Crystallization kinetics of mechanically alloyed Al₈₀Fe₂₀ amorphous powder

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Journal of Science & Technology 119 (2017) 066-070

Crystallization Kinetics of Mechanically Alloyed Al₈₀Fe₂₀

Amorphous Powder

Nguyen Thi Hoang Oanh, Tran Quoc Lap, Pham Ngoc Dieu Quynh, Le Hong Thang,

Nguyen Thi Anh Nguyet, Pham Ngoc Huyen, Nguyen Hoang Viet^*

Hanoi University of Science and Technology – No. 1, Dai Co Viet Str., Hai Ba Trung, Ha Noi, Viet Nam

Received: June 15, 2016; accepted: June 9, 2017

Abstract

The crystallization kinetics of an Al₈₀Fe₂₀amorphous powder alloy were investigated by thermal analysis.

Crystallization of amorphous Al₈₀Fe₂₀during continuous heating undergoes four stages. The first-stage

crystallization leads to the formation of fcc-Al from amorphous matrix. The next stages are the

decomposition of the residual amorphous phase into several intermetallic compounds. The activation

energies of the alloy were calculated from differential scanning calorimetry data using the Kissinger, Ozawa

and Augis–Bennett models. The non-isothermal crystallization kinetics are analyzed by Johnson-Mehl-

Avrami equation. The value of the Avrami index indicated that the crystallization is interface - controlled

growth.

Keywords: amorphous alloys, mechanical alloying, crystallization kinetics, Avrami exponent

1. Introduction^*

Al-rich metallic glasses have generated

DSC has also led to the study of the crystallization

kinetics by so-called non-isothermal methods.

Several reports on the successful formation of

an amorphous phase through MA have been

published for Al₈₀Fe₂₀amorphous alloy [2, 8-10]. But

there is a lack of studies regarding the crystallization

kinetics of Al₈₀Fe₂₀amorphous alloy.

In this study, the thermal stability as well as the

crystallization kinetics of the mechanically alloyed

Al₈₀Fe₂₀amorphous powder has been investigated

using DSC in non-isothermal modes. The value of the

Avrami index is calculated by Johnson-Mehl-Avrami

equation to determine crystallization mechanism of

Al₈₀Fe₂₀amorphous powder.

considerable research interest because of the excellent

mechanical and chemical properties. Tensile strength

of Al-based amorphous alloys is 2-5 times higher

than their conventional crystalline counterparts [1-3].

Their high tensile strength can be further enhanced if

fcc-Al nano-particles are homogeneously dispersed

within a certain size and fraction range through

primary crystallization [4, 5]. One of the critical

aspects of their applications is thermal stability, as the

amorphous state is a non-equilibrium phase which

irreversibly crystallizes upon heating. The

crystallization kinetics are very important for the

development of amorphous alloys and nanocrystalline

materials, the properties of which are strongly

affected by the crystallization process. Therefore, the

crystallization kinetics of amorphous alloys have

2. Experimental

Al₈₀Fe₂₀amorphous alloy powder was prepared

via mechanical alloying process after 60h of milling

(more details in [11]). The structure of the as-

received samples was confirmed by XRD

measurements using RIGAKU RINT-2000 with

CuKα (λ=1.5405Å) radiation. Morphology of the

amorphous powder samples was observed by a field

emission scanning electron microscope (FE-SEM).

The crystallization kinetic of the powders was

evaluated by non-isothermal DSC under a continuous

flow of Ar gas (70 mL/min) at heating rates of 5, 10,

20 and 40 K/min using NETZSCH STA 409C, where

platinum cups were used as containers.

been

studied

extensively.

Controlling

the

microstructure development from the glassy

precursors requires detailed understanding of the

specific

mechanisms

influencing

structural

transformations. Moreover, crystallization studies are

essential for the proper choice of the consolidation

parameters in order to maximize densification and, at

the same time, retaining the desired microstructure [6,

7].

Differential scanning calorimetry (DSC)

technique allows a rapid and precise determination of

crystallization temperatures of amorphous materials.

3. Results and disscution

Fig. 1 shows the XRD pattern of Al₈₀Fe₂₀

powder mixture presented

structure after 60 hours of milling.

fully amorphous

^*Corresponding author: Tel.: (+84) 904.777.570

Email: viet.nguyenhoang@hust.edu.vn

Journal of Science & Technology 119 (2017) 066-070

crystallization peaks shift to higher temperatures. The

peak temperature (T_p) values at different heating rates

are summarized in Table 1.

Table 1. Characteristic temperature at crystallization

peaks of Al₈₀Fe₂₀powder at different heating rates

Heating rate,

T_p1,

°C

360.9

366.1

371.7

373.6

T_p2,

°C

412.0

424.0

438.2

445.9

T_p3,

°C

486.0 576,.9

496.5

506.8

512.5

T_p4,

°C

K/min

587.6

596.4

601.3

Fig. 1. X-ray diffraction patterns of Al₈₀Fe₂₀

amorphous powder.

Similar observation for the temperature peak for

the first crystallization peak of those amorphous

samples were made by F. Zhou [8] with T_p1about 400

^oC. These amorphous alloys have crystallization

temperature range from 300 C to 640 ^oC by F. Zhou

and from 350 ^oC to 630 ^oC in this study.

The activation energy of the crystallization

process gives important information regarding the

thermal stability of the sample. It can be evaluated

from constant-rate heating DSC curves taken at

different heating rates using the Kissinger Ozawa and

Augis-Bennett equations, as given by equation (1),

(2), (3), respectively: [12]

5µm

Fig. 2. FE-SEM image of Al₈₀Fe₂₀amorphous

powder after 60h of milling.





E_a



T_p

 

 const

(1)

(2)





RT_p





E_a

ln()  

 const

RT_p





E_a



 

 const

(3)





T_pT_o

RT_p





where β is the heating rate, T_pis the temperature

at the exothermal peak, R is the gas constant and E_ais

the activation energy of crystallization. Figure 4-6

show that Kissinger plot ln(β/T_p) versus 1000/T_p,

Ozawa plot ln(β) versus 1000/T_p, Augis-Bennett plot

ln(β/T_p-T_o) versus 1000/T_p, which yields straight lines

with a good fit, respectively. Table 2 presents results

of the activation energy calculated through three

methods.

Fig. 3. DSC curves of Al₈₀Fe₂₀amorphous powder at

various heating rates.

Fig. 2 illustrates the SEM/EDS observation for

as-received Al₈₀Fe₂₀amorphous powder. It can be

seen that fine powder particles, the particle size

mostly below 15 µm, were agglomerated to form

larger particles

Table 2. Activation energy (E_a[kJ/mol]) of Al₈₀Fe₂₀

amorphous powder for the crystallization stages

determined via three methods

Fig. 3 presents the DSC diagram for the Al₈₀Fe₂₀

amorphous powder as a function of temperature taken

at different heating rates. As can be seen, this powder

has four crystallization peaks, which means that

powder undergoes four crystallization stages.

Moreover, increasing the heating rate from 5 to 40

^oC/min caused all position of the exothermic

Active Energy, kJ/mol

Methods

Peak 1

510.1

520.7

515.4

Peak 2

230.2

241.8

236.0

Peak 3

362.6

375.4

369.0

Peak 4

493.2

507.6

500.4

Kissinger

Ozawa

Augis-Bennett

Journal of Science & Technology 119 (2017) 066-070

crystalline grains during the phase transition, which

can be obtained by Johnson-Mehl-Avrami (JMA)

equation: [12]

x(t) 1 e^k

(4)

where x is the crystallization volume fraction at

time t, n is the Avrami exponent and k is the reaction

rate constant related to absolute temperature

described by Arrhenius equation:

E_a

k  k_oe^

(5)

Fig. 4. Kissinger plots of the Al₈₀Fe₂₀amorphous

powder.

where

is a constant,

is the activation

energy, R is the gas constant and T is the absolute

temperature.

There are 2 methods to determine the Avrami

parameter. The first method was proposed by Ozawa.

We have:

d ln(ln(1 x))

 n

d ln 

(6)

The value of x at any selected T is calculated

from the ratio of the partial area of the crystallization

peak at the selected temperature T to the total area of

the exothermic peak. Fig. 7 shows diagram of

crystallized volume fraction for Al₈₀Fe₂₀amorphous

powder.

Fig. 5. Ozawa plots of the Al₈₀Fe₂₀amorphous

powder.

Fig. 7. Crystallized volume fraction x for Al₈₀Fe₂₀

powder at different heating rates.

Fig. 6. Augis-Bennett plots of the Al₈₀Fe₂₀

amorphous powder.

Combining equation (6) and plot (7), at any

fixed temperature, we can consider the Avrami

parameter to be 0.91 in the first crystallization event.

It can be seen, the values of the activation

energies calculated from three models are

approximate. Therefore, we can use one of the three

methods to calculate the activation energy.

The second method to calculate Avrami

parameter is through the activation energy calculated

by Kissinger method, as following

The Avrami index (n) gives detailed information

on the nucleation and growth mechanism of new

Journal of Science & Technology 119 (2017) 066-070

amorphous alloy were made by F. Zhou et al. [8],

Rln(ln(1 x))

n(x)  

(7)

and M. Krasnowski [2].





E_x









The crystallized volume fraction is also

determined by measuring the corresponding partial

area of the exothermic peak. Plotting ln[-ln(1-x)]

versus ln(1/T) with x between the range of 15% to

85% of transformed fractions, the JMA plots at

different heating rates are obtained as in Fig. 8.

Fig. 9. XRD patterns from amorphous Al₈₀Fe₂₀alloy

after heat treatment at temperatures at (a) 413, (b)

468, (c) 535 and (d) 670 °C.

4. Conclusion

Crystallization kinetics of mechanically alloyed

Al₈₀Fe₂₀amorphous powder have been investigated

using DSC in non-isothermal modes. The

crystallization behavior of amorphous powder occurs

in four stages in the temperature range of 350 and 630

^oC. The primary phase of fcc Al together with

maintaining amorphous phase in the first

crystallization event followed by formation of

Al₁₃Fe₄, Al₃Fe and Al₆Fe intermetallic phases in the

second crystallization event. At the higher

crystallization temperature in the third crystallization

stage, intermetallic phases of Al₁₃Fe₄and Al₆Fe

occurred. In the final exothermic event, phases of fcc-

Al, Al₁₃Fe₄and AlFe₃can be realized. The values of

activation energy calculated from three methods

Kissinger, Ozawa and Augis-Bennett are almost

same. The Avrami exponent is less than 1 for the first

Fig. 8. JMA plots for 1st crystallization peaks of

Al₈₀Fe₂₀amorphous alloys at different heating rates.

The Avrami index was obtained by the slopes of

these plots. The Avrami index (n) is 0.80 in the first

crystallization process. According to calculated

Avrami index calculated by 2 methods is approximate

to 1. The Avrami index usually between 1 and 4 if the

growth of the crystal is diffusion controlled. With n

less than 1, the crystal growth has been shown to be

interface controlled [13]. A low value of n has also

been reported by other investigators in the primary

crystallization of amorphous alloys. This value

suggesting that the transformation in this stage is

interface-controlled growth [14].

In order to determine the products of

crystallization, milled powders were annealed in the

DSC by heating at 20 °C/min to temperature in the

range of 413 and 670 °C, coressponding to the end

temperatures of four crystallization reactions. Fig. 9

shows XRD spectra from the amorphous Al₈₀Fe₂₀

alloy after heat treatment at different temperatures.

After heating to 413 °C, the amorphous alloy began

to crystallize into fcc-Al phase and remain

amorphous phase. After increase heating temperature

to 468 °C intermetallic phases of Al₁₃Fe₄, Al₃Fe and

Al₆Fe can be detected from XRD pattern in Fig. 8 (b).

At higher temperature of 535 °C cleary diffraction

peaks of Al₁₃Fe₄and Al₆Fe phases can be seen Fig. 8

(c). At the final heating temperature of 670 °C, no

amorphous phase can be retained, phases of fcc-Al

and Al₁₃Fe₄can be obtained. Similar observation

regarding products of structural changes for the

crystallization

peak,

suggesting

that

the

transformation was interface - controlled growth.

Acknowledgments

This research is funded by Vietnam National

Foundation for Science and Technology

Development (NAFOSTED) under grant number

103.02-2012.19.

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