Ion-beam-induced crystallization of Ni50Al50 amorphous films

Materials Science and Engineering, 69 ( 1985 ) 117 - 121

1] 7

Ion-beam-induced Crystallization of Ni5oAlso Amorphous Films* J. DELAFOND, C. JAOUEN, J. P. RIVIl~RE and C. FAYOUX Laboratoire de M~tallurgie Physique, Laboratoire associ~ au C N R S 131, 40 avenue du Recteur Pineau, 86022 Poitiers (France)

(Received September 17, 1984)

ABSTRACT

The crystallization o f a n equiatomic N i - A l amorphous alloy is continuously followed by in situ electrical resistivity measurements at 300 K during 340 k e V Xe ÷ ion irradiation. The crystalline structure is analysed by transmission electron microscopy experiments and indicates a complete transformation to the equilibrium B2 type o f ordered structure with a small grain size for a fluence o f 2 X 1014 Xe ÷ ions cm -2. The crystallization kinetics deduced from the resistivity measurements are in agreement with a nucleation and growth mechanism controlled by diffusion. Thermal crystallization occurs at temperatures o f 380 K or higher with a complex microstructure.

1. INTRODUCTION

Amorphous materials are basically "out-ofequilibrium" phases which will tend to relax towards the crystalline state more or less rapidly depending on the temperature. It has also been k n o w n for a long time that the irradiation of such materials with energetic projectiles (neutrons, electrons or ions) can induce crystallization of the system at temperatures much lower than those required for a purely thermal crystallization process. A considerable a m o u n t of work has been done in the last 10 years on the physical properties of amorphous materials; however, there are fewer studies concerning binary intermetallic alloys [1-3]. NisoA15o alloy was chosen for this s t u d y because it has been shown that it transformed to the amorphous phase during ion irradiation *Paper presented at the International Conference on Surface Modification of Metals by Ion Beams, Heidelberg, F.R.G., September 17-21, 1984.

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at 77 K [4] and that it remains in a crystalline B2 t y p e of ordered structure when irradiated at 300 K. A high density of elastic energy deposited in collision cascades is obtained when irradiation is performed with Xe ÷ heavy ions, producing very high localized defect concentrations which can destabilize the amorphous structure. In such a case, it is important to k n o w h o w the radiation-induced crystallization occurs. Is it a direct ion impact process with progressive accumulation and overlapping of (elementary) crystalline regions, or is it a nucleation and growth process? In this work, we tried to answer these questions b y studying in situ the radiation-induced kinetics using electrical resistivity measurements. The microstructural state of the crystalline phase obtained is studied b y transmission electron microscopy (TEM).

2. EXPERIMENTAL DETAILS

Ni-A1 multilayered samples were prepared b y sequential deposition in an oil-free vacuum system at pressures lower than 10 -8 Torr. The individual thickness of the layers ranged between 90 and 150 )k and the total thickness was 725 A. The composition was accurately determined b y X-ray microanalysis measurements and was NisoA15o. The crystalline Ni-A1 alloy produced b y ion beam mixing at 300 K was irradiated at 77 K with 360 keV Xe ÷ ions, resulting in a complete transformation of the alloy to the amorphous phase for a dose as low as 2 X 1014 Xe + ions cm -2 [4]. The preparation of the electrical resistivity samples and the measurement procedure have been described elsewhere [5]. For TEM observations, freshly cleaved NaC1 crystals were used as substrates. The Ni-A1 amorphous samples obtained b y the previously described treatQ Elsevier Sequoia/Printed in The Netherlands

118 m e n t were then irradiated at 300 K with 340 keV Xe ÷ ions. The electrical resistivity of the samples was continuously measured in situ during the ion bombardment.

be continuous at the interfaces between a given phase (1 or 2) and the surrounding medium (called the "effective medium"), the conductivity is given b y O'm

=

t

[ ( 3 X 2 - - 1 ) O 2 "t- (3xz-- 1)oz +

-F {(3x2 --1)02 -t- (3x 1 -- 1)ol} 2 -t-

3. RESULTS AND ANALYSIS 3.1. Electrical resistivity The change in resistivity measured at 300 K as a function of the Xe ÷ ion fluence is presented in Fig. 1. A continuous (and progressive) decrease in the resistivity is observed with progressive irradiation, and the final value reached ( 1 1 4 / ~ 2 cm) is almost identical with that measured (112/~2 cm) after ion beam mixing at 300 K where the alloy is in a crystalline ordered state. It is worth noting that the fluence necessary to obtain complete crystallization of the alloy is very low (about 101¢ Xe + ions c m -2 corresponding to 1 displacement per atom). This value is similar to that required for amorphization [4]. The parameter characterizing the crystallization kinetics is the volume fraction of crystallized material at a given time t or, since we operate with a constant Xe ÷ ion beam (flux), at a given fluence ¢. This parameter can be deduced from resistivity measurements. Landauer [6] has developed a theory and established a relation between the conductivity om of a heterogeneous medium and the conductivities ol and 02 of the t w o c o m p o n e n t phases 1 and 2. If the electrical field is assumed to

~k

-~- 801021/2 ] where xz and x2 are the volume fractions of the component phases 1 and 2. In our case, oz and 02 are the conductivities of amorphous and crystalline Ni~oA]5o respectively. We must emphasize the good agreement obtained between this model and computer simulations performed to study percolation problems. Moreover, Criado et al. [7] have also recently applied this model to study the thermal crystallization of F e - C o - S i - B metallic glasses using electrical resistivity measurements. Figure 2 shows the volume fraction of crystallized material as a function of the ion fluence. The experimental curves exhibit a sigmoidal shape which is characteristic of nucleation and growth kinetics. Phase t r a m formations which involve nucleation and growth processes are usually described b y the Johnson-Mehl-Avrami [8, 9] equation C = 1 -- exp(--Kt") where C is the crystalline volume fraction and t = ¢/¢ is the time which is proportional to the fluence. Generally, the value of n ranges between 1.5 and 4 and characterizes the nature of the transformation. In Fig. 3 we have plotted log[log{I/(1 -- C)}] as a function of log ¢. The curve exhibits two

III B lID ¢..) D

~ -48

N

mm mmmmm

8.8

m

~B

~ 0.6

m °o

tJ_

B

~

m m

~,, 6. 4 -88 8

i

i

i

I

i

I

86

FLUENCE¢ (10]2ionB vm-2) Fig. 1. Decrease in the electrical resistivity at 3 0 0 K of an Nis0A150 amorphous alloy as a function of the Xe + ion fluence. T h e initial resistivity p at 3 0 0 K o f the alloy is 194 p ~ cm.

~0.

Z

166

izl

0.0

100.0 FLUENCE $ (lol2iona cm-2>

Fig. 2. Crystalline fraction C as a f u n c t i o n o f the X e + ion fluence.

119

linear parts of very different slopes which can be analysed in the following way. For low fluences (10 la Xe + ions cm -2 or less) the curve should correspond to the incubation time in a classical nucleation and growth model. During this stage there is u n d o u b t e d l y the formation of crystalline nuclei, producing a resistivity change; nevertheless it is likely that another contribution to the resistivity change could be produced by the relaxation of the amorphous alloy at the start of irradiation. It is worth noting that, if this second term is predominant, C cannot be determined from the resistivity change. Therefore we shall not discuss this part of the kinetics. However, the total resistivity change during this stage is very low (about 5% of the total change). Thus

0 t

..--.

g .=0.5 -2 12

14 lOG (FLUENCE $ IONS cm-2}

Fig. 3. E x p e r i m e n t a l variation in the crystalline fraction C shown in a plot of l o g [ l o g { 1 / ( 1 - - C ) } ] as a f u n c t i o n of log ~ : - - , fit with linear variations, giving slopes n of 0.5 and 1.9.

its influence on the evaluation of the crystalline fraction during the second stage (10 la Xe ÷ ions cm -2 or higher) can be neglected. For higher fluences (10 la Xe ÷ ions cm -2 or higher) the crystalline volume fraction increases according to a Johnson-Mehl-Avrami transformation equation with a power n very close to 2. Such a value for thermal crystallization corresponds to a transformation with diffusion-controlled growth [8, 9] and a decreasing nucleation rate. 3. 2. Transmission electron microscopy The microstructural state of our samples before and after the radiation-induced crystallization was determined b y TEM measurements. The starting material can be identified as an amorphous Ni5oAlso alloy in Fig. 4. The final state in Fig. 5 corresponds to a fully and perfectly crystallized B2 t y p e of ordered alloy after irradiation with a fluence of 2 X 1014 Xe ÷ ions cm -2. The size of the crystallites formed during the radiation-induced crystallization is very small (of the order of 200 A) and homogeneous, indicating a very high nucleation rate. This result suggests, as previously emphasized b y Brimhall [1 ], that dense collision cascades such as those produced b y Xe ÷ ions in which a high degree of localized atomic displacements exists can act as preferential nucleation sites. We also performed classical thermal crystallization experiments without any ion bombardment b y annealing identical amorphous samples inside the electron microscope. It was observed that the crystallization process starts at a b o u t 380 K b y the formation of crystal-

Fig. 4. Initial microstructural state of the NisoAI50 amorphous alloy. The electron diffraction pattern exhibits diffuse rings which are characteristic of amorphous materials.

120

Fig. 5. Final B2 type of ordered crystallized state obtained at 300 K by radiation-induced crystallization with a fluence of 2 × 1014Xe + ions cm -2.

Fig. 6. Microstructure of thermally crystallized Ni50A15o after aging in the microscope for 30 rain at 650 K.

line nuclei. The diffraction patterns also reveal the presence o f intermediate phases which are different from the equilibrium B2 t y p e of ordered phase (Fig. 6). These intermediate phases are stable up to relatively high temperatures (770 K). The size of the crystals obtained after thermal crystallization is much larger than that of the crystals found under ion bombardment. 4. DISCUSSION

First of all, it is necessary to emphasize that we can exclude beam-heating effects as possible artefacts in such experiments since we used extremely low ion beam fluxes, i.e. as low as 10 n A c m -2. It is then possible to consider t w o models of radiation-induced crystallization. The first is direct ion impact crystallization in which each incoming ion produces a crystalline volume; such a mecha-

nism is expected to occur at a very high density of deposited energy in the collision cascade. The second model is the classical nucleation and growth p h e n o m e n o n as generally observed during thermal annealing of amorphous materials. Direct ion impact crystallization should imply a crystalline volume fraction proportional to the dose, giving a p o w e r n of unity at low doses. Our results are in contradiction with such a model since t h e y exhibit a sigmoidally shaped curve. They can be explained on the basis of a radiation-enhanced nucleation and growth crystallization model, nucleation enhanced inside the displacement cascades and growth enhanced via the high defect concentrations which increase the diffusion rate. The power in the Johnson-Mehl-Avrami equation suggests diffusion-controlled growth. For NisoA150, we have a homogeneous alloy in the amorphous phase, and an ordered alloy in the

121

crystalline phase. Consequently, no important composition changes are required locally to initiate the crystallization, and the diffusion will occur only over short distances. In this way we can explain the low dose of 1014 Xe ÷ ions cm -2 (1 displacement per atom) necessary for complete crystallization. For more complicated systems such as Fe-Cr-Ni-W which was studied b y Brimhall [10] and which exhibited many intermediate phases, higher doses corresponding to a b o u t 20 displacements per atom are necessary for crystallization because, in this case, diffusion occurs over long distances. It was f o u n d that thermal crystallization starts at a b o u t 80 K above r o o m temperature where radiation-induced crystallization is observed. A general approach using phase stability under irradiation has recently been proposed b y Martin [11 ]. It is predicted that the equilibrium configuration of a system under irradiation at a temperature T is identical with the configuration at ¢ = 0 and T' = T(1 + A) where A is a function which decreases with temperature and increases with chemical energy diffusion. This model could explain qualitatively the radiation-induced crystallization if the temperature shift is greater than 80 K.

5. CONCLUSIONS

(1) A decrease in the crystallization temperature for an amorphous NisoA15o alloy b o m b a r d e d with 340 keV Xe + ions is observed. (2) The radiation-induced crystallization kinetics are studied using in situ electrical

resistivity measurements and are explained b y a nucleation and growth mechanism controlled b y diffusion. (3) The amorphous phase is transformed directly to the equilibrium B2 type of ordered structure while more complex crystalline structures are formed during purely thermal crystallization.

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