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Designing new quasicrystalline compositions in Al-based alloys
Journal Pre-proof Designing new quasicrystalline compositions in Al-based alloys Witor Wolf, Claudemiro Bolfarini, Claudio S. Kiminami, Walter J. Botta PII:
S0925-8388(20)30128-6
DOI:
https://doi.org/10.1016/j.jallcom.2020.153765
Reference:
JALCOM 153765
To appear in:
Journal of Alloys and Compounds
Received Date: 26 September 2019 Revised Date:
18 December 2019
Accepted Date: 8 January 2020
Please cite this article as: W. Wolf, C. Bolfarini, C.S. Kiminami, W.J. Botta, Designing new quasicrystalline compositions in Al-based alloys, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2020.153765. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Designing new quasicrystalline compositions in Al-based alloys Witor Wolfa, Claudemiro Bolfarinib, Claudio S. Kiminamib and Walter J. Bottab,*
a
Departamento de Engenharia Metalúrgica e de Materiais, Universidade Federal de Minas Gerais, Av. Antônio Carlos, 6627, Belo Horizonte, MG, 31270-901, Brasil. b
Departamento de Engenharia de Materiais, Universidade Federal de São Carlos, Rod.Washington Luiz, Km 235, São Carlos, SP, 13565-905, Brasil. *Corresponding Author. Tel +55 16 33519481 / Fax +55 16 3361-5404
e-mail addresses: [email protected] (Witor Wolf), [email protected] (Claudemiro Bolfarini), [email protected] (Claudio S. Kiminami), [email protected] (Walter J. Botta).
Abstract In the present work, we present and discuss the most important results that were obtained in the past few years related with the development of new quaternary quasicrystalline alloys and composites. We present the alloy design strategies that we have used and that led to the discovery of over 60 new quasicrystalline compositions, 2 new quasicrystal-forming systems and 1 new Al-matrix composite. New results are also presented and discussed here. Results of interest discussed in this work include: reassessment of quasicrystal and approximant phase formation on the Al-Co-Fe-Cr system; influence of Cr and Ni additions on the icosahedral Al-Cu-Fe quasicrystal stability; discovery of a decagonal Al-Ni-Co-Cr quasicrystal; composition range of formation of the decagonal Al-Cu-Fe-Cr quasicrystal using combinatorial strategies; fabrication of aluminum matrix composites reinforced with quasicrystals using conventional metallurgy fabrication methods.
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Keywords: Quasicrystals; Al alloys; Alloy design; Al-matrix composites.
Highlights We present and discuss a successful alloy design approach applied to discover new QCs. Al-Cu-Fe-Cr was the system with larger composition range of formation among the alloys studied. Al-Cu-Fe-Cr system is adequate to produce Al-matrix composites reinforced with QCs by conventional casting.
1. Introduction Quasicrystalline (QC) and approximant phases present distinct physical and mechanical properties associated with the complexity of their unit cell, typically containing large number of atoms [1]. Properties such as low thermal conductivity, low friction coefficient and wear resistance (despite brittle behavior) attracted important efforts in the development of alloys containing such phases for protective coating applications [2– 6]. One of the main challenges to design, or discover, new QC-forming alloys are related to the narrow composition ranges, in which such phases form and/or are stable [7]. In our group we have revisited different Al-based alloys, known to form QC phases, to study the effects of alloying and metastable processing on the formation of new QC or approximant phases [2,8–12]. We have also designed Al-matrix composites reinforced with QCs [13–15]. The systems we have studied are Al-Cu-Fe, Al-Cu-Fe-Cr, Al-Cu-Fe-Ni, Al-Ni-Co, AlNi-Co-Cr, Al-Ni-Co-Cu, Al-Fe-Cr-(M=Ce,Ti,Mn,V) and Al-Co-Fe-Cr. Processing 2
routes that we have approached includesd melt-spinning, gas-atomization, HVOF spraying, arc melting, powder metallurgy and magnetron sputtering. We have performed full structural and microstructural characterization in several compositions, which led to discovery of over 60 new QC-forming alloys and 2 new QC-forming metallic systems (Al-Cu-Fe-Ni and Al-Ni-Co-Cr) [8,10,11]. We have also discovered the first metallic system, Al-Cu-Fe-Cr, forming QCs embedded in Al-FCC matrix which can be fabricated by conventional metallurgical fabrication processes [13]. In addition, functional properties such as friction coefficient, wear resistance and thermal conductivity were assessed in coatings fabricated by HVOF [2]. Mechanical properties of composites were evaluated as well [14,15]. We also suggested and applied combinatorial strategies, involving magnetron co-sputtering and high-throughput XRD and EDX analysis, to map and design new QC compositions in the Al-Cu-Fe and AlCu-Fe-Cr systems [11]. We present the rationale behind the alloy design strategies that we have used in the development of new quaternary QC alloys. New results are also presented and discussed here, which includes QC phase formation on arc-melted Al-CuFe-Cr and on melt-spun Al-Ni-Co-Cu alloys. In addition we show the as-cast microstructure of the Al85Cu6.75Fe3.375Cr4.875 alloy, the first conventionally casted alloy forming a composite microstructure containing QCs and Al-FCC (which such microstructure only forms after an appropriate annealing) [13]. 2. Designing new quasicrystalline compositions 2.1 Revisiting QC forming systems QCs were first reported by Daniel Shechtman in an Al-Mn melt-spun alloy [16]. Although refuted at first, his discovery lead to huge efforts in the sense of understanding their atomic structure and physical properties.
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Since this first metallic QC-forming system, many others have been discovered, most of them Al-based, such as Al-Cu-Fe, Al-Cu-Fe-Cr, Al-Ni-Co, Al-Fe-Cr-(M= transition metal), Al-Pd-Mn, Al-Mn-Si, Al-Li-Cu among others [17–24]. Some QC phases are considered thermodynamically stable while others can only be obtained with metastable processing routes. The interest on stable QCs is related to the ability to fabricate them using conventional casting techniques. In the past few years, our work has been focused on developing new QC alloys, based on traditionally known QC forming systems by adding alloying elements on them. Table 1 shows the previously known QC systems that we have revisited, the chemical compositions we fabricated, processing routes used and the phases that were obtained. In this table we did not add the Al-Cu-Fe-Cr system because, although this system was already known to form QCs, we have made an extensive work on it to find its compositional range of formation and thus it will be discussed in more detail in the sequence. The references where phase formation, shown in Table 1, can be found are in the Table´s 1 legend. XRD patterns of alloys that have not been published yet are available in Supplemental Material. Three ternary systems have been studied and formation of QC was observed in all of them, as expected from previously published studies. High energy ball milling to alloy Al, Cu and Fe requires posterior annealing to form QCs. We confirm the influence of cooling rates on the icosahedral phase formation from the Al-Cu-Fe system, which is favored by high cooling rates, > 104 °C/s. In the supplemental material, three XRD patterns of an Al65Cu22.5Fe12.5 alloy fabricated in three different processing conditions (a 10g arc-melteding a 10 g button, a melt-spun ribbon and a 1 mm arc-melteding a 1 mm thick plate sample that was Cu-quenched) are shown. The cooling rates in each case were in the range of 102 °C/s, 106 °C/s and 104 °C/s respectively and it can be observed 4
that the relative peak intensity of the QC phase increases with increasing cooling rates. This was expected as reported in previous studies [17,25]. These cooling rates values were calculated using the same procedure of reference [26]. The Al-Pd-Mn alloy studied easily formed a QC phase even for a sample which solidified with cooling rates in the range of 102 °C/s. However, the presence of high Pd contents is a huge obstacle for applying this QC system due to its cost. The Al-Ni-Co alloy formed a decagonal QC with some Al-Ni based intermetallic phases. High cooling rates did not prevent formation of such phases, as opposed to what was observed in the Al-Cu-Fe alloys. This can be observed by comparing the XRD pattern of this ternary alloy fabricated by meltspinning (see reference [10]) and by arc-melting a 1 mm thick plate sample that was Cuquenched (see Supplemental Material). The quaternary system Al-Co-Fe-Cr was studied aiming applications as thermal barrier coatings. The literature information about the most famous alloy of this system, Al71Co13Fe8Cr8, was somewhat contradictory. We showed, by fabricating this alloy with different processing routes, that the microstructure of as-cast samples is basically always composed by a hexagonal, µ-Al5Co2, and a monoclinic Al13Co4 phases. Both are QC approximants and exhibit interesting physical properties for application as protective coatings. Aiming to find new QC compositions, two other alloys from this system were fabricated by rapid solidification processes. However, no QC phases were found. The chemical compositions were selected based on the valence electron concentration (e/a) which would be closer to the usually found on ternary QCs [27], which however, was unsuccessful.
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Table 1 Previously known QC-forming systems that we have revisited with their chemical compositions, processing routes used, and the phases obtained. Results taken from references: [2,9,10,12,15].
Metallic system Al-Cu-Fe
Alloy Al65Cu20Fe15
Al65Cu22.5Fe12.5
Processing route
Phases
High energy ball milling
Al, Cu, Fe, θ-Al2Cu
Arc-melting + annealing at 700°C
i-QC
Arc-melting
i-QC + β-AlCu(Fe) + λ-Al13Fe4 + Al2Cu
Melt-spinning
i-QC + τ-AlCu(Fe)
1 mm plate - Cuquenched
i-QC + τ-AlCu(Fe)
Al-Pd-Mn
Al70.5Pd21Mn8.5
Arc-melting
i-QC
Al-Co-Fe-Cr
Al71Co13Fe8Cr8
1 mm plate - Cuquenched
µ-Al5Co2 + MAl13Co4 + Cr-richunknown
Melt-spinning
µ-Al5Co2 + MAl13Co4
Gas-atomization
µ-Al5Co2 + MAl13Co4
Gas-atomization + annealing at 1000 °C
µ-Al5Co2
HVOF
µ-Al5Co2 + MAl13Co4
1 mm plate - Cuquenched
M-Al13Co4 + OAl13Co4
Melt-spinning
M-Al13Co4 + OAl13Co4 + Y-Al13Co4
1 mm plate - Cuquenched
M-Al13Co4 + Al9Co2
Melt-spinning
M-Al13Co4
Melt-spinning
d-QC + Al3Ni(Co) Al3Ni2(Co)
1 mm plate - Cuquenched
d-QC + Al3Ni(Co) Al3Ni2(Co)
Al77Co11Fe6Cr6
Al76Co19Fe4Cr1
Al-Ni-Co
Al71Ni20Co9
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2.2 Designing new quaternary QC alloys based on Al-Cu-Fe and Al-Ni-Co systems From the systems studied in Table 1, two were selected based on the ability to form QCs and costs related to the alloying elements. These systems were Al-Cu-Fe and AlNi-Co. Based on selected ternary compositions, we have systematically added Cr and Ni on the Al-Cu-Fe system and Cr and Cu on the Al-Ni-Co system aiming to retain the QC atomic structure even after the alloying additions. The choice of chemical composition of each alloy was based on trying to increase the fourth element fraction and selecting values of valence electron concentration (e/a) as close as possible to the values found for Al-Cu-Fe and Al-Ni-Co QCs. Table 2 shows the quaternary alloys that were studied, including the fabrication processes and the phases identified. The references where details about phase formation, shown in Table 2, can be found are in the Table´s 2 legend. XRD patterns of alloys that have not been published yet are available in Supplemental Material. Cr addition on the Al-Cu-Fe QC leads to a gradual change on the atomic structure of the QC phase. Small additions, ~3 at. % Cr results oin the coexistence of the icosahedral and decagonal QCs, with some presence of the cubic τ-AlCu phase when the alloy is fabricated using lower cooling rates, as shown in the XRD pattern in Supplemental Material. Figure 1 shows an energy dispersive X-ray spectroscopy (EDX) mapping of a melt-spun Al65Cu22Fe10Cr3 alloy, which forms an icosahedral QC with small amounts of the decagonal QC (Cr-enriched). More details of this alloy´s phase formation and characterization can be found in reference [8].
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Figure 1 STEM EDX mapping from the Al65Cu22Fe10Cr3alloy. The matrix is the icosahedral QC and the Cr-rich phase is the decagonal QC. Details of the phase formation in this alloy can be found elsewhere [8].
Increasing Cr content above a certain limit leads to only one QC phase, with decagonal structure. The Al67Cu20Fe5Cr8 alloy was processed by fast and low solidification methods and in all the experiments, a decagonal phase co-existed with a cubic phase, with the same structure of the τ-AlCu(Fe) phase forming in ternary rapid solidified AlCu-Fe QC forming alloys. The same cubic phase was observed in reference [28]. Figure 2a shows an EDX mapping of a melt-spun Al67Cu20Fe5Cr8 alloy, which is formed by a decagonal QC matrix with the presence of the τ-AlCu(Fe) phase (phase enriched in Cu) forming at the grain boundaries. Details of this alloy´s phase formation can be found in reference [8]. In figure 2b, the same EDX mapping analysis-type was performed in the ternary melt-spunAl65Cu22.5Fe12.5 alloy which has an icosahedral QC matrix but also forms the τ-AlCu(Fe) phase, in the same way observed in the quaternary alloy. Figure 2c shows a TEM bright-field micrograph and the selected area electron diffraction pattern (SAED) of this τ-AlCu(Fe) phase taken along the [111] zone axis, from the
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ternary alloy. This phase is cubic with a CsCl-type structure and lattice parameter a = 2.94 Å. Figure 2d, also from the ternary alloy, shows an interesting “star-like” morphology of the icosahedral QC phase, with the respective SAED, taken along the 5fold axis of rotational symmetry. The bright-field micrograph was taken from the same the QC phase of the SAED pattern with the particle oriented in the 5-fold symmetry axis. This explains the “star-like” morphology, which reveals this material´s 5-fold symmetry and its influence on the growth of the solid phase.
Figure 2 (a) EDX mapping of a melt-spun Al67Cu20Fe5Cr8 alloy, showing the τ-AlCu(Fe) phase, which is located in the Cu-rich regions. (b) EDX mapping from the ternary melt-spun Al65Cu22.5Fe12.5 alloy also showing the τ-AlCu(Fe) phase. (c) Bright-field TEM micrograph of the ternary alloy with a SAED pattern of the τ-AlCu(Fe) phase taken along the [111] zone axis. (d) Bright-field TEM micrograph of the ternary alloy, displaying a star-like morphology of showing the icosahedral QC and the SAED pattern taken along the 5-fold axis.
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In addition to the interesting results we obtained regarding the relatively easiness of fabricating QCs in the quaternary Al-Cu-Fe-Cr system, formation of QC phases in this system has also been reported by other authors and with a number of different atomic compositions [6,29–35]. This motivated an extensive study we performed using combinatorial strategies [11]. This included fabrication of compositional libraries by magnetron co-sputtering of Al, Cu, Fe and Cr targets on a Si wafer, which produced samples with gradient composition containing hundreds of alloys from this system. The samples were than characterized by scanning X-ray diffraction and EDX with fast data acquisition to identify phases in the libraries. Using this strategy, we were able to screen the decagonal QC range of formation on the Al-Cu-Fe-Cr system for Al atomic contents from 65 to 70 %. We have studied over 300 different alloy compositions and found 59 new decagonal QC dominant compositions. The QC composition range of formation can be found in Wolf et al. [11]. On the other hand, Ni additions on the Al-Cu-Fe QC did not lead to formation of a decagonal QC. We found that the icosahedral QC can dissolve up to 4 at. % Ni in its atomic structure. However, additions superior to that led to destabilization of the QC in favor of a B2 cubic phase, which was the major phase in the higher Ni-containing alloy, Al64Cu20Fe13Ni9. It is also interesting to comment on the morphological differences that QC phases can be produced, which strongly depends on the cooling rates applied during solidification and on the QC atomic structure. For instance, the Al-Cu-Fe icosahedral QC can be formed with a nanometric QC grain size, if fast cooling rates are applied (Fig. 2d), or it can form an envelope-like and micrometric grain-sized microstructure, which is a consequence of the peritectic reaction that results in its formation under low cooling rates during solidification [17]. The Al-Cu-Fe-Cr decagonal QC also forms as 10
nanometric QC grains if fast cooling rates are applied (Fig. 2a) or it forms as elongated micrometric-sized columnar grains if low cooling rates are applied [11]. Cr additions on the Al-Ni-Co decagonal QC led to discovery of a new quaternary QC, also decagonal, which formed in two melt-spun alloys. The composition of this phase was measured by EDX and it was found to be Al78.2Ni8.7Cr10Co3.1. However, the alloys studied also formed two Al-Ni based intermetallic phases, even when applying rapid solidification processes. The thermal stability of this new QC continues for future elucidation. Cu additions enhanced intermetallic phase formation in detriment of the QC phase. This can be seen in the XRD patterns of melt spun Al71Ni20Co7Cu2 and Al70Ni20Co5Cu5 alloys, shown in the Supplemental Material. If compared to XRD patterns of the Al-Ni-Co-Cr melt spun alloys (see reference [10]), the Cu-containing alloys show significantly higher contents of Al3Ni and Al3Ni2 phases. For this reason, this system was the less promising one and was not further studied. The design strategies used here to produce new QC compositions proved to be efficient and led to the discovery of more than 60 new QC-based alloys. The main idea behind this strategy is: define a ternary QC to be used as a base alloy. Using empirical QCforming rules, such as the valence electron concentration, add a fourth element in different proportions. Applying rapid solidification fabrication processes is the most adequate way to produce the quaternary alloys, because since QCs usually form around a number of several intermetallic phases, whose formation can be suppressed by fast cooling rates can suppress their formation. If this fourth element addition leads to QC formation, then this quaternary system is a potential candidate for the second step of the alloy design. This second step would be applying a method, in which the fabrication and characterization of hundreds of alloy compositions can be performed so that the compositional range where the quaternary phase can be fabricated will be disclosed. 11
This can be done by magnetron co-sputtering [36–38] or by using diffusion multiples [39]. Table 2 New quaternary alloys that were studied, including the fabrication processes used and the phases observed [8,10,11].
Metallic system Al-Cu-Fe-Cr
Alloy Al65Cu22Fe10Cr3
Al-Ni-Co-Cr
Al-Ni-Co-Cu
Phases
Melt-spinning
i-QC + d-QC
1 mm plate - Cuquenched
i-QC + d-QC + τAlCu(Fe)
Arc-melting + annealing at 600 °C
d-QC + τ-AlCu(Fe)
Melt-spinning
d-QC + τ-AlCu(Fe)
Al63Cu18Fe10Ni3
Melt-spinning
i-QC + 2 B2 cubic phases + λ-Al13Fe4
Al64Cu20Fe13Ni9
Melt-spinning
i-QC + 2 B2 cubic phases
Al72Ni19Co7Cr2
Melt-spinning
d-QC + d-QC(Crrich) + Al3Ni(Co) Al3Ni2(Co)
Al72Ni20Co5Cr3
Melt-spinning
d-QC + d-QC(Crrich) + Al3Ni(Co) Al3Ni2(Co)
Al71Ni20Co7Cu2
Melt-spinning
d-QC+ Al3Ni(Co) Al3Ni2(Co)
Al70Ni20Co5Cu5
Melt-spinning
d-QC + Al3Ni(Co) Al3Ni2(Co)
Al67Cu20Fe5Cr8
Al-Cu-Fe-Ni
Processing route
3. Al-matrix composites reinforced with quasicrystals In addition to discovering new QC forming compositions, our research has also focused on developing Al-matrix composites reinforced with QCs. Although QCs show remarkable functional physical properties that could be used for tribological protection [40,41], Due to thetheir inherent brittleness of QCs, makes their use becomes very 12
limited, especially for structural applications. To overcome this difficulty, QC phases must be embedded in a ductile matrix. There are two traditional methods to produce such composites. The first one involves rapid solidification techniques, such as melt-spinning, Cu-mold quenching and gasatomization [14,22,42–45] and can lead to the fabrication of composites reinforced with nanosized QCs, which is very interesting and can result in a high strength composites. The main drawbacks are related to the complex fabrication processes needed and the fact that QCs forming in these systems are metastable and usually transform to crystalline phases after heating around 350 to 500 °C. The second way to fabricate theise composites is based on powder metallurgy [15,46]. This process allows fabrication of composites reinforced with stable QCs, however, nanosized QC particles embedded in the ductile matrix are hard to be obtained. Following the results obtained in our combinatorial studies of the Al-Cu-Fe-Cr system and from literature data regarding previously published Al-Cu-Fe-Cr QC compositions, we have developed an Al-matrix composite reinforced with QCs by arc-melting an Al85Cu6.75Fe3.375Cr4.875 alloy and subsequently annealing it at 600 °C. The details of the annealed alloy´s microstructure and fabrication can be found in reference [13]. This was the first time such microstructure could be obtained by conventional fabrication methods. Here we will show the details of the as-cast alloy, which was the precursor of the QC composite. Figure 3 shows the XRD pattern and SEM micrograph of the Al85Cu6.75Fe3.375Cr4.875 arc-melted alloy in the as-cast condition. The XRD pattern of the as-cast sample is very different from the one of the annealed sample, from reference [13].
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In the as-cast sample, the decagonal QC could not be identified and intense peaks of the Al2Cu phase can be seen. The XRD pattern also show some peaks of the Al7Cu2Fe phase. The Al-FCC phase can be seen in the SEM micrograph (phase with the darker contrast) and the Al2Cu is observed, forming an eutectic constituent with the Al-FCC phase. In addition, a third phase (the one with intermediate contrast) seems to solidify with acicular morphology. EDX analysis was performed in this phase showing an average atomic composition of Al80.79±0.27Cu2.05±0.11Fe5.16±0.52Cr12.0±0.35. This is an AlFe-Cr-based phase and its atomic structure could not be identified with the analysis performed in this work. The Al-Fe-Cr phase diagram [47] shows the formation of a significant number of complex ternary phases around this composition and its proper identification would require an extensive electron diffraction analysis (which was not done for this work).
Figure 3 XRD pattern and SEM micrograph of the Al85Cu6.75Fe3.375Cr4.875 arc-melted alloy in the as-cast condition.
Figure 4 shows the results from DSC analysis of the arc-melted sample in the as-cast and after annealing (at 600 °C, see reference [13]) conditions; both curves are shown in the same scale for comparison. It is interesting to note that in the as-cast condition, an intense endothermic peak around 550 °C is found, which is due to the eutectic Al-Al2Cu 14
reaction (see Figure 3). This peak practically vanishes after the an annealing at 600 °C for 10 hours. The microstructure of the annealed sample is shown in reference [13]. In the annealed sample, no eutectic constituent was observed, and the dominant phases are Al-FCC and the decagonal QC. These results show that, during the annealing of the Al85Cu6.75Fe3.375Cr4.875 arc-melted alloy, there was liquid phase formation (from the eutectic constituent) which reacted with the Al-Fe-Cr-based phase (Figure 3) leading to a Cu enrichment of this phase and then resulting in the QC phase after the completion of the reaction. The high-Cu content of this liquid phase lead to formation of the Al7Cu2Fe after the QC phase reached its maximum Cu solubility and this phase was formed at the interfaces of the QC-Al-FCC phases (see reference [13] for further details). Thus, such QC composite can now be fabricated using simple fabrication processes that are low-cost and allow fabrication of large volume ingots containing this microstructure, which previously was only possible using expensive and nonconventional fabrication processes. This was all achieved due to the alloy design developed.
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Figure 4 DSC analysis of the as cast and annealed Al85Cu6.75Fe3.375Cr4.875 arc-melted alloy.
4. Conclusions In this work we present and discuss a successful alloy design approach that can be applied for the discovery of new QC alloys. Among the studied QC forming systems, Al-Cu-Fe-Cr was the most promising one. QCs can be formed in this system over large composition ranges, it forms under fast or slow solidification conditions and has an equilibrium phase field with Al-FCC, which allows the fabrication of a composites containing both phases by conventional casting. Due to the large number of known ternary QC-forming systems, future applications of the here proposed strategy for alloy design will certainly lead to discovery of new Almatrix composites reinforced with QCs that can be fabricated in the same way as the Al-Cu-Fe-Cr system.
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5. Acknowledgments The authors are grateful for the financial support granted by FAPESP (processes n° 2016/19314-3, n° 2015/09008-0 and n° 2013/05987-8), CAPES and CNPq. The authors are also grateful for the technical assistance on TEM, SEM analysis and sample preparation from Laboratory of Structure Characterization of Federal University of São Carlos (UFSCar). Authors are also grateful for all the collaborators that were involved in the course of this research and played an important role for its success. Declarations of interest: none. Author Contributions W.W., C.B., C.S.K and W.J.B designed the study. W.W. fabricated and characterized the samples. All authors contributed on data analysis, discussing and manuscript writing.
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Caption List Figure 1 STEM EDX mapping from the Al65Cu22Fe10Cr3alloy. The matrix is the icosahedral QC and the Cr-rich phase is the decagonal QC. Details of the phase formation in this alloy can be found elsewhere [8].
Figure 2 (a) EDX mapping of a melt-spun Al67Cu20Fe5Cr8 alloy, showing the τAlCu(Fe) phase, which is located in the Cu-rich regions. (b) EDX mapping from the ternary melt-spun Al65Cu22.5Fe12.5 alloy also showing the τ-AlCu(Fe) phase. (c) Brightfield TEM micrograph of the ternary alloy with a SAED pattern of the τ-AlCu(Fe) phase taken along the [111] zone axis. (d) Bright-field TEM micrograph of the ternary alloy, displaying a star-like morphology of showing the icosahedral QC and the SAED pattern taken along the 5-fold axis.
Figure 3 XRD pattern and SEM micrograph of the Al85Cu6.75Fe3.375Cr4.875 arc-melted alloy in the as-cast condition.
Figure 4 DSC analysis of the as cast and annealed Al85Cu6.75Fe3.375Cr4.875 arc-melted alloy.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(20)30128-6
DOI:
https://doi.org/10.1016/j.jallcom.2020.153765
Reference:
JALCOM 153765
To appear in:
Journal of Alloys and Compounds
Received Date: 26 September 2019 Revised Date:
18 December 2019
Accepted Date: 8 January 2020
Please cite this article as: W. Wolf, C. Bolfarini, C.S. Kiminami, W.J. Botta, Designing new quasicrystalline compositions in Al-based alloys, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2020.153765. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Designing new quasicrystalline compositions in Al-based alloys Witor Wolfa, Claudemiro Bolfarinib, Claudio S. Kiminamib and Walter J. Bottab,*
a
Departamento de Engenharia Metalúrgica e de Materiais, Universidade Federal de Minas Gerais, Av. Antônio Carlos, 6627, Belo Horizonte, MG, 31270-901, Brasil. b
Departamento de Engenharia de Materiais, Universidade Federal de São Carlos, Rod.Washington Luiz, Km 235, São Carlos, SP, 13565-905, Brasil. *Corresponding Author. Tel +55 16 33519481 / Fax +55 16 3361-5404
e-mail addresses: [email protected] (Witor Wolf), [email protected] (Claudemiro Bolfarini), [email protected] (Claudio S. Kiminami), [email protected] (Walter J. Botta).
Abstract In the present work, we present and discuss the most important results that were obtained in the past few years related with the development of new quaternary quasicrystalline alloys and composites. We present the alloy design strategies that we have used and that led to the discovery of over 60 new quasicrystalline compositions, 2 new quasicrystal-forming systems and 1 new Al-matrix composite. New results are also presented and discussed here. Results of interest discussed in this work include: reassessment of quasicrystal and approximant phase formation on the Al-Co-Fe-Cr system; influence of Cr and Ni additions on the icosahedral Al-Cu-Fe quasicrystal stability; discovery of a decagonal Al-Ni-Co-Cr quasicrystal; composition range of formation of the decagonal Al-Cu-Fe-Cr quasicrystal using combinatorial strategies; fabrication of aluminum matrix composites reinforced with quasicrystals using conventional metallurgy fabrication methods.
1
Keywords: Quasicrystals; Al alloys; Alloy design; Al-matrix composites.
Highlights We present and discuss a successful alloy design approach applied to discover new QCs. Al-Cu-Fe-Cr was the system with larger composition range of formation among the alloys studied. Al-Cu-Fe-Cr system is adequate to produce Al-matrix composites reinforced with QCs by conventional casting.
1. Introduction Quasicrystalline (QC) and approximant phases present distinct physical and mechanical properties associated with the complexity of their unit cell, typically containing large number of atoms [1]. Properties such as low thermal conductivity, low friction coefficient and wear resistance (despite brittle behavior) attracted important efforts in the development of alloys containing such phases for protective coating applications [2– 6]. One of the main challenges to design, or discover, new QC-forming alloys are related to the narrow composition ranges, in which such phases form and/or are stable [7]. In our group we have revisited different Al-based alloys, known to form QC phases, to study the effects of alloying and metastable processing on the formation of new QC or approximant phases [2,8–12]. We have also designed Al-matrix composites reinforced with QCs [13–15]. The systems we have studied are Al-Cu-Fe, Al-Cu-Fe-Cr, Al-Cu-Fe-Ni, Al-Ni-Co, AlNi-Co-Cr, Al-Ni-Co-Cu, Al-Fe-Cr-(M=Ce,Ti,Mn,V) and Al-Co-Fe-Cr. Processing 2
routes that we have approached includesd melt-spinning, gas-atomization, HVOF spraying, arc melting, powder metallurgy and magnetron sputtering. We have performed full structural and microstructural characterization in several compositions, which led to discovery of over 60 new QC-forming alloys and 2 new QC-forming metallic systems (Al-Cu-Fe-Ni and Al-Ni-Co-Cr) [8,10,11]. We have also discovered the first metallic system, Al-Cu-Fe-Cr, forming QCs embedded in Al-FCC matrix which can be fabricated by conventional metallurgical fabrication processes [13]. In addition, functional properties such as friction coefficient, wear resistance and thermal conductivity were assessed in coatings fabricated by HVOF [2]. Mechanical properties of composites were evaluated as well [14,15]. We also suggested and applied combinatorial strategies, involving magnetron co-sputtering and high-throughput XRD and EDX analysis, to map and design new QC compositions in the Al-Cu-Fe and AlCu-Fe-Cr systems [11]. We present the rationale behind the alloy design strategies that we have used in the development of new quaternary QC alloys. New results are also presented and discussed here, which includes QC phase formation on arc-melted Al-CuFe-Cr and on melt-spun Al-Ni-Co-Cu alloys. In addition we show the as-cast microstructure of the Al85Cu6.75Fe3.375Cr4.875 alloy, the first conventionally casted alloy forming a composite microstructure containing QCs and Al-FCC (which such microstructure only forms after an appropriate annealing) [13]. 2. Designing new quasicrystalline compositions 2.1 Revisiting QC forming systems QCs were first reported by Daniel Shechtman in an Al-Mn melt-spun alloy [16]. Although refuted at first, his discovery lead to huge efforts in the sense of understanding their atomic structure and physical properties.
3
Since this first metallic QC-forming system, many others have been discovered, most of them Al-based, such as Al-Cu-Fe, Al-Cu-Fe-Cr, Al-Ni-Co, Al-Fe-Cr-(M= transition metal), Al-Pd-Mn, Al-Mn-Si, Al-Li-Cu among others [17–24]. Some QC phases are considered thermodynamically stable while others can only be obtained with metastable processing routes. The interest on stable QCs is related to the ability to fabricate them using conventional casting techniques. In the past few years, our work has been focused on developing new QC alloys, based on traditionally known QC forming systems by adding alloying elements on them. Table 1 shows the previously known QC systems that we have revisited, the chemical compositions we fabricated, processing routes used and the phases that were obtained. In this table we did not add the Al-Cu-Fe-Cr system because, although this system was already known to form QCs, we have made an extensive work on it to find its compositional range of formation and thus it will be discussed in more detail in the sequence. The references where phase formation, shown in Table 1, can be found are in the Table´s 1 legend. XRD patterns of alloys that have not been published yet are available in Supplemental Material. Three ternary systems have been studied and formation of QC was observed in all of them, as expected from previously published studies. High energy ball milling to alloy Al, Cu and Fe requires posterior annealing to form QCs. We confirm the influence of cooling rates on the icosahedral phase formation from the Al-Cu-Fe system, which is favored by high cooling rates, > 104 °C/s. In the supplemental material, three XRD patterns of an Al65Cu22.5Fe12.5 alloy fabricated in three different processing conditions (a 10g arc-melteding a 10 g button, a melt-spun ribbon and a 1 mm arc-melteding a 1 mm thick plate sample that was Cu-quenched) are shown. The cooling rates in each case were in the range of 102 °C/s, 106 °C/s and 104 °C/s respectively and it can be observed 4
that the relative peak intensity of the QC phase increases with increasing cooling rates. This was expected as reported in previous studies [17,25]. These cooling rates values were calculated using the same procedure of reference [26]. The Al-Pd-Mn alloy studied easily formed a QC phase even for a sample which solidified with cooling rates in the range of 102 °C/s. However, the presence of high Pd contents is a huge obstacle for applying this QC system due to its cost. The Al-Ni-Co alloy formed a decagonal QC with some Al-Ni based intermetallic phases. High cooling rates did not prevent formation of such phases, as opposed to what was observed in the Al-Cu-Fe alloys. This can be observed by comparing the XRD pattern of this ternary alloy fabricated by meltspinning (see reference [10]) and by arc-melting a 1 mm thick plate sample that was Cuquenched (see Supplemental Material). The quaternary system Al-Co-Fe-Cr was studied aiming applications as thermal barrier coatings. The literature information about the most famous alloy of this system, Al71Co13Fe8Cr8, was somewhat contradictory. We showed, by fabricating this alloy with different processing routes, that the microstructure of as-cast samples is basically always composed by a hexagonal, µ-Al5Co2, and a monoclinic Al13Co4 phases. Both are QC approximants and exhibit interesting physical properties for application as protective coatings. Aiming to find new QC compositions, two other alloys from this system were fabricated by rapid solidification processes. However, no QC phases were found. The chemical compositions were selected based on the valence electron concentration (e/a) which would be closer to the usually found on ternary QCs [27], which however, was unsuccessful.
5
Table 1 Previously known QC-forming systems that we have revisited with their chemical compositions, processing routes used, and the phases obtained. Results taken from references: [2,9,10,12,15].
Metallic system Al-Cu-Fe
Alloy Al65Cu20Fe15
Al65Cu22.5Fe12.5
Processing route
Phases
High energy ball milling
Al, Cu, Fe, θ-Al2Cu
Arc-melting + annealing at 700°C
i-QC
Arc-melting
i-QC + β-AlCu(Fe) + λ-Al13Fe4 + Al2Cu
Melt-spinning
i-QC + τ-AlCu(Fe)
1 mm plate - Cuquenched
i-QC + τ-AlCu(Fe)
Al-Pd-Mn
Al70.5Pd21Mn8.5
Arc-melting
i-QC
Al-Co-Fe-Cr
Al71Co13Fe8Cr8
1 mm plate - Cuquenched
µ-Al5Co2 + MAl13Co4 + Cr-richunknown
Melt-spinning
µ-Al5Co2 + MAl13Co4
Gas-atomization
µ-Al5Co2 + MAl13Co4
Gas-atomization + annealing at 1000 °C
µ-Al5Co2
HVOF
µ-Al5Co2 + MAl13Co4
1 mm plate - Cuquenched
M-Al13Co4 + OAl13Co4
Melt-spinning
M-Al13Co4 + OAl13Co4 + Y-Al13Co4
1 mm plate - Cuquenched
M-Al13Co4 + Al9Co2
Melt-spinning
M-Al13Co4
Melt-spinning
d-QC + Al3Ni(Co) Al3Ni2(Co)
1 mm plate - Cuquenched
d-QC + Al3Ni(Co) Al3Ni2(Co)
Al77Co11Fe6Cr6
Al76Co19Fe4Cr1
Al-Ni-Co
Al71Ni20Co9
6
2.2 Designing new quaternary QC alloys based on Al-Cu-Fe and Al-Ni-Co systems From the systems studied in Table 1, two were selected based on the ability to form QCs and costs related to the alloying elements. These systems were Al-Cu-Fe and AlNi-Co. Based on selected ternary compositions, we have systematically added Cr and Ni on the Al-Cu-Fe system and Cr and Cu on the Al-Ni-Co system aiming to retain the QC atomic structure even after the alloying additions. The choice of chemical composition of each alloy was based on trying to increase the fourth element fraction and selecting values of valence electron concentration (e/a) as close as possible to the values found for Al-Cu-Fe and Al-Ni-Co QCs. Table 2 shows the quaternary alloys that were studied, including the fabrication processes and the phases identified. The references where details about phase formation, shown in Table 2, can be found are in the Table´s 2 legend. XRD patterns of alloys that have not been published yet are available in Supplemental Material. Cr addition on the Al-Cu-Fe QC leads to a gradual change on the atomic structure of the QC phase. Small additions, ~3 at. % Cr results oin the coexistence of the icosahedral and decagonal QCs, with some presence of the cubic τ-AlCu phase when the alloy is fabricated using lower cooling rates, as shown in the XRD pattern in Supplemental Material. Figure 1 shows an energy dispersive X-ray spectroscopy (EDX) mapping of a melt-spun Al65Cu22Fe10Cr3 alloy, which forms an icosahedral QC with small amounts of the decagonal QC (Cr-enriched). More details of this alloy´s phase formation and characterization can be found in reference [8].
7
Figure 1 STEM EDX mapping from the Al65Cu22Fe10Cr3alloy. The matrix is the icosahedral QC and the Cr-rich phase is the decagonal QC. Details of the phase formation in this alloy can be found elsewhere [8].
Increasing Cr content above a certain limit leads to only one QC phase, with decagonal structure. The Al67Cu20Fe5Cr8 alloy was processed by fast and low solidification methods and in all the experiments, a decagonal phase co-existed with a cubic phase, with the same structure of the τ-AlCu(Fe) phase forming in ternary rapid solidified AlCu-Fe QC forming alloys. The same cubic phase was observed in reference [28]. Figure 2a shows an EDX mapping of a melt-spun Al67Cu20Fe5Cr8 alloy, which is formed by a decagonal QC matrix with the presence of the τ-AlCu(Fe) phase (phase enriched in Cu) forming at the grain boundaries. Details of this alloy´s phase formation can be found in reference [8]. In figure 2b, the same EDX mapping analysis-type was performed in the ternary melt-spunAl65Cu22.5Fe12.5 alloy which has an icosahedral QC matrix but also forms the τ-AlCu(Fe) phase, in the same way observed in the quaternary alloy. Figure 2c shows a TEM bright-field micrograph and the selected area electron diffraction pattern (SAED) of this τ-AlCu(Fe) phase taken along the [111] zone axis, from the
8
ternary alloy. This phase is cubic with a CsCl-type structure and lattice parameter a = 2.94 Å. Figure 2d, also from the ternary alloy, shows an interesting “star-like” morphology of the icosahedral QC phase, with the respective SAED, taken along the 5fold axis of rotational symmetry. The bright-field micrograph was taken from the same the QC phase of the SAED pattern with the particle oriented in the 5-fold symmetry axis. This explains the “star-like” morphology, which reveals this material´s 5-fold symmetry and its influence on the growth of the solid phase.
Figure 2 (a) EDX mapping of a melt-spun Al67Cu20Fe5Cr8 alloy, showing the τ-AlCu(Fe) phase, which is located in the Cu-rich regions. (b) EDX mapping from the ternary melt-spun Al65Cu22.5Fe12.5 alloy also showing the τ-AlCu(Fe) phase. (c) Bright-field TEM micrograph of the ternary alloy with a SAED pattern of the τ-AlCu(Fe) phase taken along the [111] zone axis. (d) Bright-field TEM micrograph of the ternary alloy, displaying a star-like morphology of showing the icosahedral QC and the SAED pattern taken along the 5-fold axis.
9
In addition to the interesting results we obtained regarding the relatively easiness of fabricating QCs in the quaternary Al-Cu-Fe-Cr system, formation of QC phases in this system has also been reported by other authors and with a number of different atomic compositions [6,29–35]. This motivated an extensive study we performed using combinatorial strategies [11]. This included fabrication of compositional libraries by magnetron co-sputtering of Al, Cu, Fe and Cr targets on a Si wafer, which produced samples with gradient composition containing hundreds of alloys from this system. The samples were than characterized by scanning X-ray diffraction and EDX with fast data acquisition to identify phases in the libraries. Using this strategy, we were able to screen the decagonal QC range of formation on the Al-Cu-Fe-Cr system for Al atomic contents from 65 to 70 %. We have studied over 300 different alloy compositions and found 59 new decagonal QC dominant compositions. The QC composition range of formation can be found in Wolf et al. [11]. On the other hand, Ni additions on the Al-Cu-Fe QC did not lead to formation of a decagonal QC. We found that the icosahedral QC can dissolve up to 4 at. % Ni in its atomic structure. However, additions superior to that led to destabilization of the QC in favor of a B2 cubic phase, which was the major phase in the higher Ni-containing alloy, Al64Cu20Fe13Ni9. It is also interesting to comment on the morphological differences that QC phases can be produced, which strongly depends on the cooling rates applied during solidification and on the QC atomic structure. For instance, the Al-Cu-Fe icosahedral QC can be formed with a nanometric QC grain size, if fast cooling rates are applied (Fig. 2d), or it can form an envelope-like and micrometric grain-sized microstructure, which is a consequence of the peritectic reaction that results in its formation under low cooling rates during solidification [17]. The Al-Cu-Fe-Cr decagonal QC also forms as 10
nanometric QC grains if fast cooling rates are applied (Fig. 2a) or it forms as elongated micrometric-sized columnar grains if low cooling rates are applied [11]. Cr additions on the Al-Ni-Co decagonal QC led to discovery of a new quaternary QC, also decagonal, which formed in two melt-spun alloys. The composition of this phase was measured by EDX and it was found to be Al78.2Ni8.7Cr10Co3.1. However, the alloys studied also formed two Al-Ni based intermetallic phases, even when applying rapid solidification processes. The thermal stability of this new QC continues for future elucidation. Cu additions enhanced intermetallic phase formation in detriment of the QC phase. This can be seen in the XRD patterns of melt spun Al71Ni20Co7Cu2 and Al70Ni20Co5Cu5 alloys, shown in the Supplemental Material. If compared to XRD patterns of the Al-Ni-Co-Cr melt spun alloys (see reference [10]), the Cu-containing alloys show significantly higher contents of Al3Ni and Al3Ni2 phases. For this reason, this system was the less promising one and was not further studied. The design strategies used here to produce new QC compositions proved to be efficient and led to the discovery of more than 60 new QC-based alloys. The main idea behind this strategy is: define a ternary QC to be used as a base alloy. Using empirical QCforming rules, such as the valence electron concentration, add a fourth element in different proportions. Applying rapid solidification fabrication processes is the most adequate way to produce the quaternary alloys, because since QCs usually form around a number of several intermetallic phases, whose formation can be suppressed by fast cooling rates can suppress their formation. If this fourth element addition leads to QC formation, then this quaternary system is a potential candidate for the second step of the alloy design. This second step would be applying a method, in which the fabrication and characterization of hundreds of alloy compositions can be performed so that the compositional range where the quaternary phase can be fabricated will be disclosed. 11
This can be done by magnetron co-sputtering [36–38] or by using diffusion multiples [39]. Table 2 New quaternary alloys that were studied, including the fabrication processes used and the phases observed [8,10,11].
Metallic system Al-Cu-Fe-Cr
Alloy Al65Cu22Fe10Cr3
Al-Ni-Co-Cr
Al-Ni-Co-Cu
Phases
Melt-spinning
i-QC + d-QC
1 mm plate - Cuquenched
i-QC + d-QC + τAlCu(Fe)
Arc-melting + annealing at 600 °C
d-QC + τ-AlCu(Fe)
Melt-spinning
d-QC + τ-AlCu(Fe)
Al63Cu18Fe10Ni3
Melt-spinning
i-QC + 2 B2 cubic phases + λ-Al13Fe4
Al64Cu20Fe13Ni9
Melt-spinning
i-QC + 2 B2 cubic phases
Al72Ni19Co7Cr2
Melt-spinning
d-QC + d-QC(Crrich) + Al3Ni(Co) Al3Ni2(Co)
Al72Ni20Co5Cr3
Melt-spinning
d-QC + d-QC(Crrich) + Al3Ni(Co) Al3Ni2(Co)
Al71Ni20Co7Cu2
Melt-spinning
d-QC+ Al3Ni(Co) Al3Ni2(Co)
Al70Ni20Co5Cu5
Melt-spinning
d-QC + Al3Ni(Co) Al3Ni2(Co)
Al67Cu20Fe5Cr8
Al-Cu-Fe-Ni
Processing route
3. Al-matrix composites reinforced with quasicrystals In addition to discovering new QC forming compositions, our research has also focused on developing Al-matrix composites reinforced with QCs. Although QCs show remarkable functional physical properties that could be used for tribological protection [40,41], Due to thetheir inherent brittleness of QCs, makes their use becomes very 12
limited, especially for structural applications. To overcome this difficulty, QC phases must be embedded in a ductile matrix. There are two traditional methods to produce such composites. The first one involves rapid solidification techniques, such as melt-spinning, Cu-mold quenching and gasatomization [14,22,42–45] and can lead to the fabrication of composites reinforced with nanosized QCs, which is very interesting and can result in a high strength composites. The main drawbacks are related to the complex fabrication processes needed and the fact that QCs forming in these systems are metastable and usually transform to crystalline phases after heating around 350 to 500 °C. The second way to fabricate theise composites is based on powder metallurgy [15,46]. This process allows fabrication of composites reinforced with stable QCs, however, nanosized QC particles embedded in the ductile matrix are hard to be obtained. Following the results obtained in our combinatorial studies of the Al-Cu-Fe-Cr system and from literature data regarding previously published Al-Cu-Fe-Cr QC compositions, we have developed an Al-matrix composite reinforced with QCs by arc-melting an Al85Cu6.75Fe3.375Cr4.875 alloy and subsequently annealing it at 600 °C. The details of the annealed alloy´s microstructure and fabrication can be found in reference [13]. This was the first time such microstructure could be obtained by conventional fabrication methods. Here we will show the details of the as-cast alloy, which was the precursor of the QC composite. Figure 3 shows the XRD pattern and SEM micrograph of the Al85Cu6.75Fe3.375Cr4.875 arc-melted alloy in the as-cast condition. The XRD pattern of the as-cast sample is very different from the one of the annealed sample, from reference [13].
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In the as-cast sample, the decagonal QC could not be identified and intense peaks of the Al2Cu phase can be seen. The XRD pattern also show some peaks of the Al7Cu2Fe phase. The Al-FCC phase can be seen in the SEM micrograph (phase with the darker contrast) and the Al2Cu is observed, forming an eutectic constituent with the Al-FCC phase. In addition, a third phase (the one with intermediate contrast) seems to solidify with acicular morphology. EDX analysis was performed in this phase showing an average atomic composition of Al80.79±0.27Cu2.05±0.11Fe5.16±0.52Cr12.0±0.35. This is an AlFe-Cr-based phase and its atomic structure could not be identified with the analysis performed in this work. The Al-Fe-Cr phase diagram [47] shows the formation of a significant number of complex ternary phases around this composition and its proper identification would require an extensive electron diffraction analysis (which was not done for this work).
Figure 3 XRD pattern and SEM micrograph of the Al85Cu6.75Fe3.375Cr4.875 arc-melted alloy in the as-cast condition.
Figure 4 shows the results from DSC analysis of the arc-melted sample in the as-cast and after annealing (at 600 °C, see reference [13]) conditions; both curves are shown in the same scale for comparison. It is interesting to note that in the as-cast condition, an intense endothermic peak around 550 °C is found, which is due to the eutectic Al-Al2Cu 14
reaction (see Figure 3). This peak practically vanishes after the an annealing at 600 °C for 10 hours. The microstructure of the annealed sample is shown in reference [13]. In the annealed sample, no eutectic constituent was observed, and the dominant phases are Al-FCC and the decagonal QC. These results show that, during the annealing of the Al85Cu6.75Fe3.375Cr4.875 arc-melted alloy, there was liquid phase formation (from the eutectic constituent) which reacted with the Al-Fe-Cr-based phase (Figure 3) leading to a Cu enrichment of this phase and then resulting in the QC phase after the completion of the reaction. The high-Cu content of this liquid phase lead to formation of the Al7Cu2Fe after the QC phase reached its maximum Cu solubility and this phase was formed at the interfaces of the QC-Al-FCC phases (see reference [13] for further details). Thus, such QC composite can now be fabricated using simple fabrication processes that are low-cost and allow fabrication of large volume ingots containing this microstructure, which previously was only possible using expensive and nonconventional fabrication processes. This was all achieved due to the alloy design developed.
15
Figure 4 DSC analysis of the as cast and annealed Al85Cu6.75Fe3.375Cr4.875 arc-melted alloy.
4. Conclusions In this work we present and discuss a successful alloy design approach that can be applied for the discovery of new QC alloys. Among the studied QC forming systems, Al-Cu-Fe-Cr was the most promising one. QCs can be formed in this system over large composition ranges, it forms under fast or slow solidification conditions and has an equilibrium phase field with Al-FCC, which allows the fabrication of a composites containing both phases by conventional casting. Due to the large number of known ternary QC-forming systems, future applications of the here proposed strategy for alloy design will certainly lead to discovery of new Almatrix composites reinforced with QCs that can be fabricated in the same way as the Al-Cu-Fe-Cr system.
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5. Acknowledgments The authors are grateful for the financial support granted by FAPESP (processes n° 2016/19314-3, n° 2015/09008-0 and n° 2013/05987-8), CAPES and CNPq. The authors are also grateful for the technical assistance on TEM, SEM analysis and sample preparation from Laboratory of Structure Characterization of Federal University of São Carlos (UFSCar). Authors are also grateful for all the collaborators that were involved in the course of this research and played an important role for its success. Declarations of interest: none. Author Contributions W.W., C.B., C.S.K and W.J.B designed the study. W.W. fabricated and characterized the samples. All authors contributed on data analysis, discussing and manuscript writing.
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Caption List Figure 1 STEM EDX mapping from the Al65Cu22Fe10Cr3alloy. The matrix is the icosahedral QC and the Cr-rich phase is the decagonal QC. Details of the phase formation in this alloy can be found elsewhere [8].
Figure 2 (a) EDX mapping of a melt-spun Al67Cu20Fe5Cr8 alloy, showing the τAlCu(Fe) phase, which is located in the Cu-rich regions. (b) EDX mapping from the ternary melt-spun Al65Cu22.5Fe12.5 alloy also showing the τ-AlCu(Fe) phase. (c) Brightfield TEM micrograph of the ternary alloy with a SAED pattern of the τ-AlCu(Fe) phase taken along the [111] zone axis. (d) Bright-field TEM micrograph of the ternary alloy, displaying a star-like morphology of showing the icosahedral QC and the SAED pattern taken along the 5-fold axis.
Figure 3 XRD pattern and SEM micrograph of the Al85Cu6.75Fe3.375Cr4.875 arc-melted alloy in the as-cast condition.
Figure 4 DSC analysis of the as cast and annealed Al85Cu6.75Fe3.375Cr4.875 arc-melted alloy.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: