Effect of heat treatment on microstructure and mechanical properties of Al 2024 matrix composites reinforced with Ni60Nb40 metallic glass particles

Journal Pre-proof Effect of heat treatment on microstructure and mechanical properties of Al 2024 matrix composites reinforced with Ni60Nb40 metallic glass particles Onur Ertugrul, Tianbing He, Rub-Nawaz Shahid, Sergio Scudino PII:

S0925-8388(19)32965-2

DOI:

https://doi.org/10.1016/j.jallcom.2019.151732

Reference:

JALCOM 151732

To appear in:

Journal of Alloys and Compounds

Received Date: 6 April 2019 Revised Date:

17 July 2019

Accepted Date: 5 August 2019

Please cite this article as: O. Ertugrul, T. He, R.-N. Shahid, S. Scudino, Effect of heat treatment on microstructure and mechanical properties of Al 2024 matrix composites reinforced with Ni60Nb40 metallic glass particles, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.151732. 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. © 2019 Published by Elsevier B.V.

Effect of heat treatment on microstructure and mechanical properties of Al 2024 matrix composites reinforced with Ni60Nb40 metallic glass particles Onur Ertugrul a, *, Tianbing He b, Rub-Nawaz Shahid c, Sergio Scudino b a

Department of Materials Science and Engineering, Izmir Katip Celebi University, 35620 Izmir, Turkey

b

Solidification Processes and Complex Structures, Institute for Complex Materials, IFW Dresden, D-01069

Dresden, Germany c

Department of Metallurgy and Materials Engineering, Pakistan Institute of Engineering and Applied Sciences,

Islamabad, Pakistan * Corresponding author: [email protected]; [email protected] (Onur Ertugrul)

Abstract Al 2024 matrix composites reinforced with 20 and 40 vol% Ni60Nb40 metallic glass particles were synthesized through powder metallurgy using hot pressing and the effect of heat treatment (solution and artificial aging, T6) on microstructure and mechanical properties was examined. The microstructure of the unreinforced matrix shows the formation of CuAl2 and Al2CuMg precipitates after heat treatment, while the composites show the formation of additional CuNiAl and NbNiAl phases due to the reaction between the matrix and glassy particles. The yield strength is improved by the Ni60Nb40 addition in both as-sintered and heat-treated conditions, whereas the ductility is reduced for the composites with 40 vol% of reinforcement. The microstructural modifications due to the interfacial reactions in heat-treated composites result in a significant strengthening contribution.

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Keywords: Al 2024; Ni60Nb40 metallic glass; heat treatment; powder metallurgy; mechanical properties; interfacial reaction

1. Introduction Aluminum alloy matrix composites (AMCs) have attracted significant interest in recent years due to their superior properties, such as low density, high specific strength and stiffness, improved fatigue and wear resistance, which have led to extensive applications in automobile, aerospace and military industries [1-3]. Different materials, such as ceramics [4-6] , and carbon based materials (carbon fibers [7], carbon nanotubes [8], graphene [9]) have been used as reinforcements in AMCs in the form of fibers, flakes or mostly particulates. Since the early 2000s, metallic glasses have progressively become one of the most popular reinforcement materials in pure Al matrix composites because of their extremely high strength and hardness, along with good corrosion resistance [10-15]. They have been also proved to be compatible with the metal matrix and thus provide improved interface bonding between the matrix and the reinforcement [16-18]. Several studies have reported on composites with increased mechanical properties using Al-, Zr-, Fe-, Ni- and Cu-based metallic glass particles or ribbons [10-12, 19-21] as a reinforcement. In the metallic glass family, Ni-based glasses are of particular interest as a reinforcement due to their superior hardness, high corrosion resistance, and relatively low costs [12, 17, 22, 23]. More importantly, because metallic glasses are in a non-equilibrium state (which means crystallization may easily occur during heating), the high crystallization temperature of Ni-based glasses makes them an advantageous reinforcement in the heat-treatable matrices. It is also known that Ni-based particles 2

are compatible with Al matrix and, according to previous studies [12, 22], a detectable crystallization and interfacial reactions may occur after long time annealing at very high temperatures (more than 853K). Heat treatment (solution and natural/artificial aging) plays a pivotal role in the strengthening of aluminum alloys, while the use of metallic glass reinforcements in heat-treatable AMCs has barely been reported [23, 24]. This can be ascribed to the metastable nature of metallic glasses. Devitrification of metallic glasses starts to occur at the crystallization temperature (Tx) or even at temperatures below Tx but above the glass transition temperature (Tg) after a certain incubation time [25, 26]. The solution treatment of Al alloys requires annealing at relatively high temperatures (737 – 837 K), which limits the use of glassy reinforcements exhibiting high crystallization temperatures to materials [27-30]. In addition, the atomic diffusion increases at elevated temperature, which may promote the interfacial reaction between the matrix and reinforcements. The nature of the metallic reinforcements (metallic glasses or intermetallic compounds) on one hand may lead to improved interface compatibility with metallic matrix in comparison to ceramic or carbon-based reinforcements. On the other hand, the tendency to form intermetallics at matrix-reinforcement interface through diffusion mechanism at high temperature seems to be increased to some extent. For example, Yu et al. [22] reported that Ni60Nb40 metallic glass reinforcement reacted with the Al matrix immediately after glass crystallization with the formation of Al3Ni and Al3Nb intermetallics. It is also well-documented that the interfacial products have significant influence on physical and mechanical properties of the composites. For example, strengthening by interfacial reaction through the formation of Al5Fe2 and Al13Fe4 phases has been reported for the Al-Fe3Al system [31]; a similar 3

effect has been observed in the Al-Mg system due to the formation of Al3Mg2 and Al12Mg17 intermetallics [32]. In contrast, Barta et al. [33] showed that the formation of a thick intermetallic layer (~10 µm) in AMCs was detrimental to the tensile properties. This aspect becomes of primary importance when heat-treatable matrices are used because the degree of interfacial reaction and/or glassy phase crystallization is expected to significantly affect the properties of the composites. In this study, Al 2024 matrix composites reinforced with 20 and 40 vol% of Ni60Nb40 metallic glass particles have been synthesized using powder metallurgy (PM) and heat treatment has been applied to further improve the mechanical properties. Al 2024 alloy was selected as a heat-treatable matrix and Ni-based metallic glass powder was chosen as the reinforcement because of its high crystallization temperature. The effects of heat treatment and microstructural changes on the mechanical properties were evaluated under compressive load, and the strengthening mechanisms were investigated.

2. Experimental procedures Al 2024 (4.40 Cu, 1.42 Mg, 0.45 Mn, 0.2 Fe, <0.1 Si, balance Al, wt. %) powder with particle sizes in the range of 2-30 µm (average particle size ~9.1 µm) was used as the matrix (Fig. 1(a)). Glassy particles with nominal composition of Ni60Nb40 (at.%) and size range of 20-45 µm (average particle size ~33 µm) were produced by mechanical alloying of elemental powders using a Retsch PM400 planetary ball mill and hardened steel balls and vials (Fig. 1(b)). The powders were milled for 50 h at a milling speed of 150 rpm and at a ball-to-powder mass ratio (BPR) of about 13:1. No process control agent was used. 2024 matrix particles and Ni60Nb40 glassy particles (20 and 40 vol%) 4

were mixed by ball milling for 1 h using a Retsch PM400 planetary ball mill equipped with hardened steel balls and vials (BPR = 10:1, milling speed = 100 rpm). To avoid or minimize possible atmosphere contamination during milling, the vials were sealed in a MBraun 150B-G glove box under purified argon atmosphere (less than 0.5 ppm O2 and H2O). Consolidation of the powder mixtures was carried out by uniaxial hot pressing (HP) under argon atmosphere at 673 K and 640 MPa with a holding time of 10 min. The sample dimensions are 10 mm in diameter and 10 mm in height. Al 2024 bulk samples were produced by HP under the same condition as a reference group. The heat treatment (T6) consists of a solutionizing step at 773K for 1h (followed by water quenching) and an aging step at 423K for 18h. The T6 heat treatment was conducted under Ar atmosphere.

Fig. 1. SEM images of the (a) Al 2024 powder and (b) Ni60Nb40 powder.

The thermal behavior of the metallic glass powders was investigated by differential scanning calorimetry (DSC) using a Perkin-Elmer DSC7 calorimeter at a heating rate of 20 K/min under a continuous flow of purified Ar. Phase analysis of the powders and consolidated (as-sintered and heat-treated) samples was performed by X-ray diffraction (XRD) using a PANalytical X’Pert PRO D3290 diffractometer with Co-Kα radiation (λ=0.17889 nm) operating at 40 kV and 40 mA. 5

Microstructural characterization and elemental mapping of the consolidated samples were carried out by scanning electron microscopy (SEM) using a Zeiss Gemini 1530 field emission microscope equipped with energy-dispersive X-ray spectrometer (EDX). Quantitative analysis of the optical and SEM images was performed using the ImageJ software. In order to analyze the percentage of cracked and debonded particles on the fracture surfaces, the particles were counted using five different images of 100X magnification which represents nearly the whole cross-section. For phase analysis, ten SEM images were analyzed for each sample. The relative density of the consolidated samples was measured by the Archimedes principle, all samples giving yielding a relative density exceeding 99 % for all specimens. Brinell hardness measurements of the samples were conducted using an Emco-test Duravision hardness tester by applying 62.5 kgf load using 2.5 mm steel ball intender. Cylindrical samples for compression tests having 3 mm diameter and 6 mm height were prepared from the as-sintered and heat-treated samples using wire-cutting. The dimensions of the samples (aspect ratio length/diameter ratio of 2) are in accordance with the ASTM standart for compression testing [34]. Both ends of the samples were carefully grinded and polished to make them parallel to each other, and also the longitudinal surfaces were polished using emery paper. The compression tests were performed at room temperature using an Instron 5862 testing facility with a strain rate of 1 × 10-4 s-1. The strain was measured directly on the samples using a Fiedler laser-extensometer. The compression tests were intentionally stopped after the strain reached 20%. Five samples were tested for each condition in order to ensure the reproducibility of the results.

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3. Results and discussion 3.1 Microstructural evaluation Fig 2(a) illustrates the XRD pattern of the mechanically-alloyed Ni60Nb40 metallic glass powder. The glassy powder shows the typical broad maximum characteristic for glassy materials at 2θ = 51°. Fig. 2(b) shows the DSC scan of the Ni60Nb40 glassy powder. According to the DSC scan, crystallization temperature is at about 803 K, which is higher than the selected solutionizing temperature of the T6 heat treatment process (773 K).

Fig. 2. (a) XRD pattern, and (b) DSC scan of the Ni60Nb40 powder.

Fig. 3(a) shows the XRD patterns of the as-sintered Al 2024 unreinforced matrix and the composite samples. The XRD pattern of pure Al2024 sample reveals the presence of α-Al phase together with intermetallic phases (CuAl2 and Al2CuMg). The formation of these precipitates are in agreement with previous reports on Al 2024 alloys [24, 35, 36]. The XRD patterns of the composites show the existence of the α-Al, CuAl2, and Al2CuMg phases in addition to a small peak of Ni3Nb phase at 2θ = 51o. The presence of the Ni3Nb phase in the as-sintered composites suggests that partial crystallization of the glassy particles took place during the fabrication of the samples, despite broad 7

diffraction maxima at about 51o which belongs to the glassy phase. This suggests that glassy particles retained their amorphous structure during hot pressing as stated in previous studies [11, 12].

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Fig. 3. XRD patterns of the unreinforced Al 2024 matrix and composite samples: (a) as-sintered and (b) heat-treated.

Fig. 3(b) shows the XRD patterns of the heat-treated Al 2024 matrix and composite samples. The heat-treated Al 2024 shows CuAl2 phase as already observed in the as-sintered sample, along with a smaller amount of Al2CuMg. The XRD patterns of the heat-treated composites show the presence of CuAl2 and nearly no Al2CuMg precipitates. The disappearance of the Al2CuMg phase is similar to the findings reported in previous studies [37]. According to Jafari et al. [37], who found that the Al2CuMg precipitates disappear when the annealing temperature is higher than 623 K and suggested that, since Mg is larger than Cu, and Mg atom has lower diffusivity than Cu atoms the formation of CuAl2 phase is more favorable. Additionally, the heat-treated composites show the presence of the CuNiAl and NbNiAl phases, suggesting atomic diffusion between the Al 2024 matrix and Ni-Nb reinforcement particles. The heat-treated 20 vol% reinforced sample shows small amounts of CuNiAl and NiNbAl, while the XRD pattern for the 40 vol% reinforced sample shows considerable amounts of CuNiAl and NbNiAl phases. The corresponding XRD patterns of the heat-treated 20 vol% reinforced composites do not show the phases CuNiAl and NbNiAl; this is most likely due to the extremely small amount of these phases below the detection limit of the device. Fig. 4(a-b) show the SEM micrographs of the as-sintered Al 2024 samples. The samples show the aluminum matrix surrounded by a network of Cu-rich based secondary phases and also containing small size Al2CuMg particles. The micrographs of the heat-treated Al 2024 samples (Fig. 4(c,d)) indicate that the microstructure consists of the aluminum matrix containing micron and 9

sub-micron sized precipitates. These precipitates are of two types: the large ones are CuAl2 based (~1-3 µm) and the smaller and round ones are Al2CuMg based (~100 nm) as shown in the Fig. 4(d).

Fig. 4. SEM micrographs of the unreinforced Al2024 (a,b) as-sintered and (c,d) heat-treated.

Fig. 5 presents the EDX elemental maps analysis of the as-sintered unreinforced Al 2024 samples in order to further analyze the microstructure. The elemental maps EDX point analysis in Fig. 5(a) confirm that the white phases particles in the as-sintered Al 2024 are a network of Cu-rich intermetallics (points labelled by 2) based secondary phases together with homogeneously distributed Al2CuMg particles (points labelled by 3), which corroborates the XRD results. Fig. 5(b) shows the elemental mapping EDX point analysis of the heat-treated Al 2024 sample containing white particles 10

which are Cu based precipitates. It is obvious that CuAl2 based precipitates (points labelled by 2) and Al2CuMg based very fine precipitates (points labelled by 3).

Fig. 5. Elemental maps of the unreinforced Al 2024 (a) as-sintered and (b) heat-treated samples.

Fig. 6 shows the SEM micrographs of the as-sintered and heat-treated composites. The microstructure is rather homogeneous and the distribution of metallic glass particles is good for both composites with 20 and 40 vol% reinforcement; furthermore, a clean interface between matrix and reinforcement particles can be seen in Fig. 6(b) and (f) for the as-sintered composites. In the heat-treated composites, interfacial regions between matrix and reinforcement particles, which are

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signs of diffusion, are visible (Fig. 6(d-h)). This interfacial reaction is more pronounced for the heat-treated 40 vol% reinforced composite samples.

Table 1 gives the amounts of matrix, reinforcement and interfacial phases present in the composites evaluated from SEM analysis. The amount of the matrix phase in the as-sintered 20 vol% composites is about 76 %. This small matrix material loss is probably due to preferential sticking of the softer matrix to the balls and inner walls of the vials during the mixing process carried out by ball milling. The amount of 2024 matrix in the heat-treated 20 vol% composites further decreases to about 73.5 % because of the interfacial reactions. The amount of glassy reinforcement does not change significantly in the as-sintered and heat-treated 20 vol% composites (24.3 and 23.8%), indicating that interfacial regions are generated at the expense of the 2024 matrix. Matrix loss is aggravated in 40 vol% composites as a result of the enlarged interfacial regions. In these composites, the amounts of reinforcement are also reduced (36.7 and 38.7%), suggesting that the diffusion due to interfacial reactions involves the Ni60Nb40 glass as well as the matrix phase. Similarly, the amount of interfacial regions after heat treatment increases significantly with increasing Ni60Nb40 volume fraction. The amount of matrix phase decreases again which is directly proportional to increased interfacial regions.

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Fig. 6. SEM micrographs of the composites (a,b) 20 vol% Ni60Nb40 as-sintered (c,d) 20 vol% Ni60Nb40 heat-treated (e,f) 40 vol% Ni60Nb40 as-sintered (g,h) 40 vol% Ni60Nb40 heat-treated.

Table 1. Amount of phases present in the composite samples. T6 indicates the heat-treated samples. Material

Matrix (vol %)

Reinforcement

Interfacial

(%)

region (%)

Al2024-20Ni60Nb40

75.7 ± 2.7

24.3 ± 2.3

Negligible

Al2024-20Ni60Nb40+T6

73.5 ± 2.2

23.8 ± 2.0

2.7 ± 1.1

Al2024-40Ni60Nb40

63.3 ± 1.9

36.7 ± 2.1

Negligible

Al2024-40Ni60Nb40+T6

56.9 ± 2.7

38.7 ± 1.8

4.3 ± 0.9

Fig. 7 presents high magnification SEM micrographs of the composites. The interfaces of the as-sintered composites are sharp without any visible reaction products (Fig. 7(a,c)). The interfacial reaction between matrix and reinforcement is easily observed in the heat-treated composites (Fig. 7(b) and (d)); here, a thin interfacial region surrounds the Ni-Nb particles in the heat-treated composites with 20 vol% and 40 vol% Ni60Nb40. These regions have a rather homogenous thickness of 2-3 µm in 13

the 20 vol% composites, which becomes heterogeneous (2-10 µm thickness) in the 40 vol% composites.

Fig. 7. High magnification SEM micrographs of the (a) 20 vol% Ni60Nb40 as-sintered (b) 20 vol% Ni60Nb40 heat-treated (c) 40 vol% Ni60Nb40 as-sintered (d) 40 vol% Ni60Nb40 heat-treated.

Fig. 8(a) shows the EDX point analysis of the thin interfacial regions present in the heat-treated 20 vol% reinforced sample. The results indicate that the interfacial region is rich in Al, Cu, and Ni with a minor amount of Nb; this suggests that the interface most probably consists mainly of the CuNiAl and NbNiAl phase observed in the XRD patterns (Fig. 3(b)). These results are confirmed by the EDX elemental maps for the same sample shown in Fig. 8(b,c), which indicate that Cu and Al 14

diffuse from the matrix to the reinforcement and Ni diffuses from the reinforcement to form the CuNiAl interfacial region. These findings are similar to the findings reported in the study of Yu et al. [22], in which a similar Al/Ni60Nb40p composite system was investigated. In that study, diffusion controlled interfacial reaction of Ni and Al started above 873K, after the crystallization of the metallic glass particles which occurred at temperatures higher than 853K. In our study, the partial crystallization of the metallic glass occurred around 773K, which is the solutionizing temperature of T6 heat treatment. The low crystallization temperature can be attributed to the strong atomic diffusion of Al, Cu and Ni atoms near the interfacial zones which in turn would reduce the onset of crystallization.

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Fig. 8. (a) EDX point analysis, (b) elemental maps, (c) elemental concentration profiles of the interfacial region in the heat-treated 20 vol% sample.

3.2 Mechanical properties The representative room-temperature stress-strain curves under compression for the as-sintered and heat-treated composites are shown in Fig. 9 and the corresponding values of yield strength are summarized in Table 2. The yield strength (YS) of the as-sintered Al 2024 is 204 ± 4 MPa which is very close to previous findings for 2024 alloy ([16], [38], [39]). T6 heat treatment increases the YS of Al 2024 to 273 ± 5 MPa, corresponding to an improvement of about 33 %. Heat treatment keeps 16

the plastic deformability constant as both of the samples show the plastic strain of more than 20% (where the test was intentionally stopped). This improved mechanical behavior is in agreement with the results reported by Emamy et al. [40] and Carreno-Gallardo et al. [41], and are caused by the fine precipitates which effectively block dislocation motion. The compressive strength (CS, the highest compressive stress that the material is able to withstand [34]) also increased with T6 heat treatment due to visible strain hardening. The yield strength of the 20 vol% composite is 229 ± 9 MPa, therefore increased by about 25 MPa compared to the as-sintered unreinforced Al 2024 matrix (Table 2). This result is similar to the strength improvements reported in earlier studies for Ni-Nb metallic glass reinforced composites. For example, in the study of Yu et al. [12], 111 MPa YS is obtained for Al-30wt.% Ni70Nb30 composites which is 8 MPa more than that the pure Al matrix (103 MPa). In the study of Jayalakshmi et al. [28], Al-based composites reinforced with Ni60Nb40 glassy particles were synthesized using microwave-assisted rapid sintering followed by hot-extrusion without changing the amorphous structure of the reinforcement. The compressive YS was improved significantly with increasing Ni60Nb40 volume fraction, while the tensile YS began to improve with a minimum of 25 vol. % addition. Also, the failure strain of the 25% Vp composites are is still more than 50% under compression loading means amorphous reinforcements can be superior than to ceramics in terms of ductility. Finally, Lichtenberg et al. [23] have studied the cast AlSi10Mg-NiNbTa system, and found a significant increase of compression properties, which was attributed to the pore-free microstructure and strengthening contribution of the glassy flakes.

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The T6 heat treatment significantly increases the yield strength from 229 to 323 MPa (about 100 MPa) in Al2024-20Ni60Nb40 composites. One interesting point is that the plastic strain of the 20 vol % composite samples does not decrease below 20% (where the tests were stopped), in both as-sintered and heat-treated states. Therefore, it can be said that the addition of 20 vol% metallic glass has no adverse effect on the plastic deformability in the strain range studied in this work. The increase of reinforcement amount from 20 to 40 vol% increases the yield strength of the as-sintered composites from 229 ± 9 MPa to 292 ± 15 MPa. The strength further increases to 389 ± 10 MPa after T6 heat treatment. However, the plastic strain is reduced to about 14 % and 8 % (when the time the samples were fractured) for the as-sintered and heat-treated 40 vol% composite samples, respectively.

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Fig. 9. Stress-strain curves of the samples under compression. Note that the compression tests of the unreinforced Al2024 matrix and Al2024-20Ni60Nb40 composite were intentionally stopped after the strain reached 20%.

Table 2. Compression test results of the composite samples. T6 indicate the heat-treated samples. Material

Yield Strength

True Strain

(MPa)

(%)

Al2024

204 ±4

>20

Al2024+T6

273 ±5

>20

Al2024-20Ni60Nb40

229 ±9

>20

Al2024-20Ni60Nb40+T6

323 ±5

>20

Al2024-40Ni60Nb40

292 ±15

14.2 ±1.4

Al2024-40Ni60Nb40+T6

389 ±10

7.6 ±0.4

Fig. 10 presents the Brinell hardness values of the samples. Precipitation strengthening not only can improve the strength but also can enhance the hardness of the material. It is clear that the T6 treatment is successful since the hardness of as-sintered Al 2024 matrix is increased from 89 to 93 HBW when heat-treated which is in agreement with the results reported by other researchers [35, 36]. The hardness values of the as-sintered composites with 20 and 40 vol% metallic glass are 104 and 132 HBW, respectively. This improvement is due to the good bonding between the matrix and the reinforcement, and the high hardness of Ni60Nb40 particles which is similar to the hardness increase observed in the Mg-Ni60Nb40 system [29]. The hardness of the heat-treated 20 vol% reinforced 19

sample is 142 HBW which corresponds to a high increase of 38 HBW resulted from T6 heat treatment. Moreover, the heat-treated samples reinforced with 40 vol% of Ni60Nb40 particles showed the highest hardness of 194 HBW, which can be attributed to the increased amount of glassy particles together with interfacial regions. It is also evident from Fig. 10 that there is a good correlation between yield strength and hardness of the samples, which confirms that heat treatment results in a significant increase of both strength and hardness.

Fig. 10. Brinell hardness and yield strength of all samples

3.3 Strengthening contributions The three main factors that may affect the mechanical behavior of the heat-treated composites are (i) load bearing and dislocation strengthening resulting from the addition of the hard second phase reinforcement [42-47], (ii) precipitation strengthening of the matrix [23, 48] and (iii) the 20

possible strengthening effect induced by the interfacial regions [32, 49, 50]. The T6 heat treatment has a positive effect on the strength of the composites, as also reported by other authors. Lichtenberg et al. [23] observed that the T6 heat treatment increases the compressive strength of AlSi10Mg matrix MMCs. Cooke et al. [51] studied the effect of T6 tempering on an Al-2.3Cu-1.6Mg alloy as it provided an increase of ~90 MPa in yield strength combined with a small decrease (~2.5%) in ductility. In this sense, the relatively high values of improvement in YS (~90 MPa) was accompanied by a decrease in plastic deformation (~7%) in 40 vol% reinforcement as expected from precipitation strengthening. Microstructural modifications (formation of new phases at the matrix-reinforcement interface) can also give a significant strengthening contribution, as seen in other studies [31, 32]. For example, The CuNiAl/NbNiAl interfacial regions between the matrix and the reinforcing particles observed in this work may enhance the interfacial bonding of the composites and, as a result, they can carry extra load during mechanical tests. The positive effect of these regions on the yield strength can be approximately estimated if one considers the strengthening contributions (∆σy) from reinforcement addition and heat treatment individually and then evaluates the contribution from the interfacial regions by subtraction. The strengthening contribution due to the addition of the glassy particles can be estimated by analyzing the increment of strength for the as-sintered specimens, where no contributions from heat treatment and interfacial reactions are expected. In this case, the contributions are ∆σy = 25 MPa and ∆σy = 88 MPa for the 20 vol% and 40 vol% composites, respectively. The contribution from heat treatment (∆σy = 69 MPa) can be measured via the strength increase observed in the unreinforced 2024 matrix, where no contributions from load bearing, dislocation strengthening and interfacial reactions are expected. The overall strength improvement of 21

the heat-treated 20 vol% and 40 vol% composites with respect to the as-sintered unreinforced matrix are respectively ∆σy = 119 MPa and ∆σy = 185 MPa, which gives a strengthening contribution from the interfacial reactions of ∆σy = 25 MPa and ∆σy = 28 MPa; these are plausible values if one considers the uncertainty of the amount of phases shown in Table 1. The SEM images of polished cross-sections of the composite samples after compression tests are shown in Fig 11, and the corresponding amount of cracked and debonded particles are given in Table 3. Some particle cracking in addition to decohesion (particle/matrix interface debonding) is visible for the composites (Fig. 11(a,e)). The amount of particle cracking is higher for the 20 vol% addition composite and it gets lower for the 40 vol% addition (Table 3), which is probably because of the reduced tendency of smaller glassy particles do not tend to crack but than the larger ones, similar to the results of Kapoor et al. [52]. The decohesion is in the vicinity of some particles together with small voids nucleated at the particle/matrix interfaces for the as-sintered composites, while the degree of decohesion is more pronounced for 40 vol% (Fig. 11e,f). There are also some deformation marks in the matrix of as-sintered samples showing the presence of plastic deformation (Fig. 11f). In the heat-treated composites, strong particle cracking is the main damage mechanism as it is seen in most of the reinforcement particles and there is little debonding between matrix and reinforcement particles (Fig. 11(c,g)). This is due to the interfacial regions in the heat-treated composites, which improve the interfacial bond strength, thus maintaining critical cohesion of particles and the matrix (preventing decohesion during cracking), empowering the superior load transfer capability of the particles, and reducing microvoid penetration into the matrix. Although these interfacial regions are more pronounced for the heat-treated 40 vol% reinforced composites, some amount of void 22

formation between the reinforcing particles is seen with increasing reinforcement amount (Fig. 11h). Particle clustering is also observed for 40 vol% addition which results in decohesion (Fig. 6(f)) and inter-particle crack propagation (Fig. 6(h)) which is dangerous for load transfer, and thus deteriorated toughness behavior.

(a)

(b)

(c)

(d)

23

(e)

(f)

(g)

(h)

Fig. 11. The fracture surfaces of the as-compressed composites (a,b) 20 vol% Ni60Nb40 as-sintered (c,d) 20 vol% Ni60Nb40 heat-treated (e,f) 40 vol% Ni60Nb40 as-sintered (g,h) 40 vol% Ni60Nb40 heat-treated.

Table 3. Percentage of cracked and debonded particles on the fracture surface of the composites Material

Cracked

Debonded

Particles (%)

Particles (%)

Al2024-20Ni60Nb40

25.4 ±3.7

9.0 ±3.9

Al2024-20Ni60Nb40+T6

65.8 ±1.9

1.9 ±0.9

Al2024-40Ni60Nb40

9.9 ±4.3

20.2 ±4

24

Al2024-40Ni60Nb40+T6

36.7 ±3.1

13.1 ±2.2

4. Conclusions Al 2024 matrix composites reinforced with 20 and 40 vol% Ni60Nb40 glassy particles were successfully produced and the effect of heat treatment on mechanical properties was examined. As a result, the following conclusions can be drawn: 1. XRD analyses of the composites confirm suggest that crystallization of the glassy particles retained their amorphous structure took place during hot pressing. SEM micrographs of the as-sintered composites show a homogenous microstructure and there is a clean interface without any visible reaction products. 2. The as-sintered Al 2024 samples contain two types of precipitates: CuAl2 and Al2CuMg; yet heat-treated Al 2024 shows the same precipitates with few Al2CuMg phase. The heat-treated composites show very little amount of CuAl2, but additional CuNiAl and NbNiAl phases different than as-sintered composites. 3. In the heat-treated composites, an interfacial region rich in Al, Cu and Ni is formed. It is clear that Cu diffuses from the matrix to the interface and Ni diffuses from the glass to form the CuNiAl phase. 4. 20 vol% Ni60Nb40 addition increases the yield strength of Al 2024 matrix about 25 MPa. The yield strength of the as-sintered 40 vol% Ni60Nb40 composites increases to 292 MPa (about 90 MPa higher than matrix), yet the plastic strain is reduced to 14 %. 5. T6 heat treatment improves the yield strength of Al 2024 to 273 MPa (about 70 MPa), and do not reduce the plastic deformability. T6 heat treatment results in significant increase in the yield strength 25

of 20NiNb vol% reinforced composites to 323 MPa (about 94 MPa), without decreasing the plastic deformability giving a very good strength-ductility combination. When the addition is 40 vol %, the contribution of the heat treatment in the strength is similar which is about 100 MPa (to 389 MPa). Contrarily, the strain of these composites is getting lower (7.6 %). 6. Formation of new phases (CuNiAl/NbNiAl) at the interfaces results in significant strengthening contribution in the composites after T6 heat treatment is due to the formation of new phases (CuNiAl/NbNiAl) at the interfaces. The interfacial regions are beneficial to some extend of Ni60Nb40 addition (20%), yet have brittleness effect in 40vol% additions.

Acknowledgements The authors thank B. Opitz, A. Schultze and H. Merker for technical support. Onur Ertugrul acknowledges the financial support provided by TUBITAK (The Scientific and Technological Research Council of Turkey) with 2219-International Postdoctoral Research Fellowship Programme (Grant No. 1059B191601037).

Data availability The raw/processed data required to reproduce these findings are available upon request by contact with the corresponding author.

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Highlights 1. 2. 3. 4.

With heat treatment, Cu diffuses from the matrix and Ni diffuses from the glass. Heat treated 20 vol% Ni60Nb40 composites show significant mechanical properties. Interfacial regions results in strengthening contribution in the composites. Interfacial regions of heat-treated composites prevent decohesion during cracking.