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AMORPHIZATION OF NiZr INTERFACES UNDERCONTROLLED CONDITIONS OF DEFORMATION

AND TEMPERATURES. Martelli, G. Mazzone, A. Montone, M. Vittori Antisari

To cite this version:S. Martelli, G. Mazzone, A. Montone, M. Vittori Antisari. AMORPHIZATION OF NiZr INTER-FACES UNDER CONTROLLED CONDITIONS OF DEFORMATION AND TEMPERATURE.Journal de Physique Colloques, 1990, 51 (C4), pp.C4-241-C4-247. �10.1051/jphyscol:1990429�. �jpa-00230789�

COLLOQUE DE PHYSIQUE Colloque C4, supplement au n014, Tome 51, 15 juillet 1990

AMORPHIZATION OF NiZr INTERFACES UNDER CONTROLLED CONDITIONS OF DEFORMATION AND TEMPERATURE

S. MARTELLI, G. MAZZONE, A. MONTONE and M. VITTORI ANTISARI

Divisione Scienza dei Materiali, ENEA-TIB C.R.E. Casaccia, I-2400 ROma, Italy

Resume-Nous proposons une nouvelle methode de deformation mecanique d'une couple massif de diffusion qui permet de separer le role de la deformation plastique sur la cinetique de reaction de l'effet thermique simultan8.Des echantillons tricouches prepares en placant une feuille de Ni entre deux feuilles de Zr de 25 mm2 sont soumis A une charge normale g l'interface allant jusqu'a 220 kN appliquee en 0.1 s environ. Le temperature effective de l'interface pendant le processus tourne autour de 100 C: elle est mesuree sur l'echantillon lui-meme en utilisant l'interface NiZr comrne le point chaud d'une thermocouple NiZr verifie par ailleurs sur des echantillons quelconques en utilisant une thermocouple trks mince NiCr-NiAl. La section transverse des echantillons est preparee pour la microscopie dlectronique h transmission par decapage ionique sur un porte-objet refroidi l'azote 1iquide.Les premiers resultats montrent la presence d'une couche amorphe a l'interface dont l'epaisseur depend de l'importance de la deformation plastique. La comparaison avec des vitesses de croissance de la phase amorphe observees apres des traitements isothermes montre que la deformation mecanique augmente fortement le processus d'interdiffusion.

Abstract-We propose a novel method of mechanically deforming a bulk diffusion couple which allows to separate the effects of plastic deformation on the reaction kinetics from the concurrent thermal effects. Trilayer specimens prepared placing a Ni foil between two Zr sheets and having an area of about 25 mm2 were subjected to a load normal to the interfaces of up to 220 KN applied in about 0.1 S. The effective interface temperature during this process, which was found to be of the order of 100 C, was measured on the sample itself using the NiZr interface as the hot junction of a NiZr thermocouple and checked on a dummy sample using a very thin NiCr-NiAl thermocouple. Cross sectional specimens for TEM were prepared by Ar ion beam milling on a liquid N2 cooled stage.First results show the presence of an amorphous layer at the Ni-Zr interface whose thickness depends on the amount of plastic deformation. Comparison with the growth rates of amorphous NiZr derived from isothermal annealing shows that the mechanical deformation strongly enhances the interdiffusion process.

The formation of amorphous alloys by solid state reactions induced by heat treatment or by plastic deformation has received a great deal of attention in recent years after the first experimental reports of this kind of phenomena (112) Glassy phases generally form by solid state diffusion at a relatively low temperature in a suitable-elemental couple whose thermodynamic properties like for instance the heat of mixing have been widely investigated. Experimental studies have often been carried out on artificial multilayers with a periodicity of a few tens of nm obtained by vacuum evaporation.Therma1 processing under controlled conditions and subsequent characterization has allowed the measurement of several properties of the interdiffusion process and of the amorphous phase formation,both thermodynamic and kinetic.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990429

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As far as the plastic deformation process is concernedlit has been shown that elemental multilayers with spacing in the same range can be obtained by ball milling (3) or cold rolling (4,5).This kind of mechanically prepared multilayers can be transformed to a homogeneous glassy phase by heat treatment ( 4 , 5 ) or by further mechanical processing (2,6).In the last case however,the nature of the process makes it difficult to control accurately the experimental conditions.In particular, both the peak and the average temperature of the interfacerwhile the powder particles are subjected to the ball collision,can be only approximately estimated (7) so that it is practically impossible to separate the thermal and the deformation effects ( if any ) on the kinetics of the glassy phase.Actually it is well known that a heavy plastic deformation can generate a large amount of point defects (8),which,at low temperature can modify the diffusion behavior and consequently the reaction kinetics.Furthermore,during mechanical alloying, the amount of plastic deformation and the strain rate can influence the sample temperature through the milling strength (7) making it difficult to separate the effect of experimental conditions on the amorphization process. The purpose of this paper is to describe a new experimental approach to the problem of studying the influence of plastic deformation on the kinetics of solid state reactions between bulk elemental foils.The method allows an accurate control of both the mechanical parameters and the interface temperature during plastic deformation. Moreover the specimen after mechanical processing is suitable to be prepared in cross section for detailed TEM characterization of the reacted interface.

2-EXPERIMENTAL METHODS

The f i r s t step of the experimental procedure consisted in the preparation of a macroscopic trilayer by insertion of an elemental foil between two sheets of the second element of the couple under study,all foils having a thickness in the mm range.This trilayer was subsequently subjected to a load normal to the interfaces in a fatigue machine operated in the single cycle compressive mode. The applied load,as high as 250 kN,could be applied following several time functions in times ranging from about 50 ms,at the maximum load,to several hundreds of seconds.The actual stress experienced by the sample depended upon its surface area during plastic deformation. A crucial problem in this kind of experiments is the measurement of the interface temperature while the sample is being plastically deformed.This temperature is determined by the balance between the amount of heat generated by the work done to deform the sample,which in a hypotetically adiabatic experiment can rise the temperature up to roughly one thousand Cland the heat conduction from the specimen surface in contact with the machine pistons.The heat conduction, in turn, depends on the thermal properties of sample and piston,as well as on the specimen thickness (9). The sample size appears to play an important role in this kind of experiments since it affects both the stress experienced by the sample and the interface temperature.Another requirement concerns the preparation of cross-sectional TEM specimens for which the final sample thickness can be not less than about 1 mm. All the results reported in this paper have been obtained on samples obtained placing a 0.5 mm thick Ni foil between two 1.6 mm thick Zr sheets;the as prepared samples had a square shape with a 5 mm side.This size allowed to meet all the requirements listed so far,in fact the load capability of our machine was large enough to deform considerably the trilayer giving good bonding between Zr and Ni;the interface temperature,as it will be shown later,could be kept at a reasonably low 1evel;TEM specimens could be prepared from the deformed composite. Ni and Zr have been selected as a diffusion couple owing to the ability of these materials to form an amorphous phase by interdiffusion and to the large body of work done on this system. The true temperature of the interface during the plastic deformation process was measured taking advantage of the large thermoelectric power shown by this elemental couple.Two of the three foils have therefore been used as a thermocouple during the deformation process itself, the hot junction being the reacting interface.To provide a reliable cold contact for the temperature difference measurements,the sheets used for this set of experiments were

machined to a particular shape such that a square of the usual size was connected to a three cm long slab-since the maximum temperature increase was expected for the fastest deformations,for which the load was applied in about 50 ms,this length was sufficient to ensure that the cold contact was not heated by conduction from the hot thermocouple junction.In fact the characteristic heat diffusion time t=x2/2~,~ being the thermal diffusivity, was higher than several tens of seconds for both elements.The thermocouple output was recorded by a digital oscilloscope and the actual thermoelectric power of the thermocouple was measured later with suitable hot baths. To confirm the values of the interface temperature a second set of measurements was performed inserting a thin (0.08 mm) NiCr-NiAl calibrated thermocouple between the Ni foil and one of the Zr sheets. The thermocouple was spot welded in a groove machined on the Zr surface, in order to avoid wire fracture during the sample deformation.The measurement procedure was the same as in the previous set.The results of the two methods were in agreement within the experimental error of 10-20 C mainly due to electronic noise in the recorded oscilloscope trace. To examine by TEM the reacted Ni-Zr interfaces,thin cross sectional specimens were prepared following a procedure already developed in semiconductor characterization (10).The deformed samples were sliced normal to the interface by a diamond saw and subsequently reduced to a thickness of about 0.07 mm by careful mechanical grinding.The resulting sections were then glued between two single hole Cu grids fitting the TEM specimen holder.Fina1 thinning was carried out by Ar ion beam milling at 5 kV with an ion current of about 0.2 mA. To avoid specimen heating by ion bombardment a liquid N2 stage was used.Moreover to prevent strong preferential etching of the Ni layer the ion beam was set to impinge at a glazing angle of 18 degrees normal to the interface without any specimen rotation while final cleaning was performed at a lower energy on the sample in rotation.

3-RESULTS AND DISCUSSION

The first experimental finding concerns the adhesion between Ni and Zr.Samples having an area of 25 mm2 give after deformation a strongly bonded composite when the applied load is higher than about 50 kN.This load which is the minimum required for reproducible results gives a maximum stress of about 900 MPa, with a reduction of the total thickness by a factor of 2.As one can expect from the different mechanical properties of the two metals,the deformation process affects the Ni thickness about 50% more than it does Zr; in addition we have been able to establish that strong bonding is obtained for this couple only when the load is high enough to deform.macroscopically both the Ni foil and the more resistant Zr sheet. The typical structure of the deformed trilayer can be seen in fig 1 where a backscattered electron SEM image of the specimen cross section is reported.

Fig.1 SEM image of the deformed trilayer with the Ni foil sandwitched between the Zr sheets.

C4-244 COLLOQUE DE PHYSIQUE

Fig 2 shows the red6ction in thickness as a function of loadlapplied in this set-up with a sinuisodal rate in about 60 ms. AS far as the interface temperature is concerned,a typical Ni-Zr thermocouple output is reported in fig 3.The peak temperature depends on applied load as shown in fig 4 while its full width at half maximum is roughly constant with a value of about 50 ms.This is consistent with the hypothesis that the temperature increase is controlled both by the energy input and by the heat conduction from the surfaces,which ,in this caselappears to have a characteristic time shorter than the power input time.

A P P L I E D L O A D K N

Fig.2 Final thickness of the deformed trilayer versus the applied load.

With our experimental set up, it is possible to change the interface temperature from the outside and in a way completely uncorrelated with the deformation process.In particular an increase of the sample temperature can be obtained either by heating the system with a suitable RF coil or by making the pistons with a badly conducting material.

Fig.3 Thermocouple output for the trilayer deformed at 227 kN.

The Ni-Zr interface was characterized by TEM on cross sectional specimens.A typical result is shown in fig 5 where a bright field, image is rep0rted.A featureless layer about 5 nm thick,is observed between the Ni and the Zr crystals.The amorphous nature of the reaction product has been checked in several ways.Electron microdiffraction patterns from this region have shown only diffuse rings.Equally,the dark field contrast was characteristically fine and consistent with the amorphous structure of the interface 1ayer.In a few cases

crystalline interface compounds were found,whose composition and structure have not been yet investigated.To rule out the presence of a large amount of crystalline nuclei, some specimens were heat-treated at 300 C ,for 1 h.The thickness of the interface layer increased to about 50 nm retaining its original structure and with a growth rate consistent with previous measurements (4,ll)'. As in previous observations (4,ll) Kirkendall voids where found after heat treatment at the Ni-NiZr interface confirming that Ni is the dominant diffusing species in this reaction.

A P P L I E D L O A D K N

Fig.4 Peak temperature increase of the Ni-zr interface versus the applied load.

Fig.5 TEM bright field image of the ~i-Zr interface in the trilayer deformed with 204 kN.

Fig 6 shows a TEM lattice image of the Zr-NiZr interface.The interface appears to be quite smooth and no patches of fringes can be seen in the reaction product. Considering the large lattice parameters of the Ni-Zr intermetallic compounds and the microcrystalline structure with random orientation which probably arises from the interdiffusion process,the absence of fringes in the reaction product confirms the amorphous structure of the interface layer. The width of the amorphous layer without any further heat-treatment depends on the multilayer deformation process in a way which is currently under investigation.First results,reported in fig 7,show the effect of the applied

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load on the interface layer thickness while all other deformation parameters like loading time and temperature are kept constantlapart for the small temperature increase due to the process itself and shown in fig 4. The values of interface thickness show that the. interdiffusion rate is substantially enhanced by the process of mechanical deformation. In fact,if we admit that the process is controlled by Ni diffusion through the NiZr glass,as it is generally accepted for isothermal interdiffusion in this couple, so that the diffusion length is linearly dependent on the square root of the treatment time at constant temperature, the glass growth rate measured from isothermal annealing experiments (12) is not consistent with the thickness of the amorphous layers observed in our samples.Even if the peak temperature measured by the thermocouple is assumed constant during the whole deformation timerthe isothermal annealing times required according to Ding and coworkers (12) to form the observed amounts of amorphous phase range from about 1 hour to several hours, that is well above all possible heating times in our set-up.0n the other hand the small body of data relative to isothermal annealinq of our samples after def ornation gi;e interdif fusion rates in agreement with ~Gblished results (4,ll) .

Fig.6 TEM lattice image of the Zr-NiZr interface in the specimen deformed with 227 kN.

A P P L I E D L O A D K N

Fig.7 Width of the interface layer versus the applied load

We are therefore led to conclude that the process of plastic deformation can substantially increase the glassy phase growth rate, at least at near room temperature.It is interesting to notice that heavy ion bombardment can enhance observed growth rates by a few orders of magnitude (12,13) .One could then make the hypothesis that point defects created by plastic deformation play a role similar to that played by defects created by ion irradiation.0f course further work is necessary to elucidate the mechanism responsible for the observed high growth rate of the amorphous phase.

A novel experimental method to study solid state reactions between bulk elemental foils has been developed.This method allows to change in a wide range the deformation parameters like applied stress and relative strain rate while keeping the interface at a relatively low temperature which in any case can be accurately measured. As an alternative the interface temperature can be fixed in a wide range and in a way completely uncorrelated with the deformation process. Cross sectional TEM specimens can be prepared from the deformed samples for a detailed characterization of the interfaces. First results show the presence of an amorphous interface layer between Zr and Ni crystals after heavy plastic deformation at near room temperature. The thickness of this layer can not be explained by thermal effects alone (12), so that one has to consider the possibility that mechanical deformation affects also the kinetics of the glassy phase formation.

ACKNOWLEDGMENTS

We acknowledge the skillful assistance of Mr M.Traficante in the preparation of the samples and in the operation of the fatigue machine.

REFERENCES

(1) R.B.Schwar z and W.L.Johnson, Phys.Rev.Lett. 51, 415 (1983) (2) C.C.Koch, O.B.Calvin, C.G.McKamey, J.O.Scarbrough, Appl.Phys.Lett. 43, 1017

(1983) ( 3 ) E .~elistern and L. schultz, ~ p p l .~hys. ~ e t t . 48, 124 (1986) (4) L.Schultz, Mat.Res.Soc.Symp.Proc.Vol.80, 97 (1987) (5) M.Atzmon,J:D.Verhdeven,E.D.Gibson,W.L.Johnson Appl.Phys.Lett. 45, 1052

(1984) (6) M.Atzmon,K.M.Unruh,C.Politis,W.L.Johnson, Mat.Res.Soc.Symp.PrOC.Vo1.58, 27

(1986) (7) R.B.Schwar z and C.C.Koch, Appl.Phys.Lett. 49, 146 (1986) (8) R.W.K.Honeycombe, The plastic deformation of metals,Edward Arnold, London

(1977) (9) H.S.Carslaw and J.C.Jaeger, Conduction of heat in solids, Clarendon Press,

Oxford, (1959) (10) A.Garulli,A.Armigliato,M.Vanzi J.Micros.Spectrosc.Electron. 10,135 (1985) (11) S.B.Newcomb and K.N.Tu, Appl.Phys.Lett. 48, 1436 (1986) (12) F.R.Ding,P.R.Okamoto,L.E.Rehen, J.Mater.Res: 6, 1444 (1989) (13) F.R.Ding,R.S.Averback,H.Hahn, J.Appl.Phys. 64, 1785 (1988)