Crystallization kinetics of Fe79B13Si8 metallic glass

Volume

4, number

MATERIALS

4

CRYSTALLIZATION H. MIRANDA, Departamento Received

KINETICS OF Fe,9B,,Si,

C. CONDE,

A. CONDE

LETTERS

METALLIC

GLASS

and R. MARQUEZ

de Fisica del Estado Sblido, Facultad de Flsica, hive&dad

10 January

June 1986

de Sevilla, Seville, Spain

1986

Crystallization of 26058-2 metglass kinetics was derived from isothermal

occurs in two stages as revealed by exothermic peaks at 825 and 840 K. Crystallization DSC runs and the activation energies were determined from an Arrhenius fit. Values

found are 460 and 390 kJ/mol.

1. Introduction

2. Experimental

Crystallization behaviour of metallic glasses is a subject of rapid growth in interest in the last years. The kinetics of the thermal evolution and the crystallization depend on composition, sample homogeneity, concentration of nucleation sites, the existence of and possible separation of amorphous phases, ... . An understanding of the processes involved is important from the practical standpoint (stability) as well as the scientific interest and also in the search for favourable crystalline microstructures produced by the controlled up-quenching of glasses. Fe-based metal glasses have been investigated extensively in recent years and the crystallization processes associated with each of the exothermic peaks showing up in DSC (differential scanning calorimetry) experiments have been identified [ 1,2]. Studies of isothermal kinetics of the processes separately are usually difficult to be carried out if the characteristic temperatures are close because of overlap of the peaks

Metglas ribbons with nominal composition Fe,,B,,Si, (26053-2) were supplied by Allied Chemicals Co. Both types of experiments, continuous heating and isothermal annealing, were carried out in a Perkin-Elmer DSC-IIC calorimeter. Heating rates of 2.5,5,10,20,40, and 80 K/min were used in continuous heating experiments and the starting temperature was 625 K in all the runs. The specimens were heated up to that temperature at maximum available rate from room temperature to prevent unwanted annealing. In isothermal scans the specimens were also heated up to the annealing temperatures at maximum available rate. The fraction transformed at any time t was determined as the ratio A (t)/A , where A (t) and A are respectively the areas under the isothermal exotherm up to time t and the total area under the exotherm. The areas were measured with an image analyzer and the time origin was taken for the first exotherm at the point where the plot deviates from the base line. For the second exotherm the time origin was taken at the initial point of the calculated plot from a mathematical separation of the two peaks.

131. The results of this study indicate that Metglas 26058-2 crystallized in two subsequent stages and the two exothermic crystallization peaks are separated by 15 K at a heating rate of 20 K/mm. From isothermal anneals at various temperatures an approach to isothermal kinetics is made and its validity should be judged in the light of the resulting kinetic data.

3. Results Fig. la represents the DSC dynamic scan obtained by continuous heating at a heating rate of 20 K/min

226

0 167-577x/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

June 1986

MATERIALS LETTERS

Volume 4, number 4

+ a

650

+790

K

0795

K

0 800

K

750

A 805

K

T(K)--

~010

K

al

t (min) a)

I;

;

;I

;

1;

1;

t (min) b)

Fig. 1. (a) Dynamic DSC curve at 20 annealingrecord at 810 K.

K/m& (b) Isothermal

. 795 K 0 800 K

showing the two exothermic crystallization peaks at 825 and 840 K respectively. The areas under the peaks yield the enthalpies of the transformation once instrumental calibration factors have been determined and the resultingvalueswere 3.4 and 5.3 kJ/mol respectively. The activation energies of the crystallization processes were determined by the Kissinger’s peak shift method [4] and were found to be 448 kJfmo1 for the first and 378 kJ/mol for the second crystallization stage., Fig. I b shows an isothermal exotherm corresponding to an annealing temperature of 810 K, and the sigmoidal pattern typical of Johnson-Mehl-Avrami (JMA) transformations for the two crystallization events and different temperatures are shown in fig. 2. The kinetics of the transformation was analyzed in terms of the JMA equation:

Fig. 2, Sigmoidal curves for the first (a) and the second (b) crystallization stage.

x(t) = 1 - exp(- bt”) ,

n=a+bp,

where x(t) is the volume fraction transformed at time t, n is the Avrami exponent which varies from n = 4 for a constant nucleation rate to n = 3 for a zero nucleation rate 151.If a In-ln plot of -ln(l -x) against t yields a straight line one can assume that the transformation is not inconsistent with the JMA theory. Fig. 3 shows that the experimental data from

where a accounts for the nucleation rate and varies from zero (for quenched-in nuclei) to 1 (for a constant nucleation rate), b defines the dimensionality of the growth (b = I,2 or 3), p has the value I for interfacial control of growth (assumed linear with the time), and p = 0.5 for diffusion controlled growth [6]. Thus, for the first stage a value of n = 1 + 3/2 = 2.5

4805

10

5

K

15

t (min)

bl

the two successive processes fit to a single straight line from which n and b can be determined. The values of the Avrami exponent for different temperatures are indicated in table 1.The results can be examined in terms of the approximate model:

227

MATERIALS LETTERS

Volume 4, number 4

June 1986

2

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0

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Fig. 4. Arrhenius plot (3 = 80%) for determination of activation energies: (a) first and (b) second crystallization stage.

a)

b

2

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tn t b) Fig. 3. Plots of the JMA equation fit for the different isothermal runs: first (a) and second (b) crystallization events.

should account for a precipitation governed by homogeneous nucleation and a diffusion controlled (parabolic law) growth. For the second event the exponent

value n = 3 agrees with values obtained for processes involving interface controlled growth [7] as is usual in polymorphic crystallization of metallic glasses. Fig. 4 shows plots of l/Tversus In t&80), where t,(SO) is the time corresponding to an 80% transformed fraction from the sigmoidal curves at the various temperatures T.Reasonable straight line fits (I-> 0.995) seem to justify the Arrhenius relation, and the slopes of the lines were then used to calculate the activation energies, now related to the isothermal processes. The values found were 460 and 390 kJ/mol (4.8 and 4.1 eV/at) respectively, in very good agreement with the values obtained by Kissinger’s method. Further studies including transmission electron microscopy are in progress, aiming at a structural characterization of crystallization events and to complete their kinetic description.

References Table 1 Values of Avrami exponents at various annealing temperatures

228

T(K)

Primary crystallization

Polymorphic crystallization

185 790 795 800 805 810

2.73 3.03 2.65 2.52 2.67 2.96

2.98 3.03 2.96 2.96 3.21 3.17

[l] H.S. Chen, Rept. Progr. Phys. 43 (1980) 380. [2] U. Koster and U. Herold, in: Topics in applied physics, Vol. 46 eds. H.J. Guntherodt and H. Beck (Springer, Berlin, 1981). [3] E.G. Baburaj, G.K. Dey, M.J. Patni and R. Krishnan, Scripta Met. 19 (1985) 305. [4] H.E. Kissinger, Anal. Chem. 29 (1957) 1702. [5] J.N. Christian, The theory of transformation in metals and alloys, 2nd Ed. (Pergamon Press, London, 1975). [6] V.R.V. Ramanan and G. Fish, J. Appl. Phys. 53 (1982) 2273. [7] B.G. Bagley and E.M. Vogel, J. NonCryst. Solids 18 (1975) 29.