Mechanical alloying of Fe, Al and TiO
2system to produce (Fe,Ti)
3Al-Al
2O
3nanocomposite
Mahdi Rafiei1*, Hossein Mostaan2
1Advanced Materials Research Center, Department of Materials Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran.
2Faculty of Engineering, Department of Materials and Metallurgical Engineering, Arak University, Arak, Iran.
*Email: [email protected]
Abstarct
In this study, Fe, Al and TiO2 powder mixture with molar ratio of 6:7:3 was mechanically alloyed to produce (Fe,Ti)3Al-Al2O3 nanocomposite powder containing 40 vol.% Al2O3. The structural evolution and morphological changes of powder particles during mechanical alloying were studied by X-ray diffractometery (XRD) and scanning electron microscopy (SEM) analysis. It was found that at early stages of milling, TiO2 gradually reacted by Al and as a result crystalline Ti and amorphous Al2O3 phases formed. Further milling led to the dissolution of Ti and remaining Al into Fe lattice and therefore formation of Fe(Al,Ti) solid solution. This structure transformed to (Fe,Ti)3Al intermetallic compound withdisordered DO3 structure at longer milling times. Heat treatment of this structure at 900º C for 1h led to the crystallization of Al2O3 and ordering of (Fe,Ti)3Al phases. The crystallite size of (Fe,Ti)3Al and Al2O3 phases after heat treatment were about 70 and 55 nm respectively.
Keywords: (Fe,Ti)3Al-Al2O3 Nanocomposite, Nanostructure Materials, Mechanical Alloying.
1. Introduction
Fe3Al based intermetallics indicate attractive properties including, high hardness, high melting point, and relatively low density compared with Fe- and Ni-based alloys and excellent oxidation and corrosion resistance [1]. As a result these compounds are useful for many structural applications including, gas metal filters, heating element, heat treatment fixtures, high temperature dies and molds and cutting tools [2,3].
Addition of third alloying element such as Ti, Cr,… to Fe3Al intermetallic compound can lead to the improvement of mechanical properties including ductility at room temperature and creep resistance at high temperature, chemical stability and tribological behavior by solid solution and/or precipitation hardening as well as grain boundary strengthening [4-6].
Moreover it was shown that the dispersion of a hard second phase material (e.g.
Al2O3) within the matrix can enhance the mechanical properties [7]. Intermetallic
2
reinforced alumina composites exhibit several advantages such as; high strength, good wear resistance and improved fracture toughness [8,9].
Mechanical alloying (MA) is an adaptable method for in-situ fabrication of nanocomposites [10]. This technique has some advantages such as: it is an inexpensive method, performed at room temperature using simple equipment, it can yield relatively large quantities of powders, and most importantly, the mechanically alloyed powder generally consists of nanometer-sized grains, which is expected to improve the room temperature brittleness generally exhibited by most intermetallics [11].
There have been a number of investigations on preparation of titanium aluminides and iron aluminieds matrix nanocomposites by MA. Zhang et al. [12] reported that mechanical milling of Al-TiO2 powder mixture leads to the formation of a range of Ti-based composites including Ti(Al,O)/Al2O3, TixAly(O)/Al2O3 and Ti3Al/TiAl.
Forouzanmehr et al. [13] synthesized the TiAl/Al2O3 nanocomposite by MA of Al and TiO2 powder mixture and reported that TiO2 is gradually reduced by Al during ball milling. Khodaei et al. [14] synthesized Fe3Al-Al2O3 nanocomposite by MA of Al and Fe2O3 powder mixture and reported that prior to the Fe2O3-Al combustion reaction, powder particles attained a nanocrystalline structure which promotes the Fe2O3-Al reaction by providing high diffusivity paths.
In our previous work [15] the formation of (Fe,Ti)3Al intermetallic compound was discussed. In this work the formation mechanism of (Fe,Ti)3Al-Al2O3 nanocomposite by MA starting from Fe, Al and TiO2 powder mixture is reported.
2. Materials and methods
Fe, Al and TiO2 powder mixture with purity of 99.8, 99.5 and 99% respectively were used as starting materials. Fig. 1 shows SEM micrographs of starting materials. As seen the Fe particles had a nearly uniform size of ~ 100 µm. Al particles were irregular in shape with a size distribution of 50-100 µm and TiO2 particles were very small in size with a size distribution of 200-300 nm.
Fig. 1: SEM images of as-received powder particles (a) Fe, (b) Al and (c) TiO2.
reaction (1).
MA was carried out in a high energy planetary ball mill, nominally at room temperature and under Ar atmosphere. The milling media consisted of five 20 mm diameter balls confined in a 120 ml volume vial. The ball and vial materials were hardened chromium steel. Ball to powder weight ratio and rotation speed of vial were 10:1 and 500 rpm respectively. The total powder mass was 18 gr and 0.3 wt.% stearic acid was added as a process control agent (PCA). Samples were taken at selected time intervals and characterized by XRD in a Philips X' PERT MPD diffractometer using filtered Cu Kα radiation (λ= 0.1542 nm). Morphology of powder particles was characterized by SEM in a Philips XL30. The crystallite size of powders was estimated using the Williamson-Hall method [16]:
( )ε θ θ λ
βcos 2A 2sin D
K +
=
Where θ is the Bragg diffraction angle, D the crystallite size, ε the average internal strain, λ the wave length of the radiation used, β the diffraction peak width at half maximum intensity, K the Scherrer constant (0.9) and A is the coefficient which depends on the distribution of strain; it is near to unity for dislocations.
Isothermal annealing was carried out to study the thermal behavior of milled powders. A small amount of powder was sealed and isothermally annealed at 900º C for 1h in a conventional furnace and then cooled in air. The structural transition occurred during annealing were determined by XRD.
3. Results and discussion
3.1. Structural evolution
(Fe,Ti)3Al- 40 vol.% Al2O3 nanocomposite can be synthesized by reaction of Fe, Al and TiO2 powder mixture according to the following reaction:
3 2 2
2 3( , ) 2
3 7
6Fe+ Al+ TiO = Fe Ti Al+ AlO (1)
Fig. 2 shows XRD patterns of Fe, Al and TiO2 powder mixture as-received and after different milling times. XRD results suggest that the reaction (1) takes place in two subsequent stages;
[ ]
2 32 3 2
3
4Al+ TiO = TiFe+ AlO (2)
Al Ti Fe Ti
Al
Fe 3[ ]Fe 3[ ]Fe 3( , )
6 + + = 2 (3)
As can be seen with increasing milling time the intensity of Fe, Al and TiO2
diffraction peaks decreases and their width increases due to the internal strain induced by lattice defects and refinement of crystallite size. It should be noted that (Fe2,Ti)Al can be wrriten as (Fe,Ti)3Al which is a ternary intermetallic compound.
4
Fig. 2: XRD patterns of Fe, Al and TiO2 powder mixture at different milling times.
Fig. 3: Displacement of Fe (110) diffraction peak.
Moreover as seen in Fig. 3 Fe (110) diffraction peak gradually displaced toward lower angles. During milling Al can react with TiO2 according to the reaction (2).
∆Gº298 for reaction (2) has a very high negative value (-498212 J/mol) indicating that reaction (2) can thermodynamically take place at room temperature. However reactions with negative ∆Gº298 do not necessarily occur at room temperature because of their slow kinetic. In this regard MA can enhance kinetics of reactions by creating
30 40 50 60 70 80 90 100
2θ (degree)
Intensity
100h
60h
40h
20h
10h
5h
2h
0h
● ● ●
▼ ●
▼
▼ ▼
■ ■ ■
■
■ ■
■ ■
■
■
■ ■ ■
▼
■■■
■ ♦
♦ ♦
Fe ● TiO2 ■ Al ▼ (Fe,Ti)3Al ♦
43 43.5 44 44.5 45 45.5 46 46.5 47 47.5
20h
10h
5h
2h
cold welding of powder particles and local temperature rise [17].
The displacement of Fe (110) peak toward lower angles (Fig. 3) after 20h of MA suggests that Ti formed by reaction (2) and remaining Al dissolve into Fe lattice according to the reaction (3) and therefore a Fe(Al,Ti) solid solution forms during milling. Increasing milling time to 40h led to the transformation of Fe(Al,Ti) solid solution to (Fe,Ti)3Al intermetallic compound. Similar behavior was observed during MA of Fe, Al and Ti ternary system [15]. It should be noted that the Al2O3 XRD peaks formed by reaction (1) can not be observed on XRD patterns (Fig. 2). In fact in-situ formed Al2O3 particles are expected to have a very small size with an amorphous structure [18,19]. Formation of amorphous Al2O3 phase during ball milling of Al and TiO2 powder mixture has been previously reported [13].
Lack of superlattice diffraction peaks for (Fe,Ti)3Al phase indicates that this phase has a disordered crystal structure. Increasing milling time up to 100h has no significant effect on phase composition, but increased the broadening of XRD peaks.
3.2. Thermodynamics aspects
As mentioned reaction (1) takes place in two subsequent stages. First stage has negative ∆Gº298 and ∆Hº298 values indicating that this reaction can occur at room temperature and is highly exothermic. The adiabatic temperature (Tad) is often used as a criterion for anticipation of modality of reactions [20]. Tad is the highest temperature that the reaction system can reach, provided that all the released heat is used to increase the temperature of system after the initiation of reaction [21]. It has been empirically demonstrated that the reaction will be self-sustaining only if
ad C
T ≥1527 [22].
The value of Tad for reaction (2) was calculated to be about 1532º C [21]. Therefore reaction (2) is expected to occur with a combustion mode during ball milling. The addition of diluents will however decrease the Tad [21]. In this work extra Fe and Al were added to the starting powder mixture to obtain (Fe,Ti)3Al matrix. In this case Tad was calculated using following relation:
∫
∫
∫
∫
∫
+
∆Η
× + +
+
+
=
∆Η
−
ad ad
ad ad
T All
p
m Al s
Al p T Fe
p
T Ti
p O
T Al p
dT C
dT C dT
C
dT C dT
C
933 ) (
) 933 ( 298 )
(
298
298 ) ( 298
298
3
3 3
6
3 2
3 2
α α
(4)
Where in this equation ∆Hº298 is the enthalpy change of reaction at room temperature,
Cp the heat capacity and ∆ΗAlm the enthalpy of fusion of Al. The Tad in presence of extra Fe and Al was calculated to be 787º C indicating that the modality of reaction (2) is gradual (progressive) rather than self propagating combustion.
As seen in Fig. 4 after isothermal annealing (900º C, 1h) of 100h-milled sample the Al2O3 peaks are observed on XRD pattern. The appearance of crystalline Al2O3 peaks
6
can further confirm that the Al2O3 formed during ball milling has an amorphous structure which subsequently transforms to a crystalline structure during annealing.
The crystallite size of (Fe,Ti)3Al and Al2O3 phases after heat treatment calculated using Williamson-Hall method were about 70 and 55 nm respectively.
Fig. 4: XRD patterns of powder mixture (a) after 100h of milling time and (b) after subsequent isothermal annealing at 900ºC for 1h.
3.3. Morphological studies
Morphological changes of powder particles during milling were analyzed by SEM observations. As can be seen in Fig. 5 after 2 and 5h of milling times the powder particles were irregular in shape with non-uniform size distribution.
Fig. 5: Secondary electron SEM images of powder particles after (a) 2h, (b) 5h, (c,d) 60h and (e,f) 100h of milling times.
30 40 50 60 70 80 90 100
2θ (degree)
Intensity
♦
♦
♦
♦
♦ ♦
▼
▼
▼ ▼ ▼ ▼
▼▼ ▼
▼ ▼
▼ ▼ ▼
▼
(Fe,Ti)3Al ♦ Al2O3 ▼
b ▼
a
(Fe,Ti)3Al ♦ Al2O3 ▼
particle size continuously decreased and became more uniform so that after 100h of milling time the mean powder particle size was about 10 µm. As seen in Fig. 5 d, f the larger particles are an agglomaration of many smaller particles.
4. Conclusion
Mechanical alloying of Fe, Al and TiO2 powder mixture led to the formation of (Fe,Ti)3Al-Al2O3 nanocomposite. It was found that the TiO2 oxide gradually reacts with Al during MA. This reaction led to the formation of crystalline Ti and amorphous Al2O3. On further milling Ti and remaining Al dissolved into Fe lattice and formed a Fe(Al,Ti) solid solution which transformed to (Fe,Ti)3Al intermetallic compound with disordered DO3 structure at longer milling times. Annealing of final structure led to the crystallization of amorphous Al2O3 and ordering of (Fe,Ti)3Al matrix.
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