X-ray Thermo-Diffraction Study of the Aluminum-Based Multicomponent Alloy Al58Zn28Si8Mg6

Citation: Bilbao, Y.; Trujillo, J.J.; Vicario, I.; Arruebarrena, G.; Hurtado, I.; Guraya, T. X-ray Thermo-Diffraction Study of the Aluminum-Based Multicomponent Alloy Al58Zn28Si8Mg6.Materials 2022, 15, 5056. https://doi.org/10.3390/ ma15145056 Academic Editor: Lijun Zhang Received: 21 June 2022 Accepted: 19 July 2022 Published: 20 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article X-ray Thermo-Diffraction Study of the Aluminum-Based Multicomponent Alloy Al58Zn28Si8Mg6 Yoana Bilbao 1,* , Juan JoséTrujillo 2, Iban Vicario 3, Gurutze Arruebarrena 2, Iñaki Hurtado 2 and Teresa Guraya 1 1 Department of Mining and Metallurgical Engineering and Materials Science, Faculty of Engineering of Bilbao, University of the Basque Country (UPV/EHU), 48013 Bilbao, Spain; [email protected] 2Mechanical and Manufacturing Department, Faculty of Engineering, Mondragon Unibertsitatea, 20500 Arrasate/Mondragon, Spain; [email protected] (J.J.T.); [email protected] (G.A.); [email protected] (I.H.) 3Manufacturing Processes and Materials Department, Tecnalia, Basque Research and Technology Alliance (BRTA), 48160 Derio, Spain; [email protected] *Correspondence: [email protected] Abstract: Newly designed multicomponent light alloys are giving rise to non-conventional microstructures that need to be thoroughly studied before determining their potential applications. In this study, the novel Al 58 Zn 28 Si 8 Mg 6 alloy, previously studied with CALPHAD methods, was cast and heat-treated under several conditions. An analysis of the phase evolution was carried out with in situ X-ray diffraction supported by differential scanning calorimetry and electron microscopy. A total of eight phases were identified in the alloy in the temperature range from 30 to 380 ◦ C: α -Al, α ’-Al, Zn, Si, Mg 2 Si, MgZn 2 , Mg 2 Zn 11 , and SrZn 13 . Several thermal transitions below 360 ◦ C were determined, and the natural precipitation of the Zn phase was confirmed after nine months. The study showed that the thermal history can strongly affect the presence of the MgZn 2 and Mg 2 Zn 11 phases. The combination of X-ray thermo-diffraction with CALPHAD methods, differential scanning calorimetry, and electron microscopy offered us a satisfactory understanding of the alloy behavior at different temperatures. Keywords: lightweight multicomponent alloys; X-ray thermo-diffraction; differential scanning calorimetry; Al–Zn; Zn precipitation; Mg–Zn phases; strontium modification 1. Introduction Historically, metallic alloys have been developed by selecting one or two major components and adding several minor ones that confer specific properties, such as corrosion resistance or higher mechanical properties. The multicomponent alloy concept, however, is based on the design of alloys where there are several main components that cover the central areas of phase diagrams [1]. Initial developments in the field focused mainly on steel-like alloys for industrial applications. They were based on equiatomic and near-equiatomic compositions of Co, Cr, Cu, Fe, Mn, or Ni, sometimes adding Al, Ti, or Zr, that resulted in single or dual phase microstructures [ 2 – 7 ]. Yeh et al. suggested that the prevalence of solid solutions over intermetallic phases could be explained by the high mixing entropy generated by the multiple components in the alloy, hence the term “high-entropy alloys” (HEAs) [ 8 ]. The conditions that the alloys should satisfy to be considered HEAs may be found in [9]. Over the past decade, research has been extended to other alloy classifications that have evolved from the original HEA concept: “medium-entropy alloys” (MEAs) [ 9 ], “nonequiatomic HEAs”, or “multi-phase HEAs” that may contain bulky secondary phases [ 10 ]. In fact, the idea of intentionally having secondary phases in this type of alloy was first suggested by Miracle et al. [ 11 ]. Other alloy families have also been explored, leading Materials 2022,15, 5056. https://doi.org/10.3390/ma15145056 https://www.mdpi.com/journal/materials Materials 2022,15, 5056 2 of 14 to refractory metal HEAs for high temperature structural applications based on Cr, Hf, Mo, Nb, Ta, Ti, V, and W or lightweight multicomponent alloys in the aeronautical field involving Al, Li, Mg, or Zn. Several studies may be found in the literature as proof of this tendency [12–17]. Among the lightweight multicomponent alloys, Yang et al. explored the Al–Li–Mg– (Zn, Cu, Sn) system, obtaining structures dominated by intermetallic compounds. The aluminum face-centered cubic (FCC) structure predominated only in selected alloy compositions [ 18 ]. In fact, Sanchez et al. highlighted the difficulty of forming solid solutions in medium entropy alloys based on aluminum (65–70 at. %) with elements such as Cu, Mg, Cr, Fe, Si, Ni, Zn, or Zr. The magnitude of the negative mixing enthalpy of aluminum with transition metals gave rise to intermetallic phases [ 19 ]. The presence of intermetallics was also reported by Tun et al. [ 20 ]. Only the most recent research suggests that rapid solidification processes may enhance single phase microstructures in this type of alloy [ 21 ]. However, in a previous work by Nagase et al. on Al–Mg–Li–Ca equiatomic and non-equiatomic alloys, a single solid solution could not be obtained, even with rapid solidification [22]. Asadikiya et al. considered that the application of the entropy concept in aluminum alloys may be the answer to the challenge of developing novel Al alloys with improved properties [ 10 ]. Therefore, multicomponent lightweight alloys continue to be researched for their potential applications. In the present study, the objective was to characterize the novel Al 58 Zn 28 Si 8 Mg 6 cast alloy. It was designed to obtain as much solid solution of aluminum and zinc as possible, reinforced with intermetallics based on Zn, Mg, and Si. On the one hand, zinc is highly soluble in aluminum, enhancing the obtention of a solid solution matrix. On the other hand, Mg–Zn phases are the usual precipitates in 7xx.x aluminum cast alloys, while Mg–Si phases are common in 3xx.x alloys. In addition, Al–Zn-based alloys have attracted the interest of researchers beyond their usual use as coatings. In fact, Al–Zn cast alloys have potential applications where tribological and damping properties are required [ 23 – 25 ]. In terms of entropy, our multicomponent alloy would be classified as a multi-phase MEA. The approach was explored by the CALPHAD (calculation of phase diagrams) method. This technique requires databases that are valid in composition ranges that may not be found in conventional alloys, thus demanding further experimental verification [ 26 ]. However, it is considered the most direct method for compositional design [ 27 ] and has already been used in the design of lightweight multicomponent alloys with differing degrees of success [28–30]. The study is focused on identifying and evaluating the effect of the temperature on the phases that are generated at different initial thermal conditions. Differential scanning calorimetry (DSC), electron microscopy, and X-ray thermo-diffraction are the techniques used in this evaluation. X-ray thermo-diffraction, also known as “high temperature X-ray diffraction” (HT-XRD), enables the in situ study of the solution and precipitation phenomena in the alloys [31–33]. 2. Materials and Methods 2.1. Material Manufacturing Aluminum was melted at 750 ◦ C in a resistance furnace with forced convection; silicon and zinc were subsequently added. Magnesium followed, and once all the elements were melted, strontium was added as a silicon modifier. Aluminum, magnesium, and silicon were of commercial purity, whereas zinc was incorporated by adding a Zamak Zn4Al1Cu alloy so that the final alloy composition contained some residual copper. Samples were obtained to determine the chemical composition by inductively coupled plasma mass spectrometry (ICP) (Table 1). The metal was gravity cast into a graphite mold, and samples were obtained that were 50 mm long, 22.5 mm wide, and 4 mm thick. Materials 2022,15, 5056 3 of 14 Table 1. Chemical composition of the alloy analyzed by ICP. Al Zn Mg Si Cu Fe Sr wt. % 41.15 48.40 4.11 5.83 0.43 0.05 0.03 at. % 57.56 27.93 6.38 7.83 0.26 0.03 0.01 2.2. Selection of Sample Thermal Treatments and Study Temperatures In order to select the temperatures of interest, DSC tests were performed on as-cast samples (Figure 1). A Netzsch STA 449 Fe Jupiter calorimeter was used, and measurements were made under argon atmosphere in a temperature range between 25 and 675 ◦ C with a heating rate of 10 ◦ C/min. Samples were then cooled down back to room temperature under these same conditions. Materials 2022, 15, 5056 3 of 15 Table 1. Chemical composition of the alloy analyzed by ICP. Al Zn Mg Si Cu Fe Sr wt. % 41.15 48.40 4.11 5.83 0.43 0.05 0.03 at. % 57.56 27.93 6.38 7.83 0.26 0.03 0.01 The metal was gravity cast into a graphite mold, and samples were obtained that were 50 mm long, 22.5 mm wide, and 4 mm thick. 2.2. Selection of Sample Thermal Treatments and Study Temperatures In order to select the temperatures of interest, DSC tests were performed on as-cast samples (Figure 1). A Netzsch STA 449 Fe Jupiter calorimeter was used, and measurements were made under argon atmosphere in a temperature range between 25 and 675 °C with a heating rate of 10 °C/min. Samples were then cooled down back to room temperature under these same conditions. Figure 1. DSC curves for the as-cast sample. Peak temperatures during heating were considered for the thermal treatment selection of the samples. We decided to subject the samples to seven different thermal conditions to try to separate and simplify the identification of the different phases appearing and disappearing during the heating process (Table 2 and Figure 2). Table 2. Heat treatments for each sample condition. The two-step solution treatment in Q360 and Q380 was applied to prevent any partial melting during one-step solution treatment at the solution temperature. Sample Condition Heat Treatment as-cast None. Eq280 Heated to 280 °C and immediately slowly cooled (10 °C/min) to room temperature. Eq360 Heated to 360 °C and immediately slowly cooled (10 °C/min) to room temperature. Q280 Solution treated for 24 h at 280 °C, then water quenched to room temperature. Q310 Solution treated for 24 h at 310 °C , then water quenched to room temperature. Q360 Solution treated for 24 h at 325 °C, then heated and kept at 360 °C for 24 h and water quenched. Q380 Solution treated for 24 h at 325 °C, then heated and kept at 380 °C for 24 h and water quenched. Figure 1. DSC curves for the as-cast sample. Peak temperatures during heating were considered for the thermal treatment selection of the samples. We decided to subject the samples to seven different thermal conditions to try to separate and simplify the identification of the different phases appearing and disappearing during the heating process (Table 2and Figure 2). Materials 2022, 15, 5056 4 of 15 (a) (b) Figure 2. Heating and cooling sequences for the samples, in addition to the as-cast sample. (a) Slowly cooled (10 °C/min) samples (Eq280 and Eq360). (b) Quenched samples (Q280, Q310, Q360, and Q380). 2.3. Thermodynamic Simulations Equilibrium and Scheil non-equilibrium solidification simulations were carried out with the CALPHAD method for the cast alloy composition with FactSage 7.3 software, along with the FTlite (2021) database. Only the four main elements in the alloy were considered. 2.4. Microstructural Observations As-cast and Q380 samples were observed with scanning electron microscopy (SEM of Shottky field emission, JEOL JSM-7000F) and energy dispersive X-ray spectroscopy (INCA EDX detector X-sight Serie Si (Li) pentaFET Oxford) at an electron beam voltage of 5.0 kV at room temperature. Specimens had been previously cleaned, ground, and polished to obtain a proper surface finish for the analysis. 2.5. X-ray Thermo-Diffraction Tests The equipment used for the X-ray thermo-diffraction tests was a Bruker D8 Advance diffractometer that operated at 30 kV and 20 mA for reflection measurements. It was equipped with a copper anode (λ = 1.5418 Å), a Vantec-1 PSD detector, and an Anton Parr HTK2000 high temperature furnace. The sample holder used, on which the test temperature was controlled, was made of platinum. The seven specimens, which were 10 × 10 mm2 with a thickness between 1 and 2 mm, were subjected to a heating cycle from 30 to 360 °C and cooling again to 30 °C in the diffractometer. Diffraction tests were performed at room temperature (30 °C), at three temperatures during heating (260, 320, 360 °C), and at three temperatures during cooling (260, 180, 30 °C), based on the temperatures of interest found in the DSC curves (Figure 3). The measurements were recorded in the range 10° ≤ 2θ ≤ 100° at increments of 0.033°, with each stage lasting 0.8 s. 240 260 280 300 320 340 360 380 400 Temperature (°C) Time Eq280 Eq360 240 260 280 300 320 340 360 380 400 Temperature (°C) Time Q310 ≈≈ ≈ ≈ ≈ Q280 Q360 Q380 Figure 2. Heating and cooling sequences for the samples, in addition to the as-cast sample. ( a ) Slowly cooled (10 ◦ C/min) samples (Eq280 and Eq360). ( b ) Quenched samples (Q280, Q310, Q360, and Q380). Materials 2022,15, 5056 4 of 14 Table 2. Heat treatments for each sample condition. The two-step solution treatment in Q360 and Q380 was applied to prevent any partial melting during one-step solution treatment at the solution temperature. Sample Condition Heat Treatment as-cast None. Eq280 Heated to 280 ◦C and immediately slowly cooled (10 ◦C/min) to room temperature. Eq360 Heated to 360 ◦C and immediately slowly cooled (10 ◦C/min) to room temperature. Q280 Solution treated for 24 h at 280 ◦C, then water quenched to room temperature. Q310 Solution treated for 24 h at 310 ◦C, then water quenched to room temperature. Q360 Solution treated for 24 h at 325 ◦C, then heated and kept at 360 ◦C for 24 h and water quenched. Q380 Solution treated for 24 h at 325 ◦C, then heated and kept at 380 ◦C for 24 h and water quenched. 2.3. Thermodynamic Simulations Equilibrium and Scheil non-equilibrium solidification simulations were carried out with the CALPHAD method for the cast alloy composition with FactSage 7.3 software, along with the FTlite (2021) database. Only the four main elements in the alloy were considered. 2.4. Microstructural Observations As-cast and Q380 samples were observed with scanning electron microscopy (SEM of Shottky field emission, JEOL JSM-7000F) and energy dispersive X-ray spectroscopy (INCA EDX detector X-sight Serie Si (Li) pentaFET Oxford) at an electron beam voltage of 5.0 kV at room temperature. Specimens had been previously cleaned, ground, and polished to obtain a proper surface finish for the analysis. 2.5. X-ray Thermo-Diffraction Tests The equipment used for the X-ray thermo-diffraction tests was a Bruker D8 Advance diffractometer that operated at 30 kV and 20 mA for reflection measurements. It was equipped with a copper anode ( λ = 1.5418 Å), a Vantec-1 PSD detector, and an Anton Parr HTK2000 high temperature furnace. The sample holder used, on which the test temperature was controlled, was made of platinum. The seven specimens, which were 10 × 10 mm 2 with a thickness between 1 and 2 mm, were subjected to a heating cycle from 30 to 360 ◦ C and cooling again to 30 ◦ C in the diffractometer. Diffraction tests were performed at room temperature (30 ◦ C), at three temperatures during heating (260, 320, 360 ◦ C), and at three temperatures during cooling (260, 180, 30 ◦ C), based on the temperatures of interest found in the DSC curves (Figure 3). The measurements were recorded in the range 10 ◦≤ 2 θ≤ 100 ◦ at increments of 0.033 ◦ , with each stage lasting 0.8 s. Thermo-diffraction tests were performed three months after the samples were prepared. Twelve months after the preparation; that is, nine months after being subjected to the thermal cycle in the thermo-diffractometer, samples were retested in the same conditions as before but only at 30 ◦ C, in order to observe whether natural precipitation had taken place. The X-ray diffraction patterns were indexed with the PDF-4+ 2021 database from the International Center for Diffraction Data (ICDD). For the search of non-indexed phases, least squares-based Rietveld refinement was carried out in selected patterns with the FullProf software (FullProf.2k Version 7.40, January 2021, J. Rodriguez-Carvajal, ILL, Grenoble, France). The shape of the Bragg peaks was represented by a Pseudo-Voigt function. Conventional R-values, corrected for background, are given in the figures as agreement of the fitting to the observed values [ 34 , 35 ]. The term “intensity” is used to refer to the “integrated intensity”. Materials 2022,15, 5056 5 of 14 Materials 2022, 15, 5056 5 of 15 Figure 3. Thermal cycle of the samples in the thermo-diffractometer. Measurements were performed at the temperatures indicated in each step. Thermo-diffraction tests were performed three months after the samples were prepared. Twelve months after the preparation; that is, nine months after being subjected to the thermal cycle in the thermo-diffractometer, samples were retested in the same conditions as before but only at 30 °C, in order to observe whether natural precipitation had taken place. The X-ray diffraction patterns were indexed with the PDF-4+ 2021 database from the International Center for Diffraction Data (ICDD). For the search of non-indexed phases, least squares-based Rietveld refinement was carried out in selected patterns with the FullProf software (FullProf.2k Version 7.40, January 2021, J. Rodriguez-Carvajal, ILL, Grenoble, France). The shape of the Bragg peaks was represented by a Pseudo-Voigt function. Conventional R-values, corrected for background, are given in the figures as agreement of the fitting to the observed values [34,35]. The term “intensity” is used to refer to the “integrated intensity”. 3. Results 3.1. Thermodynamic Simulation Results Thermodynamic simulations performed with FactSage for equilibrium cooling conditions (Figure 4a) predicted a high proportion of the FCC aluminum solid solution at temperatures between 360 and 380 °C, with the Si and Mg2Si phases being precipitated at these temperatures. As cooling went on, the solid solution decomposed and around 350 °C a second aluminum phase (Al#2) was generated but disappeared soon after. This phase would correspond to the zinc-rich α’ aluminum phase of the miscibility gap in the Al–Zn system [36,37]. At about 340 °C, the intermetallic phase Mg2Zn11 was formed, and MgZn2 precipitated from Mg2Zn11 at around 140 °C. The simulation under non-equilibrium conditions (Scheil approximation) predicted the precipitation of Mg2Zn11 and MgZn2 at about 370 °C and that of hexagonal zinc at 350 °C (Figure 4b). 30 260 320 360 260 180 30 0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 Temperature (°C) Time 10 °C/min Figure 3. Thermal cycle of the samples in the thermo-diffractometer. Measurements were performed at the temperatures indicated in each step. 3. Results 3.1. Thermodynamic Simulation Results Thermodynamic simulations performed with FactSage for equilibrium cooling conditions (Figure 4a) predicted a high proportion of the FCC aluminum solid solution at temperatures between 360 and 380 ◦ C, with the Si and Mg 2 Si phases being precipitated at these temperatures. As cooling went on, the solid solution decomposed and around 350 ◦ C a second aluminum phase (Al#2) was generated but disappeared soon after. This phase would correspond to the zinc-rich α ’ aluminum phase of the miscibility gap in the Al–Zn system [ 36 , 37 ]. At about 340 ◦ C, the intermetallic phase Mg 2 Zn 11 was formed, and MgZn 2 precipitated from Mg 2 Zn 11 at around 140 ◦ C. The simulation under non-equilibrium conditions (Scheil approximation) predicted the precipitation of Mg 2 Zn 11 and MgZn 2 at about 370 ◦C and that of hexagonal zinc at 350 ◦C (Figure 4b). Materials 2022, 15, 5056 6 of 15 (a) (b) Figure 4. Thermodynamic simulations with FactSage for the studied alloy considering (a) equilibrium solidification conditions and (b) non-equilibrium solidification (Scheil model). 3.2. Microstructure of the Samples Depending on the Initial Thermal Condition The microstructure resulting from the as-cast state was heterogeneous, with different phases distributed throughout the interdendritic region depending on the solidification rate (Figure 5a). In the Q380 condition (Figure 5b), the globulization and reduction in the size of the phases after the solution treatment were remarkable. The Si, Mg–Si, and Mg–Zn phases were found by EDX measurements. The Si phase solidified in certain areas as eutectic and in other areas as primary silicon. In addition, isolated Al–Fe– Mg–Si phases were detected. (a) (b) Figure 4. Thermodynamic simulations with FactSage for the studied alloy considering ( a ) equilibrium solidification conditions and (b) non-equilibrium solidification (Scheil model). Materials 2022,15, 5056 6 of 14 3.2. Microstructure of the Samples Depending on the Initial Thermal Condition The microstructure resulting from the as-cast state was heterogeneous, with different phases distributed throughout the interdendritic region depending on the solidification rate (Figure 5a). In the Q380 condition (Figure 5b), the globulization and reduction in the size of the phases after the solution treatment were remarkable. The Si, Mg–Si, and Mg–Zn phases were found by EDX measurements. The Si phase solidified in certain areas as eutectic and in other areas as primary silicon. In addition, isolated Al–Fe–Mg–Si phases were detected. Materials 2022, 15, 5056 6 of 15 (a) (b) Figure 4. Thermodynamic simulations with FactSage for the studied alloy considering (a) equilibrium solidification conditions and (b) non-equilibrium solidification (Scheil model). 3.2. Microstructure of the Samples Depending on the Initial Thermal Condition The microstructure resulting from the as-cast state was heterogeneous, with different phases distributed throughout the interdendritic region depending on the solidification rate (Figure 5a). In the Q380 condition (Figure 5b), the globulization and reduction in the size of the phases after the solution treatment were remarkable. The Si, Mg–Si, and Mg–Zn phases were found by EDX measurements. The Si phase solidified in certain areas as eutectic and in other areas as primary silicon. In addition, isolated Al–Fe– Mg–Si phases were detected. (a) (b) Materials 2022, 15, 5056 7 of 15 Figure 5. SEM micrographs of the material with x1000 magnification (a) As-cast. (b) Q380. Numbers 1 to 4 refer to the EDX results provided below. 1: Al-Zn matrix, 2: Mg-Zn phases, 3: Si phases and 4: Mg-Si phases. As for the matrix, it showed a two-phase microstructure of aluminum and zinc. Precipitation of the Zn phase was observed in the as-cast material, unlike in sample Q380, where the Zn phase was not detected, indicating that it was dissolved within the matrix (Figure 6). (a) (b) Figure 6. SEM micrographs of the Al–Zn matrix. (a) As-cast. (b) Q380. The identification of the phases present in each initial thermal condition was performed by room temperature X-ray diffraction (before the heating cycle). As is shown in Figure 7, in addition to the Al phase (PDF: 00-004-0787) and the Pt phase from the sample holder (PDF: 04-013-4766), which are not indicated for clarity, the phases detected were Zn (PDF: 01-078-9363), Si (PDF: 00-027-1402), MgZn 2 (PDF: 04-003-2083), Mg 2 Zn 11 (PDF: 04-007-1412), and Mg 2 Si (PDF: 01-083-5235). Figure 5. SEM micrographs of the material with × 1000 magnification ( a ) As-cast. ( b ) Q380. Numbers 1 to 4 refer to the EDX results provided below. 1: Al-Zn matrix, 2: Mg-Zn phases, 3: Si phases and 4: Mg-Si phases. As for the matrix, it showed a two-phase microstructure of aluminum and zinc. Precipitation of the Zn phase was observed in the as-cast material, unlike in sample Q380, Materials 2022,15, 5056 7 of 14 where the Zn phase was not detected, indicating that it was dissolved within the matrix (Figure 6). Materials 2022, 15, 5056 7 of 15 Figure 5. SEM micrographs of the material with x1000 magnification (a) As-cast. (b) Q380. Numbers 1 to 4 refer to the EDX results provided below. 1: Al-Zn matrix, 2: Mg-Zn phases, 3: Si phases and 4: Mg-Si phases. As for the matrix, it showed a two-phase microstructure of aluminum and zinc. Precipitation of the Zn phase was observed in the as-cast material, unlike in sample Q380, where the Zn phase was not detected, indicating that it was dissolved within the matrix (Figure 6). (a) (b) Figure 6. SEM micrographs of the Al–Zn matrix. (a) As-cast. (b) Q380. The identification of the phases present in each initial thermal condition was performed by room temperature X-ray diffraction (before the heating cycle). As is shown in Figure 7, in addition to the Al phase (PDF: 00-004-0787) and the Pt phase from the sample holder (PDF: 04-013-4766), which are not indicated for clarity, the phases detected were Zn (PDF: 01-078-9363), Si (PDF: 00-027-1402), MgZn 2 (PDF: 04-003-2083), Mg 2 Zn 11 (PDF: 04-007-1412), and Mg 2 Si (PDF: 01-083-5235). Figure 6. SEM micrographs of the Al–Zn matrix. (a) As-cast. (b) Q380. The identification of the phases present in each initial thermal condition was performed by room temperature X-ray diffraction (before the heating cycle). As is shown in Figure 7, in addition to the Al phase (PDF: 00-004-0787) and the Pt phase from the sample holder (PDF: 04-013-4766), which are not indicated for clarity, the phases detected were Zn (PDF: 01078-9363), Si (PDF: 00-027-1402), MgZn 2 (PDF: 04-003-2083), Mg 2 Zn 11 (PDF: 04-007-1412), and Mg2Si (PDF: 01-083-5235). Materials 2022, 15, 5056 8 of 15 (a) (b) Figure 7. Diffraction patterns of the samples in each thermal condition at 30 °C prior to the heating cycle in the thermo-diffractometer. Indexation is shown above the patterns of samples Eq280 (Mg 2 Zn 11 and Mg 2 Si phases) and Eq360 (Si, Zn and MgZn 2 phases). Peaks corresponding to Al and Pt phases (the latter from the sample holder) are omitted for clarity. (a) As-cast samples and those cooled slowly. (b) Quenched samples. However, the microstructure obtained depended on the applied treatment; that is, the temperature at which cooling had started and the cooling rate. In as-cast conditions Zn precipitated, as did both MgZn 2 and Mg 2 Zn 11 to a lesser extent. When slowly cooling from 280 °C (Eq280 sample), MgZn 2 was obtained again, as in the previous case, but now Mg 2 Zn 11 precipitated preferentially, while HCP Zn was hardly detected. When the cooling began at 360 °C (Eq360 sample), on the other hand, no precipitation of Mg 2 Zn 11 was observed and zinc was present in the HCP Zn and MgZn 2 phases. MgZn 2 phases were found in greater quantities than in the as-cast or Eq280 conditions. As for the quenched samples, the Mg 2 Zn 11 phase was dissolved when reaching 360 °C. Regarding the Fe-bearing quaternary phases observed by SEM, it was not possible to confirm them by X-ray diffraction. The most intense Bragg peak for Al 8 FeMg 3 Si 6 (PDF: 03-065-5936) would overlap with the Al (111) reflection. Given its condition as a minor phase, further peaks could not be detected. Therefore, if other Cuand Fe-bearing phases found in aluminum alloys containing Zn, Mg, Si, and/or Cu [38,39] were present in very small amounts in this alloy, specific X-ray diffraction conditions and equipment would be required to identify them. The appearance of the Mg 2 Si phase and the dissolution and precipitation of the Zn phase are discussed in the following section. 3.3. Evolution of the HCP Zn and Intermetallic Phases with Temperature The profiles obtained for the as-cast sample are representative of the evolution of the zinc-containing phases with temperature (Figure 8). The description is thus valid for the rest of the samples, while the matrix will be dealt with in the next section. This evolution is summarized below. Figure 7. Diffraction patterns of the samples in each thermal condition at 30 ◦ C prior to the heating cycle in the thermo-diffractometer. Indexation is shown above the patterns of samples Eq280 (Mg 2 Zn 11 and Mg 2 Si phases) and Eq360 (Si, Zn and MgZn 2 phases). Peaks corresponding to Al and Pt phases (the latter from the sample holder) are omitted for clarity. ( a ) As-cast samples and those cooled slowly. (b) Quenched samples. However, the microstructure obtained depended on the applied treatment; that is, the temperature at which cooling had started and the cooling rate. In as-cast conditions Zn precipitated, as did both MgZn 2 and Mg 2 Zn 11 to a lesser extent. When slowly cooling from 280 ◦ C (Eq280 sample), MgZn 2 was obtained again, as in the previous case, but now Materials 2022,15, 5056 8 of 14 Mg 2 Zn 11 precipitated preferentially, while HCP Zn was hardly detected. When the cooling began at 360 ◦ C (Eq360 sample), on the other hand, no precipitation of Mg 2 Zn 11 was observed and zinc was present in the HCP Zn and MgZn 2 phases. MgZn 2 phases were found in greater quantities than in the as-cast or Eq280 conditions. As for the quenched samples, the Mg2Zn11 phase was dissolved when reaching 360 ◦C. Regarding the Fe-bearing quaternary phases observed by SEM, it was not possible to confirm them by X-ray diffraction. The most intense Bragg peak for Al 8 FeMg 3 Si 6 (PDF: 03-065-5936) would overlap with the Al (111) reflection. Given its condition as a minor phase, further peaks could not be detected. Therefore, if other Cuand Fe-bearing phases found in aluminum alloys containing Zn, Mg, Si, and/or Cu [ 38 , 39 ] were present in very small amounts in this alloy, specific X-ray diffraction conditions and equipment would be required to identify them. The appearance of the Mg 2 Si phase and the dissolution and precipitation of the Zn phase are discussed in the following section. 3.3. Evolution of the HCP Zn and Intermetallic Phases with Temperature The profiles obtained for the as-cast sample are representative of the evolution of the zinc-containing phases with temperature (Figure 8). The description is thus valid for the rest of the samples, while the matrix will be dealt with in the next section. This evolution is summarized below. Materials 2022, 15, 5056 9 of 15 (a) (b) Figure 8. Diffraction patterns of the as-cast sample showing the evolution of the intermetallic phases, Zn, and Si with temperature (a) during the heating cycle (from 30 to 360 °C) (b) and cooling cycle (from 360 to 30 °C) in the thermo-diffractometer. The main Bragg peaks for the SrZn 13 phase are identified. At 30 °C, Zn, MgZn 2 , and Mg 2 Zn 11 phases were found. At 260 °C, the intensity of Zn peaks decreased while two additional Bragg peaks were detected around 2θ = 35.9° and 2θ = 54.0°. These peaks did not belong to any of the phases already indexed. Assuming they belonged to a new phase, it was clear that it arose at a temperature between 30 and 260 °C and likely dissolved between 260 and 280 °C, since it was absent in the Q280 sample at room temperature and in all the samples at any other temperature during the heating cycle. Indexing was performed considering minor elements present in the alloy, such as Cu, Fe, and Sr, and finally the SrZn 13 phase was identified (PDF: 04-013-4885). At 320 °C, both Zn and SrZn 13 were dissolved. In addition, between 30 and 320 °C the intensity of Mg 2 Zn 11 increased and then became negligible at 360 °C. From the increase in the intensity of the MgZn 2 peaks at this temperature, it followed that Mg 2 Zn 11 had not completely dissolved in the matrix and may have become the MgZn 2 phase. Regarding the cooling cycle, the onset of the precipitation of the Mg 2 Zn 11 phase was observed at 260 °C, while that of the Zn phase was not detected until 180 °C. At this temperature, the peaks belonging to SrZn 13 showed slightly and disappeared again with further cooling. It should be noted that in the final measurement at 30 °C, the distribution of precipitated phases was different from what it had been at the beginning. The proportion of MgZn 2 obtained at 360 °C remained stable during cooling and was higher than that found during the initial measurement. No evolution with temperature was observed for the Mg 2 Si phase. There were difficulties with detecting it in some of the measurements (see differences in Figure 7), but this was related to the specific sample (local segregations or inhomogeneities) and not to transformations taking place with temperature. 3.4. Evolution of Aluminum Phases As was previously mentioned, a two-phase Al–Zn matrix was found. However, a detailed observation of the indexed profiles led to the detection of some peaks whose intensity was higher than expected. These observations were confirmed when Figure 8. Diffraction patterns of the as-cast sample showing the evolution of the intermetallic phases, Zn, and Si with temperature ( a ) during the heating cycle (from 30 to 360 ◦ C) ( b ) and cooling cycle (from 360 to 30 ◦ C) in the thermo-diffractometer. The main Bragg peaks for the SrZn 13 phase are identified. At 30 ◦ C, Zn, MgZn 2 , and Mg 2 Zn 11 phases were found. At 260 ◦ C, the intensity of Zn peaks decreased while two additional Bragg peaks were detected around 2 θ = 35.9 ◦ and 2θ= 54.0◦. These peaks did not belong to any of the phases already indexed. Assuming they belonged to a new phase, it was clear that it arose at a temperature between 30 and 260 ◦ C and likely dissolved between 260 and 280 ◦ C, since it was absent in the Q280 sample at room temperature and in all the samples at any other temperature during the heating cycle. Indexing was performed considering minor elements present in the alloy, such as Cu, Fe, and Sr, and finally the SrZn13 phase was identified (PDF: 04-013-4885). Materials 2022,15, 5056 9 of 14 At 320 ◦ C, both Zn and SrZn 13 were dissolved. In addition, between 30 and 320 ◦ C the intensity of Mg 2 Zn 11 increased and then became negligible at 360 ◦ C. From the increase in the intensity of the MgZn2peaks at this temperature, it followed that Mg2Zn11 had not completely dissolved in the matrix and may have become the MgZn2phase. Regarding the cooling cycle, the onset of the precipitation of the Mg 2 Zn 11 phase was observed at 260 ◦ C, while that of the Zn phase was not detected until 180 ◦ C. At this temperature, the peaks belonging to SrZn 13 showed slightly and disappeared again with further cooling. It should be noted that in the final measurement at 30 ◦ C, the distribution of precipitated phases was different from what it had been at the beginning. The proportion of MgZn 2 obtained at 360 ◦ C remained stable during cooling and was higher than that found during the initial measurement. No evolution with temperature was observed for the Mg 2 Si phase. There were difficulties with detecting it in some of the measurements (see differences in Figure 7), but this was related to the specific sample (local segregations or inhomogeneities) and not to transformations taking place with temperature. 3.4. Evolution of Aluminum Phases As was previously mentioned, a two-phase Al–Zn matrix was found. However, a detailed observation of the indexed profiles led to the detection of some peaks whose intensity was higher than expected. These observations were confirmed when performing a Rietveld fitting on one of the profiles (Eq280 sample at 30 ◦ C, before heating). It was verified that some of the peaks could not be fitted with the original model and there was a phase missing (Figure 9a). The addition of a phase with the same spatial group as aluminum ( Fm3m ) but a smaller lattice parameter managed to solve the structural model with satisfactory precision (Figure 9b). Due to the smaller atomic size of zinc compared to aluminum, a zinc-rich aluminum phase would show a smaller lattice parameter than α -Al and thus its Bragg peaks would shift to greater angles [ 32 ]. Therefore, the new phase observed could be the zinc-rich α ’ aluminum metastable phase of the miscibility gap in the Al–Zn system. The samples were retested with room temperature X-ray diffractometry nine months later with the aim of determining whether this was the case, and it was found that precipitation of the Zn phase from the α’ metastable phase had taken place. Materials 2022, 15, 5056 10 of 15 performing a Rietveld fitting on one of the profiles (Eq280 sample at 30 °C, before heating). It was verified that some of the peaks could not be fitted with the original model and there was a phase missing (Figure 9a). The addition of a phase with the same spatial group as aluminum (𝐹𝑚3 𝑚) but a smaller lattice parameter managed to solve the structural model with satisfactory precision (Figure 9b). Due to the smaller atomic size of zinc compared to aluminum, a zinc-rich aluminum phase would show a smaller lattice parameter than α-Al and thus its Bragg peaks would shift to greater angles [32]. Therefore, the new phase observed could be the zinc-rich α’ aluminum metastable phase of the miscibility gap in the Al–Zn system. The samples were retested with room temperature X-ray diffractometry nine months later with the aim of determining whether this was the case, and it was found that precipitation of the Zn phase from the α’ metastable phase had taken place. (a) (b) Figure 9. Rietveld fitting of the Eq280 sample at 30 °C (before heating) with FullProf software. Red dots: experimental data. Black line: Fitting data. (a) The following six phases are considered Al (a = 4.0428 Å), Zn, Si, MgZn 2 , Mg 2 Zn 11 , and Mg 2 Si. The Pt phase comes from the sample holder. R-values with background correction: R p = 36.6 %, R wp = 38.6 %, R exp = 10.72 %, χ 2 = 12.91. (b) An α’ phase with a lattice parameter a = 4.0089 Å is added to the previous case. Al phase in (a) is now labeled as α-Al. R-values with background correction: R p = 20.5 %, R wp = 18.7 %, R exp = 10.54 %, χ 2 = 3.15. The evolution of the intensity of the Bragg peak corresponding to the (101) plane of the Zn phase was observed (Figure 10). The reason for choosing this peak is simple: it is the one with the maximum intensity of the Zn phase and it does not overlap with signals belonging to any other phase. For these reasons, this reflection is one of those taken as a reference in precipitation studies of Al–Zn alloys [31]. Intensity increased in all cases, although only four of them are shown in the figure. Figure 9. Rietveld fitting of the Eq280 sample at 30 ◦ C (before heating) with FullProf software. Red dots: experimental data. Black line: Fitting data. ( a ) The following six phases are considered Al (a = 4.0428 ˚ A), Zn, Si, MgZn 2 , Mg 2 Zn 11 , and Mg 2 Si. The Pt phase comes from the sample holder. R-values with background correction: R p = 36.6 %, R wp = 38.6 %, R exp = 10.72 %, χ2 = 12.91. ( b ) An α ’ phase with a lattice parameter a = 4.0089 ˚ A is added to the previous case. Al phase in (a) is now labeled as α -Al. R-values with background correction: R p = 20.5 %, R wp = 18.7 %, R exp = 10.54 %, χ2= 3.15.