thermally activated amorphous phase formation in cold-rolled multilayers of al–ni, al–ta,...
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Thermally activated amorphous phase formation in cold-rolledmultilayers of Al±Ni, Al±Ta, Al±Fe and Zr±Cu
H. Sieber *, G. Wilde, J.H. Perepezko
University of Wisconsin-Madison, Department of Materials Science and Engineering, 53706 Madison, WI, USA
Abstract
Intermixed structures of di�erent binary systems (Al80Ni20, Al75Ta25, Al75Fe25 and Zr67Cu33) were prepared by re-
peated cold rolling with intermediate folding of elemental foils. The microstructural development during the rolling
procedure and after annealing were measured by X-ray di�raction, scanning and transmission electron microscopy.
While the rolling procedure yielded mostly nanometer sized crystalline (nanocrystalline) morphologies, amorphous
regions were formed in all investigated samples after thermal exposure of the samples under argon atmosphere at
temperatures between 560°C and 1400°C. The amorphous phases are formed in localized sample regions of up to 20 lm
in size which had local oxygen levels between 10 and 40 at.%. While amorphous areas of an Al80Ni20 sample crystallized
on heating in the beam of a transmission electron microscope with rejection of oxygen, thicker amorphous regions were
una�ected by this thermal treatment. The thermal stability of the amorphous regions together with the observation of
the in situ crystallization of thinner sample areas indicate that the oxygen content acts to stabilize the amorphous phase
and prevents the growth of intermetallic crystals. Ó 1999 Published by Elsevier Science B.V. All rights reserved.
1. Introduction
The formation of amorphous phases by a solidstate amorphization reaction (SSAR) has recentlyattracted increased attention due to the potentialof producing these advanced materials by a rathersimple processing strategy [1]. The two mainmechanisms for the SSAR are the mechanicallyactivated and the thermally activated modes. Theformation of amorphous phases by mechanicalalloying was reported for di�erent binary and
multicomponent systems mainly by ball milling ofpowder samples, and also in multilayer samplesprepared by cold rolling (see Refs. [2,3] and in-cluded references). Compared to ball milling, thecold rolling procedure provides a smaller strainrate mechanical deformation of the elementalmultilayer samples at ambient temperatures. Phaseformation and microstructural morphologies caneasily be monitored by microstructural methods(e.g. scanning and transmission electron micros-copy) even at the early stages of the reaction [4].
Al80Ni20, Al75Ta25, Al75Fe25 and Zr67Cu33
multilayers were cold rolled for di�erent defor-mation levels from elemental foils. None of theinvestigated binary systems showed the formationof an amorphous phase after low levels of me-chanical deformation. Yet, after annealing, mic-rostructural evidence for a thermally activated
Journal of Non-Crystalline Solids 250±252 (1999) 611±615
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* Corresponding author. Present address: University of
Erlangen-Nuernberg, Materials Science (III) Glass and Ceram-
ics, Martensstrasse 5, 91058 Erlangen, Germany. Tel.: +49-9131
85 7553; fax: +49-9131 85 8311; e-mail: [email protected]
erlangen.de
0022-3093/99/$ ± see front matter Ó 1999 Published by Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 1 4 2 - 8
SSAR in localized regions of the multilayers wasfound in all of the samples. The amorphousphases that formed during the thermal treatmentwere all oxygen-rich, while the crystalline fractionof the samples contained much smaller amountsof oxygen. The results are examined with regardto the importance of the impurity concentrationon the reaction pathway especially in mechani-cally driven systems.
2. Experimental
Elemental foils of Al, Zr, Ta, Cu, Ni and Fe(purity > 99.9%) each with a thickness of about 25lm were stacked to form an array of the selectedbinary compositions and then folded approxi-mately 4 times to yield a 10 ´ 10 mm2 multilayersandwich. The folded samples were then manuallyrolled in air to a thickness of approximately 80 lmat a strain rate of about 0.1 sÿ1. After di�erentnumbers of folding and rolling (F&R) passes smallpieces of the samples were removed for furtheranalysis. The microstructural changes during roll-ing were investigated by X-ray di�raction (XRD),scanning electron microscopy (SEM) and trans-mission electron microscopy/selected area electrondi�raction (TEM/SAED). For TEM investiga-tions, 3 mm in diameter plan view samples werepunched mechanically and then ion-beam thinned(Ar�, 5 kV, 1 mA) in a liquid-N2-cooling stage.The TEM investigations were performed with aPhilips CM200, which was equipped with an en-ergy dispersive X-ray system (EDS) for elementalanalysis. The annealing treatment was performedby di�erential scanning calorimetry (DSC) com-puter controlled system (Perkin Elmer DSC7).
3. Results
The thermally activated formation of anamorphous phase after annealing will be describedin detail for the Al75Ta25 system. The microstruc-tural properties of the amorphous phases in theother systems are similar. A summary of thecompositions and the annealing temperatures usedin the experiments is given in Table 1.
3.1. Al75Ta25 system
During the cold rolling process, a multilayeredstructure is developed in the cold-rolled Al75Ta25
samples [5]. After 40 F&R passes, SEM investiga-tions indicate that large Ta particles remain em-bedded in an aluminum matrix. After 80 F&Rpasses, the Ta particle size is less than 1 lm and amultilayered structure of thin Ta and Al elementallayers with a thickness of less than 100 nm is formed.XRD measurements indicated that the averagegrain size after 80 F&R passes is >50 nm and alsoshowed that no new phases have been formed dur-ing the ambient temperature rolling procedure.
TEM investigations show that after 80 F&Rpasses, the sample consists only of a polycrystal-line structure with an average grain size in therange of 50 nm. An annealing treatment at atemperature of 560°C formed a nanocrystallineAl3Ta phase, but no amorphous phase could bedetected. On further heat treatment, an amor-phous phase was formed at temperatures of about800°C (see Fig. 1). While the major fraction ofthese samples crystallized as Al3Ta with a graindiameter of about 500 nm on average, a minorvolume of the sample formed amorphous regions.
TEM/EDX measurements revealed a di�erencebetween the compositions of the crystalline and theamorphous regions. In the crystalline state, the ra-tio of Al and Ta was found to be 3:1 in agreementwith the overall sample composition of Al75Ta25
but, the Al±Ta ratio is �2:1, i.e. Al67Ta33 in theamorphous regions. Additionally, the amorphousregions contain oxygen amounts between 20 to 40at.% and also some argon, molybdenum and copperas a result of the ion milling procedure (see Fig. 2).High resolution (HR) TEM investigations of the
Table 1
Composition of the amorphous phase determined by TEM/
EDX measurements
System Composition a Annealing temperature
Al±25Ta Al±33Ta, 20±30 at.% O 800±1400°C
Al±20Ni Al±25Ni, 20±40 at.% O 560°C
Al±25Fe Al±33Fe, 10±30 at.% O 560°C
Zr±33Cu Zr±30Cu, 10±20 at.% O 560°C
a All amorphous phases showed minor compositions of Ar, Mo
and Cu.
612 H. Sieber et al. / Journal of Non-Crystalline Solids 250±252 (1999) 611±615
amorphous regions (named as Al±Ta±(O) phase)indicate nanocrystalline grains inside the amor-phous matrix (see Fig. 3). These grains, which aremost likely crystals of an intermetallic Al±Ta-phaseare less than 5 nm in diameter and seem to becompletely embedded in the amorphous matrix.Larger particles were not detected inside theamorphous matrix.
3.2. Al80Ni20, Al75Fe25 and Zr67Cu33 systems
TEM investigations have shown that after an-nealing, the samples consist mostly of crystallineintermetallic phases or residual crystalline ele-ments (all without a detectable level of contami-nants) and also some amorphous phase regions.The microstructure of the amorphous phases,which formed in these systems, is similar to thatobserved for the Al±Ta±(O) phase. However, thethermally induced amorphization reaction devel-oped in these systems already at 560°C, i.e. atconsiderably lower temperatures compared to theAl±Ta-system. The amorphous phases are alwaysformed in separated regions that amount to only afew percent of the entire sample and largeramounts of oxygen besides some argon, molyb-denum and copper. Similar to the observations onthe amorphous Al±Ta±(O) phase, nanocrystalswith an average diameter of <5 nm have beenidenti®ed within the amorphous sample regions.
4. Discussion
TEM investigations con®rmed that a thermallyinduced amorphous state occurred in all the foursystems in localized sample regions while the majorfraction of the sample material formed crystalline
Fig. 3. HRTEM image of the amorphous phase in Fig. 1. Inside
the amorphous matrix, nanocrystalline grains below 5 nm in
size are visible.
Fig. 2. TEM/EDX spectra of the intermetallic Al3Ta phase and
the amorphous region. The EDX analysis shows a composition
ratio of about Al75Ta25 for the Al3Ta phase and of about
Al67Ta33 for the amorphous phase which also contains oxygen
of about 20 at.% and small amounts of argon and molybdenum.
Fig. 1. TEM overview of an amorphous region of an Al75Ta25
multilayer sample after 80 F&R passes and heat treatment up to
800°C (20 K/min). As inset in the lower left corner is shown the
electron di�raction pattern.
H. Sieber et al. / Journal of Non-Crystalline Solids 250±252 (1999) 611±615 613
phases of intermetallic structure. While the stoi-chiometry of the crystalline phases corresponds tothe equilibrium composition of the intermetallicphases [6], the amorphous phases in the Al-systemsshowed a smaller amount of Al (see Table 1) andcontained always larger amounts of oxygen besidessome contamination due to ion milling. The exis-tence of measurable traces of these elements isconsistent with the well-known fact that amor-phous phases have a greater solubility for solutes ingeneral than crystalline compounds [7]. In contrastto the smaller amounts of Ar, Mo and Cu, thelarger amounts of oxygen may result from an oxidecontamination in localized sample regions. Thus,as indicated by the nomenclature used here, theamorphous Al (Zr)±TM±(O) phases (TM: transi-tion metal) should be regarded as a ternary non-crystalline phase. The free energy of this phase isstill larger than the free energy of the binary equi-librium intermetallic phase and free oxygen, but thethermodynamically favored transition is kineticallyhindered. Evidence for this interpretation is givenby the observed in situ crystallization of thinneramorphous Al80Ni20 sample areas on heating withthe TEM electron beam (the sample was heated toapproximately 300°C) [8]. In this experiment,crystalline grains of a B2 structure up to 100 nm insize grew rejecting oxygen, but with the Al:Ni
composition ratio of the amorphous matrix (seeFig. 4). The crystallization front stopped (Fig. 5) inthicker amorphous areas, where the fraction ofoxygen that can be transported by the electronbeam enhanced surface di�usion is smaller than inthe thinner areas. This change also explains thepresence of the nanocrystals inside the amorphousphases: the equilibrium phase of low solubility foroxygen nucleated and started to grow. The com-plete transformation appears to be inhibited due tothe con®ned high oxygen content between the in-termetallic phase regions, which have also a verylimited solubility for oxygen.
5. Conclusion
Thermally induced SSAR have been observed indi�erent binary systems (Al80Ni20, Al75Ta25,Al75Fe25 and Zr67Cu33) after low mechanical de-formation and heat treatment that was promotedby high oxygen levels in separated sample regions.The amorphous phase formation mechanism canbe described as a thermally induced amorphizationby supersaturation with oxygen. The presence ofmetastable phases may alter the slope of the com-mon tangent to the free energy curves and thus leadto di�erent phase equilibria (i.e. a di�erent mixtureof phases with the lowest free energy) leading to anincrease of the solubility for gases (here: oxygen)in the sample. These e�ects, in turn, correspond toa stabilization of the metastable phases by thepresence of oxygen. A transformation of the
Fig. 4. TEM bright ®eld image of a region in an Al80Ni20
sample that was in situ crystallized in the TEM. The insets on
the right side show the low index electron di�raction pattern of
the B2 phase.
Fig. 5. TEM bright ®eld image of an in situ TEM crystallized
region in an Al80Ni20 sample.
614 H. Sieber et al. / Journal of Non-Crystalline Solids 250±252 (1999) 611±615
amorphous oxygen-rich phases to the equilibriumcrystalline structures is kinetically hindered by thelow solubilities of oxygen in the intermetallicphases and by a smaller ¯ux of oxygen to the sur-face in the bulk samples compared to electrontransparent foils which lead to a high temperaturestability for the observed amorphous phases.
Acknowledgements
The ®nancial support from the Alexander vonHumboldt Foundation via the Feodor-LynenProgram (G.W., V-3-FLF-1052606), ONR(N00014-92-J-1554) and ARO (DAAG 55-97-1-0261) is gratefully acknowledged.
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