Crystals and nanocrystals in rapidly solidified AlSm alloys

NtuwStructurcd Materi&. Vol. 10,No. 5. pp.767-776.1998 Elsevia scienceLtd @ 1998ActaMetahgica Inc. RintediIlthelJsA. AllrightsraKNd 0965~9773/98 919.00+ .OO

Pergarnon

PII SO9659773(98)001147

CRYSTALS

AND NANOCRYSTALS IN RAPIDLY SOLIDIFIED AI-Sm ALLOYS

P. Rizzi*, C. Antonione+, M. Baricco*, L. Battezzati*, L. Armelaot, E. Tondello’, M. Fabrizio”, and S. Daolio” *Dipartimento di Chimica FM, Universita di Torino, V. Giuria 9,10125 Torino, Italy +Dipartimento di Ingegneria Chimica e Scienza dei Materiali, Politecnico di Torino, C.so Duca degi Abruzzi 24,10129 Torino, Italy ODipartimento di Chimica Inorganica, Metallorganica ed Analitica, Universita di Padova, V. Loredan 4,35 13 1 Padova, Italy TCSSRCC, CNR, V. Marzolo 1.35131 Padova, Italy *Istituto di Polarografii ed Elettrochimica Preparative, CNR, C.so Stati Uniti 4,35127 Padova, Italy (AcceptedJune 22,1998)

Abstract,- Theformation, size and morphologyof crystalsin Alloo.,Smx(8a<12) alloys either melt spun or crystallizedfrom the glass was analyzedwithSEM, APM, TEM, DSC, XPS, SIMS. APM and SEM show crytallinefractions even in some ribbons appearing amorphous at XRD. On the su#ace which was in contact with the wheel, colonies of crystals were found, apparentlynucleatedon heterogeneities.In A192Sm.s theywere identifiedas a-Al, inAlsaSml2and Al&mlo as AltlSm3. On the oppositesurface of Al&ms ribbons, APM reveals crytalsof 100200 nm, regularly distributed,nucleated in the undercoolingregime. In glass samples annealed between423 K and 444 K, TEM showsnanocrystalsreaching a densityof 1g2 rnT3in less thana minute withmaximumsize of 20 nm. The isothermaltransformedfractionindicatesa two step process of crystallization.Thefindings are discussedconsideringnucleationin varioustemperature ranges. @1998 Acta MetallurgicaInc.

INTRODUCTION New Al-Rare Earth based alloys containing Al nanocrystals embedded in an amorphous matrix display interesting mechanical properties (tensile strength of 1.6 GPa, about 1.5 times that of the corresponding fully amorphous alloys and 3 times that of conventional precipitation hardened alloys) (1,2). They can be obtained by rapid solidification of the melt to give a glass which is then partially devitrified by thermal treatment. In order to produce materials with such a microstructure, it is necessary to understand the mechanism of nucleation and growth of the nanocrytalline phase (3,5). In this work, we focus our attention on the simplest binary system (Al767

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Sm), in which nanocrystalscan be obtained, comparing the phases produced by devitrification and during rapid solidification. Al-Sm is a marginal glass-former so amorphisation is not always achieved in rapid solidification experiments. Partially crystalline materials are nevertheless useful to study the formation of crystals from the melt. Various microscopy and analytical technique are used in order to detect crystals nucleated on the surface or in the bulk of the ribbons at different levels of resolution. Their morphology, number density and constitution allow to trace out the thermal history of the sample and to obtain hints on the nucleation and growth processes.

EXPERIMENTAL Master alloys were prepared by arc melting the pure elements (Al 99.999 %wt, Sm 99.9% wt.). The alloys were rapidly solidified by melt-spinning under an atmosphere of pure argon producing both amorhpous and partially crystalline ribbons of 40 lrm thickness and 2-3 mm width. The wheel speed was 150 ms-l unless otherwise stated in the text. Identification of the sample structure was made by X-ray diffraction (XRD) using CoKa radiation and transmission electron microscopy (T&l) in a Jeol JEM 2000 EX, operating at 200 kV TBM specimens were prepared by chemical etching in 1: 1 HCl in HzO. Differential scanning calorimetry (DSC) was performed at 20 K/min under flowing Ar using a PE7 instrument. The microstructure of the surfaces was examined by scanning electron microscopy (SEM) (Leica Stereoscan 420) with energy dispersive X-ray spectroscopy (BDS), and atomic force microscopy (AIM). Topographic AFM images of the samples were obtained on a Park Scientific Instruments (PSI) Autoprobe CP instrument using contact AFM in air at room temperature (RT) adopting the constant force mode (the force being ca. 1-2 nN). The cantilever used is a PSI microlever: a gold coated silicon nitride cantilever with a silicon tip (0.6 microns). The images am presented with elaboration a background subtraction and noise filtering. The samples were glued to the AFM sample holder using a cyanoacrilate glue. The composition of the samples at the surface and in the bulk was obtained by X-ray photoelectron spectroscopy (XPS). The spectra were run on a Perkin-Elmer @ 56OOcispectrometer using non-monochromatized Mg-Ku radiation (1253.6 ev). The working pressure was less than 5 10s8Pa. Survey scans were obtained in the 0- 1100 eV range. Detailed scans were recorded fortheA12p,Sm3d,ClsandOlsregions. Theatomiccomposition wasevaluatedusingtheoretical cross-sections (6). Depth profiles were carried out by Ar+ sputtering at 2.5 keV with an argon partial pressure of 5 . lo4 Pa. Under these conditions we estimated a sputtering rate around 3-4 nm/min. Further information was obtained by secondary ion mass spectroscopy (SIMS). A custom-built instrument was used for SIMS analysis. Amonochromatic (l-10 keV) 02+ ion beam collimated to 50 m was generated in a mass-filtered duoplasmatron ion gun (model DPSOB, VG Fisons). The secondary ion optics were of three-lens design with a central stop interfaced with a Balzer QMA 400 quadrupole massanalyzer. A secondary electron multiplier (90” off-axis) was used for positive and negative ion detection in the counting mode. Sputtering of the Altoo_xSm, ribbons was carried out with a 2 keV a+ primary ion energy at an ion current of 400-800 nA. During the experiment the vacuum chamber was kept at 5. lo4 Pa. RESULTS Morphological features of the ribbons are reported below for each composition. A common aspect for all ribbons revealed by SIMS and XPS is the presence of impurities. Besides the metallic

CRYSTALSANDNANOCRYSTALS IN RAPIDLYSOLIMFIEDAI-Sm ALLOYS

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Figure 1. SEM micrograph of an AlssSm12 ribbon, side opposite to the wheel.

Al and Sm, both techniques show oxidized species (i.e. AlO+, Al20+, SmO+, AlO-, Al&-) in the sameamountoneitbertbesurfaceincontact with the wheelandtheoppositeone. Theribbons were obtained under Ar after evacuation of the spinning chamber, however gases adsorbed on the wheel are not likely tobe removed. After quenching, the ribbons were exposed to air.Amounts of nitrogen and fluorine as trace impurities were found by SIMS profiling up to a thickness of about 1 pm.

This alloy was only partially amorphised. It is described at first because it provides a useful benchmark for identification of phases and crystal morphologies. In X-ray diffraction ribbons show AllISlms peaks. SEM analyses performed on the side opposite to the wheel using a backscattered electron detector (Figure 1) show dendritic crystals that appear bright with respect to the amorphous matrix, i.e. they contain a larger amount of the heavier element Sm. They are associated to the AlrrSma crystalline phase identified by X-ray analysis. Their maximum dimension is 2 l.urt - 3 l.tm, but smaller crystals (500 nm) are also evident. AlItSm3 particles are unevenly distributed along the ribbon length, being concentrated in strips 10 ltm wide. The microstructure is similar also on the side in contact with the wheel. So the nucleation of crystals in colonies is probably linked to heterogeneities of the wheel, causing local decrease in cooling rate across the whole ribbon. AFM analysis (Figure 2) extends the results achieved by SEM, showing smaller crystals on the side of the ribbon opposite to the wheel. They are between 100 mn and 200 nm in size. A compositional check of the two ribbon sides made by XPS revealed a loss of metallic Al with respect to the nominal composition, probably due to oxidation and preferential segregation of the samarium with respect to aluminum. To thisrespect we observe that theAlSm03 mixed oxide: has been found by XRD in the surface- oxide layer which always forms during arc melting the elements.

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Figure 2. AFM micrograph for the AlssSmlz ribbons, side opposite to the wheel. The surface morphology and the presence of nanocrystallitesare apparent.

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Figure 3. X-ray diffraction patterns of: (a) AlgoSrnlo ribbon wheel side; (b) %zSms ribbon obtained with 50 msJ wheel speed side opposite to the wheel; and (c) Al92Smsribbon obtained with 150 rnsl wheel speed side opposite to the wheel.

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sputtering time (min) Figure 4. XPS depth profile for the AlguSmtu alloy, side opposite to the wheel. For Al and Sm the atomic percentages refer to the respective metallic state.

Ribbons were obtained appearing amorphous to X-ray diffraction on both sides (Figure 3, pattern a). Investigations with different techniques were made on the ribbon side solidified in contact with the wheel, where only the roughness due to the polishing ridges was evident. In SEE/I, dendritically shaped crystals appear bright with respect to the amorphous matrix. TheyaresimilarinsizeshapeandcontrasttotheAlttSm~cryStalsdescribedforthe~ggSmt~a~oy. EDS analysis shows that they are enriched in Sm, but a precise composition cannot be determined due to the presence of the amorphous matrix between the arms of the dendrites. The crystals appear to concerttram on a strip along the ribbon, probably because of roughness on the wheel: the largest are 1 pm - 2 pm in size, but smaller ones are evident. AFM analysis shows also the occurrence of smaller crystals. It should be emphasized that the devitrification of the amorphous phase produces a metastable compound with a polymorphic transformation (7,8). The phase was later shown to be cubic with a = 1.9154 nm (9). The composition of the ribbon surface was determined by XPS (Figure 4). A constant decrease of the oxygen concentration was found as a function of sputtering time. The oxygen content became undetectable (i.e. < 1% at.) after sputtering for 40 min. A carbon contamination on the very surface was detected, decreasing rapidly with depth. In the XPS spectra the ratio of the content of metallic Al to that of Sm appears lower than expected from the nominal composition of the alloy as in the previous alloy.

Both partially crystalline and amorphous ribbons were produced by melt-spinning according to the wheel speed. Using a low wheel speed (50 ms“),XRD evidenced crystalline phases such

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Figure 5. SEM micrograph of an Al92Sms ribbon, side opposite to the wheel.

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Figure 6. AFM micrograph for the A192Smgribbons, side opposite to the wheel. The surface morphology and the presence of nanoqstallites are well apparent.

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as Al and AlttSm3 (Figure 3, pattern b) on both sides of a sample. Dark spots and light crystals dendritic in shape are assigned to Al and AltrSm3 phases respectively (Figure 5). These phases must have been nucleated in the liquid during undercooling. Samples prepared with 150 ms“ wheel speed display XRD patterns (Figure 3, pattern c) characterized by a large halo typical of the amorphous phase, and a peak attributed to a small amount of Al crystals grown on the surface of the ribbon opposite to the wheel. The latter are not manifest in SEM micrographs, but AFM analysis shows crystals of 500 nm in diameter on the side opposite to the wheel; again they appear aligned in rows along the ribbon (Figure 6). TEM analysis of samples thinned on the side in contact with the wheel in order to observe the opposite side (Figure 7) extends the AFM observations, showing agglomerates of crystals of 500 mn size. The crystals were identified as Al by electron diffraction. Continuous heating of amorphous ribbons produces two DSC peaks (7). The first exotherm has its onset at about 453 K when heating at 40 K/min and is related to the primary crystallization of a-Al nanocrystals. With TEM analysis on samples isothermally annealed at different temperatures for various times, it is found that the density of crystals is immediately high after the first minutes of annealing and then remains almost unchanged (the density of crystals is4.1@’ rns3and 9-1021 mm3alfter 300 s of annealing respectively at 423 K and 439 K, 1.2. l@* rnT3and 1.4 - lo** rnw3in the range from 700 s to 3600 s annealing at 444 K). The crystals are homogeneously distributed across the ribbon with no detectable effect of the local composition change on the surface (Figure 8). The size of nanocrystals increases rapidly during the first minutes of annealing

Fig,ure 7. TEM micrograph of an A192Smsribbon, side opposite to the wheel.

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Figure 8. TEM microgmph of an Alg2Sms ribbon annealed at 423 K for 3 min.

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Figure 9. The transformedfraction for primarycrystallization in &2Sms as a function of annealing time at 444 K.

CRYSTMSANDNANOCRYSTALSINRAANYSOUD~FIED

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and remains almost constant for longer times, reaching the value of about 10 nm for annealing at 444 K. The crystalline fraction of u-Al was determined at different annealing times by comparing the amount of heat released after the isothermal treatments to the amount of heat evolved from a totally amorphous sample. It displays a composite behavior as a function of time, suggesting an overlap of at least two different processes (Figure 9). DISCUSSION The surface analyses performed by different techniques show crystals that are not detected in XRD patterns becaue of their low volume fraction. These crystalline phases are probably solidified from the melt during the process of rapid quenching in an undercooling regime; in fact, their dendritic shape is typical of a primary solidification from the liquid. The AlssSmlz alloy producesAlllSm3 crystals of different size, indicative of nucleation events from the melt occurred at various temperatures. As quenched Altim ribbons contain a small fraction of crystals of AlllSm3 as well. Al&ms as quenched ribbons contain a small fraction of Al crystals, the same phase produced by devitrification of the amorphous phase. In this case the crystals size is different: a few nanometers for the latter and about 500 nm for the particles nucleated during the undercooling of the melt. Moreover, the growth of nanocrystals from the amorphous matrix becomes blocked after annealing for a short time in a rather wide temperature range (423 K - 444 K). These contrasting findings point to different mechanisms of nucleation and growth for the same phase in this alloy. Large crystals formed directly from the melt are expected if the undercooling is limited, i.e. if the nucleation rate is low and the growth rate is high. This is actually found here so we conclude that there is no hindrance to growth from the liquid phase. On the other hand, it is apparent that growth or ripening within the amorphous phase is very slow, well beyond what is expected as an effect on diffusivity of lowering the temperature. The present observations may support the suggestion that soon after nucleation, which is expectedly copious, concentration gradients established in the matrix hind’er the growth of Al nanocrystals (10). Note that after homogenization at higher temperatures the amorphous matrix will crystallize to a single metastable phase observed also in the alloys having 10% and 12% Sm. Compositional effects may bring as a consequence stress gradients wlhich also oppose diffusional growth (11). Of course, this does not occur in the melt where homogenization is fast. The two steps seen in the transformed faction could then be attributed to nucleation and growth of the a-Al phase and relief of compositional gradients and stress fields in the matrix. Nucleation from the melt is very often heterogeneous. We have evidence here of the e:ffect on crystallization on the surface imperfections of the quenching tool where also inclusion or impurities may concentrate It is not clear, however, whether the nucleation of nanocrystals is heterogeneous as well. Being socopious, they might be homogeneously nucleated. From a CALPHAD calculation (12), the chemical driving force for nucleation of primary Al crystals, i.e., the difference in chemical potential of Al between metastable and stable phases, was shown to be comparable to that of intermetallic compounds in the high undercooling regime and the glassy state. The classical theory of nucleation cannot be applied quantitatively since the interfacial free energy between liquid and each crystal phase is not known. However, we made the observation a posteriori that, instead of AlllSm3, in AlQSms Al crystals nucleate from the melt. and in AlmSmlu, a metastable phase nucleate from the glass. So interfacial effects must play a

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decisive role in favoring the homogeneous formation of a-Al at the lowest Sm content and a polymorphic reaction in Aldrnlo. On the other hand, the nanocrystallization of a-Al in Al-Ni-Y and Al-Ni-Nd alloys was interpreted as a process governed by transient heterogeneous nucleation (13). Since the Al nanocrystals were homogeneously distributed in the volume as in our alloys, they should originate from a corresponding distribution of nucleants. However, the nature and origin of such a high number of seed could not be specified. In our case we have shown that impurities (N, F) were present in all the examined alloys which can originate from the preparation process of the pure elements Al and Sm. The data collected in this work tend to rule out the possibility of nucleation assisted by these impurities. In fact, we have observed a variety of phases and microstructures in a limited composition range in the presence of the same contaminants in comparable concentrations. Also, there is no apparent effect of the oxide surface layer on phase se& tion. ACKNOWLEDGMENT Ms. Sandra Boesso, Dipartimento CIMA, University of Padova, is acknowledged for the technical support. Dr. Renzo Bertoncello, Dipartimento CIMA, University of Padova is acknowledged for the discussion of the results. Part of the TBM work was made during a stay (Paola Rizzi) at the Laboratoire de M&allurgie Physique, University of Poitiers. REFERENCES 1.

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