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AMORPHIZATION IN COLD-ROLLED MULTILAYER COMPOSITES AND THE
ORIGINALITIES OF THE PROCESS
F. Bordeaux, A. Yavari
To cite this version:
F. Bordeaux, A. Yavari. AMORPHIZATION IN COLD-ROLLED MULTILAYER COMPOSITES
AND THE ORIGINALITIES OF THE PROCESS. Journal de Physique Colloques, 1990, 51 (C4),
pp.C4-249-C4-257. �10.1051/jphyscol:1990430�. �jpa-00230790�
AMORPHIZATION IN COLD-ROLLED MULTILAYER COMPOSITES AND THE ORIGINALITIES OF THE PROCESS
F. BORDEAUX and A.R. YAVARI*
INRS, 1650 Mont6e Sainte Julie, CP 1020, Varennes, J3XlS2 Quebec, Fanada
LTPCM-CNRS U A 29, Institut National Polytechnique de Grenoble, BP.75, F-38402 Saint Martin d l H e r e s , France
RCsumC - Des phases amorphes de AI-Pt et Ni-Zr ont ktk obtenues par rkaction en phase solide dans des composites de multicouches fines metal-mCtal fabriquks par laminage B froid. Les matkriaux obtenus ont kte caractCrisCs par diffraction X, microscopie klectronique B balayage et en transmission, mesures d'aimantation, calorimCtrie differentielle B balayage et thermogravimktrie. Nous avons montrk pour les systkmes prkcites, l'existence de la rCaction d'amorphisation dkjB pendant la dkfomation mkcanique B des tempkrature proches de l'ambiante. La rkaction d'amorphisation revkle de plus une forte dkpendance avec l'atmosphkre de recuit et les inCgalitCs en kpaisseur des couches dans les materiaux bruts de fabrication. Le r6le des inhomogkn6itks en kpaisseur et de l'oxygkne dissous sur les mesures cinktiques est envisagk pour valider nos rksultats expkrimentaux qui montrent des Cnergies d'activation nettement plus faibles dans les composites fabriquks par deformation mkcanique par rapport B ceux fabriquks par d'autres techniques.
Abstract
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Amorphous phases of AI-Pt and Ni-Zr have been synthesized by solid state reaction (SSR) of composite metal multilayers produced by cold rolling. The resulting material was investigated by means of X-ray diffraction, electron microscopy (SEM and TEM), magnetization measurements, differential scanning calorimetry and thermogravimetry. The amorphization reaction was observed to occur already during the mechanical deformation near ambiant temperature and to depend strongly on the nature of the annealing atmosphere and on the homogeneity of the as-prepared materials.The presence of dissolved gases and thickness in homogeneities are suggested to contribute to the observed accelerated amorphisation kinetics as compared to similar composites prepared by other techniques.
1- INTRODUCTION
Amorphous alloys can be formed from pure components in the solid state by spontaneous atomic interdiffusion in metal-metal multilayer composites /l/. Amorphous alloys prepared by SSR such as Au-Y, Au-La /2/ or Zr-Ni 131 all exhibit a fast interstitial diffusion of the smaller transition or noble metal atom in the other with much larger atomic volume /4/ and large negative heat of mixing.
Several systems selected on the basis of these two criteria and prepared by cold rolling were partially or fully amorphized by a fast diffusion reaction in the solid state : Ni-Nb, Ni-Zr /S/, Cu-Zr, Cu-Ti, Fe-Ti 161. We have also reported complete amorphization of the AI-Pt composites fabricated by the same technique 171. The amorphization in this,couple was first obtained by Legresy et all81 in an A1 Pt bilayer deposited by electron beam evaporation. Amorphization by SSR in AI-Pt is consistent with its very large negative heat of mixing, however, it has not been previously demonstrated that any interstitial mechanism intervenes in atomic diffusion in this couple where atomic radii are nearly equal and so inhibit interstitial site occupancy of one in the fcc lattice of the other.
The aim of this paper is to review the phenomenon of amorphization by cold rolling. The role of dissolved gases, thickness inhomogeneities and defects are discussed in order to explain the observed amorphization during cold rolling, the low activation energies and the dependence of the reaction kinetics on the annealing atmosphere.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990430
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2
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EXPEFUMENTAL METHODSThe nature of the microstructure obtained by codeformation of multilayers by cold rolling depends on the nature of the constituent elemental layers. Extensive deformation down to submicron thicknesses resulting in multilayer type microstructure during cold rolling is predicted for all the cited systems /9/.
Ribbons of pure elements such as A1 and Pt were obtained by cold rolling bulk ingots or purchased (Ni, Zr.. .) in high purity with layer thicknesses ranging from 10 to 50 micrometers. Elemental ribbons were then cold rolled separatly to reach the chosen thickness required to obtain the appropriate volume fraction (or composition) in the composite. Each pair of chosen foils is then wound in a spiral form and the resulting sample cold rolled under air, between two steel sheets of 0.5 mm thickness.
The average layer thickness is estimated from SEM electron micrographs obtained on the cross sections of the composites or directly from X-ray diffraction spectra by measuring the widths at half maxima of the Bragg peaks as a function of their angles (see ref 171 for more details).
3
-
RESULTS AND DISCUSSION3-a Inhomoeeneities in laver thicknesses- Role on the kinetic measurements
Measurements of layer thicknesses and their evolution during laminating were presented in a previous paper 191.
The secondary SEM micrographs (Fig.1) for A1-Pt and Ni-Zr systems after 50 passes in the rolling mill show an average layer thickness of about 60 nm as confirmed by measurements of full widths at half maximum of the Bragg peaks of pure Pt, Ni and Zr 161.
Fig.1
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SEM micrographs on the cross sections of A166Pt33 (left) and Ni64Zr36 (right) after 50 passes in the rolling mill.There is unfortunately no way of determining exactly the distribution in layer thicknesses because of the amorphization reaction taking place during cold-rolling as will be seen later. Nevertheless, measurements of layer thicknesses at the early stage of the deformation show, for the number of layers investigated (typically loo), a satisfactory representation by a Gaussian type distribution. Such distributions are shown in Fig.2 for different variations of layer thicknesses around the average value of 500
A.
From the values represented above, calculation of the remaining volume fraction, v(tj), of pure elements at a given time tj become possible using, for exemple, a simple diffusion law such as e = n twith e the layer thickness, D the diffusion coefficient and t the reaction time.
with tj the time necessary for complete reaction of a layer of initial thickness ej n the total number of layers
ni the number of layers with thickness ei
The first term between parenthesis indicates the complete reaction of layers with thicknesses below ej and the second one, the partial reaction of the others. The results of such calculations are shown in Fig.3.
The deviation from the normal diffusion law (linear in this representation) becomes smnger as the inhomogeneity of the layer thickness increases. In addition to this "thickness effect", we will see later that the accumulation of dissolved oxygen in the non reacted layers can act as a diffusion banier when the amorphization progresses.
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m (cm)
Fig.3 - ESfect of the Gaussian distribution of the layer thicknesses on kinetic curves of the remaining pure element versus the reaction time for the d8usion law e = n t
From the present discussion, it can be seen that ulterior kinetic measurements should rather be done for short reaction times to avoid effects due to thickness inhomogeneities in cold-rolled samples.
3 b - Amomhization durine cold rolling
Fig.4 - Typical cross-sectional bright field image of m-prepared Ni-Zr multilayer prepared by cold rolling. Grey areas are predominantly amorphous as indicated by their e-d.iflaction pattern (inset) showing amorphous halos.
(white bar represents 50 nm)
The selected area diffraction pattern (inset, Figure 4) taken from the featureless grey areas indicates clearly that these zones are already amorphous. The presence of these large amorphous areas is consistent with X-ray diffraction patterns and saturation magnetisation measurements at 4.2 K at which temperature the amorphous phase is paramagnetic 1101. Only 32 at% pure nickel is detected by this last technique in the as-prepared composite with nominal composition Ni64Zr36. So the amorphization during cold rolling occurs at temperatures well below those required for usual solid state amorphization for deposited multilayers (near 560 K /l 14 even though local heating to 450 K can occur during rolling for cumulative times of the order of a few seconds (see ref I101 for more details). The accelerated diffusion kinetics are atmbuted in part to the continuous creation of fresh surfaces between crystalline layers during cold rolling and to the dislocation pipe diffusion mechanism in presence of some 1013Icm2 of dislocation lines I121 generated by severe deformation (thickness reduction of 99.75% in the case of Ni-Zr composite).
In contrast to previous results /13,14,15/, no Kirkendall holes were observed next to the amorphous layers. The absence of holes which are likely to be continuously crushed during cold rolling is also expected to help maintain a high Ni flux into the amorphous phase.
We have also observed arnorphization during cold rolling in Ni-Nb 161, Mg-Ni 1161 and AI-Pt 171. This last system shows great interest because in contrast to the other mentioned couples, no interstitial diffusion of one constituent element in the matrix of the other is expected here. Figure 5 shows the complete arnorphization in this system after 130 passes in the rolling mill.
& I '
' 4 5 '
'$ 5 '
' 6 5 7k2 0 (DEGREES)
Fig.5 - X-ray diffraction spectra (Cu-Ka) of for different stages of deformation (from 50 to 130passes in the rolling mill). The 3 clearly visible cvstalline peaks in the 50 times laminated composite are
gram
left to right) Pt[l l l ] , P1[200] and Pt[220] peaks.COLLOQUE DE PHYSIQUE
It is also interesting to note that in the related preparation technique of ball milling (mechanical alloying), depending on the milling intensity, temperatures are variously estimated to be from 100 to 300 'C (373 to 673 K) during contact or collision time /17,18,19/. So it would appear that temperatures attained during cold rolling correspond to those of "slow" ball-milling.
3 c
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Amomhization durine annealing-
Kinetic measurementsAmorphization can also be observed during annealing
,
following the first partial amorphization during cold rolling /5,7,20/.The quantities of remaining pure elements were estimated from the integrated intensities of the Bragg peaks on the X-ray diffraction spectra or directly by magnetisation measurements in some cases (Ni in NiZr or NiNb). The results were plotted as a function of the square root of the reaction time as can be seen in Fig.6 for the AI-Pt system.
Fig.6
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Quantity of remaining pure Pt (in arbitrary units) as a function of the square root of the reaction time at different temperatures. 100 is the quantizy of remaining Pt after the partial amorphization which already rook place during rolling.It can be noted that there is no evidence of dependance in a t1I2 law for the reaction time used here as was expected due to thickness inhomogeneities in partially amorphised multilayers prepared by cold rolling (see figure 3). In addition, A1-Pt system shows the appearance of an intermetallic compound after a few minutes annealing at temperatures higher than 400 K. The appearance of the new phase (A12Pt) is likely to change the reaction kinetics and measurements are made only at the beginning of the reaction during in-situ heating in the diffractorneter.
We used the "Cross-Cut" method 1211 (model using constant energy and single activation process) for kinetic interpretations of isothermal annealings. A related method was used for interpretations of isochronal DSc scans (constant rate of heating) /7/.
Here we present the results obtained by a method fust described by Highmore et al./22/ who assume that for at least part of the reaction the amorphous layers grow by a diffusion controlled process 1231, with constant concentration at the interfaces between the amorphous and elemental layers and a linear concentration profile accross the amomhous laver. Thev then calculate the rate of change of thickness of an amomhous laver with
where A(T) is the area of the amorphization peak on a D S c trace integrated from the onset of reaction to temperature T, dA(T)/dt is the heat flow in the D S c at temperature T and E is the activation energy. C is a constant depending on :
-
the atomic fraction range over which the amorphous phase exists-
the average interdiffusion coefficient-
the total interfacial area between the pure components for the starting material (non reacted)-
the initial proportions of pure elements-
the enthalpy release per unit volume of amorphous phase formed.They obtained a straight line for a large range of temperatures in plots of Ln(A.dA/dt) against 1/T for the Ni-Zr system and were able to deduce a pre-exponential and an activation energy for interdiffusion in the amorphous alloy from a single DSc trace.
Similar calculations were made for our D S c curves and the results obtained are presented in Fig.7 for 3 different rates of heating.
W 1 0 K l m i n
* 4 0 K l m i n 8 0 K l m i n
1 . S 2 2 . 5
103IT
(K-')
Fig.7
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Plots of In(A.dAldt) versus the reciprocal of absolute temperature deduced from D S c traces at 3 different rates of heating. The units of A are ml and the units of dAldT are mW.For the above mentioned reasons (thickness inhomogeneities, pre-amorphization during cold rolling) we could not expect good correlations with the results obtained in deposited Ni-Zr with regular modulation wavelengths.
However, activation energies can be deduced from the approximately linear region of the plots and range from 0.5 to 0.66 and 0.72 eV respectively for heating rates of 10,40 and 80 Wmin. Measured activation energies are far
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lower than litterature values of the order of 1.1 eV 1241 and could confirm the role of the high concentration of defects induced by the mechanical deformation and that of dissolved gases 1261.
The dependence of the activation energy on the heating rate was not as expected next by Highmore et al. This phenomenon was not observed in the AI-Pt system 161, which has little solubility for oxygen.
3 d - Role of dissolved eases
Oxygen is known to be present in high concentrations at equilibrium in some pure elements such as titanium or zirconium (more than 20 at% Oxygen in Zirconium for example 1251 at 25°C and normal pressure) and so is present in some of our multilayers prepared under air. Yavari et a1 1261 have arqued that this dissolved oxygen facilitates amorphisation of the Zr layers.
Even though oxygen dissolves in the growing amorphous phase 1261, some of it is likely to be redistributed in the remaining Zr films after the onset of amorphisation. This effect called the "snowplow effect" by Kang et a1 1271 can explain why the amorphous phase growth is almost stopped at longer annealing times. The accumulation of oxygen in the remaining pure element (Zr) could explain the vanishing of Ni diffusion and the subsequent appearance of Bragg peaks due to the formation of ZrOz 120,261.
The partial elimination of dissolved gases is illustrated in the thennogrammetric weight profile of figure 8 for a NiZr composite prepared by cold-rolling in air. Although the precision of these weight change measurements is rather low, this experiment performed under a vacuum of 10-3 tort shows a large weight loss due to reduction of dissolved gases followed by a weight increase due to oxydation at higher temperatures.
0 2 0 0 4 0 0
TEMPERATURE (C)
Fig.8 - Weight loss versus the temperature for a Ni6&rj6 multilayer composite after 50 rolling passes. The heating rate is I0 Klmin.
4 - CONCLUSION
Cold rolling has many effects on the solid state amorphization reaction in multilayer composites. High concentration of defects due to the deformation and continuous creation of fresh surfaces lead to higher diffusion rates that permit amorphization already during cold rolling near room temperature. Inhomogeneities in crystalline
continuous creation of fresh surfaces and defects induced by deformation are also present.
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