A R C H I V E S O F M E T A L L U R G Y A N D M A T E R I A L S

Volume 58 2013 Issue 2

DOI: 10.2478/v10172-012-0195-0

K. STAN∗, L. LITYŃSKA-DOBRZYŃSKA∗, P. OCHIN∗∗, A. WIERZBICKA-MIERNIK∗, A. GÓRAL∗, J. WOJEWODA-BUDKA∗

EFFECT OF ALLOYING ELEMENTS ON MICROSTRUCTURE AND PROPERTIES OF Al-Mn-Fe RIBBON

WPŁYW DODATKÓW STOPOWYCH NA MIKROSTRUKTURĘ I WŁASNOŚCI SZYBKO CHŁODZONYCH TAŚM STOPU Al-Mn-Fe

Influence of Ti, V, Cr, Zr, and Mo additions on microstructure and mechanical properties of the Al91Mn7Fe2 quasicrystallinealloy produced by the melt spinning technique has been studied. It was found that the microstructure of obtained all ribbons wassimilar and consists of spherical or dendritic icosahedral quasicrystalline particles embedded in an aluminium matrix coexistingwith small fraction of intermetallic phase. Comporing DSC curves obtained for each sample it was observed that the alloy withMo addition exhibits the best thermal stability among prepared alloys. Addition of molybdenum caused a significant shift ofthe main exothermic peak corresponding to temperature of quasicrystalline phase decomposition from 450◦C for ternary alloyto about 550◦C for quaternary composition. Microhardness measured for all prepared alloys were similar with the mean valueof about 200 HV only alloy with Zr addition exhibited higher microhardness of about 270 HV caused by strengthening effectof Zr localized in the grains of aluminium matrix.

Keywords: quasicrystals, aluminium alloys, Al-Mn-Fe, rapid quenching, TEM, DSC

W niniejszej pracy badano wpływ dodatków stopowych: Ti, V, Cr, Zr i Mo na mikrostrukturę oraz własności mechanicz-ne kwazikrystalicznego stopu Al91Mn7Fe2 przygotowanego metodą odlewania na wirujący walec (melt spinning). Uzyskanewyniki pokazały, że mikrostruktura wszystkich otrzymanych taśm była zbliżona i zawierała sferyczne lub dendrytyczne cząstkikwazikrystaliczne umieszczone w osnowie aluminiowej wraz z niewielką ilością fazy międzymetalicznej. Porównując krzyweDSC dla każdej z próbek zaobserwowano, że stop z dodatkiem molibdenu wykazuje najlepszą stabilność termiczną pośródodlanych stopów. Dodatek tego pierwiastka spowodował znaczące przesunięcie głównego piku egzotermicznego związanegoz rozpadem fazy kwazikrystalicznej o około 100◦C, to jest od temperatury 450◦C dla stopu trójskładnikowego do temperatury550◦C dla stopu czteroskładnikowego. Wartości mikrotwardości zmierzone dla każdego ze stopów były zbliżone a ich średniawartość wynosiła 200 HV. Wyjątek stanowił stop z dodatkiem Zr dla którego średnia wartość mikrotwardości wynosiła 270 HVze względu na efekt umacniający, związany z obecnością cyrkonu w ziarnach osnowy.

1. Introduction

Recently, a group of high strength and thermally stablealuminium base alloys with microstructure containing fine-ly dispersed nano-size quasicrystalline particles embedded inan aluminium solid solution matrix has been strongly inves-tigated [1-3]. Some of the first results concerning these al-loys reported the value of tensile fracture strength exceeding1000 MPa for Al-Mn-Ce alloy [4]. Alloys with mentionedstructure can be obtained for composition with more than 90at.% of Al and with addition of two or more transition ele-ments. Production of such materials involves rapid quenchingmethods which enable formation of the metastable quasicrys-talline phase in Al-TM systems. Addition of transition metalsas alloying elements also provides good thermal stability ofthe alloy considered as maintaining its specific microstructure,and therefore the mechanical properties even after prolongedheating at elevated temperatures [5]. This effect is connectedwith low diffusivity of the transition elements in Al which

is responsible for retarded decomposition of the strengthen-ing phase during heating - in this case quasicrystalline phase.Combination of high strength and ability to work at highertemperature range without deterioration of alloy mechanicalproperties is of great interest in terms of potential industrialapplications. At present 7xxx series alloys with the best me-chanical properties (tensile strength of about 700 MPa) usuallyto operate at room temperatures. For these alloys deteriorationof mechanical properties occurs at about 150◦C [6]. Even forAl-Si base alloys commonly used at higher temperature, thevalue of UTS at 300◦C is about 200-250 MPa [7-9], whilefor nanoquasicrystalline aluminium alloys reported value at350◦C was 440 MPa for Al-Fe-Cr-Ti alloy [1]. The tempera-ture of quasicrystalline phase decomposition should be above400◦C when considering a route of formation of bulk materi-als by compaction of atomized powders or pulverized ribbonse.g. by extrusion or hot pressing is considered. With a propercombination of alloying elements, the stability of quasicrys-talline phase in the alloy can be maintained up to 550◦C for

∗ INSTITUTE OF METALLURGY AND MATERIALS SCIENCE POLISH ACADEMY OF SCIENCES, 30-059 KRAKOW, 25 REYMONTA ST., POLAND∗∗ INSTITUT DE CHIMIE ET DES MATERIAUX PARIS EST, CNRS-UNIVERSITE PARIS XII, 2-8 RUE HENRI DUNANT, 94320 THIAIS, FRANCE

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some compositions [10]. In the literature, quasicrystalline al-loys based on Al-Cr [2-3, 11] and Al-Fe [12-17] systems aremost extensively investigated, in particular the effect of subse-quent alloying elements on their microstructure and properties.The most common additions are V, Ti, and Si [1-3]. Some ofthese alloys have been also produced in a bulk form [2, 3, 5].

For the Al-Mn system, influence of alloying elementssuch as Be [18], Sr [19] and Ce [20] has been studied. Ithas been also reported that Al-Mn base alloy with iron addi-tion exhibits promising mechanical properties with the tensilefracture strength of 1250 MPa [21]. However, the stability ofquasicrystalline phase in this alloy was the lowest among thesystems (Al-Cr and Al-V) studied in mentioned work. Sincethere is a lack of information about improvement of alloy prop-erties including its thermal stability, more detailed researchare carried out. This work is a continuation of the studies onthe quasicrystalline alloys based on Al-Mn-Fe system [22-23].In this paper an influence of addition of fourth alloying ele-ment on the microstructure and mechanical properties of theAl91Mn7Fe2 alloy, as well as its thermal stability are investi-gated. With the aim of producing desired microstructure, tran-sition metals were chosen as the subsequent alloying elementsaccording to previous considerations of their ability to formquasicrystalline phase, as well as concerning their low diffu-sion coefficient in aluminium which influences alloy thermalstability.

2. Experimental procedure

The alloys of nominal compositions Al91Mn6Fe2X1 (X=

Zr, Cr, Ti, V and Mo) were prepared using high-purity ele-ments: 99.99% Al, 99.99% Fe, 99.99% Mn (purity of fourthalloying element, marked as X, was at least 99.95%). Theternary Al91Mn7Fe2 alloy was taken as a reference material.All alloys were induction melted in vacuum and then castinto steel mould. The nominal chemical composition of theprepared alloys, together with composition measured by EDXin SEM, as well as the alloy designations are presented inTable 1. The ingots were re-melted and rapidly solidified us-

ing the melt-spinning technique. The melt was ejected froma quartz crucible with a nozzle in the bottom onto a copperwheel rotating at linear velocity v = 20 m/s under heliumatmosphere. The pressure of the gas ejecting the molten alloywas 180 mbar. The ribbons cast under these conditions were20-40 µm thick and about 10 mm broad.

Phase characterization of the melt spun ribbons was per-formed by X-ray diffraction (XRD) using Philips PW 1840X-ray diffractometer with CoKα radiation. Microstructure ob-servation and chemical composition analysis were carried outusing scanning electron microscopy (SEM, FEI E-SEM XL30)equipped with an energy dispersive X-ray spectrometer (EDX)and transmission electron microscopy (TEM, FEI Tecnai G2)operating at 200 keV and combined with high-angle annulardark field scanning transmission electron microscopy detector(HAADF-STEM) as well as energy dispersive X-ray spectrom-eter (EDAX). The Tenupol-5 double jet electropolisher wasused for the preparation of thin foils in electrolyte containingnitric acid and methanol (1:3), at the temperature of -30◦Cand voltage of 15 V. Thermal stability of melt spun ribbonswas analyzed by differential scanning calorimetry (DSC, DuPont 910) with the heating rate of 40 K/min in the argonprotective atmosphere. Microhardness was measured with theCSM Microhardness Tester with load 0.1 N (contact load –0.01 N).

3. Results and discussion

The microstructure of all investigated ribbons was studiedby SEM observation of their longitudinal cross section (Fig.1). Thickness of the ribbons measured on SEM images variesin the range 20-40 µm and changes over the ribbons length. Inmost cases the formation of two typical areas within the rib-bon was noticed. Wheel side usually contained fine sphericalparticles (less than 1 µm), while dendrites with significantlylarger size (from one to several µm) were formed at the airside. It is generally assumed that formation of these two zonesis the result of different cooling rates within the both sides ofribbon (air and wheel side) during solidification.

TABLE 1Alloys designation and nominal chemical compositions of the prepared ingots together with an average values of the elements content

measured by EDX in SEM; (X = Zr, Cr, Ti, V or Mo)

Alloy designationNominal chemicalcomposition [at.%]

Chemical composition measuredby EDX [at. %]

Al Mn Fe X

AlMnFe Al91Mn7Fe2 90.7±1.8 7.0±0.3 2.3±0.5 -

AlMnFeZr or (Zr) Al91Mn6Fe2Zr1 92.2±1.8 5.0±0.5 1.7±0.4 1.1±0.2

AlMnFeCr or (Cr) Al91Mn6Fe2Cr1 92.2±1.8 5.2±0.5 1.7±0.4 0.9±0.4

AlMnFeTi or (Ti) Al91Mn6Fe2Ti1 91.2±1.8 5.8±0.2 2.0±0.4 1.0±0.2

AlMnFeV or (V) Al91Mn6Fe2V1 92.6±1.9 4.9±0.5 1.7±0.3 0.8±0.4

AlMnFeMo or (Mo) Al91Mn6Fe2Mo1 90.7±1.8 6.1±0.2 2.0±0.4 1.2±0.2

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Fig. 1. SEM images of melt spun ribbons longitudinal cross sec-tions: a) Al91Mn7Fe2, b) Al91Mn6Fe2Zr1, c) Al91Mn6Fe2Cr1, d)Al91Mn6Fe2Ti1, e) Al91Mn6Fe2V1, f) Al91Mn6Fe2Mo1 the wheel sideof the ribbon is marked as A and air side as B

The ribbons are not homogeneous over the length andone type of microstructure within entire width of the ribbon(instead of the two zones) were observed for all samples. Someparts of ribbon contained either large or fine particles as shownin Fig. 2. Based on the observations of various parts of thesame ribbon it was noticed that the fraction of the area con-taining the small particles is higher for alloys with the additionof Zr, Ti and Cr.

Fig. 2. Examples of ribbons cross sections showing homogeneity ofthe structure within entire width of the ribbon

Results of XRD measurements show the similarity ofphase composition for all investigated ribbons (Fig. 3). Thestrongest reflections which are observed in diffractograms cor-

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respond to α(Al) solid solution and quasicrystalline phase. Theshift of the position of the quasicrystalline reflections are visi-ble in the enlargement part of the diffractograms (the high shiftobserved for the samples with V, Ti and Mo addition confirmsdissolution of these elements in quasicrystalline phase). Forthe ribbon containing Zr, the peaks do not shift even thoughthis element has the highest values of atomic radii comparingwith other element used (see Table 2, where diffusivity andsolid solubility of transition metals in Al, together with theirmelting points and atomic radius are presented). This indicatethat Zr atoms are not involved in the formation of quasicrys-talline phase in this system and remain in the matrix.

Fig. 3. Diffractograms revealing similarity in phase composition forall investigated ribbons

TABLE 2Diffusivity and solid solubility of transition metals in Al, together

with their melting points and atomic radius

ElementMeltingpoint

[◦C] [24]

D in Alat 400◦C

[m2/s−1] [25]

Atomicradius[A◦]

Maximum solidsolubility in Alin at.% [26]

Al 660.13.25x10−15

(self-diffusion) 1.820 –

Fe 1536.0 5.41x10−18 1.720 0.026

Mn 1244.0 6.24x10−19 1.790 0.92

Cr 1857.0 1.29x10−21 1.850 0.42

V 1902.0 4.85x10−24 1.920 0.2

Ti 1667.0 7.39x10−22 2.000 0.8

Zr 1852.0 1.20x10−20 2.160 0.07

Mo 2620.0 5.52x10−23 2.010 0.07

TEM studies confirm that the microstructure of all in-vestigated melt spun ribbons is composed of aluminium solidsolution matrix with the quasicrystalline particles identify asthe icosahedral type based on 2, 3 and 5-fold selected areadiffraction patterns (SADP). As an example, TEM bright fieldimage of AlMnFeCr ribbon showing the quasicrysalline parti-cles with the size of about 0.5 µm and 5-fold SADP obtainedfrom a single particle are presented in Fig. 4. Composition ofthe quasicrystalline phase and the matrix determined by theEDX for all investigated alloys is given in Table 3. For allribbons the quasicrystalline particles are enriched in Mn andFe. The content of manganese varied from about 10 at.% inthe case of Mo, Ti, Cr, V to about 12 at.% for AlMnFeZrand ternary AlMnFe alloy, while the iron content is in the

range of 3.5-5 at.% for all alloys. The obtained values clearlyrevealed that zirconium is not present in the quasicrystals aswas mentioned before, while the content of other adalitionswas about 2 at.%. The matrix was enriched only in Mn (upto 2 at.%), except of AlMnFeZr ribbon, which additionallycontained about 2 at.% of Zr.

Fig. 4. Bright field TEM image of Al91Mn6Fe2Cr1 melt spun ribbonand SADP taken from one of the observed particles revealing 5-foldsymmetry

TABLE 3Mean chemical composition in at% of the icosahedral phase andmatrix in melt spun ribbons together with the standard deviations

Alloydesignation

Investigatedarea Al Mn Fe X

AlMnFeQuasicrystal 82.8±1.4 12.5±1.0 4.7±0.4 –

Matrix 97.5±1 2.30.7 – –

AlMnFeMoQuasicrystal 84.7±1.3 9.8±1.0 3.5±0.6 2.0±0.8

Matrix 98.2±0.7 1.6±0.3 – –

AlMnFeCrQuasicrystal 82.6±1.9 10.4±1.5 5.0±0.6 2.0±0.5

Matrix 96.0±1.1 2.4±0.8 1.3±0.4 –

AlMnFeTiQuasicrystal 84.5±0.9 10.2±0.7 3.9±0.3 1.4±0.2

Matrix 97.4±1.1 2.2±0.4 – –

AlMnFeVQuasicrystal 83.7±0.9 10.6±0.6 3.8±0.5 1.9±0.2

Matrix 98.1±0.4 1.9±0.2 - -

AlMnFeZrQuasicrystal 82.5±1.6 12.0±1.4 5.3±1.0 -

Matrix 96.5±1 1.5±0.4 – 2±1.0

In order to determine thermal stability of the quasicrys-talline phase DSC analysis was performed. The as spun rib-bons were heated up to 650◦C with the rate of 40 K/min. Forall the ribbons, one well noticeable exothermic peak is visible(Fig. 5). By comparing the curves obtained for the sampleswith different composition, shifts of this main exothermic peakis observed. This peak is connected with transformation of themetastable quasicrystals into stable crystalline phase duringheating. Some of the curves have more complex run and con-tain additional small exothermic peaks revealing formation ofsome other phases during heating. Taking into account bothmain peak position and the presence of additional exothermicreactions occurring in the samples, it was concluded that thebest thermal stability exhibits alloy with Mo addition. For thisalloy the main exothermic peak shifts of about 100◦C towardshigher temperatures (in comparison with the ternary alloy)with almost no occurrence of additional reactions.

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Fig. 5. DSC curves for all of the ribbons showing shift of the mainexothermic peak position for different alloys composition

Vickers microhardness was measured on the ribbons crosssections for both typical areas observed in the sample (withsmall or large particles). Up to 20 indentations were performedfor each sample. On the graphs (Fig. 6), average values ob-tained separately for both regions are marked (A and B simi-larly as in the Fig. 1-2) together with standard deviations. Ad-ditionally the average value calculated for both measurementsis given for each sample. The region with fine particles alwaysexhibits much higher microhardness than those obtained forthe areas with larger particles. The highest microhardness forboth areas was obtained for the sample with Zr addition andreaches mean value of over 270 µ HV100. This result is dueto the fact that zirconium remains in the matrix (probably inthe form of precipitates) and causes additional strengtheningof the alloy.

Fig. 6. Microhardness tests at room temperature for the as spun rib-bons with addition of different transition metals. For each composi-tion values for area A (fine spherical quasicrystalline particles) andarea B (large particles) were measured

4. Conclusions

In this work influence of subsequent alloying elementson the microstructure and mechanical properties of the meltspun Al91Mn7Fe2 alloy has been studied. The results revealedthat the addition of fourth element (transition metal: Zr, Cr,Ti, V, and Mo) to the ternary alloy do not change the alloy mi-crostructure and phase composition. All investigated ribbons

contained quasicrystalline phase of icosahedral type in a formof spherical or dendritic particles with different sizes (fromhundreds of nm to few micrometers) embedded in aluminiummatrix. Size and shape of quasicrystalline particles dependstrongly on cooling rates within the ribbon during solidifica-tion. Chemical analysis of both phases observed in the samplesrevealed that quasicrystalline phase always contains similaramounts of Al, Fe and Mn with small amount (up to 2%)of fourth element except of alloy with Zr addition. Chemicalanalysis of the latter showed that Zr is localized in the grainsof aluminium matrix resulting in additional strengthening ofthe alloy. Therefore for this alloy the highest values of micro-hardness were obtained. DSC studies showed that the thermalstability of the alloys change with composition. Analysis ofthe main exothermic peak position as well as occurrence ofother exothermic reaction in the sample showed that the beststability exhibits alloy with the Mo addition. It is not surprisingsince this element has one of the lowest diffusion coefficientin aluminium from the used additions and also the highestmelting temperature.

Acknowledgements

The work was supported through project no. POKL.04.01.00-00-004/10 co-financed by the European Union within theEuropean Social Fund and project Polonium no.8415/2011.

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Received: 20 January 2013.