Microstructural Investigations of Rapidly Solidified Al-Co-Y Alloys

The alloys with different compositions in the Al-rich corner of the Al-Co-Y ternary system were prepared by conventional casting and further processed by melt-spinning technique. The microstructure and the thermal behavior of the alloys were analyzed by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and differential thermal analysis (DTA). It was found that only rapidly solidified Al 85 Co 7 Y 8 alloy exhibited the best glass forming ability (GFA) and a fully amorphous structure. Besides, Al 85 Co 13 Y 2 and Al 85 Co 2 Y 13 alloy ribbons were fully crystalline, whereas Al 85 Co 10 Y 5 and Al 85 Co 5 Y 10 alloy ribbons consisted of some crystalline phases within an amorphous matrix. The SEM results showed the same trend that the crystalline phase fraction decreases with the approaching into best glass former. From DSC results, only Al 85 Co 7 Y 8 amorphous alloy exhibited a glass transition temperature (T g ) at 569K, and its supercooled liquid region (ΔT x = T x − T g ) was found to be 17 K. Moreover, other calculated GFA parameters for this alloy system were also discussed.


Introduction
Because of their remarkable mechanical properties, which are useful for industrial and structural applications, Al-based amorphous alloys with high Al content (>80 at.%) have attracted considerable attention [1,2].In recent years, a series of investigations on the formation, structure, and properties of Al-based amorphous alloys have been carried out with an aim of developing high-strength materials with good ductility and high corrosion resistance [2][3][4][5][6].So far, various Al-based amorphous alloys have been produced by rapid solidification processing.Among them, Al-TM-RE alloys (TM: transition metal, RE: rare earth metal) and their derivations, such as Al-Ni-Y [2,[7][8][9], and Al-Ni-La [10][11][12], and Al-Ni-Y-Co [13][14][15], have been intensively studied with regard to the formation and stability of the amorphous phase.It was reported that Al-Ni-Y ternary amorphous alloys exhibit better mechanical properties, and their thermal stability and glass forming ability (GFA) can be substantially improved with the addition of Co.And it has been found that quaternary Al 85 Ni 5 Y 8 Co 2 (at.%) alloy has the highest GFA in Al-based amorphous alloys [13].That is why the investigations on components of Al-based amorphous alloys are focused on the Al-Ni-Y ternary system.On the other hand, just a few works have been reported for the other ternary system, Al-Co-Y alloys.In 1988, Inoue et al. [16] were the first to report that the amorphous phase was formed in the range of 2-17 at.%Y and 4-18 at.%Co for the Al-Co-Y system.Latuch and Dmowski [17]  Moreover, these results exhibited that by monitoring the microstructural changes, the best glass forming composition could be successfully found in the given alloy system.

Experimental
The nominal compositions of the investigated alloys are Al 85 Co 15− Y  ( = 2, 5, 8, 10, and 13 at.%).Designation of the studied alloy compositions and their locations in the Al-rich corner of Al-Co-Y ternary phase diagram are shown in Figure 1.As seen in Figure 1, the alloy compositions are located in the ternary triangle zone Al-Al 9 Co 2 -Al 3 Y.The alloy ingots (10 g each) were prepared from high-purity elements by arc melting (Edmund Bühler, Arc-Melter MAM-1) under argon atmosphere.The melting process was repeated four times in order to obtain chemical homogeneity, and the weight loss was less than 1 wt.%.Then, the alloy ingots were cut into suitably shaped pieces for the rapid solidification.Rapidly solidified ribbons were prepared by the single roller melt-spinning process (Edmund Bühler, Melt Spinner SC) with a copper wheel surface velocity of 30 ms −1 under argon atmosphere.The resulting melt-spun ribbons were typically 1.5-4 mm in width and 30-120 m in thickness.The phase identification of the conventional cast and rapidly solidified alloys were carried out by X-ray diffraction (XRD) using a Philips X'Pert powder diffractometer with Cu-K radiation generated at 40 kV and 30 mA.The XRD analyses were performed from 20 ∘ to 100 ∘ (2) with a step size of 0.02 ∘ and a count time of 1 s per step.The cross section of the ribbons was examined by a Zeiss Evo LS10 scanning electron microscope (SEM) with an energy-dispersive X-ray spectroscopy (EDX) analysis.Thermal behavior of the ribbons was studied by differential scanning calorimetry (Perkin-Elmer DSC 800) and differential thermal analysis (Perkin-Elmer Diamond DTA) at a heating rate of 20 ∘ C min −1 under dynamic nitrogen atmosphere.

Results and Discussion
The phase constitutions of the conventional ingot alloys were identified by XRD analysis.Figure 2 shows the peaks obtained in the analysis compared with the JCPDS database [24] [26], the increase of the lattice parameter of the -Al phase indicates the increase of the solubility of Y in the -Al phase.A similar behavior was observed for the Al-10Ni-10Ce (wt.%) alloy [27].
The XRD patterns of the melt-spun Al-Co-Y alloy ribbons are shown in Figure 3 These results indicate that the amount of crystalline phases decreases when the alloy is closer to the best glass-forming composition.Thus, the GFA of these alloys is enhanced, when the composition is away from the corresponding crystalline phases.As seen in Figure 1, the Al 85 Co 7 Y 8 alloy is located at the cross junction and represents the best glass former within the studied compositional range.Around this optimum GFA composition, it is noted that there are the composite structures with primary phases of -Al, Al 3 Y and Al 9 Co 2 .Similarly, Al 88 Co 4 Y 8 and Al 88 Co 5 Y 7 alloys were also reported as the best glass formers within different compositions in this triangle area [18][19][20].Furthermore, it was found that the optimum glass-forming region for the Al-Ni-RE (La, Ce, Pr, Nd, and Mm) systems was in the center of the composites with primary phases of -Al, Al 11 RE 3 , and Al 3 Ni.It was verified that optimum GFA compositions in such systems are sensitive to both Ni and RE contents [28].It can be concluded that the replacement between the alloy elements plays an important role in promoting glass formation.Moreover, the average crystallite sizes of the fcc-Al phase were determined from the X-ray line broadening using the well-known Scherrer equation [29].The calculated average crystallite sizes are 28.7,32.7, 33.1, and 40.5 nm for the melt-spun alloy ribbons of  = 2, 5, 10, and 13, respectively, as a result of the high cooling rate gained in the melt-spun technique.By the way, the melt-spun alloy ribbons including crystalline phases exhibit brittle behavior, while the meltspun amorphous alloy exhibits good bending ductility.
Figure 4 shows the cross sectional images of the melt-spun alloy ribbons.These images represent different microstructural features.The SEM results exhibit the same behavior in agreement with the XRD results.As seen in Figure 4(c), the Al 85 Co 7 Y 8 alloy displays the featureless morphology, which is the typical characteristic of the amorphous phase.Similar microstructural features were also reported in the literature [6,18,20,27,28].Also, Figure 4(f) shows the EDX analysis of the Al 85 Co 7 Y 8 alloy ribbon.From EDX analysis, the peaks of Al, Co, and Y are clearly seen and the percentages of element compositions are close to the nominal compositions.This indicated the minimal loss of solute elements during alloy preparation and processing.
In order to examine the crystallization and melting behavior of the melt-spun Al 85 Co 15− Y  ( = 2, 5, 8, 10, and 13) alloys, the DSC and DTA analyses were carried out, as shown in Figures 5 and 6, respectively.Thermal properties of these alloys, including   (glass transition temperature),   (onset crystallization temperature),   (onset melting temperature),   (liquidus temperature), and calculated GFA parameters (  /  ,   /  and Δ  ) are listed in Table 2   Temperature (K) Al 88 Ni 4 Y 8 and Al 88 Co 4 Y 8 alloys did not exhibit a glass transition temperature (  ).It might be due to the structural relaxation on the DSC curve which could overlap with the endothermic glass transition signals.Thus,   would be hidden in the DSC curve [30].Apart from this, the chemical interaction between Al and the alloying elements plays an essential role in the practical GFA for the Al-based alloys [23].And, it is proved that in some amorphous alloy systems, higher   /  and lower Δ  usually correspond to better GFA.Moreover, the alloy with a higher   /  parameter more likely has a better GFA [32,35].According to these criterions and the calculated GFA parameters listed in Table 2, the Al 85 Co 7 Y 8 alloy has a better GFA compared to previously reported values [32,35].

Conclusions
In the present study, the Al 85 Y  Co 15− ( = 2, 5, 8, 10, and 13 at.%)alloys were prepared by conventional casting and meltspinning technique.The microstructure of the master alloy ingots was composed of three different phases, fcc -Al, Al 3 Y, and Al 9 Co 2 .It was also indicated that, the solubility of Y in the -Al phase increased with the increasing of Y amount.However, the XRD and SEM results of the melt-spun alloys revealed that only Al 85 Co 7 Y 8 have a fully amorphous structure, whereas crystalline phases were detected in others.Also, the amount of the crystalline phases decreases when the alloy is closer to the best glass-forming composition.Moreover, the melt-spun alloy ribbons including crystalline phases were brittle, whereas the melt-spun amorphous alloy was ductile.
From the thermal analysis, the Al 85 Co 7 Y 8 amorphous alloy was found to be the most thermally stable.Also, some GFArelated parameters, such as   /  ,   /  , and Δ  , were calculated.It was found that the Al 85 Co 7 Y 8 alloy had a better GFA, and it could be explained by the increased thermal stability.

Figure 1 :
Figure 1: Al-Co-Y ternary phase diagram showing alloy compositions studied in this research.
. The microstructures of the rapidly solidified alloys are almost different to the conventionally cast one.It is obvious that only Al 85 Co 7 Y 8 alloy ribbon shows a fully amorphous structure, whereas the Al 85 Co 13 Y 2 and Al 85 Co 2 Y 13 alloy ribbons are fully crystalline.And the Al 85 Co 10 Y 5 and Al 85 Co 5 Y 10 alloy ribbons consist of some crystalline phases within an amorphous matrix, shown in Figures 3(b) and 3(d).
Al 85 Co 13 Y 2 and Al 85 Co 2 Y 13 alloys display the rough and irregularly shaped domain structure with intermetallic phase precipitations.According to the XRD results of both samples, these precipitations are the Al 3 Y and Al 9 Co 2 intermetallic phases.As seen in Figures 4(b) and 4(d), the SEM images of the Al 85 Co 10 Y 5 and Al 85 Co 5 Y 10 alloys reveal the white dendritic crystalline particles, which are uniformly distributed in a featureless amorphous matrix.

the best glass formers Al 88 Co 4 Y 8 and Al 88 Co 5 Y 7 alloys did not
. The XRD patterns indicate that Al-Co-Y ingot alloys have three different phases, -Al solid solution, intermetallic Al 3 Y,Figure 3: XRD patterns of Al-Co-Y alloy ribbons, (a) Al 85 Co 13 Y 2 , (b) Al 85 Co 10 Y 5 , (c) Al 85 Co 7 Y 8 , (d) Al 85 Co 5 Y 10 , and (e) Al 85 Co 2 Y 13 .Al lattice parameter was determined by Rietveld analysis using the Maud refinement software [25].It was found that the lattice parameter of the -Al phase increased from 4.0474 Å for Al 85 Co 13 Y 2 to 4.0495 Å for Al 85 Co 2 Y 13 .By taking into account nominal atomic radii of Al (1.43 Å), Co (1.25 Å), and Y (1.80 Å) and Al 9 Co 2 .Table1summarizes the crystallographic data obtained from the XRD analysis.As indicated by the variation of diffraction peak intensities in Figure2, the peak intensities of the Al 3 Y phase grow slightly with the increasing of Y amount.For the alloy with 13 at.%Y, the XRD pattern of the sample appears to be composed of mainly Al 3 Y and fcc-Al phases with a small amount of Al 9 Co 2 phase.Similarly, with decreasing Y content in the Al-Co-Y alloys, the amount of Al 3 Y decreases, but that of Al 9 Co 2 increases.In Al 85 Co 13 Y 2 alloy, the diffraction pattern shows mainly the reflections of Al 9 Co 2 and fcc-Al phases with a small amount of Al 3 Y phase.It is clear from the peak intensities that fcc -Al is the major phase for all alloy compositions.However, the peaks of -Al phase were found to be slightly shifted to lower diffraction angles with the increase of Y content.This can be attributed to the change in the lattice parameters.Corresponding -

Table 1 :
[18]tallographic data of phases present in the Al-Co-Y ingot alloys[24].Regarding this, the Al 85 Co 13 Y 2 and Al 85 Co 2 Y 13 alloys do not exhibit any crystallization exotherms.It can be also seen in Table2that the   of the Al 85 Co 7 Y 8 alloy is higher than that of the other alloys.This increase in the thermal stability against devitrification can be attributed to the slight variation of solute concentration in the amorphous phase.Yang et al.[18]reported the same crystallization behavior for Al-Ni-Y and Al-Co-Y ternary alloys.However, they also noted that the best glass formers alloy undergoes a two-stage crystallization process.As seen in Figure5, the corresponding exothermic peaks in DSC curve become wider and weaker with both the increasing and decreasing of Y content.

Table 2 :
Thermal properties of the melt-spun Al-Co-Y alloy ribbons.
[34]19]33]t, in the present study,   was observed at 569 K in Al 85 Co 7 Y 8 alloy.The glass transition behavior for amorphous alloys is of technological and scientific importance, because the   reflects the atomic transport and viscosity properties which are dominant factors in the GFA of alloys and in the structural relaxation and thermal stability of the amorphous structure[31].Moreover, A. Inoue[2]pointed out the supercooled liquid region (Δ  =   −   ), which is an important parameter characterizing the thermal stability.The Δ  was found to be 17 K for the Al 85 Co 7 Y 8 alloy, which is higher than those of other similar systems[26,32,33].But, it is lower than that obtained in a similar composition range in the Al-Co-Y system[18,19].It is noted that Δ  is specific to the alloy composition and the heating scan rate in DSC analysis.On the other hand, it can be seen from the DTA curves that the onset melting temperature (  ) remains almost constant, while the liquidus temperature (  ) changes with the alloy composition.Using the   and   values, the reduced glass transition temperature   (=   /  ) was estimated to be about 0.48 for the Al 85 Co 7 Y 8 .It was reported that in Al-based amorphous alloys with high Al content (>80 at.%), the   values are generally less than 0.5[34].As seen in Table2,   /  ,   /  , and Δ  (=   −   ) were also calculated.