A Study of Second-Phase Precipitates and Dispersoid Particles in 2024 Aluminum Alloy After Diﬀerent Aging Treatments

materials

Article A Study of Second-Phase Precipitates and Dispersoid Particles in 2024 Aluminum Alloy after Diﬀerent Aging Treatments

Anna Staszczyk 1 , Jacek Sawicki 1,* and Boguslawa Adamczyk-Cieslak 2

1 Institute of Materials Science and Engineering, Lodz University of Technology, Stefanowskiego 1/15, 90-924 Lodz, Poland; [email protected] 2 Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland; [email protected] * Correspondence: [email protected]

Received: 7 November 2019; Accepted: 10 December 2019; Published: 12 December 2019

Abstract: Aluminum alloys such as AA2024 are popular in the automotive and aircraft industries. The application of artificial aging significantly improves their mechanical properties by precipitation hardening. However, commercial alloys very often contain different amounts of elements such as Si and Fe that make the evolution of the microstructure harder to control. Large intermetallic particles can influence the overall results of heat treatment and cause deterioration of material properties. The authors decided to examine changes in the microstructure of three commercial 2024 alloys with varying chemical compositions by applying three different types of aging treatments. The results show considerable differences in the amount, size and morphologies of the precipitates. Second-phase Al2Cu and Al2CuMg precipitates were identified in one of the alloys. Other interesting types of multiphase particles were discovered in alloys with higher Si contents. The results show that even small variations in the composition can lead to a completely different microstructure.

Keywords: precipitation hardening; microstructure; artiﬁcial aging; 2024; aluminum alloys

1. Introduction The 2024 aluminum alloy has been commonly used in the aircraft industry for many years. The most common and well-understood process of improving its mechanical properties is the artificial aging treatment (T6) at high temperature. It results in precipitation hardening following a sequence of forming S”, S’ and S (Al2Cu) phases from the super-saturated solid solution [1,2]. The phenomenon of secondary aging was observed in Al–Zn alloys by Löffler et al. [3]. It was observed that when aging was interrupted before reaching its peak, further natural hardening of the material can lead to obtaining even higher hardness and strength of the alloy. Secondary precipitation during interrupted treatments was thoroughly investigated by Lumley et al. [4–6]. It was suggested that the interruption period at a low temperature followed by another stage of aging at an elevated temperature results in finer dispersion of precipitates in the alloy [7]. It has been observed that hardening in 2024 Al alloy occurs in two stages separated by a plateau. The first, rapid increase of hardness takes place in less than 10 min when heated to a high temperature [8]. In terms of interrupted treatment of Al–Cu–Mg alloys, it is believed that the dwell period in room temperature allows more Cu–Mg clusters to nucleate at the nanometric scale of the alloy, as in the study by Marceau et al. [9]. According to the up-to-date literature studies, clusters of less than 5 nm contribute significantly to the strengthening effect; sometimes to a greater extent than S and S’ particles (S’ phase is sometimes associated with the Guinier-Preston-Bagaryatsky zones in Al alloys contrary

Materials 2019, 12, 4168; doi:10.3390/ma12244168 www.mdpi.com/journal/materials Materials 2019, 12, 4168 2 of 10 to the previous belief that S phase, with its metastable variants, nucleates on the GPB zones [10]). However, Risanti et al. applied interrupted aging on 2024 alloy and obtained no improvement in comparison to T6 treatment [11]. Conflicting results published by researchers regarding the hardening effect of 2024 alloy may be attributed to the variations in chemical composition. Silicon and iron have poor solubility in the alpha solution of an Al–Cu–Mg alloy. They form intermetallic phases with copper, magnesium and manganese during solidification of the cast. It is not possible to dissolve them entirely during solution treatment. Formation of those large dispersoid particles also extracts Cu and Mg from the supersaturated solution, decreasing the number of elements that could form clusters or particles during the strengthening phase. Mrówka-Nowotnik and Sieniawski examined precipitates of phases present in the microstructure just after casting [12]. The intermetallic particles were much harder and more brittle than the matrix or S phase, as measured by Radutoiu et al. [13]. It was discovered, e.g., in work by Boag et al. [14], that many of them were, in fact, multiphase with heterogeneous compositions. There is very little knowledge about mechanisms responsible for the formation of multiphase precipitates. Kaczmarek et al. optimized the process of heat treatment of 2024 and 7075 alloys so that they obtained particles of unique core–shell morphology [15]. This was followed by the latest study, done by Lipa et al., stating that those precipitates significantly improved the fatigue behavour of the 2024 alloy [16]. The research performed by Campestrini et al. demonstrated that intermetallic particles, especially of core–shell morphology, could contribute to pitting corrosion in the material [17]. They found out that the surface potential difference between the core and shell of the particle is higher than that typically found between precipitates and the matrix phase of the alloy. The standard assumes that the amount of silicon in 2024 alloy can vary from 0% to 0.5%. It has been proven that the amount of Si can strongly influence the precipitation process in Al alloys [18–20]. The presence of large brittle particles of intermetallic phases can lead to deterioration of mechanical properties as they serve as crack initiation points. This decrease in toughness is a serious issue faced when applying aluminum alloys for construction applications. Therefore, it is important to investigate this problem and find ways to avoid it by altering the microstructure of the material through alloying elements or heat treatment conditions. So far, not enough research has focused on finding differences in the behavior of 2024 alloy with varying amounts of alloying elements. From an industrial point of view, it is not possible to eliminate Si and Fe impurities from high-strength Al alloys. The purpose of this research was to prove that, in commercial 2024 alloy with composition in the range described by a standard, there might be considerable differences in the obtained microstructures and therefore predicted hardening response.

2. Materials and Methods Three diﬀerent 2024 alloys with varying chemical compositions were studied, denoted as A, B and C. Their compositions are shown in Table1. Each of the alloys was bought from a di ﬀerent supplier as a standard AA2024 and their chemical composition was examined using an ARL Perform’X™ Sequential WDXRF X-ray spectrometer.

Table 1. Chemical composition of three examined 2024 alloys in % mass.

Alloy Cu Mg Mn Si Fe Zn Cr Ti Ni Al A 4.76 1.36 0.79 0.16 0.12 0.04 0.02 0.02 0.01 Rest B 4.15 1.51 0.64 0.24 0.25 0.09 0.01 0.06 0.01 Rest C 4.39 1.62 0.74 0.41 0.43 0.04 0.01 0.05 0.01 Rest

All of the samples were solution treated at 500 ◦C for 4 h, then quenched in room temperature water. This process was optimized during preliminary research. Parameters of aging applied to the alloys were developed based on common industry standards and literature studies. T6 aging was performed for 10 h at 180 ◦C, which is a widely applied practice for 2024 alloy. T6I6 aging treatment Materials 2019, 12, 4168 3 of 10 was performed at the same temperature, but in two stages (1 and 5 h) interrupted by a week at room Materials 2019, 12, x FOR PEER REVIEW 3 of 10 temperature. Finally, the innovative T-DA (double aging) treatment was introduced based on the good goodresults results obtained obtained by Kaczmarek by Kaczmare et al.k [21 et]. al. The [21]. schematic The schematic representation representation of the processes of the processes is shown inis shownFigure1 in. The Figure Vickers 1. The hardness Vickers of hardness specimens of spec wasimens measured was aftermeasured solution after treatment solution andtreatment then after and theneach followingafter each process.following The process. measurements The measurements were performed were onperformed the Innovatest on the Verzus Innovatest 700AS Verzus tester (INNOVATEST700AS tester (INNOVATEST Europe BV, Maastricht, Europe BV, The Maastricht, Netherlands) The withNetherlands) a 5 kg load. with a 5 kg load.

Figure 1. SchematicSchematic representation representation of three different diﬀerent agin agingg treatments applied to the examined alloys.

All of the specimens after after heat heat treatment treatment were were pr preparedepared for for scanning scanning electron electron microscopy microscopy (SEM) (SEM) observation with traditional methods; i.e., ground with papers from 300 to 2400 grade and polished with 0.03 µµmm colloidal silica. When not under examin examination,ation, the specimens were stored in a freezer at −1818 °CC to to avoid avoid natural natural aging aging processes. processes. − ◦ Scanning electronelectron microscopy microscopy observations observations were were carried carried out on out the JEOLon the JSM-6610LV JEOL JSM-6610LV equipment (Jeolequipment Ltd., Tokyo, (Jeol Ltd., Japan). Tokyo, The 20 Japan). keV accelerating The 20 keV voltage accelerating was applied voltage with was a 10 applied mm working with a distance. 10 mm workingSEM distance. micrographs were taken of at least ten randomly chosen regions of each specimen withSEM magnification micrographs 1000 wereto taken determine of at least the amountten randomly of secondary chosen regions phases of in each the microstructure.specimen with × magnificationEnergy dispersive 1000× spectroscopy to determine (EDS) the was amount used toof determine secondary chemical phases compositionsin the microstructure. of selected Energy phases. dispersiveThese spectroscopy pictures were (EDS) then was analyzed used to with determi imagene processingchemical compositions software to of calculate selected a phases. number of precipitatesThese pictures and the areaswere occupiedthen analyzed by them. with The image obtained processing results weresoftware then to analyzed calculate to a compare number the of precipitateseffects of diff anderent the compositions areas occupied and by treatments. them. The obtained results were then analyzed to compare the effects of different compositions and treatments. 3. Results and Discussion 3. ResultsThe results and Discussion of the Vickers hardness measurements of the specimens are shown with the calculated standardThe deviationresults of inthe Table Vickers2. There hardness is a tendency measurements that alloy of A the has specimens the highest are hardness shown of with all after the calculatedevery treatment. standard Also, deviation it obtained in Ta peakble 2. hardness There is a after tendency the T-DA that treatmentalloy A has and the had highest the lowest hardness value of allafter after T6I6. every For treatment. the rest of Also, the alloys, it obtained there werepeak nohardness differences after in the hardness T-DA treatment between agingand had types. the lowest It may valuebe explained after T6I6. by comparingFor the rest the of the volume alloys, fractions there we ofre intermetallic no differences phases in hardness present inbetween the microstructures aging types. whichIt may are be onexplained the same by level comparing for all specimens the volume of alloys fractions B and of C. intermetallic phases present in the microstructures which are on the same level for all specimens of alloys B and C.

Table 2. Values of measured Vickers hardness of examined alloys after heat treatment.

Alloy T T6 T6I6 T-DA A 110 ± 2.6 HV 138 ± 2.7 HV 135 ± 1.3 HV 143 ± 2.9 HV B 102 ± 2.6 HV 134 ± 1.9 HV 132 ± 3.1 HV 133 ± 1.0 HV C 105 ± 3.2 HV 135 ± 3.2 HV 132 ± 2.6 HV 133 ± 2.4 HV

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Table 2. Values of measured Vickers hardness of examined alloys after heat treatment.

Alloy T T6 T6I6 T-DA A 110 2.6 HV 138 2.7 HV 135 1.3 HV 143 2.9 HV ± ± ± ± B 102 2.6 HV 134 1.9 HV 132 3.1 HV 133 1.0 HV Materials 2019, 12, x FOR PEER REVIEW± ± ± ± 4 of 10 C 105 3.2 HV 135 3.2 HV 132 2.6 HV 133 2.4 HV ± ± ± ± There is a considerable difference between the microstructures of alloys with different chemical compositions,There is a and considerable the tendency difference remains between the same the fo microstructuresr every heat treatment of alloys type. with The diff erentcomparison chemical of microstructurescompositions, and at the same tendency magnification remains the is samedisplayed for every in Figure heat treatment2. The fraction type. of The the comparison precipitate phaseof microstructures is shown in Figure at the same3, and magnification it was calculated is displayed from micrographs in Figure2 .as The the fraction amount of of the area precipitate occupied byphase precipitates is shown in relation Figure3, to and total it was area calculated of an image. from For micrographs alloy A, there as the is amounta small drop of area in occupiedthe amount by ofprecipitates precipitate in phase relation after to totalcombined area of treatments an image. T6I6 For alloyand T-DA, A, there in iscontrast a small to drop T6. inAlso, the the amount lowest of volumeprecipitate fraction phase corresponds after combined to the treatments peak hardness T6I6 and of T-DA, the alloy. in contrast For alloys to T6. B Also,and C the there lowest is no volume such tendency;fraction corresponds there is even to the a peak noticeable hardness increase of the alloy. in volume For alloys fraction, B and Cespecially there is no for such T-DA tendency; treatment. there Overall,is even a the noticeable differences increase between in volume the variants fraction, are especially not significant for T-DA and treatment. the values Overall, are all in the the di ffrangeerences of standardbetween thedeviation. variants are not significant and the values are all in the range of standard deviation.

Figure 2. ComparisonComparison of of microstructures microstructures of ( a)) alloy A, ( b) alloy B, and ( c) alloy C after T6 aging.

FigureFigure 3. TotalTotal volume fractions of precipitate phas phaseses for all the alloy treatment variants.

2 The calculatedcalculated number number of of precipitates precipitates per per 1 mm 1 mmis shown2 is shown in Figure in 4Figure. In the 4. case In ofthe this case parameter, of this parameter,there is a very there distinct is a very diff erencedistinct between difference the be alloys.tween The the microstructurealloys. The microstructure of alloy B has ofmuch alloy fewerB has muchparticles fewer than particles the other than two. the However, other two. the However, most drastic the most difference drastic is difference visible for is alloy visible C, wherefor alloy there C, whereare up there to 5 times are up more to 5 precipitates times more than precipitates in alloys Athan and in B. alloys Those A results and B. are Those firmly results connected are firmly with connectedthe mean radii with ofthe particles mean radii shown of particles in Figure shown5. It was in Figure calculated 5. It was as an calculated equivalent as radiusan equivalent of an average radius ofround an average precipitate round resulting precipitate from the resulting area occupied from the by ar it.ea In occupied alloy B, with by it. the In lowest alloy numberB, with ofthe particles lowest number of particles per unit of area, the precipitates are the largest, whereas in alloy C, the precipitates are numerous but very small.

Materials 2019, 12, 4168 5 of 10 per unit of area, the precipitates are the largest, whereas in alloy C, the precipitates are numerous but veryMaterials small. 2019, 12, x FOR PEER REVIEW 5 of 10

Figure 4. Number of precipitates on the examinedexamined surface of the specimen per mmmm22.

Figure 5. The equivalent radius of precipitates on the examined surfacesurface of the specimen.

Single precipitatesprecipitates were were analyzed analyzed with with EDS EDS to determine to determine their chemicaltheir chemical composition. composition. The element The weightelement % weight contents % of thecontents most representativeof the most phasesrepresentative are shown phases in Table are3. Theshown corresponding in Table spectra3. The correspondingare shown in Figure spectra6. In are alloy shown A, two in di Figurefferent 6. types In alloy of particles A, two were different observed. types The of firstparticles were largerwere particlesobserved. with The irregularfirst were shapes larger consistingparticles with of Al, irregu Cu,lar Mn, shapes Fe and consisting Si, as shown of Al, in Cu, Figure Mn,7. Fe There and isSi, no as showndifferentiation in Figure in 7. chemical There is composition no differentiation within in the chemical precipitate. composition There is alsowithin a second the precipitate. type of particle There present,is also aclose second to roundtype of in particle shape and present, visually close much to brighter,round in in shape contrast and to visually the others much onSEM brighter, images. in contrastPrecipitates to the of others this type on containSEM images. Al and Precipitates Cu with a of weight this type ratio contain of 60%:40%, Al and andCu with some a ofweight them ratio also ofhave 60%:40%, Mg in smallerand some amounts of them (Figure also have8). Based Mg in on sm thealler EDS amounts results, (Figure and on 8). their Based visual on the characteristics, EDS results, θ andthey on were their identified visual characterist as Al2Cu andics, Althey2CuMg, were identified which corresponds as Al2Cu and to the Al well-known2CuMg, whichand corresponds S phases, torespectively. the well-known Despite θ theand fact S phases, that they respectively. are usually Despite observed the with fact nanometricthat they are sizes, usually it is notobserved unheard with of nanometricto find larger sizes, particles it is not of thisunheard type, of as to reported find larger by, e.g.,particles Buchheit of this et type, al. [22 as]. reported Such precipitates by, e.g., Buchheit were not etcommonly al. [22]. Such observed precipitates in other were alloys, not where commonly higher ob contentsserved in of other Si and alloys, Fe might where have higher encouraged contents the of Siformation and Fe might of more have complex encouraged phases the instead. formation of more complex phases instead.

Table 3. Chemical compositions of the observed phases obtained by EDS. The corresponding spectra are shown in Figure 6.

wt % in Phases Element Al2Cu Al2CuMg “Bright” Phase “Dark” Phase Al 60.75 ± 0.25 60.72 ± 0.20 57.36 ± 0.28 59.84 ± 0.25 Cu 39.25 ± 0.25 28.40 ± 0.21 30.38 ± 0.29 6.20 ± 0.13 Mg 10.89 ± 0.10 Mn 2.89 ± 0.11 10.75 ± 0.17 Fe 9.36 ± 0.17 15.53 ± 0.20 Si 7.69 ± 0.13

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Table 3. Chemical compositions of the observed phases obtained by EDS. The corresponding spectra are shown in Figure6.

wt % in Phases Element Al2Cu Al2CuMg “Bright” Phase “Dark” Phase Al 60.75 0.25 60.72 0.20 57.36 0.28 59.84 0.25 ± ± ± ± Cu 39.25 0.25 28.40 0.21 30.38 0.29 6.20 0.13 ± ± ± ± Mg 10.89 0.10 ± Mn 2.89 0.11 10.75 0.17 ± ± Fe 9.36 0.17 15.53 0.20 ± ± Si 7.69 0.13 MaterialsMaterials 20192019,, 1212,, xx FORFOR PEERPEER REVIEWREVIEW ± 66 ofof 1010

FigureFigure 6. 6. SpectraSpectra fromfrom EDS EDS analysis analysis of of the the most most common common phases phases found found in in the the alloys alloys A, A, B B and and C. C.

FigureFigure 7. 7. AA precipitate precipitate found found in in the the alloy alloy A A after after T-DA T-DA aging.aging.

FigureFigure 8.8. PrecipitatesPrecipitates ofof AlAl22CuCu andand AlAl22CuMgCuMg phasesphases foundfound inin alloyalloy A.A.

ParticlesParticles ofof intermetallicintermetallic phasesphases inin alloyalloy BB areare visiblyvisibly largerlarger andand moremore elongatedelongated inin shapeshape thanthan inin thethe twotwo otherother alloys.alloys. TheyThey oftenoften formform complicatedcomplicated andand curvedcurved structuresstructures withwith ‘empty’‘empty’ placesplaces inin thethe middlemiddle ofof theirtheir cross-section.cross-section. TheirTheir chemicalchemical compositioncomposition isis veryvery closeclose toto thethe biggerbigger phasesphases

Materials 2019, 12, x FOR PEER REVIEW 6 of 10

Figure 6. Spectra from EDS analysis of the most common phases found in the alloys A, B and C.

Materials 2019, 12, 4168 7 of 10 Figure 7. A precipitate found in the alloy A after T-DA aging.

Figure 8.8. Precipitates ofof AlAl22Cu and Al2CuMg phases found in alloy A.

Particles of intermetallic phases in alloy B are visibly larger and more elongated in shape than in MaterialsParticles 2019, 12 ,of x FORintermetallic PEER REVIEW phases in alloy B are visibly larger and more elongated in shape7 than of 10 thein the two two other other alloys. alloys. They They often often form form complicated complicated and and curved curved structures structures with with ‘empty’ ‘empty’ places places in the in observedmiddlethe middle of in their ofalloy their cross-section. A, cross-section.as EDS results Their Their chemical in Figure chemical composition 9 show. composition There is very were is close virtuallyvery to close the no bigger to θ theand phases biggerS precipitates observed phases in alloy A, as EDS results in Figure9 show. There were virtually no θ and S precipitates visible. visible.

Figure 9. A precipitate found in alloy B after T-DA aging.

In alloy B, multiphase precipitates were observed, such as in Figure9 9.. OneOne phasephase hadhad moremore CuCu and another had more Si. The The Cu-rich Cu-rich phase phase appe appearedared brighter in in the image and was very often located nearnear thethe edges edges of of precipitates. precipitates. For For identification identification purposes, purposes, it is referredit is referred to as theto as “bright” the “bright” phase, whilephase, the while phase the withphase lower with Culower content Cu content was called was thecalled “dark” the “dark” phase. phase. The EDS The results EDS results for both for can both be canfound be infound Table in3 Tableand in 3 Figure and in6 .Figure 6. A fragment of the precipitate showing the boundaryboundary between the two phases is presented in greater magnificationmagnification in Figure 1010.. This phenomenon was never observed in alloy A, which leads to the conclusionconclusion that that this this specific specific morphology morphology of precipitates of precipitates is correlated is correlated with higher with Sihigher and FeSi contentsand Fe contentsin the alloy. in the alloy.

Figure 10. A closer view at the multiphase precipitate found in alloy B after T6I6 aging showing the distinction between “bright” and “dark” phases.

Materials 2019, 12, x FOR PEER REVIEW 7 of 10 observed in alloy A, as EDS results in Figure 9 show. There were virtually no θ and S precipitates visible.

Figure 9. A precipitate found in alloy B after T-DA aging.

In alloy B, multiphase precipitates were observed, such as in Figure 9. One phase had more Cu and another had more Si. The Cu-rich phase appeared brighter in the image and was very often located near the edges of precipitates. For identification purposes, it is referred to as the “bright” phase, while the phase with lower Cu content was called the “dark” phase. The EDS results for both can be found in Table 3 and in Figure 6. A fragment of the precipitate showing the boundary between the two phases is presented in greater magnification in Figure 10. This phenomenon was never observed in alloy A, which leads to theMaterials conclusion2019, 12, 4168that this specific morphology of precipitates is correlated with higher Si and8 of Fe 10 contents in the alloy.

MaterialsFigure 2019 ,10. 12 , Ax FORcloser PEER view REVIEW at the multiphase precipitate found in alloy B after T6I6 aging showing the 8 of 10 distinction between “bright” and “dark” phases. A similar diversity of phases was observed in alloy C. An interesting type of precipitate morphologyA similar was diversity observed of phases in this was alloy, observed where in one alloy particle C. An interestingconsists of typetwo ofphases precipitate merged morphology in a form wasresembling observed a core in this and alloy, shell. where The results one particle of the EDS consists analysis of two of phasesthese particles merged are in ashown form resemblingin Figure 11. a coreHere, and again, shell. the The brighter results phase of the is EDS rich analysis in copper of theseand it particles tends to are occur shown on inthe Figure circumference 11. Here, of again, the theprecipitate. brighter phase is rich in copper and it tends to occur on the circumference of the precipitate.

Figure 11. A precipitate of core–shell morphology found in alloy C after T6 aging. The results of EDS analysis show didifferentiationﬀerentiation ofof chemicalchemical compositioncomposition betweenbetween thethe twotwo phases.phases.

In thethe microstructure microstructure of alloyof alloy C, there C, there is a much is a finer much dispersion finer dispersion of particles; of they particles; are considerably they are smallerconsiderably and densely smaller distributed. and densely Again, distributed. no distinguishable Again, noθ distinguishableand S phases were θ and observed. S phases However, were allobserved. of the particles However, tended all of to the be more particles regular tended and rounderto be more than regular in the two and other rounder alloys. than The in microstructure the two other ofalloys. this alloyThe matchesmicrostructure the description of this alloy of possible matches core–shell the description particles describedof possible in core–shell previous studies particles of 2024described alloy, in though previous it was studies not possible of 2024 toalloy, find though a connection it was between not possible their to presence find a connection and the parameters between oftheir heat presence treatment. and Similar the parameters looking particles of heat with treatment. the same Similar compositions looking wereparticles found with in specimensthe same accordingcompositions to each were aging found type, in specimens but they did according not influence to each the aging hardness type,of but the they alloy. did not influence the hardness of the alloy.

4. Conclusions 1. There was a considerable difference in the microstructure of alloys containing varying amounts of added Si and Fe. The total volume fraction of intermetallics remained at the same level for all of them; huge differences in size and dispersion of particles were observed. Alloy A, with small amounts of Si and Fe, contained two distinct types of particles. Alloy B, in the middle range of the standardized composition, contained phases much bigger than the other two, with curved, complicated shapes. Alloy C, in the upper limit of alloying additions, had a high number of smaller and rounder precipitates. 2. There was no significant difference in hardness after different aging treatments for one alloy. Alloy A had the highest hardness after solution treatment, and the tendency remained the same after all processes. Process parameters did not influence the characteristics of intermetallic particles in the material. 3. Alloy A, with the lowest additions of Si and Fe, was the only one to have numerous Al2Cu and Al2CuMg precipitates. This leads to the conclusion that higher Si and Fe contents prevent the θ and S phases from growth on a microscopic scale. 4. In alloys B and C, with higher Si and Fe contents, the presence of multiphase precipitates was confirmed, as predicted by literature study. The difference between chemical compositions of those phases was most prominent in Cu contents. 5. In alloy C, some of the multiphase precipitates showed core–shell-like morphology. It looks like their presence correlated to higher Si and Fe content, but not heat treatment type. We conclude

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4. Conclusions 1. There was a considerable difference in the microstructure of alloys containing varying amounts of added Si and Fe. The total volume fraction of intermetallics remained at the same level for all of them; huge differences in size and dispersion of particles were observed. Alloy A, with small amounts of Si and Fe, contained two distinct types of particles. Alloy B, in the middle range of the standardized composition, contained phases much bigger than the other two, with curved, complicated shapes. Alloy C, in the upper limit of alloying additions, had a high number of smaller and rounder precipitates. 2. There was no significant difference in hardness after different aging treatments for one alloy. Alloy A had the highest hardness after solution treatment, and the tendency remained the same after all processes. Process parameters did not influence the characteristics of intermetallic particles in the material.

3. Alloy A, with the lowest additions of Si and Fe, was the only one to have numerous Al2Cu and Al2CuMg precipitates. This leads to the conclusion that higher Si and Fe contents prevent the θ and S phases from growth on a microscopic scale. 4. In alloys B and C, with higher Si and Fe contents, the presence of multiphase precipitates was confirmed, as predicted by literature study. The difference between chemical compositions of those phases was most prominent in Cu contents. 5. In alloy C, some of the multiphase precipitates showed core–shell-like morphology. It looks like their presence correlated to higher Si and Fe content, but not heat treatment type. We conclude that the beneficial effects connected with multiphase aging treatments could be caused by the specific chemical composition of the 2024 alloy used in a study.

Author Contributions: Conceptualization: A.S.; Investigation: A.S. and B.A.-C.; Methodology: A.S.; Resources: J.S.; Supervision: J.S.; Validation: A.S., J.S. and B.A.-C.; Writing—original draft: A.S.; Writing—review and editing: J.S. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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