Powder Metallurgy and Metal Ceramics, Vol. 59, Nos. 3-4, July, 2020 (Russian Original Vol. 59, Nos. 3-4, March-April, 2020)
THE EFFECT OF Ti ADDITION ON MICROSTRUCTURE AND MAGNETIC PROPERTIES OF NANOCRYSTALLINE FeAl
40ALLOY POWDERS PREPARED
BY MECHANICAL ALLOYING
Nadia Metidji,1,2,5 Nacer Eddine Bacha,1 Abderrahmane Younes,3 and Djaffar Saidi4
UDC 669.1’669.715+676.017.58 Recent research on nanocrystalline FeAl alloys has shown that these alloys are of high importance due to their promising structural and mechanical properties, particularly magnetic behavior. This work aims at studying the synthesis, structural and magnetic characterization of nanocrystalline FeAl alloy powders, prepared by a mechanical alloying process (MA), as well as the effect of Ti addition on the magnetic properties of a compound. The powder morphology, phase transformation, crystallite size, micro-stress evolution, and magnetic properties were investigated by X-ray diffraction (XRD), scanning electronic microscopy (SEM), and vibrating samples magnetometer (VSM). It has been found that at the final stage of mechanical alloying the bcc-disordered FeAl phase and nanocrystalline Fe(Al, Ti) solid solution occurred for the FeAl40 and FeAl40Ti3 alloys, respectively. The milling time and the addition of titanium affect the powder morphology and decrease the size of the particles. The average crystallites size of 17.2 and 11.2 nm was reached at the end of 30 h of milling, and the lattice strain increased up to 0.3 and 0.21% for the FeAl40 and FeAl40Ti3alloys, respectively. Also, the magnetic properties attributed to microstructural changes were investigated. It has been established that the change in magnetic behavior occurs mainly due to the formation of a supersaturated Fe(Al, Ti) solid solution. Magnetic properties of the samples are highly influenced by the addition of the Ti element into FeAl40alloy, as well. The magnetism of the FeAl40Ti3compound is reported to be higher than that of FeAl40.
Keywords: mechanical alloying, nanocrystalline materials, lattice strain, crystallite size, magnetic behavior.
INTRODUCTION
The nanocrystalline materials have a wide interest in recent years due to their unique structural and mechanical properties, which in particular include: elasticity, crystallite size, hardness, and magnetic behavior [1, 2]. Among enhanced properties of the nanostructured powders is its grain size, which is typically less than 100 nm.
1Laboratory of Surface Treatment & Materials University of Saad Dahleb Blida, Algeria. 2Unité de Développement des Equipements Solaires, UDES/Centre de Développement des Energies Renouvelables, CDER, Bou Ismail, 42415, W. Tipaza, Algeria. 3Research Center in Industrial Technologies (CRTI), P.O.Box 64,Cheraga 16014, Algiers, Algeria. 4Nuclear Research Center, Draria, 16050, Algiers, Algeria.
5To whom correspondence should be addressed; e-mail: nadia_science@yahoo.fr.
Published in Poroshkova Metallurgiya, Vol. 59, Nos. 3–4 (532), pp. 55–68, 2020. Original article submitted June 18, 2019.
DOI 10.1007/s11106-020-00148-3
For the synthesis of fine crystalline grains, different techniques are applied. One of them is mechanical alloying (MA) or high-energy ball milling, which allows the manufacturing of nanostructured materials on an industrial scale. Major process variables involved in MA, such as milling time, milling speed, ball-to-powder weight ratio, and process control agent (PCA), are of high importance for the structure and morphology of the final powders [3].
The possibility of FeAl-based intermetallics fabrication by powder metallurgy (PM), including hot isostatic pressing (HIP) [4], extruding [5], hot forging or pressing [6, 7], and powder injection molding [8], is reported in numerous studies. The FeAl powders were prepared by mechanical alloying (MA) [5–7, 9]. The FeAl alloys are very important for high-temperature applications due to their low density, high strength, good oxidation and corrosion resistance, and high melting temperatures [10, 11].
Iron aluminide is intended for high-temperature applications to replace superalloys in the aerospace and automotive industry [12]. However, its industrial appliance is limited due to high fragility at room temperature and low creep resistance, which also complicates the manufacturing processes. As possible solution we suggest the reduction of the grain size by the MA process [13] and metal elements addition [14–17], which may improve the mechanical and physical properties of the FeAl alloys. Also, our attention is paid to the dependence of magnetic properties on the milling times and the change in the chemical composition of FeAl nanocrystalline materials during mechanical alloying.
Oanha et al. [18] reported that the saturation magnetization of the Al82Fe16Ti2, Al82Fe16Ni2, and Al82Fe16Cu2 alloys milled from 5 to 60 h depend on the decrease of the ferromagnetic phase referred to as bcc-FeAl and AlFe3(D03-ferro) and the increase of paramagnetic phase referred to as bcc-Al(Fe)-para.
The coercivity is strongly affected by the powder morphology, microstructural characteristics, and the level of lattice strain occurring during mechanical milling with increasing milling time, which results in a decrease of grain size and an increase in the internal microstrain. Also, the magnetic properties of nanostructured materials are influenced by the nanocrystalline state, such as saturation magnetization (Ms), and the ferromagnetic/
paramagnetic transition temperature, which decreases considerably [19, 20].
The maximum acquired magnetization value that can be achieved for a sample in an applied magnetic field is saturation magnetization (Ms). It implies that all the magnetic domains are aligned in the direction of the field.
When the applied field returns to zero, a certain magnetization remains. This phenomenon is called remanent magnetization (Mr).
This work aims to characterization of nanocrystalline FeAlTi alloy through the alloying process. Studies were performed with the help of X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). For measuring the magnetism values, we used a vibrating sample magnetometer (VSM). Subsequently, we study structural transformations and magnetic properties of FeAl nanocrystalline alloy in relation to Ti addition and milling time.
EXPERIMENTAL PROCEDURES
Elemental powders of Fe, Al, and Ti of 99.98, 98, and 98% purity, respectively, were weighed and mixed to obtain the desired composition of FeAl40 and Fe40AlTi3 alloys. The chemical composition of the mentioned alloys is shown in Table 1.
TABLE 1. Alloy Chemical Compositions
Samples
Fe Al Ti Fe Al Ti
at.% wt.%
FeAl40 60 40 0 75.63 24.36 0
FeAl40Ti3 57 40 3 72.24 24.49 3.26
Mechanical alloying was carried out in a Vario-Planetary mill (Fritsch P4) at room temperature with the ball-to-powder weight ratio of 10 : 1. The 0.3 wt.% stearic acid was added as a control agent to the blend before placing the sample into the vials to prevent sticking within the milling media. Afterward, the mixture of powders was milled for 2, 10, 20, and 30 h with the rotational speed of the disk = 250 rpm. Every 15 minutes the machine was paused to avoid the increase of temperature during milling. The microstructures of milled powders were characterized by X-ray diffraction method (XRD) and the scanning electron microscopy (SEM). Microstructural parameters were calculated from obtained XRD data with the help of High Score Plus software [21].
XRD analysis is often used to characterize the microstructure of nanocrystalline metals [22, 23], in particular, to determine mean grain size (D), average microstrain value (), and the line broadening at half the maximum intensity (). Mean grain size (D) was calculated according to Scherrer equation included in the software and defined by [23]:
cos D K
FWHM
, (1)
where D is the size of crystallite (in nm); K is a shape factor, which can be smaller or equal to the grain size; is the wavelength of the Cu-K1 radiation; is the Bragg angle; and FWHM is the peak broadening at half the maximum of intensity plotted against the 2 profile.
The average microstrain value () was calculated with tangent formula included in the software and described by [21]:
4 tan
FWHM
. (2)
W
hen employing the vibrating sample magnetometer (VSM) for studying the magnetic properties of the material, the appearance of hysteresis loops in the external magnetic field and temperature increase in a vacuum can be observed. Hysteresis loops show the relationship between the induced magnetic flux density (M) and the magnetizing force (H). VSM was also used to measure the isothermal ascent and the demagnetization curves. The measurement is performed in such a way that the sample is saturated by a high magnetic field (1.6 MA/m) and then progressively demagnetized to zero. The demagnetized sample is exposed to the magnetic field for a short time (H1 = 1.6 kA/m), and after the field is reduced to zero, the remanent magnetization is measured. Afterward, the magnetic field was first increased to a slightly elevated level of (H2 = 2H1), then decreased to zero and, the Jr (H2) magnetism was measured. Thus, measurements are made with the increasing magnetic field H up to the level required for saturation (1.6 MA/m), resulting in obtaining the entire J (H) curve. The Jd (H) curve is measured similarly but the first applied high field is negative to obtain approximately half of the particles and/or domain. The field is then strongly reduced to zero and the same procedure as for the Jr (H) curve is applied up to the maximum magnetic field.RESULTS AND DISCUSSIONS
Morphology and Particle Size Analysis. Figure 1 depicts the morphology of the powder mixture obtained during 2, 20, and 30 h of mechanical milling. The particles received in 2h-milling were of an irregular shape and with an average size of 43 ± 13 m (Fig. 1a). With increasing the time of milling up to 20 h, the mean particle size of the powder decreased to about 30 ± 8 m (Fig. 1b). This indicates a predominant effect of fracturing rather than welding, although the last one still has a place at this time of milling. After 30 h of milling, the shape of the particles became spherical, and the size decreased to 20 ± 5 m. Such a significant reduction in size can be attributed to that large particles tend to fracture under stable conditions (Fig. 1c).
SEM micrographs of the mechanically alloyed powders are shown in Fig. 2. The addition of 3 at.% Ti to the FeAl40 alloy decreases the grain size up to 39 ± 6 m after 2 h of milling treatment (Fig. 2a). Prolonging milling for 20 h led to a further reduction of the particle size to 20 ± 3 m (Fig. 2b). After 30 h of milling time and under intensive fracturing and welding processes induced by MA, most crystallites exhibit flattened or rounded shape, and
a b c Fig. 1. Morphology of the FeAl powder mixture milled for (a) 2, (b) 20, and (c) 30 h
a b c Fig. 2. Morphology of the FeAlTi powder mixture milled for (a) 2, (b) 20, and (c) 30 h
its size decreased to about 9 ± 1 m (Fig. 2c). These results indicate that the kinetics of FeAl mechanical alloying is sensitive to the milling conditions and any changes of it (i.e., type of milling device, ball-to-powder weight ratio, temperature, time or intensity of milling, etc.). Besides, the addition of extra elements can lead to significant differences in the final structure. Zhu and Iwasaki [24] investigated the effect of Fe substitution with Ti in the Fe3Al system during the MA process and reported that mechanical alloying of blended elemental powders FexAl25Ti75–x (x = 75, 70, 65, 60) resulted in the formation of nanocrystalline Fe(Al, Ti) solid solution with based centered cubic (bcc) structure and reduced particle size of the final powder.
Fe40Al40Mn20 intermetallics were further subjected to mechanical alloying for 40 h with subsequent hot pressing at 650°C under 450 MPa for 1 h, as described by Maziarz et al. [25]. The Fe(Al) solid solution was reported to be disordered. After 40 h of milling treatment, chemically homogenous spherical particles with a size of approximately 5 m were identified in alloys by scanning analytical electron microscopy.
Structural Analysis. X-ray diffraction patterns for FeAl40Ti3 and FeAl40 powders before and after milling treatment for different periods (2, 20, and 30 h) are presented in Fig. 3. Samples without Ti exhibit typically bcc- disordered FeAl phase, whereas samples with 3 at.% Ti concentration have both the FeTi phase and the Fe(Ti, Al) solid solution. Unmilled FeAl40 alloy powders (t = 0 h) build a complex of body-centered cubic (bcc) Fe and face- centered cubic (fcc) Al, in which (222), (220), and (200) Al-peaks overlap with (110), (200), and (211) Fe-peaks.
The (111) and (311) Al diffraction peaks remain unaffected. During the milling process they disappear and are not observed in XRD patterns obtained after 2 h of milling time. The reason for this can be, among others, that aluminum remains elemental with small particle size and completely diffuse into the bcc lattice of -Fe to form ordered or disordered solution.
Even though the lattice parameter could increase evidently, the 2 h milled powders still display the presence of pure -Fe. The disappearance of Al diffraction peaks after 2 h of milling indicates that the aluminum particles are located on the grain boundary of iron. An identical phenomenon is observed in other mechanically
Fig. 3. X-ray diffraction patterns of powder mixture as a function of MA time: a) FeAl40, b) FeAl40Ti3
alloyed system Zr–Al [26] when increasing milling time results in the absence of Al diffraction peaks to Al minor particle size. A similar observation was also stated in [27].
The first 10 h of FeAl40mechanical alloying resulted in the disappearance of Fe and Al peaks. It caused the formation of a single-phase Fe(Al) solid solution with a bcc crystal structure, where the peak corresponding to Fe(Al) became the most intense (Fig. 3a). With increasing MA time, the diffraction peaks of the solid solutions became wider and less intense due to transformed crystallites size and increased strain at the atomic level as a sequence of severe plastic deformation. The same trend is noted by Mhadhbi [28], who studied the effect of milling time (from 2 to 20 h) on the crystallite size, lattice strain, dislocation density, and lattice parameters of Fe–40 at.%
Al powder mixture. The single phase of disordered Fe(Al) solution formed after 14 h of milling and the final powder exhibited a nanometer-scale structure with an average crystallite size of about 8 nm and a dislocation density of 2.9 · 1016 m–2. Obtained results demonstrate that the strain broadening during high-energy mechanical alloying can be attributed to severe plastic deformation (SPD).
During the MA from 10 to 20 h, the Fe(Al) peak shifted to a lower Bragg angle. M.M. Rajath Hegde et al.
and R. Bernal-Correa et al. [29, 30] have reported that the Fe(Al) solid solution is formed continuously by dissolving of Al in the Fe lattice, which is evinced by slow deposition of the Fe diffraction peaks at lower angles with longer milling duration. This shift indicates that the solid solution Fe(Al) has been completely formed. A similar phase evolution was observed for the Fe52Al48powder alloy [29]. At the end of 30 h of milling, the diffraction peaks increased in width (Fig. 3a), which indicates the phase transformation to bcc Fe(Al) solid solution.
The broadening of diffraction peaks implies the crystallite size refinement and the increase of lattice strain in the alloy. Similar observations were noted by Jiraskova et al. [31] when disordered Fe(Al) solid solution transformed into the ordered FeAl intermetallic after 20 h of milling time.
X-ray diffraction patterns of FeAlTi powder mixture during different milling times are shown in Fig. 3b.
The evolution of the diffractograms as a function of milling time demonstrates how the structure of elemental powders (iron, aluminum, and titanium) changed compared to that of nanostructured alloys. According to performed measurements, show aluminum and titanium diffraction peaks tend to gradually vanish with prolonged
milling time. Figure 3b depicts the progression of the alloy formation by milling recognizable by X-rays (Cu-K1,
= 0.15406 nm). The aluminum peak is not visible after 10 h of milling as, supposedly, it has formed an alloy or a solid solution with iron, while the titanium diffraction peaks disappear after 10 h of milling. The same observations are reported by H.M. Enayatim et al. and S. Haixia et al. [32, 33].
The progressive disappearance of Al and Ti peaks evidently show that these elements formed a solid solution with iron followed by long milling times until the complete alloy constitution. The deformation and various MA-associated defects serve as diffusion pathways for small aluminum and titanium particles or atoms into the iron to form an alloy. With continued milling time, the mixture of powders becomes more homogeneous and occurs on the atomic level.
After 30 h of milling, only the diffraction peaks of the alloy are observed in the diffractograms, which shows that the mentioned milling interval is sufficient to form the alloy within the limits of this method [31]. When titanium peaks disappear, the iron diffraction peaks become larger, which can be attributed to the bcc Fe(Al, Ti) solid solution.
Figure 4 presents the variation of the lattice parameters for FeAl40 and FeAl40Ti3 alloys during milling. It can be seen that after 2 h of MA, the lattice parameter of FeAl40 alloy increases unto 0.28646 nm and remains constant with increasing milling time up to 30 h. Similar results for the Fe–Al powders were established by Hamlati et al. [34] and can be attributed to the ordered structure of the alloy. Enhanced diffusion of Al atoms during milling can induce the decrease in lattice parameter and, thus, weaken the Al content in the bcc-Fe–Al phase [35].
Figure 4b shows the variation of the lattice parameters for FeAlTi powder alloy. Thus, the maximum value of 0.2881 nm is reached after 2 h of milling time. The increase in the lattice parameter is explained by the fact that the early stage of milling gives a start to the formation and growth of the compound. Once the compound is formed after more than 10h-milling, the lattice parameter increases slightly with the continuation of the milling due to stresses and defects, which occur in the lattice during milling. Compared to FeAl mixture powder, the presence of titanium even in small amount considerably modifies the behavior of mechanical alloying [36–39].
a b Fig. 4. Evolution of lattice parameters as a function of MA duration for FeAl40 and FeAl40Ti3
powders
a b Fig. 5. Variation of lattice strain and crystallite size parameters during milling for: a) FeAl40 and b)
FeAl40Ti3powder alloys
The method of Scherrer and Williamson–Hall is applied [22, 23] for the calculation of the grain size. The evolution of the micro-strains and the average crystallite sizes of FeAl40 and FeAl40Ti3 powders as a function of mechanically alloying time are shown in Fig. 5.
From Fig. 5a it is visible that the grain size of FeAl40 decreases with further milling, which is typical for mechanical alloying [40]. The calculated grain size of the 30h-milled sample is 17.2 ± 5 nm, which is slightly larger than the values presented in references [9, 40]. This can be attributed to the fact that powder particles are subjected to severe plastic deformation and extreme cold operation during MA. The dislocation density increases to a considerably high level resulting in formation of shear bands with high dislocation density. With continuous milling, the strain increases with higher dislocation density, which results in the formation of dislocation and sub- grain cells separated by low-angle grain boundaries. After sufficient milling duration, the mobility of low-angle limits is much lower than that of wide ones [41].
By applying the XRD analysis and methods of Williamson–Hall and Scherrer [22, 23], the grain size of 30h-milled FeAl40Ti3 composite was determined to be approximately 11.2 nm (Fig. 5b). These values are slightly smaller than those obtained by other researches [41, 42]. It has been reported that the final size of crystallites in the bcc metals is determined by the evolution of the dislocation cell structure [9, 43]. The average crystallite size of FeAl40 is 70% bigger than that of the FeAl40Ti3 alloy. Diffraction lines attributed to FeTi and supersaturated Fe(Al) solid solutions are too weak and, thus, can hardly be used to estimate the average crystallite size and the average network voltage. The analysis of the diffraction peaks allows determining the grain size and lattice strain of the powders.
The microstrain values of the powders are shown in Fig. 5. First, the microstrain of FeAl40 enlarged sharply during longer milling and reached a value of about 0.3% at the end of 30 h of milling time (Fig. 5a). The evolution of the microstrain of FeAl40Ti3 solid solution as a function of milling times is shown in Fig. 5b. It demonstrates that the microstrain was gradually increasing from 2 to 10 hours of milling followed by a slight increase during 10–20 h and rapidly grew after 30 h of milling resulting in a value of 0.21%.
Magnetic Behavior. The samples of 0.09 g powders were placed into a 35 mm3capsule to perform the magnetic measurement in the vibrating sample magnetometer (VSM). The magnetization variation of the ball- milled samples versus the applied magnetic field is shown as a function of milling time in Fig. 6. It can be seen that the magnetization of FeAl40increases with rising the milling time up to 20 h and decreases again after 30 h of milling. The decrease can be related to the enrichment of the aluminum lattice with atoms of iron and to lower magnetic moment of Fe due to weak ferromagnetic interactions and antiferromagnetic super-exchange interaction between Fe sites mediated by Al atoms, as suggested by Plascak et al. [43]. The magnetization hysteresis loops of FeAl40 and FeAlTi alloys were obtained at room temperature using VSM.
Figure 7 depicts the saturation magnetization (Ms) and the coercive field (Hc) obtained from previously shown hysteresis loops, and Fig. 8 illustrates Ms and Hc as functions of mechanical alloying time for FeAl40and FeAl40Ti3 powders, respectively. The addition of Ti into FeAl powder alloy modifies the magnetic behavior intensifying the magnetic properties of the FeAl40Ti3 compound in comparison with FeAl40.
Rapidly increased coercivity (Hc) of FeAl and FeAlTi powder alloy, the squareness (Mr/Ms), and remanent magnetization (Mr) of FeAl powder alloy after 20 h of milling can be mainly attributed to the formation of supersaturated solid solutions Fe(Al) and Fe(Al, Ti) as the interaction between Fe atoms. At that, Fe(Al) solid solution becomes less intense (Fig. 8). Magnetic properties depend on the structure, composition, defects, crystallite size, and internal strain [44]. The coercive field (Hc) weakens due to the increase of stress, impurities concentration, and the decrease of saturation magnetization (Ms) [45].
According to the Stoner–Wohlfarth single-domain theory [23], Hc decreases with the reduction of crystallite size when it has a single-domain structure. Soft magnetic materials undergo a magnetic hardening according to the random anisotropy model [46], which implies that ferromagnetic exchange length (L0) must be bigger than the mean crystallite size (D).
Fig. 6. Hysteresis loops of ball-milled powder samples at different milling duration
Fig. 7 Fig. 8
Fig. 7. Variation of coercivity and saturation magnetization of FeAl and FeAlTi powder alloys during different milling time
Fig. 8. Variation of remanent magnetization and squareness of FeAl and FeAlTi powder alloys during mechanical milling
The value L0 for Fe-based alloys corresponds approximately to the D value of the Fe(Al) and Fe(Al, Ti) alloy at 20 h of MA treatment for FeAl40 and FeAl40Ti3 powder alloys, respectively. However, Ms slightly decreased for FeAl40Ti3 powder during milling for less than 20 h and increased at the end of 30 h milling time. The main reason for this is the dilution of the Fe magnetic lattice caused by Al, as noted from the XRD analysis.
Aluminum has been reported to decrease the magnetic moment of individual Fe sites due to lower direct ferromagnetic interaction between Fe–Fe sites and antiferromagnetic super-exchange interaction between Fe sites mediated by Al atoms [36–43].
CONCLUSIONS
The mechanical alloying process has been applied to synthesize a nanocrystalline Fe(Al) and Fe(Al, Ti) solid solutions from Fe, Al, and Ti elemental powders. The addition of 3 at.% Ti element to FeAl40 powder alloy induces the modification of measured parameters.
It can be noticed that a longer duration of the MA up to 30 h reduces the particle size of FeAl40 (20 ± 5 m) and leads to the spherical agglomeration of particles being distributed in FeAl40Ti3 powder within the range of 9–
1 m. Obtained results indicate that the milling time and addition of Ti affects the powder morphology and decreases the size of particles.
However, the samples without Ti show the presence of a bcc-disordered phase FeAl, whereas the samples with 3 at.% Ti concentration exhibit both the FeTi phase and the Fe(Ti, Al) solid solution.
Thus, the addition of Ti and the elevated content of mechanically-alloyed powders increase the lattice parameters and the strain values but decrease the crystallite size.
The magnetic properties are highly influenced by the addition of the Ti element into FeAl40alloy. It is stated that the magnetic properties of the FeAl40Ti3 compound are higher than those of FeAl40. The change in magnetic behavior occurs mainly due to the formation of a supersaturated Fe(Al, Ti) solid solution as a sequence of the interaction between the Fe atoms, which are ferromagnetic with Ti and Al atoms.
ACKNOWLEDGMENTS
The authors would like to thank Dr.A. Guittoum (Nuclear Research Centre, Algiers, Algeria) for assistance in X-ray patterns analysis by the High Score Plus software.
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