diffraction patternsfor HfNbTaTiZr alloy during HPT straining for variousnumber of HPT revolutions N .Figure 3 shows results of the Rietveld refinement fittingof XRD diffraction patterns of HfNbTaTiZr samples. Thelattice parameter slightly increases in the early stages ofHPT processing. The size of CDD substantially decreasesafter deformation and saturates for N ≥ 1. Moreover,the microstrain e0 induced by deformation increases withHPT straining and reaches a saturated value in order of3.5×10−3. Note that unusually high defect concentrationwas reported in HfNbTaTiZr alloy deformed by compres-sion using plastic strain values between 2 and 20% [12],i.e. subjected to significantly lower deformation com-pared to HPT. Similarly, a high dislocation density inthe order of 1016 m−2 was determined by X-ray line pro-file analysis in HPT-deformed CrMnFeCoNi high-entropyalloy [12].Fig. 3. Results of the Rietveld refinement fits forHfNbTaTiZr alloy.Figure 4 shows the mean positron lifetime measuredfor all samples studied. The as-cast sample exhibits themean positron lifetime of 159(1) ps. The as-cast samplemay contain defects introduced during casting and shap-ing. In order to recover these defects the as-cast sam-ple was annealed at 1000 ◦C for 2 h sealed in evacuatedquartz ampoule. After annealing the surface of the sam-ple was chemically etched to remove a thin oxide layerformed during annealing. The mean positron lifetime ofthe annealed sample is 146(1) ps. This value is close tothe bulk lifetime of HfNbTaTiZr alloy obtained by by ourpreliminary ab-initio theoretical calculations. Hence, thedensity of defects in the annealed sample is very low andalmost all positrons are annihilated in the free state. Onthe other hand, the as-cast sample contains defects in-troduced by cutting and polishing and the mean positronlifetime is increased by the presence of these open volumedefects.Fig. 4. The mean positron lifetime for HfNbTaTiZr al-loy subjected to HPT straining. The PAS measurementswere always performed in the centre of the sample disc.The mean positron lifetime for the only pressed sam-ple (N = 0) is remarkably higher than that for the as-cast sample testifying that even compression by a highpressure introduced a high density of defects. The HPTstraining led to further increase in the concentration ofdefects which is reflected by an additional increase of themean positron lifetime. For N ≥ 1 the mean positronlifetime saturates at value around 189 ps since the den-sity of defects becomes so high that virtually all positronsare trapped at defects (saturated trapping). Since severeplastic deformation introduces dislocations as dominat-ing type of defects and vacancies are created by non-conservative motion of dislocations the observed positronlifetime of 189 ps can be attributed to vacancy-like de-fects. Note that dislocation line itself is only a shallowpositron trap and positron trapping in dislocations is atwo step process where positron is firstly weakly local-ized in the dislocation line and such pre-trapped positronrapidly diffuses along the dislocation line until it is even-tually trapped at a vacancy bound to the dislocation [13].Hence the final positron annihilation site is a vacancysqueezed by the compressive elastic field of dislocation894 F. Lukáč, et al.and the lifetime of positrons trapped at dislocations is,therefore, slightly shorter than that for plain vacancies.One can expect that the positron lifetime of 189 ps isjust the mean value of certain distribution of positronlifetimes trapped in vacancies with open volume vary-ing due to variations in the proportion of the chemicalelements (Hf, Nb, Ta, Ti, Zr) surrounding the vacancy.Since vacancies are surrounded by elements with variousatomic radius, the lattice relaxations around vacancieschange with fluctuations in the local chemical composi-tion. This causes fluctuations in the open volume of va-cancies reflected by variations of the lifetime of trappedpositrons.4. ConclusionsThe structure of refractory metal high entropy alloyHfNbTaTiZr was successfully refined by HPT. The Ri-etveld refinement of X-ray diffraction patterns showedthat the mean size of coherently scattering domainsstrongly decreases during HPT straining while the mi-crostrain increases due to lattice defects created by se-vere plastic deformation. This was confirmed by positronannihilation spectroscopy. The mean positron lifetimeexhibits a pronounced increase with HPT straining andreaches a saturated value when single HPT revolution iscompleted. Similarly, the mean size of coherently scatter-ing domains and the microstrain become saturated aftersingle HPT revolution.AcknowledgmentsThis work was supported by the Czech Science Foun-dation (project 17-17016S). P.H. and M.J. acknowl-edge financial support by ERDF under the projectCZ.02.1.01/0.0/0.0/15 003/0000485. J.M. and H.S.K. ac-knowledge financial support by the Future Material Dis-covery Project of the National Research Foundation ofKorea (NRF) funded by the Ministry of Science and ICTof Korea (NRF-2016M3D1A1023383).References[1] O.N. Senkov, J.M. Scott, S.V. Senkova, D.B. Miracle,C.F. Woodward, J. Alloys Compd. 509, 6043 (2011).[2] J. Zýka, J. Málek, Z. Pala, I. Andršová, J. Vesely,in: METAL 2015 — 24th International Conferenceon Metallurgy and Materials, Brno 2015, p. 1687.[3] O.N. Senkov, S.L. Semiatin, J. Alloys Compd. 649,1110 (2015).[4] P.W. Bridgman, Phys. Rev. 48, 825 (1935).[5] A. Zhilyaev, T. Langdon, Prog. Mater. Sci. 53, 893(2008).[6] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov,Prog. Mater. Sci. 45, 103 (2000).[7] F. Lukáč, J. Čížek, J. Knapp, I. Procházka, P. Zháňal,R.K. Islamgaliev, J. Phys. Conf. Ser. 674, 012007(2016).[8] J. Čížek, M. Janeček, O. Srba, R. Kužel,Z. Barnovská, I. Procházka, S. Dobatkin, Acta Mater.59, 2322 (2011).[9] A.A. Coelho, TOPAS, Coelho Software, Brisbane2016.[10] W.A. Dollase, J. Appl. Crystallogr., 19, 267, (1986).[11] F. Bečvář, J. Čížek, I. Procházka, J. Janotová,Nucl. Instrum. Methods Phys. Res. A 539, 372 (2005).[12] G. Dirras, J. Gubicza, A. Heczel, L. Lilensten,J.-P. Couzinié, L. Perričre, I. Guillot, A. Hocini,Mater. Charact. 108, 1 (2015).[13] L.C. Smedskjaer, M. Manninen, M.J. Fluss,J. Phys. F Met. Phys. 10, 2237 (1980).

Defects in High Entropy Alloy HfNbTaTiZr Prepared by High Pressure Torsion

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Defects in High Entropy Alloy HfNbTaTiZr Prepared by High Pressure Torsion

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포항공과대학교