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SOLID STATE AMORPHIZATION OF MgNi MULTILAYERS
T. Ben Ameur, A. Yavari
To cite this version:
T. Ben Ameur, A. Yavari. SOLID STATE AMORPHIZATION OF MgNi MULTILAYERS. Journal
de Physique Colloques, 1990, 51 (C4), pp.C4-219-C4-226. �10.1051/jphyscol:1990427�. �jpa-00230787�
SOLID STATE AMORPHIZATION OF MgNi MULTILAYERS
T . BEN AMEUR and A . R . YAVARI
LTPCM-CNRS, U A 29, Institut National Polytechnique de Grenoble, BP. 75, Domaine Universitaire, F-38402 Saint Martin dfHeres, France
Resume
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Nous avons obtenu, pour la premikre fois, une croissance de phases amorphes au cours du colaminage des composites de Mg-Ni. En general, l'amorphisation de multicouches AB, par reaction en phase solide, intervient dans les systkmesA
forte enthalpie de melange qui prksentent une diffusion rapide d'un des ClBments composants. L'enthalpie de formation de MgNi est de l'ordre de 10 % de celle du composite Zr-Ni mais le nickel diffuse interstitiellement dans Mg. Nous avons prepare des composites MgXNil-, avec X = 0.5, 0.6, 0.7 et 0.8 par codkformation de mbans de Mg et Ni & la temperature ambiante, examinks par diffraction de rayons X, MEB et DSc. La formation de la phase amorphe a et6 dktectte durant le laminage de tous les composites, alors que le multicouches Mg0.6Ni0.4 devient presque complttement amorphe aprks de multiples passages au laminoir. Les cinktiques de croissance du volume amorphis6 semble suivre une loi en t.Abstract
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We report for the first time, the formation of amorphous MgNi alloys during cold-rolling of crystalline Mg-Ni multilayers. Amorphization of AB multilayers by solid state reaction usually occurs in systems with largely negative heats of mixing AHmix and with diffusivities DB >> DA in the amorphising A mamces. While the heat of formation of MgNi of the order of 10 % of that of ZrNi, Ni diffuses interstitiaIly in Mg. We prepared MgNi multilayer composites with nominal atomic compositions MgxNil., corresponding to X = 0.5,0.6,0.7 and 0.8 using codeformation of Mg and Ni foil sandwiches by cold-rolling at room temperature. The samples were examined by X-ray diffraction, SEM and DSc. Amorphous phase formation was detected during the cold rolling in each of the fom multilayers and amorphisation kinetics seem to follow a linear t-law. After several hundred rolling passes, Mg0.6Ni0.4 was found to be predominantly amorphous.Since the early work of Schwarz and Johnson /l/, amorphous alloys have been synthesized by solid-state reaction (SSR) in AuLa 121, ZrNi I3 I, Cu-Zr 141, NiNb, AI-Pt, FeTi /5,6,7/ multilayers and by mechanical alloying in NiTi 181, MnTi, CuTi, MnZr, FeZr 191, NbgSn, Nb3Ge /10/ systems. The most important criteria for spontaneous amorphisation by solid state reaction are a large negative heat of mixing, AHmix, for alloy formation and fast diffusion of one component in the other.
Sommer et al. /l l / obtained amorphous alloys by rapid quenching of MgNi liquid alloys with compositions in the range of 8 to 25 at % Ni. Although the heat of mixing, AHmix, for MgNi estimated by the method of Miedema 112,131 is only about 6k.Tlg.at, Ni is known to exhibit interstitial fast diffusion in the matrix of Mg /14/.Furthermore, heavy deformation such as in ball-milling is known to facilitate amorphous phase formation even in systems with only slightly negative heats of mixing of the order of that of MgNi, such as in NbgSn 1101.
We have therefore attemped and obtained amorphisation by cold-rolling in Mg-Ni multilayers.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990427
COLLOQUE DE PHYSIQUE
Amorphisation by cold rolling had previously been shown to occur in the NiZr I31 Cu-Zr and AI-Pt multilayers /6,7/.
The samples used in this study were prepared from about 20 pm thick elemental foils of magnesium and nickel with a total impurity content of 0,02 at %. ABAB-type sandwiches are repeatedly cold-rolled each time after folding the foil sandwiches as described elsewhere 1151. The samples are cold-rolled indirectly between two steel sheets.The average layer thickness is estimated from scanning electron microscopic observations (SEM) or by measuring the width at half-maximum of the Bragg peaks 1161, calorimetric measurements were performed on a Perkin-Elmer DSc-11.
The scanning electron micrographs of figure 1 show cross-sections of Mg0.52Ni0.48 multilayers after 100 and 200 rolling passes (R.P.), where the individual lamellae are from 90 nm to 200 nm thick. We do not show scanning electron micrographs of Mg0.52Ni0.48 multilayer after 300 R.P. because amorphisation reduces electron density contrast and the thin layers are not clearly perceptible.
Fig. 1 - Scanning electron micrographs of the cross-section of Mg0.52Ni0.48 after 100 (left) and 200 rolling passes (right )
.
The X-ray diffraction patterns of Figures 2, 3, 4 and 5 show the effect of number of R.P. and appearance of halos due to amorphous phase formation in multilayers with global compositions Mg0.52Ni0.48 , Mg0.6Ni0.4 , Mg0.7Ni0.3 and Mgo.gNi0.2 respectively. The intensities of the Bragg peaks of pure Mg and Ni decrease up on reaction and a broad maximum appears indicating the growth of an amorphous phase from 100 to 600 R.P.(Fig2).The evolution of the X-ray diffraction spectra during cold-rolling at near ambiant temperatures of Mg0.6Ni0.4 shows complete disappearance of pure Mg and simultaneous growth of the amorphous phase while a small quantity of pure Ni remain after extensive deformation (see fig. 3).
Fig. 2
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X-ray scattering intensity versus scattering angle 20 for Mg0.52Ni0.48 and number of rolling passes n.Fig. 3
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X-ray scattering intensity versus scattering angle 20 for Mg0.6Ni0.4 and number of rolling passes n.Here the amorphous phase composition is concluded to be close to Mg0.63Ni0.37 as compared to maximum Ni content of 20 at % achieved in fully glassy Mg0.8Ni0.2 obtained by rapid quenching /l l/. In the composite of global composition Mg0.7oNio.3 it can be seen (fig. 4) once again that the broad diffuse diffraction halo of the amorphous phase grows while the crystalline Mg and Ni peaks are considerably reduced in intensity.
Beyond 400 R.P., the X-ray patterns show the appearance of intermetallic crystalline peaks. Before this stage is attained, the amorphous phase global composition is likely to be near Mg0.74Ni0.26. A similar evolution with the number of passes occurs for Mg0.80Ni0.2 which begins to form intermetallic Mg2Ni before all of the Mg has been amorphized as witnessed by the persistance of its Bragg peaks as seen in figure 5. These preliminary results indicate a composition range 0.2 I X N ~ < 0.4 for amorphisation by solid state reaction as shown in figure 6.
In each multilayer the reaction ceases with some remaining pure Ni and an increase of the number of rolling passes does not lead to a more complete reaction and sometimes leads to the formation of Mg2Ni intermetallic compound and possibly some MgO indicated by the peak marked b y e i n fig 4.DSC scans at 40Klmin (see fig. 7) for Mg0.52Ni0.48 after 200 R.P show a broad peak in the range of 330 K to 373 K and a sharp peak near T = 427 K due to the crystallisation of the amorphous phase.
COLLOQUE DE PHYSIQUE
t ~ ~ l l l l l l l l l l l l l l ~
20 0
20 DEGREE
-
-28 DEGREE
Fig. 4 - X-ray scattering intensity versus Fig.5
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X-ray scattering intensity versus scattering angle 28 for Mg0.7Ni0.3 scattering angle 28 for Mgo.8Nio.2 and number of rolling passes n. and number of rolling passes n.4
1500 - .- z
1200 -
"
W D:
=
900-
= U a
z
W ,-
600 -
300-
o -
Mg l0 20 30 4 0 50 60 70 80 90 N i ATOM % Ni
Fig.6 - The Magnesium-Nickel equilibrium phase diagram (W.G. moffat, Binary Phase Diagram) ammrange of partially or fully glassy Mg-Ni
*range of ASSR
(Mq) + MgzN;
+
M q N i l I
...
S -
- X-
3 7
E -
-0 ..
E -
-" -
- -
- 8 r " " " ' " ' " " ' " ' ~ ~
3" TEMPERATURE (K) 496
Fig 7
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DSc thermogram obtained at 40 Wmin for Mg0.52Ni0.48 after 200 rolling passes.20
28 DEGREE
Fig.8
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X-ray scattering intensity versus scattering angle 28 for as rolled Mgo.gNi0.2 composite after n = 600 R.P.(top) and isothermally reacted at 80" C after n= 300 rolling passes (bottom).Attempts to induce further amorphisation of Mg0.8Ni0.2 after cold-rolling, by heat aeatement in the range of the broad exothermic peak of figure 7proved unsuccessful and at T 2 350 K resulted in the formation of the same MgzNi intermetallic that was found to appear after heavy deformation in Mgo.gNi0.2 (fig. 8). Before this saturation regime is attained, we have performed a deconvolution of the contributions of the sum
C I ~ H
of the (loo), (002), and (101) Mg Bragg peak intensities and that of the amorphous halo I(am) in the X-ray diffraction1 1
patterns of Mg0.52Ni0.48. Figures 9 and 10 show intensity ratios C I ~ U ( M ~ ) / I ( ~ ~ ) as a function of ;J;; and
;;
where n is the number of rolling passes. It can be shown that the volume ratio V M ~ / V ~ = a C i h ~ ( ~ g ) / ~ ( a m ) where a is an experimental constant. It is seen therefore, that the VM~/V, volume fraction, decreases linearly with the number roiling passes n, due to amorphisation.
COLLOQUE DE PHYSIQUE
Fig 9 - Plots of intensity ratios V ~ ~ / v a m = a&&t.4lI(am) as a function of 1 ( n is the number
of rolling passes) for Mgo.52Nio.48
Fig 10 -Plots of intensity ratios VM,/~, = ~ C I M I ( M ~ ) A ( ~ ~ ) as a function of
;;
1 ( n is the number of rolling passes) for Mg0.52Ni0.48While a glassy MgxNil-x phase with X r 0,8 is known to be accessible by rapid quenching (see figure 6), amorphisation by solid state reaction of pure crystalline layers is somewhat unexpected due to the small negative heat of mixing AH,ix. However, the amorphisation observed here occurs during cold-rolling and is thus strongly assisted by deformation. After several hundred rolling passes the thickness per layer 'e' is of the order of lOnm which represents a thickness reductions eo/e r 1000. When samples of intermediate thickness reductions are annealed at T < T, 430 K of the crystallisation of the amorphous phase, no progress of amorphisation is perceptible on the X-ray diffraction-patterns prior to the onset of crystallization. The amorphisation, therefore, is likely to be deformation assisted. The maximum elastic energy that can be stored through heavy deformation is only a few kJoules (and is likely to be less in Mg layers which are deformed at T 2 0.4 Tm) and only slightly modifies the free energy curves of the crystal versus amorphous phases. However, the only intermetallic nucleating phase, MgzNi, is a layered hexagonal packing of trigonal prisms and is likely to be disordered by heavy deformation 1171. On the other hand heavy deformation is likely to favor amorphous phase growth by relaxation of diffusion-induced interfacial stresses 1181 and thus enhance amorphous layer growth in the interfacial reaction barrier-controlled regime which prevails for small amorphous layer thicknesses 1191. This regime is also prolonged by the repeated creation of fresh surfaces as a result of heavy deformation.
and its rate, and m the strain rate sensitivity coefficient, maximum (adiabatic) self-heating was estimated to be of the order of 80 < AT < 160K. In the present case of formation of amorphous MgNi, since its crystallisation temperature is at about 430 K such self-heating is necessarily AT I 130 K. The cumulative annealing time for such self-heating is of the order of the number of passes, n, multiplied by deformation time per pass, At, which is of the order of 0,02s. This gives total annealing time t = nAt = 10 seconds for 500 rolling passes. Figure 10 shows a linear decrease of the volume fraction V ~ ~ / V ~ m w i t h the number of rolling passes n. Thus the amorphous volume fraction appears to increase linearly with time t. The interpretation of this preliminary result is not trivial because the number of interfaces per unit volume usually increases with the number of passes. It neverthless underlines a simple relation between the kinetics of amorphisation and both the total deformation and the deformation time.
CONCLUS ION
MgNi multilayers can be amorphized by cold-rolling even though the heat of mixing AHmix is only slightly negative. The amorphous compositions contain 20 to nearly 40 at % Ni, as compared to glassy MgsoNi20 obtained by rapid-quenching. It is demonstrated that sample temperatures remain below 130' C (430 K) during deformation. Preliminary results indicate the kinetics of amorphisation scale linearly with the number of rolling passes and consequently with cumulative effective deformation time t. It is possisble that heavy plastic deformation during thickness reduction eo/e 2 1000 is responsible for amorphous phase growth via the reestablishment of the more rapid interfacial reaction barrier-controlled growth regime but this point requires further analysis. It is also proposed that in the low temperature regime where thermal diffusion is insignificant, the heavy deformation destabilises and disorders any emerging embryos of intermetallic Mg2Ni with layered hexagonal structure in the manner demonstrated by ion beam irradiation I211 and mechanical deformation for CuTi 1221. However, at later stages, in Mg0.70Ni0,30 and Mgo.gNi0.2 composites, when nearly all of the existing Mg has been amorphised, as witnessed by the disappearance of its Bragg peaks, further codeformation of the remaining Ni with hard (high o ) amorphous MgNi layers is likely to result in increased local heating to T > Tc and explain the growth of the intermetallic Mg2Ni as observed in fig. 4 and 5. This phenomenon which needs further clarification was not observed in MgNi (fig. 2) and in Mgo.6Nio.4 (fig. 3) where we succeeded in introducing a higher Ni content in the amorphous phase by heavy deformation. We attribute this to a higher crystallisation temperature Tc of an amorphous phase rich in Ni, and of composition close to Mg0.6Ni0.4.
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