Enhanced thermal stability of amorphous Al-Fe alloys by addition of Ce and Mn

The thermal stability of mechanically alloyed amorphous Al-Fe-based alloy powders, with nominal compositions Al82Fe16Ce2 and Al82Fe14Mn2Ce2, was investigated using differential scanning calorimetry (DSC), x-ray diffraction (XRD), and scanning electron microscopy (SEM) complemented by energy-dispersive x-ray spectroscopy (EDX). Analysis through DSC indicated that both Al82Fe16Ce2 and Al82Fe14Mn2Ce2 alloys undergo a two-stage crystallization process. Notably, the initial crystallization temperatures for the Al82Fe16Ce2 and Al82Fe14Mn2Ce2 alloys were determined to be approximately 525 °C and 550 °C, respectively. This high thermal stability is attributed to the delayed nucleation process induced by the presence of Ce and Mn within the Al-Fe matrix. During polymorphic crystallization, distinct phases such as β-AlFe, Al13Fe4 for Al82Fe16Ce2, and β-Al(Fe, Mn), Al13Fe4, Al10CeMn2 for Al82Fe14Mn2Ce2 were identified. Furthermore, post-annealing of these amorphous alloy powders at elevated temperatures of 600, 700, and 800 °C led to distinct morphological characteristics based on the alloy composition. For Al82Fe16Ce2, the particles preserved a nearly spherical morphology, with size distributions ranging from 1 to 5 μm. In contrast, for Al82Fe14Mn2Ce2, the particles exhibited an irregular shape with a broader size range of 1 to 15 μm.


Introduction
Amorphous aluminum-based alloys have high specific strength, good corrosion resistance and wear resistance properties are interested for automobile and aerospace applications [1][2][3].In recent years, research efforts have been devoted to improving the mechanical properties of amorphous aluminum-based alloys.The tensile strength of Al alloys with amorphous structure can reach 1000 MPa [4].However they are quite brittle.Remarkable improvement in good ductility and high tensile strength can be obtained by partial devitrification and consolidation of Al-based amorphous alloy powders [5][6][7].Kawamura et al attributed the high compressive strength of 1420 MPa by hot compaction atomized amorphous Al 85 Ni 5 Y 8 Co 2 powders [8].Kim et al reported that a tensile fracture stress of 1560 MPa can be obtained for Al-based amorphous nanocomposite [9].The bulk density and Young's modulus of the samples increased with increasing sintering temperature.At high sintering temperatures, the inter-particle pores are filled up, and the porosity of the sample is reduced, leading to an increase in the mechanical properties of the samples.Metallic glasses, which are metastable at room temperature, undergo a transformation during the sintering process.In this process, atoms rearrange themselves to form either crystalline or quasicrystalline phases.Al-Fe amorphous alloys with low onset temperature, T x , may limit technical application because of low sintering temperature.Hence, to obtain highstrength bulk samples, it is necessary to improve the T x of metallic glass alloy powders.Furthermore, the amorphous-to-crystalline temperature transformation is induced by thermal annealing strongly depending on the chemical composition of the sample [10,11].There have been several attempts to improve the thermal stability of Al-Fe amorphous alloys by adding transition elements such as Ti, Ni, or Cu [12,13].Among these alloys, Al 82 Fe 16 Ti 2 exhibits the highest crystallization onset temperature of 398 °C and higher than that of Al 84 Fe 16 amorphous alloys (T onset = 352.2°C) [14].Besides, the addition of rare earth (RE) elements like Y, La, and Ce, having high melting points and large atomic radii, have been performed to enhance glass forming ability (GFA) and increase the crystallization temperature in Al-based metallic glasses [15,16].Guihua Li et al investigated the thermal stability of Al-Ni-La with three compositions of Al 86 Ni 9 La 5 , Al 86 Ni 9 (La 0.5 Ce 0.5 ) 5 and Al 86 (Ni 0.5 Co 0.5 ) 9 (La 0.5 Ce 0.5 ) 5 prepared by a single-roller melt-spinning technique and obtained the onset temperatures are 242, 249 and 294 °C, respectively [15].Jianqi Zhang et al reported that the onset temperature of the amorphous alloy Al 86 Ce 10 Fe 4 synthesized by the melt-spin fast-quenching technique was about 304.2 °C [17].Typically, Ce is incorporated into amorphous Al-Fe alloys through a rapid quenching process.So far, there have been few studies of GFA and thermal stability of amorphous alloys Al-TM-Ce and Al-TM-Ce-Mn prepared by mechanical alloying (MA).In MA technique, hard balls collide continuously with a mixture powders in jar to deform, fracture and cold weld.These processes are repetedly until to achieve desirable structure.MA is a suitable technique to produce equilibrium and non-equilibrium alloy phases such as nanocomposites, quasicrystals, or amorphous alloys [18][19][20].
This study focused on improving the thermal stability of Al-Fe amorphous alloys by adding cerium and manganese, as well as investigating the crystallization behavior of amorphous Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Mn 2 Ce 2 alloys prepared by mechanical alloying.

Materials and experimental methods
Amorphous alloy powders of Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Mn 2 Ce 2 were synthesized using mechanical alloying from elemental powders in a planetary ball mill, operated at a rotation speed of 350 rpm.This method follows the procedure outlined in our previous work [21].The milling process was carried out in an argon atmosphere with hexane used to prevent the powders from adhering to the milling tools.After 40 h of milling, both Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Mn 2 Ce 2 powders achieved an amorphous state and were subsequently selected for thermal stability analysis.The non-isothermal differential scanning calorimetry (DSC) studies were carried out in a DSC-1150B Differential Scanning Calorimeter at a heating rate of 20 K min −1 under a continuous flow of purified nitrogen gas.The annealing temperatures of 600, 700, and 800 °C were chosen based on the crystallization peak completion observed in both amorphous alloys.Annealing was carried out at the selected temperatures for 15 min each, within quartz tubes under flowing argon gas, using a furnace with SiC heating elements.
Phase analysis was done by x-ray diffraction (XRD) using a X'Pert PRO Powder Diffractometer (Malvern Panalytical Ltd, Malvern, United Kingdom) using Cu Kα radiation (λ = 1.5405Å).The XRD parameters were: 2θ range of 20 to 80°, a step size of 0.03°, and a scanning speed of 1°per minute.The morphology of annealed powders was characterized by scanning electron microscopy (SEM) using a HITACHI TM4000 PLUS instrument (Hitachi High-Tech Corporation, Tokyo, Japan).

Results and discussion
Figure 1 shows the DSC heating curves of Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Mn 2 Ce 2 alloys after 40 h of milling.In the DSC thermograms, no obvious glass transition can be observed for these alloys.Triveño Rios et al investigated the thermal stability of the Al-Ce-Ni system and found that the glass transition phenomenon occurred only in the composition with a Ce content above 6.0 at% [22].There are two exothermic peaks during the crystallization of these alloys.The onset crystallization temperatures of Al 82 Fe 16 Ce 2 alloys take place at 525 °C and 620 °C, respectively.The DSC profile of the amorphous Al 82 Fe 16 Ce 2 alloys is similar to that of melt-spun Al 86 Fe 4 Ce 10 studied in [17].However, the onset temperature of the melted spun Al 86 Fe 4 Ce 10 sample is about 304.2 °C, which is much lower than that of mechanically alloyed Al 82 Fe 16 Ce 2 powders (T onset = 525 °C).Al 82 Fe 14 Mn 2 Ce 2 alloy has higher onset crystallization temperatures (550 and 634 °C) compared to those of Al 82 Fe 16 Ce 2 amorphous alloys.In order to investigate the phase formation sequences during crystallization, as-milled Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Mn 2 Ce 2 powders were annealed at 600, 700, and 800 °C for 15 min.Figures 2(a), (b) presents the x-ray diffraction patterns of the amorphous Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Ce 2 Mn 2 alloy powders after annealing at these varied temperatures.
The crystallization of the Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Ce 2 Mn 2 alloys was observed above 600 °C, with the simultaneous precipitation of Al 13 Fe 4 , a solid solution β-(AlFe), and for the latter alloy, Al 13 Fe 4 , β-Al(Fe, Mn) and Al 10 CeMn 2 phases, respectively.The intermetallic Al 13 Fe 4 phase, which has a negative formation enthalpy of −18.052 kJ mol −1 [13], tends to precipitate easily from Al-Fe amorphous alloys.The presence of alloying elements such as TM and RE, which enhances the thermal stability of the Al 84 Fe 16 -based amorphous alloy, is shown in table 1.From a thermodynamic perspective, alloying with transition metallic or rare-earth elements was expected to improve the GFA and thermal stability of a marginal Al 84 Fe 16 glass forming alloy.This is in line with Inoue's three empirical rules for achieving high GFA, which includes (1) employing multi-component systems with more than three elements, (2) ensuring a significant difference in atomic size ratios, exceeding 12%, among the three primary constituent elements, and (3) achieving negative heats of mixing among these pairelements [23].Zhang et al investigated Al-Ni-RE (RE: La, Ce) alloys prepared by arc melting, noting a  pronounced impact of the RE atom size and the negative mixing enthalpy on glass formation [24].In Al-Fe-TM/RE amorphous alloys, substituting minor alloy elements ( [25].In order to enhance the thermal stability of Al-Fe amorphous alloys, it is necessary to inhibit the diffusion of atoms, particularly the more mobile Al atoms, and to stabilize the supercooled liquid region, which is the metastable state between the glass transition temperature (T g ) and the crystallization temperature (T x ).Typically, a significant atomic size mismatch and a considerable negative heat of mixing between the constituents of an alloy, such as the Al 82 Fe 16 Ce 2 alloy, contribute positively to enhancing the GFA and thermal stability due to improved atomic packing efficiency.Since the alloying element Ce has a stronger affinity for Al than for other constituents, it is expected that 'Al-Ce' clusters or partially ordered structures might form, thereby hindering the nucleation of fcc-Al crystals.
Upon adding Mn to the Al 82 Fe 16 Ce 2 amorphous alloy, the onset crystallization temperature increases to 550 °C, which is significantly higher than those of Al 82 Fe 14 Ni 2 Y 2 (380 °C) and Al 82 Fe 14 Ti 2 Y 2 (399 °C).Considering the amorphous Al 82 Fe 14 Mn 2 Ce 2 alloy, it is noteworthy that the mixing enthalpies (ΔH mix ) between  Al-Fe, Al-Ce, and Fe-Ce are −11, −38, and 3 kJ mol −1 , respectively, while the ΔH mix values for Al-Mn and Fe-Mn are −19 and 0 kJ mol −1 , respectively [26].Thus, according to Inoue's three empirical rules for bulk metallic glasses (BMGs), the addition of Mn to the Al-Fe-Ce alloy does not noticeably contribute to a more negative enthalpy of mixing.The atomic radii of Al, Fe, Ce, and Mn are 0.143 nm, 0.124 nm, 0.183 nm, and 0.112 nm, respectively [26].It is notable that the atomic radius of Mn (0.112 nm) is smaller than that of Fe (0.124 nm), which can potentially increase the atomic packing density of the Al 82 Fe 14 Ce 2 Mn 2 amorphous alloy due to the larger atomic size difference among the constituent elements.The glass-forming ability (GFA) and thermal stability of the Al 82 Fe 14 Ce 2 Mn 2 amorphous alloy are primarily governed by the atomic size mismatch among the constituent elements, even though the mixing enthalpy of the Fe-Ce pair is slightly positive (+3 kJ/mol).Liu and Lu found experimental documentation indicating that alloying additions of atoms with small atomic radii <0.12 nm (such as B and Si) or atoms with large atomic radii >0.16 nm (such as Y and Sc) were most effective in enhancing the GFA [27].The addition of Mn to the Al 82 Fe 16 Ce 2 alloy stabilizes the amorphous phase and suppresses nucleation and grain growth in crystallization process.Furthermore, the Al 82 Fe 14 Ce 2 Mn 2 alloy features a broader range of atomic sizes among its constituent elements compared to its counterpart, which may compensate the relatively less negative heat of mixing and result in improved GFA and elevated thermal stability.Al 20 powders is slower than that of RQ ribbons due to the higher disorder, finer particle size, and the presence of strains in the as-milled powders compared to ribbons [29].In addition, Lee et al stated that the glass transition temperature and the onset crystallization of several mechanically alloyed samples were higher than those of squeeze-cast specimens due to contamination during the milling process [30].During MA, the powders collide with the milling balls and jars, resulting in cold welding, fracture, and rewelding until the desired structure is obtained.The powders undergo severe plastic deformation and disorder.Contaminants, such as oxygen from the air and iron from the milling tools, can be introduced into the mechanically alloyed powders during this process, potentially affecting their thermal stability.SEM/EDX analysis is employed to examine changes in morphology and contamination in annealed powders.addition of Mn may alter the mechanical properties, such as ductility and toughness, of the alloy.These changes can enhance cold welding and promote fracture mechanisms during milling, leading to the formation of larger particles.No significant changes in particle shape or size were noted for either alloy when annealed across the temperatures from 600 to 800 °C.The EDX microanalysis, as presented in figure S1 (please, refer to Supplementary Material) confirms that the heat-treated Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Ce 2 Mn 2 alloy powders consist predominantly of their respective constituent elements Al, Fe, Ce for the former and Al, Fe, Ce, Mn for the latter.The atomic ratios determined by EDX for the Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Ce 2 Mn 2 alloy powders after annealing at 800 °C are Al:Fe:Ce = 82.45:15.92:1.63 and Al:Fe:Ce:Mn = 83.07:14.22:1.70:1.01,respectively.These chemical composition results closely match the nominal compositions of the initial powder mixtures.

Conclusions
The presence of a small amount of Ce and Mn (2 at%) plays an important role in retarding the nucleation during the annealing process, therefore, remarkably improving the thermal stability of Al-Fe amorphous alloys.The crystallization temperatures of the amorphous alloys of Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Ce 2 Mn 2 take place at high temperatures of 525 and 550 °C, respectively.After annealing at 600 and 700 °C, the amorphous phase of Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Ce 2 Mn 2 alloys partly transformed into the simultaneous precipitation of Al 13 Fe 4 , a solid solution of β-(AlFe) and Al 13 Fe 4 , β-Al (Fe, Mn), and Al 10 CeMn 2 phases, respectively.At a higher annealing temperature of 800 °C, the residual amorphous phase completely disappeared, and the diffraction peaks of the solid solution and intermetallic phases became stronger.The thermal stability of Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Ce 2 Mn 2 is much higher than that of existing Al-based metallic glasses produced by MA or RQ.The crystallization behaviors and thermal stability of the Al-Fe-based metallic glasses are very sensitive to minor element doping and preparation routes.The particle size of the amorphous Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Ce 2 Mn 2 alloys seems not to have a significant effect after annealing at 600 to 700 and 800 °C.The higher thermal stability of Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Ce 2 Mn 2 promises higher sintering temperatures and higher mechanical properties of bulk samples.Future research will be focused to consolidating the amorphous powders and studying the mechanical properties of bulk samples.The spark plasma sintering technique is suitable for densification of amorphous alloy powders.

Figure 3 .
Figure 3. Percentage mismatch of atomic radii and enthalpies of mixing (in kJ/mole) for binary systems involving Al, Fe, Ti, Ni, Mn, Y, and Ce [26].
Figures 4 and 5 show the morphologies of Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Ce 2 Mn 2 amorphous powders after a 15 min heat treatment of 600, 700, and 800 °C.Post-heat treatment, Al 82 Fe 16 Ce 2 alloy powders exhibited predominantly spherical morphologies, with particle sizes ranging from 1-5 μm, as shown in figure 4. Both alloys demonstrated the presence of agglomerated particles.Specifically, the Al 82 Fe 14 Ce 2 Mn 2 alloy powders, which are illustrated in figure 5, displayed irregularly shaped aggregates measuring approximately 1-15 μm.The
2 at%) such as TM (Ni, Ti) or RE (Y, Ce) for Al alters the crystallization products and crystallization temperatures.The thermal resistance of Al-Fe-TM/RE ternary amorphous alloys prepared by MA increases in the following order: Al 84 Fe 16 < Al 82 Fe 16 Ni 2 < Al 82 Fe 16 Y 2 < Al 82 Fe 16 Ti 2 < Al 82 Fe 16 Ce 2 .Interestingly, the amorphous Al 82 Fe 16 Ce 2 alloy exhibits a higher onset temperature compared to the Al 82 Fe 16 Y 2 alloy, even though the binary negative mixing enthalpies of the constituent elements are identical for both alloys, as shown in figure 3.Moreover, it is noted that fcc-Al crystals nucleate and precipitate easily in Al 84 Fe 16 , Al 82 Fe 16 Ni 2 , Al 82 Fe 16 Ti 2 , and Al 82 Fe 16 Y 2 amorphous alloys, which leads to relatively low crystallization temperatures.Y. Ye and K. Lu have reported that the precipitation of fcc-Al crystals by a nucleation and diffusion-controlled growth mechanism, occurs in an amorphous Al 89 La 6 Ni 5 alloy prepared by single-roller melt-spinning

Table 1 .
Crystallization temperatures and phase compositions of amorphous alloys Al-Fe-(TM, RE) produced by MA technique.
The Al 82 Fe 16 Ce 2 and Al 82 Fe 14 Ce 2 Mn 2 amorphous alloys prepared by MA exhibit higher crystallization temperatures compared to other Al-based amorphous alloys prepared by the rapid quenching (RQ) technique, as listed in table 2. The onset temperature for the first stage of crystallization of RQ Al-based amorphous alloys typically ranges from 210 to 310 °C.The crystallization behaviors of the Al-based metallic glasses are very sensitive to minor element doping and the fabrication technique employed.For instance, the crystallization temperature of RQ Al 86 Ce 10 Fe 4 amorphous alloy is only about 304.2 °C.In contrast, the onset temperature of MAed Al 82 Fe 16 Ce 2 amorphous alloy in this work is higher, specifically at 525 °C.Oleszak et al also prepared amorphous Cu 47 Ti 34 Zr 11 Ni 8 and Cu 47 Ti 34 Sn 11 Ni 8 alloys prepared by melt-spinning and MA techniques [28].The onset temperature of Cu 47 Ti 34 Zr 11 Ni 8 ribbon (739 K) is considerably lower than that of MA powders (783 K).Kuhnast et al indicated that the crystallization process of MAed Zr 55 Ni 25

Table 2 .
Crystallization temperatures Al-based amorphous alloys are prepared by rapid quenching (with a heating rate of 20 K min −1 ).