Effect of Al on structure and mechanical properties of Al x NbTiVZr ( x = 0, 0·5, 1, 1·5) high...

SPECIAL ISSUE ARTICLE

Effect of Al on structure and mechanicalproperties of AlxNbTiVZr (x ¼ 0, 0·5, 1, 1·5)high entropy alloys

N. D. Stepanov*1, N. Yu. Yurchenko1, D. G. Shaysultanov1, G. A. Salishchev1

and M. A. Tikhonovsky2

The crystal structure, microstructure, microhardness and compression mechanical properties of

AlxNbTiVZr (x ¼ 0, 0?5, 1, 1?5) high entropy alloy were examined. In the as solidified conditions,

the alloys consisted from bcc matrix and C14 Laves phase. After homogenisation, the NbTiVZr

alloy was bcc solid solution, whereas in Al containing alloys, C14 Laves phase and Zr2Al particles

were found in the bcc matrix. Volume fraction of second phase increased with Al concentration.

Increase in Al content results in gradual decrease in density of the alloys from 6?49 g cm23 of the

NbTiVZr to 5?55 g cm23 of the Al1?5NbTiVZr alloy. The microhardness of the alloys was higher in

the alloys with higher Al content and was generally proportional to the volume fraction of second

phase particles. The compression yield strength of the alloys was of 960–1320 MPa, and NbTiVZr

alloy was stronger than Al containing alloys. The ductility of the alloys gradually decreased with

increase in Al content. The factors determining phase formation in the AlxNbTiVZr alloys and effect

of phase composition and chemical composition of individual phases on the mechanical

properties are discussed.

Keywords: High entropy alloy, Crystal structure, Laves phase, Microstructure, Mechanical properties

IntroductionThe so called high entropy alloys (HEAs) is a newemerging class of metallic alloys, usually consisting fromat least five elements in approximately equimolarconcentrations.1 High entropy alloys are frequentlyconsidered as potential structural materials for hightemperature applications2 due to effect of sluggishdiffusion,3 which is expected to stabilise structure of thealloys at high temperatures. In order to fully utilisepotential of HEAs as high temperature material, Senkovet al. has introduced refractory HEAs.4 Several refrac-tory HEAs compositions have demonstrated remarkablecompressive strength at elevated temperatures5,6

superior to currently used nickel based superalloys.Therefore, a lot of efforts have been made to exploremicrostructure and properties of refractory HEAs inorder to further enhance their performance.4–17

The first developed alloys had large density of morethan *10 g cm23, which could limit their possibleapplications.5,6 The alloys of Cr–Nb–Ti–V–Zr system

with lower density of *6?5 g cm23 were introduced inorder to provide lighter materials and had attractivemechanical properties at temperatures up to 1000 uC.7,8

However, further reduction of density is desirable. Ad-dition of Al as an alloying element has the potential todecrease density of refractory HEAs as it was demon-strated.9,10,18 Moreover, increase in strength and for-mation of simple phase structure from disordered solidsolutions due to Al addition was also demon-strated.9,10,18 The latter fact, i.e. absence of intermetallicphases, may seem surprising as Al tends to formintermetallic phases with other constitutive elements ofthe studied alloys. It should be noted that, in alloysbased on transition metals like CoCrFeNiAlx orCoCrFeNiMnAlx, increase in Al content results in sub-stantial strengthening of the alloys due to formation ofordered B2 bcc phase.19,20 However, no similar sys-tematical studies of dependencies of microstructure andproperties of Cr–Nb–Ti–V–Zr system based HEAs onAl content can be found in the literature. Nevertheless,such studies could provide better understanding ofcomposition–microstructure–properties relationships inHEAs and result in development of new alloys withenhanced properties.

Therefore, in the present work, we report structureand mechanical properties of a series of AlxNbTiVZralloys. According to previously published data, the baseNbTiVZr alloy is composed mainly from disordered bcc

1Laboratory of Bulk Nanostructured Materials, Belgorod State University,Belgorod 308015, Russia2National Science Center ‘Kharkov Institute of Physics and Technology’NAS of Ukraine, Kharkov 61108, Ukraine

*Corresponding author, email [email protected]; [email protected]

� 2015 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 29 November 2014; accepted 12 February 2015DOI 10.1179/1743284715Y.0000000032 Materials Science and Technology 2015 VOL 00 NO 0 1

Materials Science and Technology mst11781r1.3d 21/3/2015 17:17:17

phase with insignificant fraction of unknown particlesand has good ductility at room temperature but limitedstrength at high temperatures.7,8 The systematic study ofa series of the AlxNbTiVZr alloys is expected to clarifyeffect of Al on phase composition of refractory HEAsand provide insights into effect of Al on mechanicalproperties.

ExperimentalThe alloys with nominal composition of NbTiVZr,Al0?5NbTiVZr, AlNbTiVZr and Al1?5NbTiVZr wereproduced by arcmelting of the elements in a low pressure,high purity argon atmosphere inside a water cooledcopper cavity. The purities of the alloying elements wereabove 99?9 at-%. To ensure chemical homogeneity, theingots were flipped over and remelted at least five times.The produced ingots of the alloys had dimensions of*6|12|40 mm. The actual chemical compositions ofthe produced alloys (in at-%; all the compositions given intext below are also in at-%) are given in Table 1. Thealloys were studied both in as solidified state and afterhomogenisation annealing. Annealing was carried out at1200 uC for 24 h, in accord with regime used previouslyfor the NbTiVZr alloy.7,8 Before homogenisation, thesamples were sealed in vacuumed (1022 torr) quartz tubesfilled with titanium chips to prevent oxidation. Afterannealing, the tubes with the samples were removedfrom the furnace and were cooled down to roomtemperature due to heat exchange with surrounding air.

Microstructure and phase composition of the alloyswere studied using X-ray diffraction (XRD) andscanning electron microscopy (SEM) techniques. TheXRD analysis was performed using a RIGAKU dif-fractometer and Cu Ka radiation. Samples for SEMobservations were prepared by careful mechanical pol-ishing. Investigations by SEM were performed utilising a

Quanta 600 FEG microscope equipped with an energy-dispersive (EDS) detector.

Density of the homogenised alloys was measured

1

hydrostatic weighting method. Vickers microhardness,HV, was measured on polished cross-section surfacesusing a 136u Vickers diamond pyramid under a 250 gload applied for 15 s. Compressive tests were performedon rectangular specimens with dimensions of6|4|4 mm using the Instron 300LX machine. Theinitial strain rate was 1023 s21.

Results

As solidified structures of AlxNbTiVZr alloysThe XRD patterns of the as solidified AlxNbTiVZralloys are shown in the Fig. 1. Apparently, all the stu-died alloys are composed from two phases, the bcc solidsolution and the hexagonal C14 Laves phase. In theNbTiVZr alloy, the bcc phase is the dominant phase,and only tiny peaks from hexagonal lattice areobserved. Increase in Al concentration results in gradualincrease in intensity of diffraction maximums fromhexagonal lattice and decrease in intensity from peaksfrom bcc lattice.

The microstructures of the as solidified AlxNbTiVZrHEAs are demonstrated in Fig. 2. The base NbTiVZralloy has predominantly single phase structure (point 1,Fig. 2a); however, some dark second phase particles areobserved both on grain boundaries (higher magnifi-cation insert in Fig. 2a) and as a separate complex shapeparticles inside the grains (point 3, Fig. 2a). The volumefraction of dark particles is estimated of 3?1%. Oneshould also note the presence of some lighter circulardots inside the grains (point 2, Fig. 2a). The results ofchemical analysis by EDS (Table 1) demonstrates thatthe grains have composition very close to exactcomposition of the alloy and are slightly enriched withTi and V and depleted of Zr. The light dots are enrichedwith Zr and depleted of V and Nb. Finally, the secondphase particles are composed mainly from V and Zr.

The Al addition to the base alloy promotes significantchanges in the microstructure, observed on the example ofAl0?5NbTiVZr alloy (Fig. 2b). The most apparent effect isincreased amount of secondphase, which is predominantlyfound as a thick layer on grain boundaries (point 3,Fig. 2b). Volume fraction of second phasewas estimated of9?4%.The layer is non-uniform; local breaks or thickenings

1 X-ray diffraction pattern of AlxNbTiVZr (x 5 0, 0?5, 1, 1?5)

alloys in as solidified condition

Table 1 Chemical composition of different structuralconstituents in as solidified AlxNbTiVZr (x 5 0,0?5, 1, 1?5) alloys, as determined by SEM-basedEDS analysis

Element/at.% Nb Ti V Zr Al

Constituent

No. Designation NbTiVZr

1 Matrix 27?0 25?9 23?5 23?6 ...2 Light dots 19?6 25?0 22?3 33?1 ...3 Second phase 13?8 17?6 31?8 36?8 ...

Alloy composition 26?6 25?0 24?6 23?8 ...NbTiVZrAl0?5

1 Light ‘‘‘cores’’’ ofthe grains

28?4 24?6 22?3 16?0 8?7

2 Dark ‘‘‘mantles’’’ ofthe grains

24?1 24?0 22?6 19?0 10?3

3 Second phase 12?7 13?0 28?1 29?3 16?9Alloy composition 23?9 22?5 21?3 21?3 11?0

NbTiVZrAl1 Matrix 22?3 22?4 20?4 14?8 20?22 Second phase 12?9 12?9 21?0 25?8 27?4

Alloy composition 20?8 19?9 18?9 18?4 22?0NbTiVZrAl1?5

1 Second phase 11?1 9?7 18?7 25?0 35?52 Matrix 21?0 22?2 18?3 12?3 26?23 Light particles 15?4 18?1 15?0 20?0 31?6

Alloy composition 18?8 17?8 16?9 17?0 29?5

Stepanov et al. Effect of Al on properties of AlxNbTiVZr HEAs

2 Materials Science and Technology 2015 VOL 00 NO 0


are often observed. Usually groups of thin elongated lensshaped particles are located around the grain boundarylayer (Fig. 2b, highermagnification insert).Particles in suchgroups are close to be perpendicular to the grain boundarylayer.However, sometimes suchgroupsare observed insidegrains and not in vicinity of grain boundaries. Some con-trast inside matrix phase is also observed: the matrix islighter near the grain boundaries, then darker ‘mantle’ isobserved (point 2, Fig. 2b), and, in many grains, there is alight ‘core’ part in the centre (point 1, Fig. 2b). The color ofthe layer beside the grain boundary layer and the ‘core’regions are nearly identical, which is reflected in theirchemical composition: they are enriched in Nb and Ti anddepleted of Al and Zr (Table 1). The darker mantle hascomposition close to the exact composition of the alloy.Finally, the second phase particles are significantly enri-ched with Zr, V and Al and depleted of Nb and Ti.

The microstructure of the equiatomic alloy AlNb-TiVZr is composed from only two phases (Fig. 2c): thelight grey matrix phase (point 1, Fig. 2c) and dark greycoarse second phase particles (point 1, Fig. 2c). Thevolume fraction of second phase is estimated to be of23?8 %. The second phase particles have complex shape;they can either form continuous networks or be locatedas separate mostly elongated curved particles. Their

average thickness is *4 mm. Inside the second phaseparticles, some light grey particles with contrast veryclose to the contrast of the matrix phase are observed.The chemical composition of the matrix phase isreasonably close to the actual chemical composition ofthe alloy (Table 1) with the only exception for lowerconcentration of Zr. The second phase particles areenriched with Al and Zr and depleted of Nb and Ti.

Further increase in Al content in the Al1?5NbTiVZralloy results in the increase in volume fraction of secondphase to 41?5% (point 1, Fig. 2d). Second phase particlesform continuous network and almost completely sep-arate grains of the matrix phase (Point 2, Fig. 2d) fromeach other. Inside the second phase, fine particles withsimilar to matrix phase contrast are observed (point 3,Fig. 2d). The results of chemical analysis (Table 1)demonstrate that the matrix phase is enriched with Nband Ti, whereas the second phase particles are enrichedwith Al and Zr.

Structure of AlxNbTiVZr alloys afterhomogenisationThe XRD patterns of the homogenised AlxNbTiVZralloys are shown in Fig. 3. Apparently, the four-com-ponent NbTiVZr alloy consists almost entirely of bcc

(a) (b)

(c) (d)

a NbTiVZr; b Al0?5NbTiVZr; c AlNbTiVZr; d Al1?5NbTiVZr2 Images (SEM-BSE) of microstructure of AlxNbTiVZr (x 5 0, 0?5, 1, 1?5) alloys in as solidified state


Materials Science and Technology 2015 VOL 00 NO 0 3


phase, although some tiny peaks from hexagonal Lavesphase can be also found. In the Al0?5NbTiVZr alloy, thereflections from bcc lattice remain dominant, but theintensity of reflections from C14 Laves phase increases.Furthermore, additional weak diffraction maximaappear, which can be attributed to hexagonal Zr2Alphase. Further increase in Al concentration in theAlNbTiVZr and Al1?5NbTiVZr alloys results in thedecrease in intensity of diffraction peaks from bcc latticeand increase in intensity of peaks from both Laves andZr2Al intermetallic phases.

Scanning electron microscopy–backscatterd electron(SEM-BSE) images of microstructure of the Alx-NbTiVZr alloys after homogenisation annealing areshown in Fig. 4. The base NbTiVZr alloy has coarsegranular single phase structure (Fig. 4a). The chemicalcomposition of the grains is close to actual chemicalcomposition of the alloy (Table 4). However, the highermagnification insert in Fig. 4a demonstrates the presenceof very thin layer of a second phase on a grain boundary.Careful inspection has revealed that similar layer ispresent on all grain boundaries. The layer was too thinto measure its chemical composition; however, EDS linescanning in direction perpendicular to grain boundary(not given) has revealed that it is enriched with V and Zrand depleted of Ti.

The microstructure of the Al0?5NbTiVZr alloy (Fig. 4b)consists from the matrix (point 1) containing two types ofsecond phase particles clearly resolved in SEM-BSEimages: dark grey particles (point 2) and light ones(point 3). The dark grey particles are usually located ongrain boundaries, and the grain boundary layer of the darkgrey phase is observed on grain boundaries (high magni-fication insert, Fig. 4b). The light particles are generallyconnected to dark grey particles. The average size of bothtypes of the particles is similar and is about 5–20 mm. Thevolume fraction of dark grey particles was estimated of13?5%, whereas the volume fraction of light particles is of3?1%. The chemical analysis (Table 2) revealed thatthe composition of the grains was reasonably close tothe actual composition of the alloy. The dark grey particlesare enriched with Zr, V and Al, and the light particles arecomposed mainly from the same elements. However, theconcentration of Zr is considerably higher, and the con-centration of V is considerably lower in light particles thanin dark grey ones.

The microstructure of the AlNbTiVZr alloy (Fig. 4c)is close to the microstructure of the previously described

Al0?5NbTiVZr alloy. The structure constitutes fromsimilar phases, namely, the matrix phase (point 1,Fig. 4c), the dark grey particles (point 2) and the lightparticles (point 3). The volume fraction and size of darkgrey and light particles is of 20?7 and 12?2% respectively.The results of chemical analysis (Table 4) demonstratethat the matrix phase is enriched with Nb and Ti anddepleted of Zr and Al. In turn, the dark grey particlesare mainly composed of Zr, Al and V, whereas the lightparticles are enriched of Zr and Al.

Finally, the microstructure of the Al1?5NbTiVZr(Fig. 4d) is similar to that of the Al0?5NbTiVZr andAlNbTiVZr alloys. However, the volume fraction oflight (point 3, Fig. 4d) particles becomes considerablyhigher and is of 30?1%, and the volume fraction of darkgrey particles (point 2, Fig. 4d) decreases to 16?1%.Chemical analysis (Table 2) demonstrates that thecomposition of the phases is similar to those of theAlNbTiVZr alloy.

Density and mechanical properties ofAlxNbTiVZr HEAsThe experimentally determined density of the of theAlxNbTiVZr alloys after homogenisation annealing isgiven in Table 3. The given values demonstrate that anincrease in Al content results in a gradual decrease indensity of the alloys from 6?49 g cm23 of the NbTiVZrto 5?55 g cm23 of the Al1?5NbTiVZr alloy. Using therule of mixtures and densities of the constitutive el-ements, the densities of disordered solid solutions withcompositions of the studied alloys were calculated(Table 3) using the following formula9

rmix ¼P

AiciP Aiciri

ð1Þ

where Ai, ci and ri are the atomic weight, concentrationand density of the i element respectively. Good corre-lation between experimentally determined, and calcu-lated values of the density is observed.

3 X-ray diffraction pattern of AlxNbTiVZr (x 5 0, 0?5, 1, 1?5)

alloys in homogenised condition

Table 2 Chemical composition of different structuralconstituents inhomogenisedAlxNbTiVZr (x 5 0,0?5,1,1?5) alloys, as determinedbySEMbasedEDSanalysis

Element/at-% Nb Ti V Zr Al

Constituent

No. Designation NbTiVZr

1 Grains 25?6 25?5 24?9 24?0 ...Alloy composition 26?6 25?0 24?6 23?8 ...

NbTiVZrAl0?51 Matrix 25?8 24?7 20?8 19?2 9?52 Dark grey particles 11?6 8?6 28?3 32?0 19?53 Light particles 9?1 12?5 20?0 41?1 17?3

Alloy composition 23?9 22?5 21?3 21?3 11?0NbTiVZrAl

1 Matrix 25?8 24?7 18?5 14?5 16?52 Dark grey particles 9?9 8?6 22?3 30?3 28?93 Light particles 9?5 8?3 3?6 39?9 38?7

Alloy composition 20?8 19?9 18?9 18?4 22?0NbTiVZrAl1?5

1 Matrix 24?9 24?7 17?6 8?3 24?52 Dark grey particles 7?9 7?6 19?9 28?4 36?23 Light particles 11?2 9 4?1 36?3 39?4

Alloy composition 18?8 17?8 16?9 17?0 29?5




The microhardness values of the AlxNbTiVZr alloysin the as solidified condition and after homogenisation isgiven in Table 3. Apparent tendency of gradual increasein microhardness of the alloys with increase in Al con-centration in as solidified state is observed; for example,the microhadrness increases from 380 to 620 HV in theNbTiVZr and the Al1?5NbTiVZr alloys respectively.Homogenisation annealing does not affect microhard-ness of the alloys substantially, except for the NbTiVZralloy: its microhardness increases from 380 HV in assolidified state to 460 HV in the homogenised condition.

The compressive stress–strain curves of the Alx-NbTiVZr alloys are shown in Fig. 5. The resultingmechanical properties, namely, yield strength s0?2, peakstress sp and fracture strain e, are summarised inTable 4. The NbTiVZr alloy demonstrates high yieldstrength of 1320 MPa, but very limited ductility of 4?2%and fractures at stress of 1470 MPa. The Al0?5NbTiVZralloy surprisingly has lower yield strain of 960 MPa andsimilar ductility of 4%. Increase in Al concentrationresults in slight increase in yield strength and decrease inductility. For example, the Al1?5NbTiVZr alloy does notexhibit plastic deformation and fractures after reachingstress of 1310 MPa. Examination of all fractured

specimens has revealed development of multiple cracks,generally aligned with compression direction or havingan angle of ,458 with compression direction.

Discussion

Phase formation in AlxNbTiVZr HEAsThe presented results clearly demonstrate that the Alconcentration has very pronounced effect on the struc-ture of the AlxNbTiVZr alloys. The main finding can besummarised as follows:

(i) in the as solidified condition, the AlxNbTiVZralloys are composed from bcc matrix and particlesof C14 hexagonal Laves phase

(ii) in the homogenised condition, the NbTiVZr alloyhas almost entirely single bcc solid solution phasestructure, whereas in the Al containing alloys,particles of C14 Laves phase and hexagonal Zr2Alphase are found in bcc matrix

(iii) in both conditions, increase in Al concentration inthe AlxNbTiVZr alloys results in substantialincrease in volume fraction of intermetallic secondphases

(a) (b)

(c) (d)

a NbTiVZr; b Al0?5NbTiVZr; c AlNbTiVZr; d Al1?5NbTiVZr4 Images (SEM-BSE) of microstructure of AlxNbTiVZr (x 5 0, 0?5, 1, 1?5) alloys in homogenised state




(iv) the second phases are composed mainly from Al,Zr and V in different proportions

(v) clear tendency of increase in Al concentration inthe second phase particles with increase in overallconcentration of Al in the alloys is observed.

All these facts imply that Al content has dominanteffect on phase composition of the AlxNbTiVZr alloys,resulting in stabilisation of intermetallic phases. It isworth to explore reasons of Al effect on intermetallicphase formation in the AlxNbTiVZr alloys.

It is well established that despite earlier suggestion ofpreferential formation of solid solutions over intermetallicphases in HEAs,21 phase structure of the HEAs complexlydependsonchemical compositionof the alloys.1Accordingto the Hume–Rothery rules22 used to predict formation ofsolid solutions in conventional single principle elementalloys, it might be expected that alloys from elements withclose atomic radii, electronegativity, vacancy electronconcentrationVEC and crystal structure would form solidsolutions readily. The individual properties of the con-stitutive elements of the AlxNbTiVZr HEAs, includingatomic radii, shear modulus, Pauling electronegativity,VEC, melting temperature Tm and crystal structure, are

summarised in Table 5.23 Brief analysis of the table showsthat Al, Nb and Ti have very similar atomic radii, whereasV has notably smaller radius and Zr a considerably higherone. All elements have similar Pauling electronegativity.Valence electron concentration of elements derives from 3of Al to 4 of Ti and Zr and 5 of Nb and V. Finally, Al hasstable fcc lattice,Nb andVhave bcc lattice, whereas Ti andZr have the same bcc lattice at temperatures higher than882 and 863 uC respectively and, at lower temperatures,have hcp lattice. Shearmodulus of the elements varies from26 GPa of Al to 47 GPa of V. Themelting temperatures ofTi, V and Zr are reasonably close, whereas Nb has some-what higher melting temperature, and Al has significantlylower melting temperature.

Another important factor that affects the formation ofintermetallic phases is mixing enthalpy. Large negativeenthalpy causes clustering of corresponding elements andformation of ordered or intermetallic phases in HEAs.25

Values of mixing enthalpies between constitutive elementsof the AlxNbTiVZr alloys are given in the Table 6.24 Thevalues of mixing enthalpy of the constitutive elements ofthe NbTiVZr alloy are only slightly negative or evenpositive; the lowest is mixing enthalpy betweenV andZr of24 kJ mol21. However, the mixing enthalpy between Aland other constitutive elements are very low: from216 kJ mol21 (Al–V) to 244 kJ mol21 (Al–Zr). Analysisof binary phase diagrams of the constitutive elements isalso useful for predictions of phase composition of multi-component alloys, as it was demonstrated recently thatphase stability for binaries can be extended to higher orderalloys.26 The stable phases of the equiatomic binary alloysare shown in Table 6.27 Binary systems not containing Alare not subject to intermetallic phase formation with theonly exception of V–Zr system where V2Zr Laves phasewith hexagonal C14 crystal lattice is formed and are com-posed from either bcc or hcp solid solutions or mixture ofboth. It should be noted, however, that the formation ofhcp structures in the binary alloys is expected only at lowtemperatures below 770 uC; at higher temperatures, singlebcc solid solution is found in all Al free binaries except forV–Zr. InAl containing binaries, formation of intermetalliccompounds is expected, and equiatomic alloys of two sys-tems, Al–Ti and Al–Zr, are composed from single inter-metallicphase. It shouldbenoted that, in eachAl–Xsystemwhere X ¼ Nb, Ti, V or Zr, several aluminide phases withdifferent compositions and crystal structures can beformed.

Interest in accurate predictions of phase composition ofHEAs with various compositions has resulted in develop-ment of specific criteria for multiple principle elementalloys.28–32 These criterions are generally based on con-ventional Hume–Rothery rules and/or thermodynamicparameters. Some of these criterions are given below.

Table 3 Density (experimental rexp and calculated using rule of mixtures rmix) of homogenised AlxNbTiVZr (x 5 0, 0?5, 1, 1?5)high entropy alloys and microhardness of alloys in as solidified and homogenised conditions

Microhardness/HVAlloy rexp/g cm23 rmix/g cm23 As solidified Homogenised

NbTiVZr 6?49 6?46 380¡10 460¡10Al0?5NbTiVZr 6?04 6?07 470¡10 500¡20AlNbTiVZr 5?79 5?76 540¡10 550¡20Al1?5NbTiVZr 5?55 5?50 620¡20 630¡30

Table 4 Compression mechanical properties of the Alx-NbTiVZr (x 5 0, 0?5, 1, 1?5) high entropy alloys inhomogenised condition

Alloy s0?2/MPa sP/MPa e/%

NbTiVZr 1320 1470 4?2Al0?5NbTiVZr 960 1100 4AlNbTiVZr 1080 1210 2?3Al1?5NbTiVZr ... 1310 0

5 Compression stress–strain curves of AlxNbTiVZr (x 5 0,

0?5, 1, 1?5) high entropy alloys in homogenised condition




DSmix ¼ 2RX

ci ln ci ð2Þ

dr ¼ 100%X

cið12 ri=�rÞ2h i1=2 ð3Þ

DHmix ¼X

4vijcicj ð4ÞV ¼ TmDSmix= DHmixj j ð5ÞVEC ¼

XciVECi ð6Þ

Dx ¼X

ciðxi 2 �xÞ2h i1=2 ð7Þ

where DSmix is the entropy of mixing of the alloyingelements,21 R is the gas constant, ci is the atomic fractionof element i, dr is the atomic size difference,28 ri is theatomic radius of element i, �r ¼ P

ciri is the averageatomic radius, DHmix is the enthalpy of mixing,28 vij is aconcentration dependent interaction parameter betweenelements i and j in a subregular solid solution model,25 Vis a specially introduced thermodynamic parameter,29

Tm ¼ PciTmi is the average melting temperature, Tmi is

the melting temperature of element i, VEC is the averagevalence electron concentration,30 VECi is the valenceelectron concentration of element i, Dx is the Paulingelectronegativity difference,31 xi is the Pauling electro-negativity of element i and �x ¼ P

cixi is the averageelectronegativity. Formation of solid solution phases isexpected when drv6?2% and DHmix is in the range from220 to 5 kJ mol21, while intermetallic phases can bepresent in HEAs for which drw3% and DHmix-

5 kJ mol21.28 Only solid solution phases were found toform in many HEAs when the conditions V>1?1 anddr<6?6% are met.29,33 Lattice type of solid solutionphase can be predicted using VEC criterion: at VEC inthe range of 6?87–8?0, a mixture of fcc and bcc phasesforms; at VEC>8?0, fcc phases are stable; and at VECsmaller than 6?87, bcc ones.30 Finally, no topologicallyclose packed phases are found in alloys with Dxv0?133,

except for the alloys containing high concentrations ofAl.31 The values of parameters dr, DHmix, DSmix, Tm, V,VEC and dx for the studied AlxNbTiVZr alloys aresummarised in Table 7.

Analysis of parameters given in Table 7 demonstratesthat addition of Al to the base NbTiVZr alloy results invery pronounced increase in absolute value of DHmix

and decrease in the value of V, which supposes that Almakes the alloys much more susceptible to formation ofintermetallic phases. On the other hand, Al additionresults in gradual decrease in atomic size difference andincrease in DSmix, which should benefit the formation ofsolid solutions. The Al0?5NbTiVZr and AlNbTiVZralloys satisfy all the requirements for solid solution as allof them have drv6?2, 220 kJ mol21vDHmix-

5 kJ mol21, Vw1?1 and Dxv0?133. However, formationof intermetallic phases can be anticipated in theNbTiVZr alloy due to high value of dr ¼ 6?29 and in theAl1?5NbTiVZr alloy, as it has highly negative value ofDHmix ¼ 221?55 kJ mol21. In addition, the value of Vof 1?16 for the Al1?5NbTiVZr alloy is very close to thecritical value of 1?1. Finally, the VEC value suggestsformation of bcc structure in all the studied alloys,which is in accordance with the bcc matrix phase of theAlxNbTiVZr alloys. It should be also noted that theformation of bcc matrix phase in the AlxNbTiVZr alloysis in agreement with crystal structures of the individualelements (Table 5) and information from the binaryphase diagrams (Table 6).

The comparison between the predictions by other cri-terions from Table 7 with the experimentally observedphases in the AlxNbTiVZr alloys shows some contradic-tions. Despite high atomic size difference, the NbTiVZralloy in homogenised state has almost purely single solidsolution phase structure with insignificant fraction ofsecond phase on grain boundaries. The atomic sizedifference value of theAl containing alloys, which containintermetallic phases both in as solidified andhomogenisedconditions, is high,w5?5%, but lower than the thresholdvalue of 6?2%.On the other hand, theAl containing alloyshave significantly lower DHmix and V values in compari-son with the NbTiVZr alloy, but these values satisfy therequirements for intermetallic phase formationonly in thecase of the Al1?5NbTiVZr alloy.

Although the criterions described above do not predictphase structure of the AlxNbTiVZr alloy, their usage canbe very helpful for understanding phase formation in thestudied alloys. It should be noted that some of the solidsolution alloys reported in the literature with somewhatsimilar compositions, namely, AlNbTiV,18 AlMo0?5-NbTa0?5TiZr and AlNb1?5Ta0?5Ti1?5Zr0?5 (Ref. 9),had DHmix of 216?25, 216?6 and 216?1 kJ mol21 andV of 1?38, 1?92 and 1?65 respectively. These values

Table 5 Atomic radius, shear modulus, electronegativity, vacancy electron concentration and melting temperature of con-stituent elements of studied alloys23

Element Al Nb Ti V Zr

Atomic radius/pm 143 146 147 134 160Shear modulus/GPa 26 38 44 47 33Pauling electronegativity 1?61 1?6 1?54 1?63 1?55VEC 3 5 4 5 4Tm/K 933 2750 1941 2183 2128r/g cm23 2?70 8?57 4?51 6?00 6?52Crystal structure fcc bcc bcc (T.882 8C), hcp (T,882 8C) bcc bcc (T.863 8C); hcp (T,863 8C)

Table 6 Values of enthalpy of mixing DHmixAB (kJ mol21),calculated by Miedema’s model for atomic pairsbetween constitutive elements of AlxNbTiVZralloys24 (in upper right triangular region) and equi-librium phases for equiatomic binary systemscontained in AlNbTiVZr alloy at room temperature27

(in lower left triangular region)

Al Nb Ti V Zr

Al ... 218 230 216 244Nb Al3Nb þ AlNb2 ... 2 21 4Ti TiAl bccþhcp ... 22 0V bccþAl8V5 bcc bcc þ hcp ... 24Zr ZrAl bccþhcp hcp hcp þ V2Zr ...




are comparable with those of the currently reported Alcontaining alloy. However, the values of dr of theAlNbTiV, AlMo0?5NbTa0?5TiZr and AlNb1?5Ta0?5Ti1?5-Zr0?5 alloys are considerably lower than that of the Alx-NbTiVZr alloys, 3?14, 4?70 and 3?99% respectively. Thefact that solid solution are formed in alloys with similarvalues of DHmix and V but lower dr suggests that highatomic size difference is required for formation ofintermetallic phases in addition to negative mixingenthalpy. Therefore, it can be concluded that highlynegative values of DHmix (and low V) due to Al additiontogether with the high dr of all the AlxNbTiVZr alloygovern formation of intermetallic phases in the studiedAlxNbTiVZr alloys.

In addition to Table 7, Table 6 also can be very usefulfor prediction of phase composition of the AlxNbTiVZralloys. For example, DHmixAB and stable phases frombinary phase diagrams suggests that, in the NbTiVZralloy, formation of Laves phase containing mostly Vand Zr is possible, which is consistent with the exper-imental data on the as solidified alloy. On the otherhand, as the other binaries do not conclude intermetallicphases, the V2Zr Laves phase can be stable only incertain temperature range, and therefore, it is almostcompletely dissolved after homogenisation. When Al isadded to the NbTiVZr alloy, it should tend to clusterwith Zr, as the Al–Zr pair has the lowest mixingenthalpy among the possible atomic pairs. Therefore,the Al segregates in the Zr rich Laves phase and mostprobably stabilises the Laves phase forming strongchemical bonds with constitutive elements. In addition,the Al has strong tendency to form aluminides with allconstitutive elements of the alloy according to binaryphase diagrams. Owing to high negative mixing enthalpybetween Al and Zr, formation of zirconium aluminidesis expected. Indeed, after homogenisation annealing,Zr2Al phase is observed in the Al containing alloys. TheZr2Al phase almost does not contain V, which is inagreement with relatively high mixing enthalpy betweenAl and V. Thus, simultaneous usage of mixing enthalpyof atomic pair and binary phase diagrams is very usefulfor understanding of phase formation in the Alx-NbTiVZr alloys, as it was recently shown for Al–Li–Mg–Zn–Sn alloy system.34

Effect of Al on mechanical properties of Alx-NbTiVZr HEAsThe mechanical properties of the AlxNbTiVZr alloysapparently are significantly affected by the concen-tration of Al. For instance, the microhardness (Table 3)demonstrates apparent tendency to increase withincrease in Al content. As Al results in increase involume fraction of second phase particles, it can besupposed that hardness can be dependent on the volumefraction of particles. Figure 6 shows the dependence ofmicrohardness of the AlxNbTiVZr alloys on the volume

fraction of second phase particles (in the as solidifiedcondition, the volume fraction of second phase particlesis equal to the volume fraction of Laves phase particles;in homogenised condition, to sum of the volume fractionof Laves phase and Zr2Al phase).

Figure 6 clearly demonstrates that there is linearproportion between microhardness and the volumefraction of second phase particles in the AlxNbTiVZralloys, which can be written as follows

HV ¼ 5·21Vsecond phase þ 398·4 ð8ÞTwo following conclusions can be made from this

dependence: first, the hardness of the AlxNbTiVZralloys can be rather precisely controlled by varyingconcentration of Al and heat treatment, as the volumefraction of second phase particles was found to bedependent on the composition of the alloy and sensitiveto annealing; second, there is no substantial differencesin hardening produced by particles of Laves phase andZr2Al particles as the sum of their volume fractions canbe successfully used to predict hardness of the Alx-NbTiVZr alloys.

However, the hardening by second phase particlesdoes not explain increase in hardness of the NbTiVZralloy after homogenisation (Table 3), as in this case, theLaves phase particles found in as solidified conditionwere almost completely dissolved after annealing.Moreover, the yield strength of the homogenisedNbTiVZr alloy is higher than the yield strength of the Alcontaining alloys despite the fact that alloys with Alcontain significant volume fraction of second phaseparticles.

It is widely accepted that the crystal lattice of the solidsolution HEAs is severely distorted due to the presenceof several types of atoms with different sizes.1 Latticedistortions are expected to cause significant solid

Table 7 Calculated parameters dr, DHmix, DSmix, T;m, V, VEC and Dx for studied AlxNbTiVZr alloys

Alloy Dr/% DHmix/kJ mol21 DSmix/J mol21 K21 Tm/K V VEC Dx

NbTiVZr 6?29 20?25 11?53 2251 103?75 4?5 0?037Al0?5NbTiVZr 5?98 210?86 13?15 2104 2?55 4?33 0?036AlNbTiVZr 5?73 217?44 13?38 1987 1?52 4?2 0?035Al1?5NbTiVZr 5?51 221?55 13?25 1891 1?16 4?09 0?034

6 Dependence of microhardness of AlxNbTiVZr alloys on

volume fraction of second phase




solution type strengthening, although in multi-component alloys, there is difficult to define ‘solvent’and ‘solute’ atoms.35 Apparently, the strengtheningcaused by atoms of different sort is proportional todifference of their atomic sizes with average atomicradii.36 With respect to the AlxNbTiVZr alloys, the Al,Nb and Ti have similar radii, Zr has considerably higherradius and V has considerably lower radius (Table 5).Therefore, it can be suggested that lattice distortions willbe generated predominantly by V and Zr atoms. In thehomogenised NbTiVZr alloy, all V and Zr atoms are inthe solid solution, whereas in as solidified state, theycluster in the Laves phase particles (Tables 1 and 2).Therefore, probably, lower hardness of the as solidifiedalloy can be attributed to the weaker solid solution typestrengthening of the bcc matrix phase. Similarly, in theAl containing alloys, the matrix phase is enriched withNb and Ti and depleted of Zr and V, which tend tosegregate in the second phase particles (Tables 1 and 2).The difference between Zr and V content in the bccphase between the NbTiVZr and the Al containingalloys is as high as 8?9–24?0% (Table 2). Thus, the solidsolution type strengthening of the matrix phase in the Alcontaining alloys can be significantly weaker than in theNbTiVZr alloy due to depletion of the bcc matrix phaseof the Zr and V atoms, presumably causing high dis-tortions of crystal lattice.

One can anticipate that the presence of second phaseparticles will cause strengthening of the AlxNbTiVZr inproportion with hardness (Fig. 5). Indeed, some increasein the strength in the Al containing alloys with increasein Al content is most likely associated with highervolume fraction of second phase particles. However, theresulting compression strength of the Al containingalloys is lower than that of the NbTiVZr alloy despitetheir significant volume fraction. It indicates that theparticles do not contribute much to compressionstrength, most probably due to their morphology, largesizes and location predominantly on grain boundaries.Moreover, the particles of second phase likely are thereason of the poor ductility, as, for example, the Lavesphase is known to be extremely brittle at room tem-perature.37 Even in the NbTiVZr alloy, the presence ofthin grain boundary layer of Laves phase can be thoughtto be the reason of relatively low ductility. In Al con-taining alloys, increase in volume fraction of secondphases results in gradual decrease in ductility, and theAl1?5NbTiVZr alloy does not exhibit plastic deformationat all. Nevertheless, it should be mentioned that the hightemperature properties of the AlxNbTiVZr alloys canbenefit from the presence of second phase particles, as itwas demonstrated in Cr–Nb–Ti–V–Zr alloys.8

ConclusionIn the present study, the structure and mechanical prop-erties of the AlxNbTiVZr (x ¼ 0, 0?5, 1, 1?5) alloys wereexamined, and the following conclusions were drawn.(1) In the as solidified condition, the AlxNbTiVZr

alloys were composed from bcc matrix and C14Laves phase particles. The Laves phase particleswere enriched of Zr and V (and Al in Al containingalloys). The volume fraction of Laves phaseincreased from 3?1% in the NbTiVZr alloy to 41?5%in the Al1?5NbTiVZr alloy.

(2) After homogenisation annealing, the NbTiVZralloy was almost entirely single bcc phase with onlya thin grain boundary layer of presumably Lavesphase. In contrast, two intermetallic phases in bccmatrix were observed in the Al containing alloys:the C14 Laves phase and Zr2Al phase. The Zr2Alphase was enriched with Zr and Al and almost doesnot contained V. The volume fraction of secondphases increased with increase in Al content. Forexample, the volume fractions of Laves and Zr2Alphases in the Al0?5NbTiVZr alloy were 13?5 and3?1% respectively, and in the Al1?5NbTiVZr alloy,the respective values were 16?1 and 30?1%.

(3) The stabilisation of intermetallic phases in the Alx-NbTiVZr alloys with higher Al concentration isfacilitated by significant decrease in mixing entropyand V parameter (V ¼ TmDSmix/|DHmix|) in Alcontaining alloys together with high values ofatomic size difference of all alloys. For example, inthe NbTiVZr alloys, the values of DHmix, V and drwere 20?25 kJ mol21, 103?75 and 6?29% respect-ively, whereas in the Al0?5NbTiVZr and Al1?5-NbTiVZr alloys, the corresponding values were of210?86 kJ mol21, 2?55 and 5?98% and221?55 kJ mol21, 1?16 and 5?51% respectively.

(4) Increase in Al content results in gradual decrease indensity of the alloys from 6?49 g cm23 of theNbTiVZr to 5?55 g cm23 of the Al1?5NbTiVZralloy.

(5) Themicrohardness of theAlxNbTiVZr alloys increasewith increase in Al concentration. For example, themicrohardness of the NbTiVZr alloy in the as solidi-fied condition was of 379 HV and that of the Al1?5-NbTiVZr alloy was of 619 HV. Homogenisationannealing has not affectedmicrohardness of the alloyssubstantially, except for the NbTiVZr alloy wheremicrohardness increased to 457 HV. Generally, linearrelationship between volume fraction of second phaseparticles andmicrohardnessof theAlxNbTiVZralloyswas found.

(6) The compression mechanical properties of thehomogenised AlxNbTiVZr alloys were found to becomplexly dependent on Al concentration. TheNbTiVZr alloy had the highest yield strength of1320 MPa, and the Al0?5NbTiVZr alloy has con-siderably lower yield strength of 960 MPa. Furtherincrease in Al concentration resulted in continuousstrengthening; however, alloys with higher Al con-tent were less ductile. For example, the compressionductility of the NbTiVZr alloy was 4?2 %, and theAl1?5NbTiVZr alloy does not exhibited plastic de-formation at all and brittle fractured when strainreached 1310 MPa.

AcknowledgementsThe authors gratefully acknowledge the financial supportfrom the Ministry of Science and Education of RussianFederation under agreement no. 11?1816?2014/K.

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Effect of Al on structure and mechanical properties of Al x NbTiVZr ( x = 0, 0·5, 1, 1·5) high...

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