Assessing microstructures and mechanical resistances of as-atomized and as-extruded samples of Al-1wt.%Fe-1wt.%Ni alloy

Current applications of Al–Fe–Ni alloys include Alnico permanent magnets, industrial furnaces


Introduction
Al-Fe-Ni alloys are considered promising materials for various electronic and electric applications instead of conventional low conductivity Al casting alloys.This is mainly due to the low solubility of Ni and Fe on α-Al matrix, which prevent reducing high-conductivity of aluminium as a function of Ni and Fe additions [1].The alloying of Fe in aluminium is also recognized by improving its strength and stiffness [2][3][4].Nickel is able to improve the temperature mechanical strength of an Al alloy as well as its elastic modulus.Due to that, research on ternary Al-Fe-Ni alloys is of great interest.
According to Premkumar et al. [5] aluminium alloys containing transition elements such as Fe, Ni, Cr and Mo are current contenders for engineering applications.Low solid solubility and low rates of diffusion in Al characterizing such elements allow the formation of very stable and coarsening-resistant IMCs.
Slow cooled Al-Fe-Ni alloys are typified by relatively coarse and brittle intermetallic particles to be formed [6].Such structural features may deteriorate the mechanical properties of these materials, which restricts the applications.Hence, a solution to this problem can be found in the production of alloy powders by powder metallurgy (PM) techniques.In particular, the atomization of Al alloy powder with its intrinsic component of rapid solidification is able to circumvent the major limitations of conventional casting.Therefore, atomization is an alternative to develop dispersion-strengthened alloys.
Spray forming is one of the routes indicated to produce the desired microstructural arrangement, which is mainly affected by the powder size.In this process a fully dense deposit is formed by a collection of powders which are contained in liquid or liquid+solid state.The powder which is not incorporated into the deposit may be accumulated on the bottom of the chamber.Rapidly solidified powders are typically compacted via hot extrusion, which may be characterized by relatively high temperatures and short times.

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3 Although Al-Fe-Ni alloys are of high commercial significance, there have been relatively few studies that characterized their microstructures during rapid solidification processing.For instance, the effects of atomizing Al-Ni based alloys powder size on joined structures (either by cold or by hot forming) remain undetermined.Only few investigations have been related with the final microstructure features, mechanical properties and corrosion resistance of these alloys.Wada et al. [7] reported a strength value of 120MPa after precipitation heat treatment at 430°C for the hypoeutectic Al-2wt%Ni alloy, which is applied as aluminum-stabilized superconductors.Melt-spinning experiments were performed by Sivtsova et al. [8]  Premkumar and collaborators [5] reported that atomized Al-Fe-Ni powders formed both coarse IMC morphology and prevalence of cellular microstructure with IMCs on cell boundaries.The only IMC identified was Al 9 FeNi, on the form of either round particles or alternated layers with Al.By analyzing the hot consolidated Al-Fe-Ni powders with three distinct volume fractions of Al 9 FeNi, it was found that as the volume fraction of the IMC increases, the diffusion fields surrounding each intermetallic particle overlap each other, enhancing grain boundary diffusion.
Up to now, research about rapidly solidified powder microstructures and their mechanical properties after consolidation for Al-Fe-Ni alloys are still restricted.Therefore, it is the objective of the present work to investigate the microstructure features and strength of the Al-1.0wt%Fe-1.0wt%Nialloy considering two conditions, i.e., i. fabrication of powders by spray forming, and ii.Rapidly solidified powders compaction followed by hot extrusion.
Initially, emphasis is given on the correlations between the microstructures, cooling rates and hardness for the as-atomized Al-Fe-Ni powders.Then, the influence of IMC size and distribution; powder size and extrusion temperature on mechanical resistance of as-extruded Al-Fe-Ni samples is investigated.

Spray-forming and collected atomized powders
The Al-1wt%Fe-1wt%Ni alloy was prepared by using commercially pure grade (c.p.): Al (99.887 wt.%), Ni (99.896 wt.%) and Fe (99.872wt.%)which were mixed in a claygraphite crucible and heated to 1000ºC by an induction furnace.A total charge of 3,420g was considered to prepare the nominal alloy composition.The main impurities found were Si (0.042wt.%),Sn (0.0073wt.%) and Pb (0.0027%), with other elements on concentrations less than 100 ppm.The molten alloy was stabilized for 30 min, which allowed a complete mixing of the elements.The spray forming process was based on a 10bar N 2 atomization of a liquid alloy stream into variously sized droplets which are then propelled away from the region of atomization by the fast flowing atomizing gas.The melt was poured via a nozzle of 4 mm diameter fixed in the bottom of a tundish when the melt temperature achieved 742ºC.The produced Al-Fe-Ni powder was accumulated at the bottom of the chamber and collected later as can be observed in Fig. 1.Then, the powder was sieved and classified in six particle size ranges: between 500 and 250 µm, 250 and 180 µm, 180 and 106 µm, 106 and 75 µm, 75 and 53 µm and particle size less than 53 µm.A vibratory separator was used for accurate grading the powders in a single operation.
For this paper, the following size ranges were examined: i. 180-106µm, 106-75µm and <53µm for the evaluation of the Al-Fe-Ni alloy powders and ii.180-106µm and 106-75µm ranges, considering powders subjected to compaction and hot extrusion.
For each of the different powder size fractions, specimens were mounted in epoxy resin, ground and polished in preparation for metallography.Further, each sample was electropolished and etched (a solution of 0.5% HF in water).

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Fig. 1.Illustrative scheme of spray forming, extrusion apparatus and their devices.Atomized powder has been collected, cold-compacted and transferred to the hot extrusion process under two temperatures, which are 350ºC and 400ºC.
Microstructural characterization of the obtained powders on overspray condition has been performed using an optical microscope associated with an image analysis system Olympus GX51 (Olympus Co., Japan) and a Scanning Electron Microscope (SEM-EDS) FEI (Inspect S50L).Vickers microhardness tests were performed on the powders by using a test load of 10 g and a dwell time of 10s.The Vickers hardness of each powder size was the average of at least 10 measurements.The triangle method proposed by Gündüz and Çadirli [14] was used in order to quantify the cell spacing (λ c ).The average cell spacing associated with each powder size was determined through at least 20 measurements.Data obtained by directional solidification was very useful as a basis to comprehend the present results.Details concerning the experimental data obtained by directional solidification of the Al-1wt%Fe-1wt%Ni alloy can be found in a previous investigation [10].The X-ray diffraction (XRD) patterns for phases formed in the Al-1wt%Fe-1wt%Ni alloy examined have been acquired by a Siemens D5000 diffractometer with a 2-theta range from 20° to 90°, CuKα radiation and a wavelength, λ, of 0.15406 nm.XRD patterns were recorded at a scan speed of 2°/min.

Compaction and extrusion of the powders
The powders for tensile tests were prepared by single punch compaction at room temperature under normal atmosphere.Powders from each size range of interest were introduced in a hollow cylindrical geometry and sandwiched by two T-shape cylinders.
Powders were thus transformed into cylindrical preforms.The uniaxial stress applied to consolidate the samples was 600MPa.The extrusions are performed at 350°C and 400ºC with ram speed of 2 mm.s -1 and extrusion ratio of 10:1 [15], resulting in cylindrical specimens as shown in Fig. 1.
At every processing stage, specimens were polished and etched (a solution of 0.5% HF in water) for metallography.In the case of the extruded samples, central transverse sections along the length of the billets were prepared for metallography.Microstructural characterization was performed using a SEM-EDS FEI (Inspect S50L).Tensile testing on the Al-Fe-Ni as-extruded samples was performed according to specifications of ASTM Standard E 8M/04 and tested in an Instron 5500R machine at a strain rate of about 1 × 10 −3 s −1 .SEM images at high magnification of 50,000x and 100,000x have been used to examine the intermetallic particles after extrusion.The intercept method was employed in order to measure the inter-particle spacing, λ, and the diameter (d) of the intermetallic particles was also At least 20 measurements were performed for each condition of extrusion, i.e., 180-106µm powder range at 350°C, 106-75µm powder range at 350°C, 180-106µm powder range at 400°C and 180-106µm powder range at 400°C.

Length scale of the cell microstructure in the as-atomized samples
Fig. 2 shows the granulometric distribution obtained for the Al-1wt.%Fe-1wt.%Nialloy powders, with a higher mass fraction of particles associated to the powder size range of 180-106µm.Further, a considerable mass of powder was obtained for smaller granulometric sizes, as for example, between 75 and 53µm.The cell growth in the present microstructures depicted in Fig. 3 and Fig. 5 is a typical rapid solidification microstructure.The growth of cells in the other metallic alloys produced under fast cooling conditions has been discussed in previous researches [16][17][18][19][20]. Indeed, the imposition of high growth rates interfere on the growth of cells and dendrites which become finer and finer until they give rise to cellular-type structures.However, those cells formed for relatively slow cooling regarding to DS samples.In this case, cooling rates as low as 4.0K/s have been determined as can be seen in Fig. 4. Prior studies in literature showed that the cellular array characterizes the binary Al-1.0wt%Fe alloy.So, the addition of 1.0wt%Ni was not enough to destabilize microstructure for dendrites start to grow despite high cooling rates Morphology and scale of the as-atomized microstructures are governed by the cooling thermal parameters such as growth rate, v and the cooling rate, Ṫ.In theory, a sequence of morphologies and transitions may be obtained as a function of increasing v, which are plane front > cells> dendrites.If growth rate continues to be increased, the dendritic front can be changed back to cellular and planar front, the latter being associated with the limit of absolute stability [16,17].Trivedi et al. [16] examined the conditions under which the high-velocity morphological transition occurs during the steady-state growth of carbon-tetrabromide.In the present research work and considering the performed atomization conditions, fast cooling rate cells are found in the microstructure of the ternary Al-1.0wt%Fe-1.0wt%Nialloy powders.
The microstructures in Al-Fe-Ni powders in Fig. 3 and Fig. 5 are very similar in appearance except for the fact that IMC rod-like structures formed at the walls may only be observed for the coarser powders with size from 180 to 106µm, which correspond to an estimated cooling rate of ~65K/s.A eutectic form of solidification structure with alternate layers of aluminium matrix and intermetallic is formed, which is so-called microeutectic structure, can be seen in Fig. 5c [5].The finer collected powders (<106µm) do not exhibited such aspects.Evidence confirming the growth of well-defined aligned rod-like morphology in Al-Fe-Ni alloy is shown in Fig. 5d.Such feature was obtained through directional solidification, being associated with a cooling rate of 33K/s.Microstructure on coarse powders and DS samples can be considered a result of moderate cooling conditions, in which a moderate undercooling will take place.α-Al and Al 9 FeNi IMC can form together as a eutectic type of structure.Large powder particles where the undercooling is not as high are characterized by relatively low solidification velocities, giving rise to a segregated structure as can be seen in Fig. 5c.On the other hand, high levels of undercooling associated with smaller powders result in fine rounded particles which have been precipitated after solidification.

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Precipitates are highlighted in Fig. 5a and 5b, which correspond to powder microstructures formed at cooling rates of ~3x10 2 K/s and ~10 2 K/s, respectively.
Under equilibrium conditions, the solid solubility of Ni and Fe in Al is very limited [10].Nevertheless, fast cooling during atomization may promote the extension of solid solubility of these elements for higher values for non-equilibrium, supersaturated solid solutions.As a consequence, a decrease in the eutectic fraction is expected to occur with the increase on cooling rate.
The XRD patterns of the three examined powder size ranges can be seen in Fig. 6.By comparing the experimentally determined peaks with XRD databases, a prevalence of Al 9 FeNi IMC is noted due to not only higher intensity but also higher number of peaks if compared to the other intermetallics.Other phases have been detected, which are Al 3 Fe and Al 3 Ni and α-Al.Furthermore, the IMC's peaks seem to be more intense when associated with the highest powder size range.In general, the distribution of these reinforcing particles is improved with the decrease in cell spacing.Rapid solidification is effective with respect to the increase in hardness in the Al-1wt%Fe-1wt%Ni alloy.In this sense, expect levels of hardness can be preprogrammed during atomization with basis on an appropriate manipulation of powder size, i.e., controlling the cooling rate with a view to obtain a desired microstructure.

Strength and ductility in relation with microstructures of the as-extruded Al-Fe-Ni samples
The cellular structure of the Al-Fe-Ni powder has not been extinguished by compaction+extrusion process, which means that there is no significant change in the core microstructure of the powders after hot consolidation.It seems that both thermal exposure and deformation were not enough to promote homogenization.A typical example of the internal structure of the Al-Fe-Ni powder preserved after hot consolidation is shown in Fig. 8a.The presence of coarser and non-homogeneously distributed grains in the case of processing larger powders (106-180µm) seems to be connected with the features observed in microstructures of the as-cast powders.Larger powders may lead to a higher fraction of eutectic along the cell walls, which can be seen in Fig. 5c and it has been detected by XRD in Fig. 6.Higher proportion of intermetallic particles is able to improve grain boundary diffusion as stated by Premkumar et al. [5], which allows the grains to enlarge under thermal exposure.
Microstructures shown in Fig. 10  As can be seen in Fig. 11 and  It is well known that dispersed particles (whether of intermetallic or non-metallic phases) can have a considerable influence on the properties of metals, e.g.strengths are increased, recrystallization can be either accelerated or retarded, and the matrix grain size is often much reduced compared with that of a single-phase alloy having the same composition as the matrix of the dispersion alloy [21].The magnitude of these effects depends on the size and spacing of the intermetallic particles.As can be seen in Table 1, lower values of diameter of the IMC and inter-particle spacing (λ) are associated with the smaller powders (75-106µm) processed by extrusion.A combination of very fine intermetallic particles and homogeneous distribution of α-Al grains seems to be the reason why the hot extruded Al-1wt%Fe-1wt%Ni alloy powders with 75-106µm in size achieved sounder properties.The role of reinforcement of α-Al matrix promoted by the finer intermetallic particles embedded in the microstructure seems to be more efficient if compared to that devoted to the presence of coarser particles.
The binary Al-3wt%Ni alloy tested in a previous study [22,23] through unsteady-state directional solidification exhibited σ u values between 116 and 123MPa and δ values from 10-11%.Results obtained for directionally solidified Al-1.5wt%Fe [2] showed σ u values of 116 and 105MPa and δ of 23%.Kim and co-authors [1] examined the tensile mechanical properties of the Al-0.5wt%Fe-1.0wt%Nialloy fabricated by a permanent mold gravity casting method.The tensile tests showed σ u values of about 90MPa and δ values of 13%.
Therefore, the production of Al-Fe-Ni alloy powders followed by pressing and extrusion allowed more reliable tensile properties to be developed especially considering the gains in

Conclusions
We studied the effect of powder size during atomization and size distributions and morphologies of second-phase particles on Al-1wt%Fe-1wt%Ni alloy.From the results obtained in this study, the following conclusions can be drawn: 1.The as-atomized microstructure for this alloy solidified in a spray forming equipment under relatively fast cooling conditions is constituted of Al-rich cells enveloped by a eutectic mixture of α-Al +IMCs.Larger powder particles showed in their microstructure the growth of rod-like intermetallic particles in the intercellular regions.The average cell spacing determined for powder size ranges of 180-106µm, 106-75µm and <53µm correspond to 4.0µm, 3.2µm and 1.7µm, respectively.Based on the present study, it appears that there is a strong link between size and distribution of Al/Fe/Ni intermetallic particles and the tensile mechanical properties of the asextruded Al-1wt%Fe-1wt%Ni alloy.It can be stressed that the combination of low Ni/Fe solute contents and processing of atomized Al-Fe-Ni alloy powders by hot extrusion may be a promising route of processing to be developed in order to obtain more reliable tensile properties referring to components of industrial interest.

Figure captions
Fig. 1.Illustrative scheme of spray forming, extrusion apparatus and their devices.Atomized powder has been collected, cold-compacted and transferred to the hot extrusion process under two temperatures, which are 350ºC and 400ºC.
in order to examine and compare the microstructure and properties of binary and ternary Al-Ni-(Cr) alloys foils.The prevalence of high-velocity cells was shown to occur in the surface microstructure of the Al-0.6 at % Ni.It was also observed that annealing in the temperature range 260-380°C causes additional precipitation of Al 3 Ni fine particles from the supersaturated solid solution.Directionally solidified eutectic Al-Ni specimens were examined by Uan et al. [9].Some of these specimens were subsequently hot extruded in different temperatures from 200 to 400°C.The final subgrains were reported to be roughly equiaxed, having sizes of 1.14 ± 0.23 µm and 1.56 ± 0.30 µm for billets extruded at 200°C and at 400°C, respectively.Rod-like Al 3 Ni intermetallic particles were found all along the microstructure growing in both intergranular and intragranular regions.Solidification conditions and alloy composition are the main factors affecting the development of IMCs on as-cast Al-Fe-Ni alloys.According to Canté et al. [10], the isolated or simultaneous growth of typical phases such as Al 3 Fe, Al 6 Fe, Al 3 Ni and Al 9 FeNi is mainly affected by growth and cooling rates during solidification.Canté et al. [10] investigated the microstructure formation of the Al-1wt%Fe-1wt%Ni alloy from 0.8K/s to 36.5K/s.It was found that an arrangement of α-Al cells prevailed for the entire range of cooling rates with Al 9 FeNi IMC prevailing on the intercellular regions.Promising applications mentioned in the M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 4 referred work include parts of industrial furnaces and electrochemical plants [11-13].Very recently Engin et al. [3] focused their work on examining microstructures and hardness of unidirectionally solidified Al-Ni-Fe eutectic alloy.By increasing growth rate around twenty (20) times, eutectic spacing values λ Al3Ni and λ Al9NiFe decrease approximately five (5) times.

Fig. 8 .
Fig. 8. SEM images of (a) Al-Fe-Ni particles preserved during extrusion of the powder with size of 75-106µm processed at 350°C and (b) intermetallics at the cell boundaries and Al/Fe/Ni precipitates found for the size range of 106-180µm processed at 350°C.

Fig. 11 .
, the combination of low Ni and Fe contents and suitable tensile properties in the ternary Al-1wt%Fe-1wt%Ni alloy seems to be an attractive characteristic with a view to indicate this alloy for applications requiring such attributes.Representative engineering stress-strain diagrams obtained for hot-extruded Al-1wt.%Fe-1wt.%Ni specimens processed from N 2 -atomizing powders with different sizes and at different temperatures.M A N U S C R I P T A C C E P T E D

2 . 3 .
A single Hall-Petch type correlation encompasses the evolution of Vickers hardness with the cell spacing for both DS and atomized Al-Fe-Ni alloy samples.Fine cell spacing and the shape of the Al/Fe/Ni IMC seem to be significant for increasing hardness corresponding with as-atomized powders.The cellular array of the Al-Fe-Ni powder was not completely eliminated by extrusion process either at 350ºC or at 400ºC.The microstructure of as-extruded samples is M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 22 characterized by very fine and rounded IMCs embedded into an Al-rich matrix.The final microstructure has shown a relatively coarse arrangement of grains.The size and spacing of IMCs have performed a considerable influence on the mechanical properties of the Al-1wt%Fe-1wt%Ni alloy.A more optimized balance of tensile mechanical properties has been observed for the hot extruded powders with 75-106µm (350ºC).Strength of 178MPa and 31% of elongation-to-fracture are exhibited.

Fig. 8 .
Fig. 8. SEM images of (a) Al-Fe-Ni particles preserved during extrusion of the powder with size of 75-106µm processed at 350°C and (b) intermetallics at the cell boundaries and Al/Fe/Ni precipitates found for the size range of 106-180µm processed at 350°C.

Fig. 10 .
Fig.10.SEM images emphasizing sizes and morphologies of the Al/Fe/Ni IMCs after extrusion at 350°C performed with the Al-1wt%Fe-1wt%Ni alloy using powder sizes of (a) 75-106µm and (b) 106-180µm.d is the mean diameter and λ is average spacing of the intermetallic particles.
in Table1the best combination strength-ductility is 5%, respectively, corresponding to the total elongation of the extruded Al-Fe-Ni powder (75-106 µm).In contrast, around 15% and 24% of ductility are obtained for the case of hot extruded samples with the Al-Fe-Ni alloy for powders varying between 106µm and 180 µm.

Table 1 .
Powder size, extrusion condition, microstructure parameters and average tensile mechanical properties of the hot-extruded Al-1wt%Fe-1wt%Ni alloy.

Table heading Table 1 .
Powder size, extrusion condition, microstructure parameters and average tensile mechanical properties of the hot-extruded Al-1wt%Fe-1wt%Ni alloy.