Crystallization Kinetics of Al-Fe and Al-FeY Amorphous Alloys Produced by Mechanical Milling

1School of Materials Science and Engineering, Hanoi University of Science and Technology, Hanoi, Vietnam 2Lavrentyev Institute of Hydrodynamics SB RAS, Lavrentyev Ave. 15, Novosibirsk 630090, Russia 3Novosibirsk State Technical University, K. Marx Ave. 20, Novosibirsk 630073, Russia 4School of Engineering Physics, Hanoi University of Science and Technology, Hanoi, Vietnam 5School ofMaterials Science and Engineering, University of Ulsan, San-29,Mugeo-2 Dong, Nam-Gu, Ulsan 680-749, Republic of Korea


Introduction
Amorphous aluminum-based alloys containing ≥80 at.%Al constitute a group of materials promising for structural applications.The main advantages of Al-based metallic glasses are high strength, low density, high elastic strain, and good corrosion resistance [1,2].At the same time, irons aluminides have attracted interest due to high specific strength and excellent corrosion resistance at elevated temperatures under oxidizing, carburizing, and sulfurizing environments [3].Although metallic glasses possess unique properties, they are metastable and tend to crystallize during continuous heating.In order to produce bulk materials retaining an amorphous structure or bulk nanostructured composites containing nanometer-sized crystals dispersed in amorphous matrices, it is necessary to determine the stability of metallic glasses during thermal treatment and evolution of their structure upon crystallization [4][5][6].The driving force for crystallization of amorphous materials is the Gibbs free energy difference between the amorphous and the crystalline states.The crystallization mechanisms and the structure and composition of the crystallization products depend on the initial chemical composition of the amorphous phase and its preparation method.Studies of the crystallization behavior of amorphous alloys are necessary for evaluating their thermal stability against crystallization, determining the structure evolution paths of the amorphous alloy powders upon consolidation and for elucidating the fundamentals of the processes of the glass formation, nucleation, and growth.In the present work, the kinetics of crystallization of the mechanically alloyed amorphous Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 alloy powders was studied using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM).

Experimental
Amorphous alloy powders with nominal compositions of Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 were synthesized via mechanical alloying [7].Mixtures of Al, Fe, and Y were milled in a planetary ball mill (P100, South Korea) using hardened steel vials and hardened steel balls of 5 mm diameter.The rotation speed of the disc was 350 rpm and the ball to powder weight ratio was 20 : 1. Hexane was used as a process control agent to prevent extensive agglomeration and contamination during mechanical milling.DSC was conducted using NET-ZSCH STA 409C at heating rates of 5, 10, 20, and 40 K/min to study the crystallization kinetics of the amorphous alloys.XRD was performed using RIGAKU RINT-2000 diffractometer with CuK  radiation ( = 0.15405 nm) to study the phase transformations during mechanical milling and upon subsequent heating.The microstructure of the powders was studied by TEM carried out using a JEOL JEM-2100 microscope.

Results and Discussion
The XRD analysis of the powders shows that the amorphous Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 alloys formed after 100 h of milling.The XRD patterns of the powders (Figure 1(a)) exhibit only broad maxima between 35 ∘ and 50 ∘ (2) indicating that amorphization took place during milling [7].In order to study the structural evolution during heating, the amorphous alloy powders were annealed in the DSC using continuous heating at 20 K/min up to different temperatures to reach different crystallization events revealed by exothermic peaks and then cooled down to room temperature at 50 K/min.The DSC scans in Figure 2 show the first crystallization peaks of the three alloys recorded at 5, 10, 20, and 40 K/min.Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 alloys that experienced the first crystallization event along with selected-area diffraction (SAD) patterns.The contrast observed in the TEM images is due to separation of the alloys into solute-enriched and solute-depleted amorphous regions.The segregation of Al was clearly observed in the TEM images of all three alloys.The -Al nanocrystals then preferentially nucleated at the interface between the separated phases.In the Al 84 Fe 16 alloy, only -Al crystals separated from the amorphous phase and had a size ranging from 10 to 30 nm (Figures 3(a In the areas that underwent crystallization, crystals of the Al 6 Fe intermetallic were found together with crystals of -Al (Figures 4(c)-4(d)).Due to a low concentration of the intermetallic compound, the Al 6 Fe phase could not be detected by the XRD [7].The concentration of the crystalline phases separated from the amorphous phase (Figure 4(a)) in the Al 82 Fe 18 is higher than in the Al 84 Fe 16 alloy.During heating of the Al 82 Fe 16 Y 2 alloy, crystals of the Fe 4 Y intermetallic formed, as can be concluded from the TEM data (Figure 5).
The nucleation and growth of the -Al phase occurred in a manner similar to that observed during crystallization of Al 88 RE 8 Ni 4 amorphous alloys [8].
It is known that the more complex the composition of the crystallites precipitating from an amorphous phase, the higher the thermal stability of the alloys [9].From the DSC analysis of the three alloys, it can be concluded that their thermal stability increases in the following row: Al 84 Fe 16 -Al 82 Fe 18 -Al 82 Fe 16 Y 2 .The thermal stability of amorphous alloys is often interpreted using the heat of mixing Δ mix .The more negative the Δ mix , the larger the atomic constraint force and the higher the thermal stability.Δ mix between Fe and Al (solvent) is −11 kJ/mol, while Δ mix between Y and Al (solvent) is −38 kJ/mol [10].As Δ mix between Y and Al as a solvent is more negative,  1 should increase with increasing Y content in the alloy.
One of the most important kinetic parameters of the crystallization process is the activation energy.The thermal stability of glassy materials is related to the activation energy values.The Kissinger [11] and Ozawa [12] methods are used to calculate the activation energy of the amorphous to crystalline phase transformation in nonisothermal crystallization processes.Kissinger proposed a method for determining the activation energy of a simple decomposition reaction using differential thermal analysis scans recorded at different heating rates [9].The heating rate of a reaction is related to the peak temperature recorded during the thermal analysis.By considering the variation of each peak temperature   with the heating rate , the activation energy  can be calculated by the Kissinger method using the following equation: where  is the activation energy of crystallization,  is the temperature of the exothermic peak,  is the heating rate, and  is the gas constant.Figure 6 presents the plots of ln(/ 2 ) versus (1000/) and straight lines are obtained with −/ slopes for the first exothermic peak.From these data, the activation energy values  for the first crystallization event of the Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 amorphous alloy alloys were calculated to be 496.0,340.8, and 565.0 kJ/mol, respectively.
In order to confirm the results obtained using the Kissinger method, another nonisothermal method was applied to calculate the activation energy, the Ozawa method, which uses the following equation: where , , , and  are the heating rate, peak temperature, activation energy, and gas constant, respectively.Figure 7 shows the plots of ln() against 1000/  , and  was calculated from the slope of the straight lines.The values of  obtained using the Ozawa equation are 506.It is known that the onset crystallization and peak temperatures are associated with the nucleation and growth processes, respectively [13].In the case of the Al 84 Fe 16 alloy, the obtained activation energy for nucleation (626.8 kJ/mol) is higher than the activation energy for growth (496.0kJ/mol), indicating that the nucleation process for the Al 84 Fe 16 alloy is more difficult than the growth process.This can be seen from the TEM image in Figure 3(a): only a few crystals of Al nucleated and their crystallite size was from 10 to 30 nm.However, in the case of the Al 82 Fe 18 and Al 82 Fe 16 Y 2 alloys, the activation energies for nucleation (284.4 and 464.9 kJ/mol) are lower than the activation energies for growth (340.8 and 565.0 kJ/mol), indicating that the nucleation process occurs easier than the growth process.As was confirmed by TEM (Figures 4(a ).An increase in the Fe content in the alloy makes the growth process easier, while partial substitution of Y for Al alloy hinders the growth process.Yttrium has a larger atomic radius (1.8 Å) than Al (1.18 Å) [10].Larger Y atoms increase the potential barrier and hinder diffusion of atoms during the crystallization process of the amorphous alloys.Consequently, the activation energy increases when the substituting atoms have larger radii, implying that, in the Al 82 Fe 16 Y 2 alloy, a higher energy barrier must be overcome for atomic rearrangement and diffusion of atoms in the crystallization process.The values of the apparent activation energies strongly depend on the type of the primary phase.In the Al 84 Fe 16 alloy, the primary phase is -Al; in the Al 82 Fe 18 and Al 82 Fe 16 Y 2 alloys, the primary phases are the -Al and the intermetallic phases.
Generally, the crystallization kinetics of amorphous alloys is studied using the John-Mehl-Avrami equation [14]: where  is the crystallization volume fraction at time ,  is the Avrami exponent, and  is the reaction rate constant related to the absolute temperature described by the Arrhenius equation: where   is a constant,   is the activation energy,  is the gas constant, and  is the absolute temperature.In nonisothermal DSC, the heating rate is controlled, so the temperature can be expressed as where   is the starting temperature.Combining (3) and ( 4), the crystallized volume fraction can be plotted against the temperature using the following equation: where   and   are the initial and final crystallization temperatures of the crystallization peak, respectively.Figure 8 presents the plots of the crystallized volume fraction, , as a function of the temperature at different heating rates.All these curves show S-shape indicating crystallization of the amorphous alloy.
A method to determine the Avrami exponent was proposed by Ozawa [12]: where the crystallized volume fraction, , at any selected temperature  is calculated from (6).
Using equation (7) and values of  at the peak temperature of the first crystallization event from Figure 8, the Avrami exponent can be determined.For the Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 alloys, the Avrami exponent was calculated to be 0.88, 0.85, and 0.83, respectively.
An alternative method [14] to obtain the Avrami exponent is through the activation energy calculated by the Kissinger method [11]: Plots of ln[− ln(1 − )] versus ln(1/) for  ranging from 15% to 85% at different heating rates are shown in Figure 9 (the Johnson-Mehl-Avrami (JMA) plots).The Avrami exponent was obtained from the slopes of these plots.The  values for the Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 alloys calculated by using (8) are 0.94, 0.90, and 0.82, respectively.The values of the Avrami exponent  obtained by these two methods are listed in Table 3.
The Avrami exponent  provides information on the nucleation and growth mechanism of new crystalline grains during the phase transition.The values of the Avrami exponent for the Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 alloys are below 1 and close to 1.The Avrami exponent  (originally expected to be  = 1, 2, 3, 4 [14,15]) can also have noninteger fractional values corresponding to physically complex

Figure 1 :
Figure 1: XRD patterns of the amorphous alloy powders produced by mechanical milling (a) and heated in DSC up to a temperature corresponding to completion of the first crystallization event (b).
) and 3(c)).The SAD pattern in Figure3(b) corresponds to an amorphous phase and that in Figure 3(d) to crystalline -Al.The crystallization behavior the Al 82 Fe 18 was slightly different from that of the Al 84 Fe 16 .The Al 82 Fe 18 alloy remained partially amorphous, as can be seen from Figures 4(a)-4(b).

Figure 3 :
Figure 3: TEM images and SAD patterns of the Al 84 Fe 16 alloy after the first crystallization peak: (a) bright-field TEM image of an amorphous area, (b) SAD pattern corresponding to the amorphous area, (c) bright-field TEM image of an area that had undergone crystallization, and (d) SAD pattern corresponding to (c) showing spots belonging to Al.

Figure 4 :
Figure 4: TEM images and SAD patterns of the Al 82 Fe 18 alloy after the first crystallization peak: (a) bright-field TEM image of an amorphous area, (b) SAD pattern corresponding to the amorphous area, (c) bright-field TEM image of an area that had undergone crystallization, and (d) SAD pattern corresponding to (c) showing rings belonging to Al.

Figure 5 :
Figure 5: TEM images and SADP of the Al 82 Fe 16 Y 2 alloy after the first crystallization peak: (a) bright-field TEM image of an amorphous area, (b) SAD pattern corresponding to the amorphous area, (c) bright-field TEM image of an area that had undergone crystallization, and (d) SAD pattern corresponding to (c) showing rings belonging to Al.

Table 1 .
The onset temperatures,  1 , of the first crystallization peak and the peak temperatures,  1 , of the Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 alloys are presented in It can be seen that a slight change in the composition of the Al-Fe amorphous alloys, from Al 84 Fe 16 to Al 82 Fe 18 , and substitution of Y for Al (2 at.%) result in an increase of both  1 and  1 .Figure 1(b) shows the XRD patterns of the Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 alloys heated in DSC up to a temperature corresponding to completion of the first crystallization event.Figures 3-5 show TEM images of the

Table 1 :
Characteristic  1 and  1 temperatures of the Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 alloys.Heating rate  Al 84 Fe 16 Al 82 Fe 18 Al 82 Fe 16 Y 2 6, 351.9, and 576.1 kJ/mol for the Al 84 Fe 16 , Al 82 Fe 16 , and Al 82 Fe 16 Y 2 amorphous alloy powders, respectively.The activation energies for crystallization of the Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 alloys calculated by the Kissinger and Ozawa methods using the onset crystallization temperature and peak temperature of the first exothermic peak are summarized in Table 2.The values of the activation energies calculated from the Ozawa

Table 2 :
Activation energies for crystallization of the Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 amorphous alloys calculated using the onset crystallization temperature and peak temperature of the first exothermic peak.Fe 16 Al 82 Fe 18 Al 82 Fe 16 Y 2 Al 84 Fe 16 Al 82 Fe 18 Al 82 Fe 16 Y 2 Kissinger plots for evaluating the activation energies related to the first crystallization event of the Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 amorphous alloy powders.Figure 7: Ozawa plots for the evaluating the activation energies related to the first crystallization event of the Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 amorphous alloy powders.

Table 3 :
The value of the Avrami exponent for the Al 84 Fe 16 , Al 82 Fe 18 , and Al 82 Fe 16 Y 2 amorphous alloys.