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Vol. 8. Issue 2.
Pages 2092-2097 (April 2019)
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Vol. 8. Issue 2.
Pages 2092-2097 (April 2019)
Original Article
DOI: 10.1016/j.jmrt.2018.12.022
Open Access
Microstructure and mechanical behavior of Al92Fe3Cr2X3 (X=Ce, Mn, Ti, and V) alloys processed by centrifugal force casting
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Guilherme Yuuki Kogaa,
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guilherme.koga@dema.ufscar.br

Corresponding author.
, Ana Martha Branquinho e Silvaa, Witor Wolfa,b, Claudio Shyinti Kiminamia, Claudemiro Bolfarinia, Walter José Bottaa,c
a Federal University of São Carlos, Department of Materials Science and Engineering, Rod. Washington Luis, CEP 13565-905, São Carlos, SP, Brazil
b Federal University of Minas Gerais, Department of Metallurgical and Materials Engineering, CEP 31270-901, Belo Horizonte, MG, Brazil
c Grenoble Alpes University, CNRS, LEPMI, F-380000 Grenoble, France
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Abstract

Microstructural and mechanical characterization of Al92Fe3Cr2X3 (X=Ce, Mn, Ti, and V) alloys were performed. The alloys were processed by a method that uses centrifugal force to cast the samples into a rotating copper mold. Microstructural characterization was carried out by means of x-ray diffraction, scanning electron microscopy, and differential scanning calorimetry. Compressive tests at room and at 300°C were performed in selected samples to evaluate their mechanical properties. Microstructural characterization showed the formation of quasicrystalline phases as well as other intermetallic phases embedded within an Al-FCC matrix. The Ce-containing alloy exhibited promising results regarding quasicrystalline phase formation and stability as well as with respect to its mechanical properties at high temperatures. The quasicrystalline phase of this alloy appears to be stable up to 545°C when the DSC reveals an exothermic transformation. In addition, the presence of a eutectic structure surrounding the Al-FCC grains enhanced the mechanical strength of this alloy. At 300°C, the Ce-containing alloy showed yield strength and ultimate tensile strength of 180MPa and 360MPa, respectively. If compared to a commercial aluminum alloy 2024 at the T6 condition, close to 300°C, the alloy studied here showed an increase of more than 4 times in the yield strength, and almost 7 times in the ultimate tensile strength. The high thermal stability and mechanical properties at high temperatures of this alloy open interesting possibilities for further studies and future applications of this Al-Fe-Cr-Ce alloy.

Keywords:
Al-alloys
Centrifugal force casting
Quasicrystal
Mechanical properties
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1Introduction

Al-based alloys represent the second most used metallic material class worldwide and the recent decades have witnessed a continuous increase in their production, expected to duplicate or triplicate by 2050 [1,2]. The versatile properties of these alloys allow their use in different industrial segments, and the current progress on Al-based alloys has extended their application domain. For instance, Al-based alloys containing icosahedral quasicrystals (QCs) embedded within Al-FCC matrix have been attracting the scientific and the industrial attention due to their combination of mechanical properties. These include high strength, hardness, and plasticity at a wide range of temperatures. Besides the attractive mechanical properties, Al-based alloys containing QCs exhibit interesting physicochemical characteristics such as low adhesion and wetting, and appropriate corrosion resistance.

Among the Al-systems with the potential to form QCs, the Al-Fe-Cr is under interest because (i) the addition of Fe produces icosahedral clusters in an Al-based liquid [3], and (ii) the presence of Cr contributes to improve the corrosion resistance and to favor the formation of QCs [4]. Other alloying elements are also recognized to induce a composite microstructure of QCs within an Al-FCC matrix. Among them, it is well known that Mn promotes icosahedral phase formation in Al-based alloys since the first system reported to contain a QC phase was the binary Al-Mn [5]. Cerium has also been reported to enhance the formation of QCs in Al-Mn alloys [6,7]; however, some recent studies have pointed out that actually, Ce destabilizes the Al-Mn QC phase [8,9]. The QC phase found in the Al-Fe-Cr system is known to be metastable [10], and additions of Ti, V, Ta and Nb alloying elements have been shown to improve the thermal stability of the icosahedral phase in melt-spun ribbons in the Al-Fe-Cr system [10–13]. Moreover, mechanical properties at high temperature are found to be enhanced by the addition of those elements [14]. However, the additions of Ce in the Al-Fe-Cr system still needs to be further studied with respect to QC phase formation and mechanical behavior.

Al-based QCs have been synthesized by rapid quenching processes [15,16], conventional solidification methods [17–19], and by mechanical alloying [20,21]. The suitable solidification technique for a given alloy depends on its composition and the stability of the QC phase, but most of the QC alloys are metastable, therefore, QC phases are normally formed in rapidly solidified alloys. In this paper, we evaluated the microstructure of Al-Fe-Cr-based alloys resulting from a centrifugal casting into a rotating copper mold. The microstructure, the formation of QCs, and the mechanical behavior of the different Al92Fe3Cr2X3 (X=Ce, Mn, Ti, and V) alloys were discussed based on the effect of different alloying addition.

2Experimental procedure

Multicomponent Al92Fe3Cr2X3 (X=Ce, Mn, Ti, and V) alloys were prepared using high purity elements (>99%) after being chemically cleaned and then degreased with a large volume of acetone in ultrasonic vibration. Master alloys of about 10g were arc melted under Ti-gettered argon atmosphere. Chemical homogeneity of the ingots was ensured by re-melting them several times. About 4g of the master alloys were used to fabricate the alloys by centrifugal force casting. The ingots were induction-melted in a quartz tube, ejected using a difference in gas pressure of 30kPa, and then rapid-quenched in a copper mold disc rotating at 26m/s, as schematically shown in Fig. 1. Details of this fabrication process can be found in previous work [22]. The resulting centrifugal cast specimens were small bars with a section of 4mm×4mm and length of about 50mm.

Fig. 1.

(a) Schematic view of the copper wheel used as the mold for centrifugal force casting, and (b) the resulting bar in which the samples were taken from region 1 was used for mechanical testing, from region 2 for microstructure characterization, and from region 3 for thermal analysis. Ruler in cm.

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The microstructure of the samples was examined by scanning electron microscopy (SEM), in a Philips XL30 FEG equipped with energy-dispersive spectroscopy (EDS) (OXFORD – LINK ISIS 300), and by x-ray diffraction in a Siemens D5005 with Mo-Kα radiation. Thermal analysis was carried out in a differential scanning calorimeter (DSC), in a Netzsch 404, at the heating rate of 20K/min.

Compression tests were performed on an Instron 5500R device at a constant rate of 1mm/min at room temperature and at 300°C. Testing samples measuring 4mm×4mm and 8mm length were extracted from the centrifugal cast specimens. The top and bottom surfaces were ground and polished before experiments to avoid deviation from orthogonality.

3Results and discussion

Fig. 2 shows the XRD patterns of the centrifugal force cast samples with the identification of phases observed. The quasicrystalline pattern used here for identification was taken from previous work by Galano et al. [13]. The samples present an Al-FCC matrix with intermetallic phases, including quasicrystals. Binary intermetallic phases such as Al6Mn, Al3Ce, Al11Ce3, Al3V, and Al3Ti were present in the samples as well as Al13Fe4 and Al13Cr2 (in Fig. 2 identified together as θ-Al13(Cr,Fe)2–4) that were identified in all of the samples except for the Al-Fe-Cr-Ce ones. The icosahedral phase could be identified on the Ti-, V-, and Ce-containing samples, and its presence is corroborated by the microstructure observed under SEM examination that will be presented in the sequence.

Fig. 2.

XRD patterns of the samples produced by centrifugal force casting.

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A variety of microstructures composed of quasicrystalline and polycrystalline aggregates embedded within an Al-FCC matrix are observed in Fig. 3. Near-spherical phases can be seen as the yellow arrows are outlining in Fig. 3. In Fig. 3a, the microstructure of the Ce-containing sample is depicted, and it is possible to observe in the insert a near-spherical particle from which, a faceted phase grows in its radial direction. It is known that the Al-Fe-Cr-X (X=transition metals) system can form metastable quasicrystals embedded within an Al-FCC matrix, which has been the object of study because of the interesting mechanical properties presented by these materials [10–13,23,24]. This metastable quasicrystal is only formed if solidification occurs with a certain cooling velocity. If the solidification is not fast enough, the intermetallic phases will form instead, such as Al13Fe4, Al13Cr2[10,13,24], and binary phases with the fourth element of the alloy, in the case of Fig. 3a, Al-Ce phases. Fig. 3a also shows that some remaining near-spherical particles are present without the radial growth of faceted intermetallic phases. The XRD pattern of this sample showed Al3Ce and Al11Ce3 phases but the presence of Al13Fe4 and Al13Cr2 was not observed. There is a strong indication that Al3Ce is the faceted phase that is growing from the spherical quasicrystalline particles. This is because previous work with Al-Fe-Cr-(Ti, Mn) alloys showed that the destabilization of the quasicrystal leads to the formation of Al3Ti and Al6Mn, and probably the same effect is observed here for the Ce-containing alloy. Another interesting microstructural feature is that the Al-FCC matrix is interconnected by another phase, forming FCC islands surrounded by intermetallic phases. The insert in Fig. 3a actually shows that the FCC islands are interconnected by what appears to be a eutectic structure, which is consistent to the fact that the Al-Ce phase diagram shows a eutectic reaction between Al and Al11Ce3[25], and both phases were identified in the XRD pattern.

Fig. 3.

SEM images of the cross-sectional region of Al92Fe3Cr2X3 bars produced by centrifugal force casting, where X stands for (a) Ce, (b) Mn, (c) Ti and (d) V.

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Fig. 3b shows the microstructure from the Al-Fe-Cr-Mn sample. This alloy shows the presence of large faceted phases and some dendrites, possibly arising from previous quasicrystalline nuclei that were destabilized during the solidification. The yellow arrow shows the presence of a remaining small near-spherical particle that is probably a quasicrystalline phase, and it is present in a much lower fraction in comparison to the other alloys (as seen in the XRD patterns). A previous work by Rios et al. [24] showed that the Al-Fe-Cr-Mn system displays quasicrystalline phase formation on melt-spun samples but for the case of thicker samples, the quasicrystal was destabilized. The cooling rate of the centrifugal force casting process is between 102 and 103K/s while for the melt-spinning process it is about 106K/s. Fig. 3c and d is representative of the microstructures shown by the Al-Fe-Cr-Ti and Al-Fe-Cr-V alloys, respectively. Both microstructures are very similar, showing near-spherical particles from which dendritic arms grow in a radial fashion. The main difference is that the microstructure of the Ti-containing alloy is more refined.

SEM images indicate that the Ce-containing alloy is the one with a higher fraction of quasicrystalline particles free from the radial growth of intermetallic phases. It means that this quasicrystalline phase should present higher thermal stability than the quasicrystals from the other alloys studied here. This higher fraction of quasicrystalline phase is further supported by the DSC results, shown in Fig. 4. The only sample that displayed an exothermic transformation was the Ce-containing sample, which is related to the transformation of the quasicrystal to a stable intermetallic phase. The other samples probably did not have enough fraction of quasicrystalline phase to be revealed by the DSC experiment. In addition, the onset of the exothermic transformation is around 545°C, confirming this high stability of the quasicrystalline phase. The previous study from Galano et al. [13] demonstrated the influence of alloying elements on melt-spun Al-Fe-Cr-(Ti,V,Nb,Ta) alloys, and showed that addition of these elements enhances the thermal stability of the quasicrystalline phase. The DSC results in that study revealed that the onset of these transformation temperatures was always below 500°C. This is another indicative that Ce additions to the Al-Fe-Cr quasicrystal enhance its thermal stability. This result is potentially interesting because the formation of quasicrystals embedded within Al-FCC matrix would be possible with lower cooling rates for the Al-Fe-Cr-Ce alloy, allowing fabrication of thicker samples.

Fig. 4.

DSC curves of Al92Fe3Cr2X3 (X=Ce, Mn, Ti, and V) alloys produced by centrifugal casting method.

(0.08MB).

Fig. 5 shows the results from compressive tests of the centrifugal force cast samples. The Ce-containing sample exhibited substantially higher yield strength, 380MPa, in comparison to the other samples. The Mn-, Ti-, and V-containing samples showed a yield strength of 260, 220 and 250MPa, respectively. This higher resistance to plastic deformation of the Ce-containing sample can be attributed to the higher fraction of quasicrystalline phase as well as for the eutectic structure that surrounds the Al-FCC grains, acting as a barrier for the dislocation movement. The ultimate tensile strength, UTS, of the Ce- and Ti-containing alloys were the highest ones, with values of 450 and 430MPa, respectively, which are consistent with bulk Al-based alloys with dispersed quasicrystalline particles [6]. The Ti-containing sample displayed a high elongation, of about 45%, and this can be attributed to the refined dispersion of the intermetallic and quasicrystalline phases, as shown in Fig. 3c.

Fig. 5.

Effect of the different alloys additions on the true stress vs. strain curves for the Al92Fe3Cr2X3 (X=Ce, Mn, Ti, and V) bars from centrifugal cast.

(0.08MB).

The Ce-containing sample was also tested at 300°C, and the result is shown in Fig. 6. At this temperature, the yield strength of the alloy was about 180MPa and the UTS was about 360MPa. This means a 20% decrease in the UTS in comparison to the room temperature test. In addition, an elongation of almost 50% was observed in this test. The results are very interesting if compared for example with high strength commercial aluminum alloys such as the AA2024 at the T6 condition. This aluminum alloy, according to ASM handbook, volume 2, [26], shows at room temperature yield strength and UTS of 393 and 476MPa, respectively. The Ce-containing sample showed results very close to the ones of the commercial alloy. The results at higher temperatures, however, show a remarkable advantage of the quasicrystalline alloy when compared to the commercial one. The AA2024 at the T6 condition, according to the ASM handbook, shows at 316°C (the closest temperature with information available for comparison) a yield strength and UTS of 41 and 52MPa, respectively. This means that the quasicrystalline sample showed an increase of more than 4 times in the yield strength, and almost 7 times in the UTS in comparison to a commercial 2024 aluminum alloy. This substantial increase on the mechanical properties at a high temperature of quasicrystalline alloys is due to the fact that these phases have a sluggish growth rate [27], and thus they have less tendency to aggregate. In other words, they are kept with a good dispersion in the Al-FCC matrix and maintain the restrictions to dislocation movements effectively. In the case of commercial aluminum alloys, the coherent particles, after been exposed to higher temperatures, grow into bigger particles, become incoherent phases and aggregate, becoming less effective to restrict dislocation movement.

Fig. 6.

Effect of the temperature on the true stress vs. strain curves for the Al92Fe3Cr2Ce3 bars from centrifugal casting.

(0.08MB).

The mechanical strength of the Ce-containing sample is slightly inferior to the results presented by Galano et al. [12], however, the samples studied in that case were produced by melt-spinning, which in turn increased the fraction of quasicrystalline phase and resulted in a more refined microstructure. Nonetheless, the results obtained here for the sample tested at 300°C show a very promising field for application of this alloy for structural applications. The Ce-containing sample not only exhibited great mechanical properties at high temperature, but it also displayed good thermal stability. This could facilitate its processing, allowing it to be fabricated as thicker samples, which is not possible for the other systems studied here. This higher thermal stability also means that this alloy could be applied at higher temperatures (up to around 500°C), still presenting good mechanical properties, since according to Fig. 4, the quasicrystalline phase is only destabilized around 545°C.

4Conclusions

Quasicrystalline phase formation and mechanical properties of Al92Fe3Cr2X3 (X=Ce, Mn, Ti, and V) alloys fabricated by centrifugal force casting were assessed. The main conclusions can be drawn as follows:

  • -

    The Ce-containing alloy was the one with the higher tendency to form quasicrystals among the studied alloys. In addition, the quasicrystalline phase formed showed remarkable thermal stability and the temperature of quasicrystalline destabilization was around 545°C, which is very high for Al-Fe-Cr-based quasicrystals.

  • -

    The microstructure of the Ce-containing alloy consisting of quasicrystalline and intermetallic particles and a eutectic structure surrounding Al-FCC grains lead to the higher yield strength among the studied alloys.

  • -

    The refined microstructure of the Ti-containing alloy leads to the highest elongation at failure among the alloys.

  • -

    At 300°C, the Ce-containing alloy showed yield strength and ultimate tensile strength of 180 and 360MPa and, when compared to a commercial aluminum alloy 2024 at T6 condition, close to 300°C, an increase of more than 4 times in the yield strength, and almost 7 times in the ultimate tensile strength is observed.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

This work was mainly supported by FAPESP through a Thematic Project (grant number 2013/05987-8) and through a post-doctoral FAPESP funding (grant number 2017/09237-4). The authors gratefully acknowledge the Brazilian financial support agencies CAPES and CNPq, and to the Laboratory of Structural Characterization of the Federal University of São Carlos (LCE/DEMa/UFSCar) for the use of electron microscopy and x-ray diffraction facilities.

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