Dynamic equilibrium induced by ball milling in the NiZr system

Materials" Science and Engineering, A119 (1989) 185-197

185

Dynamic Equilibrium Induced by Ball Milling in the Ni-Zr System E. GAFFET Centre d'Etudes de Chimie Mdtallurgique (CNRS), 15 rue G. Urbain, F94407, Vitry/Seine Cedex (France)

(Received April 5, 1989; in revised form May 25, 1989)

Abstract

The influence of the ball-milling conditions (injected power) on the state of the end-product powder has been studied using X-ray techniques. Stationary dynamic equilibrium between amorphous and crystalline phases is observed. The nature of such phases depends also on the initial composition of the powder mixture but not really on the initial state of the mixture. The observations of such a metastable dynamic equilibrium support the idea that the mechanism which is induced during ball milling is different from that occurring during classical amorphization by solid state diffusion of multilayered films.

I. Introduction

Amorphous phase formation by mechanical alloying was first reported by Yermakov et al. [1] and Koch et al. [2] for the Co-Y and Ni-Nb systems respectively. This amorphization process has been applied in many other binary alloy systems, starting from elemental crystalline powders, e.g. Fe-Zr [3], Ni-Zr [4-6], Ti-Pd [7], Ni-Ti, Co-Ti, Fe-Ti [8-10], T i - C u [11] and Co-Sn [12], or from a mixture of intermetallic compounds in the Ni-Zr system [13-15]. Amorphization induced by mechanical alloying is usually obtained for binary alloys exhibiting a large negative heat of mixing and for which one of the elements is a fast diffuser [16]. Nevertheless, such amorphization has been observed by Richards and Johari [17] in Cu-Ag alloys with a positive heat of mixing as well as in a slow-diffusing system V29Zr71 by Weeber and Bakker [ 18]. Amorphization of intermetallic compounds has been attributed to the freeenergy increase due to accumulation of point and lattice defects, leading to the destabilization of the crystalline compounds [ 19]. 0921-5093/89/$3.50

Although this argument holds for the transition from the intermetallic compound to the amorphous phase, it cannot apply to the transition from elemental powders to the amorphous phase for which amorphization is argued to result from the solid state interdiffusion reaction [20]. This explanation is based on some analogy to the classical solid state amorphization by diffusion between evaporated multilayers [21]. Nevertheless, in some cases, starting from elemental powders, the formation of crystalline intermetallic compounds precedes the formation of the amorphous phases [5, 6] or the crystalline compounds even coexist with the amorphous phases during the stationary ball-milling state [5, 6]. This is in contradiction to the investigations of the different steps of the so-called classical solid state amorphization process for which the appearance of the crystalline compounds stops the development of the amorphous phase and furthermore will reduce the thickness of the latter by a recrystallization process. Furthermore, in spite of a negative heat of mixing which exists in such systems, Loeff and Bakker [22] have reported the decomposition of the binary alloys L a - T M ( T M - A g , Ni or Co) into the elemental components, at least in their high temperature phase structures. In a recent paper [23], we reported the first observation of the crystal-to-amorphous transition induced by ball milling in a pure element: a silicon powder. The ball-milling conditions influence the critical lattice parameter of the crystalline phase which is in dynamic equilibrium with the amorphous phase. In fact, the end product of the ball-milling process, i.e. a homogeneous amorphous phase, or a mixture of glass(es) and crystalline compound(s), or purely crystalline compound(s) seems to be highly dependent on the milling conditions: milling atmosphere, balls-to-powder weight ratio, © Elsevier Sequoia/Printed in The Netherlands

186

balls and vial materials, type of ball movement, and average milling temperature. In the present paper, from an X-ray pattern investigation, we report the influence of the ballmilling conditions (ball-milling duration, injected ball power, initial mean composition and state of the powders) on the dynamic equilibrium between amorphous and crystalline phases in the Ni-Zr system.

2. Experimental details

2.1. Ball-milling conditions The chosen composition domain varies from Niz4Zr76 to Ni90Zrl0. T w o k i n d s of N i - Z r p o w d e r

were prepared for the ball-milling process. First, some alloys were directly obtained from levitation melting with the correct mean composition; some of them were annealed before ball milling in order to achieve correct homogeneity (for details, see Table 1). After annealing, the levitation ingots were broken to obtain some fragments of about 5 mm x 5 mm × 5 mm and then introduced in the container for the ball-milling process. Nevertheless such an anneal (48 h at 450 °C) was not sufficient to obtain one single phase in the case of a mean composition corresponding to

TABLE I

an intermediate compound of the equilibrium Ni-Zr phase diagram. The second type of powder was prepared starting from a mixture of some melt-spun ribbon pieces (the dimensions are about 8 m m x l 0 mm x 50/~m) and a nickel powder (the mean particle diameter is about 10-30 pro). The mass ratio of the two species was adapted in order to obtain the correct mean composition (see Table 1 ). A 10 g portion of such a pre-prepared Ni-Zr alloy was introduced into a cylindrical tempered steel (containing 12 at.% Cr and 2 at.% C) container of capacity 45 ml. This procedure occurs in a glove box filled with purified argon. Each container was loaded with five steel balls 1.5 cm in diameter and 14 g in mass. Thus the powder-toball mass ratio was equal to 1 : 7. As the containers were sealed in the glove box with a Teflon O-ring, the milling was performed in static argon. In some specific cases, the containers were loaded in the air and the ball milling was performed in a confined air atmosphere since the containers are also sealed with a Teflon Oring. This method leads to a constant oxygen-tozirconium ratio of less than 1% [6]. Ball milling was carried out using two Fritsch planetary high energy ball-milling machines (Pulverisette P7/2 and P5/2). For the latter

Summary of the various bail-milling experiments which have been performed

Alloy

Ball-milling condition

Annealing time (h)

Gas

Powder

Ni24Zr76

P5/2(5) P7/2 P5/2(10) P5/2(5) P7/2 P5/2(10) P5/2(5) P7/2 P7/2 P5/2(10) P5/2(5) P7/2 P7/2 P5/2(5) P5/2(10) P7/2 P5/2(10) P7/2 P7/2 P5/2(5) P5/2(10) P5/2(10)

96 89 88 96 95 88 96 65 95 119 96 2 x 20 3 x 20 20 119 95 70 20 90 20 87 87

Ar Ar Ar Ar Ar Ar Ar Air Ar Ar Ar Ar Ar Air Ar Ar Ar Ar Ar Air Ar Ar

Levitation Levitation 7.05 g of Ni24Zr76(MSR) + 0.69 g of Ni Levitation Levitation 7.00 g of Ni65Zr35(MSR) + 1.40 g of Ni Levitation Levitation Levitation Levitation Levitation 5.44 g of Ni + 4.56 g of Zr 5.44 g of Ni + 4.56 g of Zr Ni65Zr35 (MSR) 8.51 g Ni63.7Zr36.3(MSR) + 1.49 g of Ni Levitation 5.24g ofNi63.TZr36.3(MSR) + 2.93 gofNi Levitation Levitation Levitation 5.00 g of Ni65Zr35 (MRS) + 5.43 g of Ni 3.3 3 g of Ni63.7Zr36.3(MSR) + 7.34 g of Ni

Ni33.3Zr66.6 Ni33.3Zr66.6

Ni33.3Zr66.6 NisoZrs0 NisoZrsi~ NisoZr50 Ni55Zr45 Ni58.sZr41.2 Ni58.sZr41.2 Niss.sZr4j.2 NirsZr~5 Ni65Zr35

Ni65Zr35

Ni70Zr30 Ni77.sZr22.2 Ni78Zr22 Nis3.3Zrlr.7

Ni83.3Zrlr.7 Ni83.3Zrlr.7 Ni85Zr15 NigoZrm MSR, melt-spun ribbon.

187

machine, two intensity settings were chosen which are referred to hereafter as P5/2(10) and P5/2(5). The higher energy condition corresponds to P7/2, and the lower to P5/2(5). The durations of the continuous milling processes corresponding to the various mean compositions are given in Table 1.

2.2. X-ray investigations 2.2.1. X-ray specimen preparation After the continuous milling, a small amount of ball-milled powder (0.3-0.6 g) was extracted from the container and glued onto a S i O 2 plate for X-ray investigations. The X-ray diffraction patterns of the ball-milled powders were obtained using a (0-20) Philips diffractometer with Co Kc¢ radiation (2 = 0.178 89 nm).

The function which describes the peaks has been reparameterized in the ABFfit program as a function of the integrated intensity /, the mean position /~ of the peak and the F W H H L and is as follows: a gaussian curve [Ib/(2JrL2)-1/2] exp(-b2(O-~)2/2L 2) with b = 2 ( 2 1 n 2 ) ~/2 and L=ob (or2 is the variance of the gaussian function). According to our previous work [23], no major difference is observed using a Cauchy or a Lorentz peak model in order to fit the experimental X-ray curve. Therefore in all the present work the X-ray investigations were performed on the ball-milled powder using a gaussian peak shape deconvolution model. To determine the proper end fit, we adopted the following criteria: the r.m.s, deviation of the integrated intensity of a given peak has to be less than the value of the integrated intensity.

2.2.2. X-ray pattern deconvolution A numerical method was used to analyse the X-ray diffraction patterns and to obtain the position and the full width at half-height (FWHH) of the various peaks. In the ABFfit program [24], the spectrum is modelled by a polynomial background with a maximum degree of two plus a set of simple shaped peaks. Then the measured X-ray intensity (hereafter referred to as Y(0) with 0 the Bragg angle) is n

Y( O)= Bkg( O)+ ~ peak~(0, S~, L, 0~, L,) i=l

with i the index of the n possible peaks, 0 the Bragg angle, Bkg a polynomial with degree of 0-2 defining the background and given by Bkg(0)= bo + bl(O- 0m)+ b2(O- 0m)2, 0m the abscissa of the centre of the [0mi,, 0m~x]angular domain of the observed pattern, S~ a peak shape parameter allowing the selection of the appropriate function, and I~, 0~ and Lg the intensity, position and F W H H of the ith peak.

2.2.3. Identification of crystalline phases and/or amorphous phases The Bragg expression was applied to determine the d parameter corresponding to the various diffraction peak positions 0: 2 = 2d sin 0 with 2 the wavelength of the X-ray beam. Now let us consider the case when some amorphous phases are present. Table 2 gives the result of the ABFfit deconvolution which may be obtained from the X-ray pattern of some amorphous melt-spun ribbons. Two very similar amorphous compositions (in at.%) were chosen in order to test the resolution of such an ABFfit deconvolution: Ni(,4~)5+02oZr35.o5 _+0.20; Ni63.65 +_0.40Zr3(,.35 + 0.4o.

The energy-dispersive X-ray spectra, which were used in order to determine the composition of these amorphous ribbons, were obtained with an energy-dispersive X-ray Si-Li detector supplied with a scanning electron microscope (Zeiss

TABLE 2 ABFfit deconvolution of the X-ray patterns obtained from amorphous ribbons Ni6sZr3s and Ni63.7Zr36.3 as a function of the various peak shapes: peak position in 2 0(A, Co K a) and full width at half-height, the value of the specific parameter obtained from the Bragg's law and the corresponding composition (Vegard's law) Peak shape

Gauss Cauchy Lorentz

Ni~4.95±o.20Zr~s.o~+0.20

N i ~ 3 65 + o 40Z?~6.35 + 0.40

20

FWHH

d

Composition

20

FWHH

d

Composition

48.60 48.46 48.52

7.74 7.71 7.53

2.1737 2.1796 2.173

Ni65.oZr35.0 Ni64.2Zr35.8 Ni64.sZr35.5

48.5(I 48.35 48.40

7.92 8.12 7.82

2.1778 2.1840 2.1819

Nir,4.sZr35.5 Ni~3.TZr3~,. 3 Ni~3.~Zr3~,.L

188

DSM 950). The Chemical determinations were performed using a quantitative microanalysis program (MicroQ from TRACOR). The Zr K and Ni K peaks were taken into account in these analyses. The various peak shapes (Gauss, Cauchy and Lorentz) have been used in order to test the validity of our choice which is, as mentioned above, a gaussian deconvolution model. As observed in Table 2, the peak positions (and the specific d parameter) which are obtained from the deconvolution using the various peak shapes are very similar. The composition of the amorphous phases corresponding to such values of the d parameter was determined using a Vegard-type law which has been determined previously for amorphous Ni-Zr alloys [5]. We note from data in the literature that the nearest-neighbour distance (hereafter denoted as d, the specific parameter of the amorphous phase) in amorphous alloys increases linearly with increasing zirconium content X, at least in the range 10-80 at.% as d = 7.79 x 1 0 - 4 X + 0.1901 where d is in nanometres and X in atomic per cent. Table 3 and Fig. 1 correspond to the compiled reference values which have been used in order to establish such a law. Obviously the deconvolution using a gaussian TABLE 3 Composition dependence of the specific parameter d corresponding to the different Ni=o0-xZrx amorphous alloys

Composition

d( x 10 i nm)

Reference

Ni20Zrs0 Ni22Zr78 Niz4Zrve, NizsZr75 NizvZr73 Ni3oZr70 Ni33Zr67 Ni34Zr66 Ni35Zr65 Ni37Zr63 Ni40Zr60 Ni42.3Zrs7.7 Ni45Zr55 NisoZrs0 Ni6jZr39 Ni637Zr36.3 Ni64Zr36 Ni~,sZr35 Ni68Zr3z NiToZr30 NisoZr20 Nig0Zrl0

2.506 2.513, 2.508 2.505, 2.503, 2.496, 2.497 2.445 2.493 2.404, 2.468 2.442 2.325, 2.437 2.42 2.432 2.32, 2.405, 2.427 2.33 2.34 2.29 2.228 2.181 2.189 2.176 2.127, 2.181, 2.137, 2.151 2.108 2.04 1.99

[4] [25, 26] [25-28] [4] [26] [4, 26] [26] [29, 30] [31] [26] [4, 26, 30] [30] [30] [32] [13] [33] [34] [35] [35-38] [39] [35] [29]

peak shape is well adapted and the accuracy of the correct mean composition is better than 1 at.%. Another point to be noted is that the FWHH of the first amorphous halo is equal to 7o-8 ° (expressed in 20(2, Co Ka)) in the case of amorphous alloys obtained by the melt-spinning method which leads to a well-homogenized ribbon and thus a composition which is close to nominal. Therefore, when these preliminary investigations in the use of the ABFfit deconvolution are taken into account, a peak which exhibits (after deconvolution by the ABFfit program) a FWHH equal to or larger than 7°-8 °, will be considered as corresponding to the first amorphous halo of an amorphous phase--at least in the first part of the X-ray pattern. 3. Results

3.1. Experimental X-ray patterns Figures 2, 3 and 4 show the experimental X-ray patterns which were obtained for the various initial mean compositions and the ballmilling conditions P7/2, P5/2(10) and P5/2(5) respectively.

3.2. X-ray pattern deconvolution The deconvolution of the various X-ray patterns leads to Tables 4, 5 and 6 which correspond to the P7/2, P5/2(10) and P5/2(5) ball-milling conditions respectively. The d value which is obtained from the Bragg law is noted. The highest intensity peak is indicated by ( + + + ), the nexthighest intensity peak, by (+ +) and so on. The peaks for which the r.m.s, deviation obtained for 28

j y = 19013 + 0.0078x R = 0.98

g

24

~,

!

2.2

-~

2.0

1.8

"

210

'

410

'

610

"

810

'

10 0

Zr composition (at.%)

Fig. 1. Vegard-type law determination: value of the specific parameter as a function of the amorphous Ni-Zr phase.

189

400

m~, 300 "

200"

:k loo.

0

~oo

•

25,0

,

-

35,0

,

•

45,0

(a)

,

•

2 thsts

0

65,0

55,0

75,0

85,0

I

•

25,0

,

•

35,0

,

(C)

(degrees)

400

•

45,0

,

•

55,0

,

•

65,0

,

•

75,0

,

85,0

2 theta (degrees)

400

.,-:. :=

~

300

300'

2O0

200'

loo

loo.

m

o 25,0

(b)

0

35,0

45,0

56.0

2 theta

66.0

76,0

65,0

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(degrees)

400

400

300

~ , 300 >,

.==

~

200

X

200

X

100

100

-

0

(C)

0

25,0

36,0

45,0

65,0

55,0

2 thsts

75.0

65.0

•

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(f')

(degrees)

,

35,0

•

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•

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,

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2 theta (degrees)

400

.>.

300

200

>~ loo

0 25,0

(d)

, 35,0

•

, 45,0

-

, 55,0

2 thets

•

, 65,0

-

, 75,0

•

, 85,0

(degrees)

Fig. 2. Experimental X-ray patterns as a function of the initial mean composition of the powder (in at.%) for P7/2 ball-milling conditions: (a) Ni33.aZr66.~,; (b) Nis~Zrs~; (c) Ni55Zr4.~; (d) Niss.~Zr4J.2; (e) Ni77.~Zr22.2; (f) Nis3.aZrlt,. 7 (ball milled for 20 h); (f') Nis3.aZrj6.7 (ball milled for 90 h).

190

400

~ 300

400 ]

300 -t ~200 m

200" 100

X

X

0 25,0

35,0

45,0

55,0

2 theta

(a)

65,0

75,0

100 "

0 25,0

85,0

(e)

(degrees)

400

400 "

m~. 300

300"

•

, 35,0

•

, • , • , 45,0 55,0 65,0 2 theta (degrees)

•

, 75,0

-

, 85,0

¢

m 200-

loo

>t loo

o 25o

35,o

45,o 55,0 65,0 2 theta (degrees)

(b)

75,0

55,0

25,0

35,0

45,0 55,0 65,0 2 theta (degrees)

(f)

75,0

85,0

400 ' ~" 400 t

~

m~, 300'

300

Q 200

X 100

0 25.0

, • 45,0 55.0 65,0 2 theta (degrees)

35.0

(C)

75.0

05,0

(~)

0 25,0

•

, 35,0

-

, , • , 45,0 55,0 65,0 2 there (degrees)

-

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, 85,0

400 1

3OO "

c

200"

1oo-

0

(d)

25,0

•

,

35,0

•

,

-

,

•

,

45.0 55,0 65,0 2 theta (degrees)

•

i

75,0

-

,

85,0

Fig. 3. Experimental X-ray patterns as a function of the initial mean composition ofthe powder(in at.%) for P5/2(10)ball-milling conditions: (a) Ni3s.3Zr66.6;(b) NisoZrso; (c) Ni58.sZr41.2; (d) Ni7oZr3o; (e) Ni78Zrz2; (f) NissZrls; (g) NiguZrtu.

191

t

300 200

1

400

300 200 -

100 -

•

25,0

35,0

45,0

55,0

55,0

75,0

•

35,0

(d)

2 thets (degrees)

(a)

,

25,0

8s,o

,

•

,

45,0

•

55,0

,

•

55,0

,

•

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,

85,0

2 theta (degrees)

400 i400

1

t 300

300"

>,

200"

lOO

lOO-

o

0

25,0

35,0

46,0

(b)

55,0

65,0

75,0

65,0

•

25,0

,

•

35,0

,

•

45,0

(e)

2 there (degrees)

,

•

55,0

,

•

65,0

,

•

75,0

,

85,0

2theta(degrees)

1400' 400

1200'

"•[' 1000 '

-ff

,~300

._>,

._~800"

200

600 • ,~" 400" X

~100

200 • 0 25,0 -(C)

.

, 35,0

.

, 45,0

•

, 55,0

•

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2 theta (degrees)

0 25,0

(f)

•

, 35,0

•

, 45,0

•

, 55,0

.

, 65,0

.

, 75,0

•

, 85,0

2 theta (degrees)

Fig. 4. Experimental X-ray patterns as a function of the initial mean composition of the powder (in at.%) for P5/2(51 ball-milling conditions: (a) Ni24Zr7~; (b) Ni33.3Zr~.~,; (c) Nis~Zrs~);(d) Nis~.sZr41.2 (e) Ni6~Zr~ (f) Ni~.~~Zr~ 7.

the integrated intensity is larger than the latter are indicated by ( - ) . Thus they will be taken into account only when they may be correlated with a set of well-defined peaks which corresponds to a crystalline compound.

3.3. Identification of the various amorphous and crystalline phases Table 7 summarizes the nature of the distinct

amorphous phase(s) and/or crystalline com-

pound(s) which are in dynamic equlibrium during the ball-milling process.

3.3.1. Crystalline compounds Some of the crystalline phases have been identified as h.c.p, compounds or f.c.c, phases. In the other cases, it was not possible to determine the nature of the crystalline phases which do not correspond to some so-called simple phases (simple cubic, f.c.c, or h.c.p.) or to some

192 TABLE 4

d(×lO

X-ray decnnvolutions corresponding to P7/2 ball-milling conditions lnm)

NiZ~, 10g, levitation, ball milled for 89 h

NiZr, 10g, levitation, ball milled for 95 h

NiliZr ~, lOg, levitation, ball milled for 65 h

NiloZr7, 10g, levitation, ball milled for 95 h

NiTZ~, 10g, levitation, ball milled for 95 h

NisZr, 10g, levitation, ball milled for 90 h

2.740 ( ) 2.544(+ + +) 2.348(+ +) 2.327a(+ + +) 2.280 ( + 2.151(2.041 ( ) 1.595 ( + 1.504(+ +) 1.415(+ +) 1.315(+) 1.273 ( ) 1.163 ( ) 1.104( )

3.160(+) 2.464(+ +) 2.349 ~( + + + ) 2.077(+ +) 1.906(+ +) 1.651 (+ +) 1.386 ( + ) 1.309 ( + ) 1.262 ( + )

2.685 ( ) 2.256 a ( + + + ) 2.247 ( + + ) 1.899(-)

2.982 ( - ) 2.748 ( - ) 2.236 ( - ) 2.215"(+ + +) 1.881 ( - )

3.020 ) 2.665 ) 2.333 + +) 2.042 + +) 2.127"(++) 2.034(+ +) 2.032(+ + +) 1.660 ( + ) 1.352 ( + ) 1.242 ( ) 1.176 ( )

2.932 ( + ) 2.612(+) 2.050" ( + + + ) 1.798 ( + ) 1.54 ( + )

aAmorphous phases.

TABLE 5

X-ray deconvolutions corresponding to P5/2(10) ball-milling conditions

d( x 10 -j nm) NiZr2, NiZr, NiloZr7, MSR + Ni powder, MSR + Ni powder, l Og, levitation ball milled, ball milled, ball milled, for 88 h for 96 h for l l 9 h

NiToZt~o, NiTaZ~2, Nia:Zrl:, Ni~Zrlo, MSR + Ni powder, MSR + Ni powder, MSR + Ni powder, MSR + Ni powder, ball milled, ball milled, ball milled, ball milled, for l l 9 h for 70 h for 87 h for 87 h

2.963(-) 2.850(-) 2.4381(+++) 2.334 ( -- ) 2.137(+) 2.054(--) 2.016(--) 1.945(--) 1.898(--) 1.452(+ +)

3.120 ( ) 2.673 ( ) 2.284(+) 2.205 ( -- ) 2.189"(+++) 2.154(+) 2.089(++) 2.034(+) 1.328( ) 1.216 ( )

3.729(-) 3.272(-) 2.636(++) 2.287 ( + ) 2.229"(+++) 2.029(+) 1.915(--) 1.615(+) 1.372(+ +)

2.623(+ +) 2.298(+) 2.182"(+++) 2.030 ( + ) 1.834(--) 1.619(+) 1.369(++)

2.651 (+) 2.188(-) 2.115a(+++) 2.078 ( + + ) 2.077(--)

2.837 ( ) 2.113(+) 2.045"(++) 2.030 ( + ) 1.954 1.429 1.176

2.817(-) 2.057(+ +) 2.020"(+++) 1.968 ( + ) 1.786(+) 1.696(--) 1.263( ) 1.076( ) 1.03( )

MSR, melt-spun ribbon.

aAmorphous phases.

intermetallic phases of the equilibrium phase Ni-Zr phase diagram; they are referred to in Table 7 as crystal(?).

P 7/2 80

.......

P 5/2(1 O)

.............

P 5/2(5)

70 60

3.3.2. Amorphous phases The mean compositions of the different a m o r p h o u s phases which correspond to the peaks showing a FWHH larger than 70-8 ° (in 20 (2, Co Ka)) are determined from the above Vegard-type law. Figure 5 shows a plot of the mean nickel concentration in the amorphous phases as a function of the initial mean nickel concentration in the

40

2O

~ ,o o

•

0

,

I0

.

,

20

.

,

30 Initial

.

,

•

40 m e a n

, 50

.

, 60

.

, 70

.

, 80

.

,

90

. 100

c o m D o s t t l o n

Fig. 5. Nickel concentration in the amorphous Ni-Zr phases as a function of the initial mean concentration for the various ball-milling conditions.

193 TABLE 6

d(xl0

X-ray deconvolutions corresponding to the P5/2 (5) ball-milling conditions

lnm)

Ni24ZrT~, 10 g, levitation, ball milled for 96 h

NiZr_,, 10 g, levitation, ball milled for gO h

NiZr, 10 g, levitation, ball milled for 96 h

NimZrT, 10 g, levitation ball milled for 96 h

NiosZr~5, 5 g, MSR, ball milled for 20 h

NLZr, 10 g, levitation, ball milled Jor 20 h

3.413(-) 3.2(11 ( - ) 2.564 ( + ) 2.450" ( + + ) 2.069"(+ +) 1.600 ( + ) 1.501 ( - ) 1.397 ( + ) 1.384 ( - )

3.332 ( - ) 2.585 ( + ) 2.418"( + +) 2.352 - ) 2.062 - ) 2.051 +) 2.018 - ) 1.591 +) 1.400 + +)

2.463 ( ) 2.307 ( ) 2.296 a(+ + +) 2.050 ( - ) 1.590 ( + ) 1.380 ( + )

2.705 ( - ) 2.363 ( ) 2.294 ( - ) 2.233( ) 2.219~(+ +) 2.183(+) 1.787(-) 1.537 ( - ) 1.463(-) 1.413 ( - ) 1.357(-) 1.315()

2.943 ( + ) 2.940 ( ) 2.640 ( - ) 2.510(+) 2.191a(+ +) 2 . 1 4 6 ( + +) 2.106(-) 2.020 ( ) 1.747(-) 1.700 ( - ) 1.609( ) 1.505 ( ) 1.477 ( + ) 1.432 ( - )

3.974 ( + ) 3.338 ( ) 3.043 ( ) 2.572(+) 2.405 ( + ) 2.387(+) 2.051 (+ + +) 2.047 ( + + ) 1.953 ( + ) 1.777 ( ) 1.614( ) 1.485(+) 1.364(+) 1.285 ( + )

MRS, meLt-spun ribbon. ~Amorphous phases.

TABLE 7

The observed end-product phases as a function of the initial mean composition and of the ball-milling conditions

Initial mean composition

Amorphous phase compositions + crystalline phases P7/2

P5/2(10)

Ni24Zr76

P5/2(5) Ni2~.sZr70.5~'+ NiTsZr2z ~+ crystal (?)

NiZr2 (Ni33.3Zr66.6)

Ni45Zr55 a + h.c.p, crystal

Ni3]Zr69" + crystal (?)

Ni33.sZr~,~,f ' + crystal ( ? )

NiZr (NisoZrso)

Ni42.sZr57.5a + crystal (?)

NissZr42" + crystal (?)

Ni4~.sZrso fl + crystal (?)

Ni I iZro (Ni55Zr45)

Ni54.sZr45.5a + crystal

Ni iiiZr7 (Ni 5s.sZr412 )

Ni59Zr40.5a

Ni~4Zr36~ + f.c.c, crystal

Ni59Zr41 a + crystal (?)

Ni~sZr35

NisoZr20a + f.c.c, crystal

NiTZr2 (Ni77.sZr2~..2)

Ni viZr29a + f.c.c, crystal + crystal (?)

Ni78Zre2 NisZr (Nis3.3Zrl6.7)

Ni63Zr37 a + crystal (?) Ni~3Zr37 ~'+ crystal

NiTiiZr30

Ni72.sZr27.5" + crystal (?) NisoZr2oa (20 h) + NisZr NislZrl9 ~'(90 h) + f.c.c, crystal

NisZr crystal + crystal (?)

NissZrl5

Ni~t 5Zr185" + crystal (?)

NigoZrl o

Nis45Zrls 5" + crystal (?)

"Amorphous phases.

powder mixture for the various ball-milling conditions. For an initial composition of Ni24Zr76 and the P5/2(5) conditions, two amorphous haloes were deconvoluted by the ABFfit program. In only one case is a single amorphous phase obtained as the

end-product material; this corresponds to an initial mean composition of Niss.sZr41.2 and to the P7/2 ball-milling conditions. For the other initial compositions and/or ball-milling conditions, the amorphous phase halo is detected with the presence of some crystalline peaks.

194 4. Discussion

4.1. Stationary dynamic equilibrium between amorphous and crystalline phases From the experimental X-ray patterns and their deconvolution by the ABFfit program, it is obvious that the end-product state of the ballmilled powder which is reached in our ballmilling conditions is a mixture of crystalline and amorphous phases. The nature of the two phases depends on the ball-milling conditions and on the initial mean composition of the powder. A complementary experiment was performed in order to determine whether the equilibrium which is observed after a long ball-milling duration is stationary. Starting from an initial composition of Ni65Zr3s, the ball-milling process was performed using the P7/2 conditions. After ball-milling for 40 h, a small amount of the powder was subjected to further X-ray investigations. The phases detected are as follows: an amorphous phase with the composition Nis0.3+o.5Zr19.7+0.5; an f.c.c, crystalline phase with a lattice parameter a of (4.57 _+0.1) × 10- l nm. After 60 h, the amorphous phase composition is Nis0.7+0.5Zr19.3_+0.5 and an f.c.c, crystalline phase is always detected with a lattice parameter a equal to (4.59 + 0.1) × 10-1 rim. Therefore, after ballmilling for 40 h, the nature of the various phases is always the same, i.e. a stationary state has been reached. Thus we can refer to stationary equilibrium induced by ball-milling for the Process duration longer than 40 h (at least).

4.1.1. Influence of the initial state of the powder mixture The end product does not depend on the initial nature of the powder mixture, i.e. whether it is a pre-alloyed powder or a mixture of melt-spun ribbons plus nickel particles. Indeed, in Fig. 5, the general features of the three curves (related to the three ball-milling conditions) are not affected by the initial type of powder mixture.

4.1.2. Influence of the oxygen atmosphere content No noticeable influence of the nature of the atmosphere is found. This result, which is different from some previous results, may be explained by the fact that the ball-milling process occurs in a confined atmosphere. Therefore the oxygen content (the ratio of oxygen to zirconium which is

less than 1%) is constant during the whole process.

4.1.3. Influence of the injected power on the amorphous composition range The composition of such a dynamic equilibrium is a function of the injected power. For an initial composition with more than 83.3 at.% Ni and P5/2(5) ball-milling conditions, an amorphous phase is no longer observed; only crystalline phases are detected. However, for higher energy ball-milling conditions, the nickel concentration is extended up to 90 at.%.

4.2. Crystalline phases One point is interesting to note. Before the ball-milling process, the pieces of material which were obtained from an ingot produced by levitation contained several crystalline phases. The first step (up to several hours) of ball milling leads to homogenization of the particles around the initial mean composition and to formation of the corresponding crystalline phase which is expected from the equilibrium phase diagram. It should be noted that an anneal of 48 h at 500 °C led to an increase in the grain size but not to the total disappearance of the initial crystalline phases for the Nis3.3Zr16.7 initial mean composition. Table 7 indicates that, after ball milling for 20 h, the only crystalline phase which was detected is the NisZr intermetallic compound, i.e. the crystalline compound which corresponds to the initial composition of the powder mixture. Furthermore, the composition of the amorphous phase which is in metastable equilibrium with this crystalline phase is very close to that of the intermetallic compound. This observation supports the idea of a polymorphous transition between the two phases.

4.3. Amorphous phases Let us now consider the homogeneity of the different amorphous phases in terms of the composition. Indeed, as revealed by our preliminary experiments on some amorphous ribbons obtained by melt spinning, the FWHH of the first amorphous haloes corresponding to these amorphous phases is very close to 7o-8 °. The analysis of the FWHH corresponding to the first amorphous halo for the ball-milled powder is not so close to these values (Table 8). Furthermore, for the initial composition corresponding to NiTZr2 and for the P7/2 ball-milling conditions, the FWHH is very large, in comparison with the

195 TABLE 8 Full width at half-height of the first amorphous halo as a function of the ball milling condition and the initial composition of the powder mixture (obtained from the ABFfit program deconvolution)

P7/2

P5/2(10)

P5/2(5)

Composition

FWHtt

Composition

FWHH

Composition

FWHH

NiZr 2 NiZr Ni 1iZr~ Nil0Zr 7 NiTZr 2 NisZr NisZr

1 1.1 8.3 11.2 9.7 19.5 11.8 (20 h) 1 1.0 (90 h)

NiZr 2 NiZr Ni.~Zr 7 NiToZr30 Ni78Zr2~ NiasZrl5 NigoZrl!~

7.1 1 1.5 10.0 13.6 10.7 10.0 9.4

Ni24Zr76 NiZr~ NiZr Nil0Zr 7 Ni65Zr35

(7.8-8.2) 9.0 1 1.8 1 1.6 12.5

other initial compositions and the various ballmilling conditions (up to 19 ° in 20 (2Co Ka); see Table 8). We propose a hypothesis for such observations, i.e. the composition of an amorphous phase is not strict but has to be interpreted in terms of a range of compositions which is centred around a given composition. The latter corresponds to the position of the first amorphous halo. Such a result is supported by previous transmission electron microscopy (TEM)-energydispersive X-ray microanalyses [6] which reveal a composition range for the amorphous areas. More precisely, the amorphous local chemical compositions are clustered in well-defined fields which do sometimes correspond to the composition of some of the intermetallic compounds of the equilibrium Ni-Zr phase diagram: Nil lZr9, N i l 0 Z r 7 and NisZr 2. It is clear that, when the contribution from overlap of the crystalline peaks is taken into account, deconvolution is not able to distinguish between the two amorphous phases corresponding to the amorphous compositions N i 11Zr 9 and N i l 0 Z r 7. In this case, the results from the ABFfit deconvolution would be a single amorphous halo which is located between the two values (taking into account the relative phase proportions) with a large FWHH. The latter includes the range of compositions. It is interesting to note that the intermetallic compounds which correspond to the abovementioned compositions, have been observed by high resolution electron microscopy [6]. This observation strongly indicates that in the present work a polymorphous reversible transition occurs during the ball-milling process between the amorphous and crystalline phases as just mentioned. Therefore, taking into account the FWHH of the amorphous halo, we prefer to speak of an

amorphous composition range which may in some cases be relatively large. This result could explain the different results obtained in the published studies on the thermal stability of this type of ball-milled powder. Indeed, there will be an activation energy spectrum corresponding to the composition range of the amorphous phase which in terms of Kissinger's [40] analysis should not be properly interpreted as crystallization of a single homogeneous amorphous phase. Another point should be noted. Ball milling amorphous melt-spun ribbons with a nickel powder is one way of increasing the nickel content of the amorphous phase. This method allows us to reach some specific compositions which are difficult to obtain directly from melt spinning, i.e. line compounds and/or intermetallic compounds with a high point of fusion.

4.4. Mechanism of the phase transition induced by ball milling Results previously published by Eckert et al. [41] have revealed the influence of the choice of kinetic energy. In ref. 40, the ball-milling settings which were chosen are P5/2(3), P5/2(5) and P5/ 2(7). Nevertheless, Eckert et aL reported formation of a homogeneous amorphous alloy, starting from an initial composition close to the middle of the phase diagram. For an initial composition with less than 35 at.% Ni or more than 70 at.% Ni, they noticed an equilibrium between zirconium and an amorphous phase or nickel and an amorphous phase. This result was explained by analogy to classical solid state diffusion. They noticed that, for the higher energy ball-milling condition, some intermetallic peaks appear. Nevertheless, they interpreted the state of their ball-milled powder as transient and suggested that the end

196

product (a homogeneous amorphous alloy) was not reached after 60 h. Their explanation for the presence of some intermetallic compound is a significant increase in the temperature for the high energy ball-milling, conditions leading to recrystallization of the amorphous phases obtained in situ. This explanation is based on an analogy to the classical solid state diffusion. From our point of view, according to such a development, once the crystalline intermetallic compound has been formed, this should stop formation of the amorphous phase and even lead to destabilization of the latter. This is not the case in our experiments. The observed amorphous phase, which is obtained after ball milling an initial composition NisZr for 20 h, is still present after ball milling for longer times even if there is a change in the nature of the crystalline phase which is in equilibrium with the former. Thus the analogy to the classical solid state amorphization is not appropriate, at least in our ball-milling conditions. Furthermore, in classical amorphization by solid state diffusion, the amorphous composition range is limited to between the common tangent points of the free-energy curves corresponding to the amorphous solid solution (considered as an undercooled liquid at the working temperature) and to the elemental components. The solid state diffusion process starting from an initial composition chosen between the two values corresponding to the tangent points leads to a homogeneous amorphous phase with a composition equal to the initial mean composition at the end of the process. For a composition with less nickel than that corresponding to the first tangent point, a metastable equilibrium is reached between the elemental component and an amorphous phase whose composition is always equal to the first tangent point. This composition remains Unchanged for all the initial compositions below the tangent point. In our ball-milling conditions, this is not the case. For the P5/2(10) ball-milling conditions, a type of plateau, located at a value of about 65 at.%Ni, is observed for initial compositions of the powder mixture which are close to the middle of the phase diagram (50-75 at.%Ni). However, on the edge (in composition) of the phase diagram, the composition of the obtained amorphous phases evolves as a function of the initial composition (and of the ball-milling conditions). This result is in contradiction to what is observed

during the amorphization process by solid state diffusion. Taking into account such remarks, we prefer to talk about the notion of dynamic equilibrium between the crystalline phase(s) and/or the amorphous phase(s) induced by ball milling. Previous observations on a microscale, based on TEM investigations [6], have also revealed the presence of such a mixture. This means that both phases (crystalline and amorphous) are in contact during the ball-milling process, leading to reinforcement of the notion of "local equilibrium".

5. Conclusion In the present paper the influence of the ballmilling conditions has been studied. The nature of the phases (amorphous phase(s) and crystalline compounds) which are in stationary dynamic equilibrium induced by the ball-milling process depends on the injected power of the balls but not on the initial state of the powder. Such a stationary dynamic equilibrium tends to prove that the mechanism of such a phenomenon is different from that at the origin of amorphization by solid state diffusion.

Acknowledgments We gratefully acknowledge Dr. J. Bigot, G. Dezellus and S. Peynot (Centre d'Etudes de Chimie M6tallurgique (CNRS), France) for their help in providing us with the various melt-spun ribbons. We wish to thank Dr. A. Quivy and J. L. Pastol (Centre d'Etudes de Chimie M6tallurgique (CNRS), France) for their technical assistance in obtaining the X-ray patterns and quantitative microanalyses respectively. Dr. N. Merk (now at the National Center for Electron Microscopy, Berkeley, CA, U.S.A.) is acknowledged for her help in the determination of the Vegard-type law corresponding to the amorphous Ni-Zr alloys.

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