154 I N Ż Y N I E R I A M A T E R I A Ł O W A ROK XXXVI

Influence of silver on glass forming ability and mechanical properties of Zr–Cu–Al alloys

Piotr Błyskun*, Grzegorz Cieślak, Maciej Kowalczyk, Jerzy Latuch, Tadeusz KulikFaculty of Materials Science and Engineering, Warsaw Univeristy of Technology, *p.blyskun@inmat.pw.edu.pl

The work focuses on studying the influence of silver content on the glass forming ability and the mechanical properties of the Zr48Cu36Al16 – xAgx alloys (x = 0, 2, 4, ..., 16 at. %). Rods with a diameter of 3 mm were manufactured by the copper mould casting. X-ray diffraction studies (Fig. 1) revealed that samples with 6÷14 at. % of silver content were fully amorphous. Differential scanning calorimetry (Fig. 3) allowed selecting the alloy that possessed the best glass forming ability on the basis of the supercooled liquid region width (ΔTx). The Zr48Cu36Al6Ag10 alloy exhibited ΔTx = 91 K. Mechanical properties of the alloys were characterized by means of Vickers microhardness (Fig. 5) and room temperature compression tests (Fig. 6). The highest value of microhardness was detected for the partially crystalline Zr48Cu36Al16 alloy (791 HV). However, the highest compression strength was measured for the Zr48Cu36Al12Ag6 alloy (σc = 1881 MPa). It should be noticed that a plastic strain was observed in the fully amorphous alloys. On the other hand, partially crystalline samples cracked catastrophically without any observable plastic strain. These studies revealed that the silver content increase resulted in the microhardness and the compression strength decrease. Good mechanical performance and satisfying glass forming ability of the fully amorphous alloys examined at this work seems to be promising set of properties for structural applications. However, the Zr48Cu36Al4Ag12 is the most promising one.

Key words: bulk metallic glasses, Zr–Cu–Al alloys, silver addition, glass forming ability, mechanical properties.

Wpływ srebra na zdolność do zeszklenia i właściwości mechaniczne stopów z układu Zr–Cu–Al

Praca dotyczy określenia wpływu dodatku srebra na zdolność do zeszklenia i właściwości mechaniczne stopów Zr48Cu36Al16 – xAgx (x = 0, 2, 4, ..., 16% at.). Pręty o średnicy 3 mm odlewano do formy miedzianej. Badania dyfrakcyjne promieni rentgenowskich wykazały, że próbki o zawartości srebra 6÷14% at. są w pełni amorficzne. Pozostałe stopy miały zbyt małą zdolność do zeszklenia, aby ulec pełnej amorfizacji w formie o średnicy 3 mm. Różnicowa kaloryme-tria skaningowa pozwoliła wskazać stopy o największej zdolności do zeszklenia wyznaczanej na podstawie szerokości zakresu cieczy przechłodzonej (ΔTx). Stop Zr48Cu36Al6Ag10 cechował się największą zdolnością do zeszklenia ΔTx = 91 K. Właściwości mechaniczne stopów scharakteryzowano za pomocą po-miarów mikrotwardości i statycznej próby ściskania w temperaturze pokojowej. Największszą mikrotwardość zmierzono dla próbki częściowo krystalicznej Zr48Cu36Al16 (791 HV). Jednak największą wytrzymałość na ściskanie uzyskano dla próbki w pełni amorficznej Zr48Cu36Al12Ag6 (σc = 1881 MPa). Należy podkreślić, że dla wszystkich próbek amorficznych odnotowano pewien zakres odkształcenia plastycznego. Natomiast wszystkie próbki częściowo krysta-liczne pękały katastroficznie bez odkształcenia plastycznego. Badania wykazały, że wzrost zawartości srebra powoduje zmniejszenie zarówno mikrotwar-dości, jak i wytrzymałości na ściskanie, ale w pewnym zakresie (6÷15% at.) wyraźnie poprawia zdolność do zeszklenia. Dobre właściwości mechaniczne i zadowalająca zdolność do zeszklenia stopów w pełni amorficznych, badanych w tej pracy, wydają się obiecującym zestawieniem właściwości w przypadku przyszłych zastosowań konstrukcyjnych. Stop o składzie Zr48Cu36Al4Ag12 okazał się optymalnym kandydatem.

Słowa kluczowe: masywne szkła metaliczne, stopy Zr–Cu–Al, dodatek srebra, zdolność do zeszklenia, właściwości mechaniczne.

Inżynieria Materiałowa 4 (206) (2015) 154÷159DOI 10.15199/28.2015.4.1© Copyright SIGMA-NOT

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MATERIALS ENGINEERING

1. INTRODUCTION

Bulk metallic glasses (BMG) are currently being examined with a lot of interest from material engineers. There are some trends al-ready settled in this area, e.g. searching for new alloys with bet-ter glass forming ability (GFA) [1, 2], enhancing GFA of known compositions by adding more elements [3], improving mechanical properties of BMGs [4] or even computer predictions of BMGs me-chanical behaviour on the atomic level [5]. There are also many new directions, such as producing porous BMGs [6], toxic elements elimination and introducing these materials into the biomedical ap-plication zone [7, 8].

There have already been discovered many of good glass forming alloys that are suitable for structural applications. They are mainly based on zirconium or copper. It is well known that the more ele-ments an alloy consists of, the more difficult the crystallization is and the better GFA it usually exhibits. The elements amount and dif-ferences in their atomic radii are important. However, a significant role is also played by the enthalpy of mixing for the main alloying elements [9].

Zr-based bulk metallic glasses exhibit unique combination of good GFA and mechanical properties, such as: high yield strength, high hardness, high specific strength, superior elastic limit, very good wear resistance and excellent formability [10]. Because of

such an extraordinary combination of properties, these materials have already been used as sporting goods, microgears, jewellery and even as some military use parts [11].

This work is focused on determining the silver influence on the GFA and the mechanical properties of Zr48Cu36Al16 – xAgx alloys. Some similar works on the Zr–Cu–Ag–Al system have already been done by other authors. The Zr to Cu content ratio and the total amount of equimolar Ag–Al additions have already been examined for this system in order to find the best glass formers [10, 12÷15]. However, the influence of Ag to Al content ratio variations on the GFA in this group of alloys remains uncharted, proving the pre-sented idea to be innovative and carrying the potential of expanding this particular branch of knowledge a little further. This approach should allow improving the GFA of this system even further and indicating the alloy or group of alloys representing the optimum set of the GFA and the mechanical properties which could be offered to the industry.

2. EXPERIMENTAL

It was reported in the literature that the Zr48Cu36Al8Ag8 alloy pos-sesses good GFA [16], therefore this alloy was selected as the ini-tial one to modify the content of silver and aluminium. The atomic radii of Zr, Cu, Al and Ag are of 0.160 nm, 0.128 nm, 0.143 nm

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and 0.145 nm, respectively [3]. The enthalpies of mixing are of: Zr–Cu = –23, Zr–Al = –44, Zr–Ag = –20, Cu–Al = –1 and Cu–Ag = –4.4 kJ/mol [17]. To determine the influence of silver on the GFA and the mechanical properties the following alloy formula was applied: Zr48Cu36Al16 – xAgx (x = 0, 2, 4, 6, 8, 10, 12, 14 and 16 at. %).

The master alloys were prepared from pure elements (at least 4N purity). The elements were melted in the arc furnace under an argon protective atmosphere. The ingots were remelted three times to en-sure chemical homogeneity. The rod shape samples with a diameter of 3 mm were also manufactured under the argon atmosphere by the copper mould casting. Structural examination was performed on the cross-section of samples by the X-ray diffraction (XRD) method on Rigaku MiniFlex2 diffractometer with Cu Kα radiation. Differen-tial scanning calorimetry (DSC) (Perkin Elmer DSC8000) allowed recording thermal effects during heating the samples with heating rates of 0.33, 0.67 and 1.22 K/s. The activation energies of crystal-lization were calculated using the Kissinger method.

Microhardness was measured with the Vickers method on Hane-mann microhardness tester under the load of 0.49 N. Examination was performed five times for each sample, always on the cross-section near the symmetry axis of the rods in order to eliminate the influence of cooling rate differences. The average values were calculated. For mechanical strength investigations cylindrical specimens of 3 mm in diameter and 4.5 mm long were prepared from the cast rods and tested with constant strain rate of 5·10–4 s–1 at room temperature. The tests were performed on five samples for each chemical composition to improve statistics as it is well known that plastic deformation in BMGs varies drastically from sample to sample and mechanical strength is also very sensitive to inter-nal defects. Moreover, working surfaces of the rod-shaped samples should be parallel what is difficult to obtain in satisfying accuracy on such small samples.

After the compression tests the scrap surfaces were investigated by the scanning electron microscopy (SEM) in the secondary elec-trons mode in order to observe potential differences between fully amorphous and partially crystalline alloys.

3. RESULTS

XRD patterns of the examined alloys series are compared in Fig-ure 1. It is clear that alloys containing 0 and 16 at. % of silver are partially crystalline what proves poor GFA of these alloys. On the other hand, the alloys containing from 6 to 14 at. % of silver are

fully amorphous. The XRD patterns of the Zr48Cu36Al14Ag2 and Zr48Cu36Al12Ag4 alloys should be questioned since the patterns are unclear. The subsequent studies revealed that these alloys were partially crystalline. The uncertain presence of crystalline peaks is probably caused by the tiny size of the crystallites which are well dispersed in the amorphous matrix.

The Zr48Cu36Al16 alloy was examined more precisely in order to identify the crystalline phase. Two rods with the diameters of 2 and 3 mm and arc molten ingot were further examined by the XRD method (Fig. 2). It was possible to determine the CuZr phase with certainty. Other phases (AlCu2Zr, Cu10Zr7, CuZr2 [18]) that are usu-ally met in such compositions did not match the diffraction patterns with satisfying accuracy.

The entire series was examined by the DSC method for thermal effects. The glass transition temperature (Tg) was considered as the inflection point not as the onset deflection point. Tx stands for the crystallization onset temperature, whereas Tp is the crystallization maximum rate temperature. Figure 3 presents the comparison of DSC curves recorded for all studied alloys. On the contrary to the other alloys, those containing 0, 2, 4 and 16 at. % of silver exhibit very weak exothermic effect representative for crystallization pro-cess. This phenomenon may have resulted from low amorphous phase content. This outcome dispels doubts about Zr48Cu36Al14Ag2 and Zr48Cu36Al12Ag4 alloys — they were partially crystalline and possessed poor GFA.

Fig. 1. XRD patterns of Zr48Cu36Al16 – xAgx rods with a diameter of 3 mmRys. 1. Dyfraktogramy uzyskane dla prętów Zr48Cu36Al16 – xAgx o śred-nicy 3 mm

Fig. 2. XRD patterns of Zr48Cu36Al16 alloy in various formsRys. 2. Dyfraktogramy uzyskane dla różnych postaci stopu Zr48Cu36Al16

Fig. 3. DSC curves of Zr48Cu36Al16 – xAgx alloysRys. 3. Krzywe DSC uzyskane dla stopu Zr48Cu36Al16 – xAgx

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The characteristic temperatures of the alloys series are plotted in Figure 4a. It can be observed that with the silver content in-crease the Tg monotonically decreases. However, both Tx and Tp behave differently. These values initially rise to reach maximum at about 8÷10 at. % of silver and then rapidly drop with further silver content growth. Subtraction of Tg from Tx gives the param-eter of ΔTx which describes the supercooled liquid region. The ΔTx is widely used to characterize the GFA of metallic glasses, this one is not the most accurate parameter though. The ΔTx depend-ence of the examined alloys on the silver content is illustrated in Figure 4b. The fully amorphous alloys (x = 6÷14 at. %) exhibited the highest ΔTx values (>70 K). The Zr48Cu36Al6Ag10 reached the maximum ΔTx value of 91 K. The ΔTx of the Zr48Cu36Al8Ag8 alloy determined by the authors is of 85 K, whereas the literature reports on the ΔTx parameter value of very similar alloys ranging from 67 K [16] to above 100 K [19] what remains in satisfying correla-tion with our results.

Figure 4c shows the enthalpy of crystallization (Hcr) versus sil-ver content of Zr48Cu36Al16 – xAgx alloys. The partially crystalline alloys (0, 2, 4 and 16 at. % of silver) exhibited heat of crystallization higher than –20 J/g. This is consistent with the expectations since those alloys were already partially transformed. The minimum Hcr is observed for the Zr48Cu36Al4Ag12 alloy (–63.1 kJ/mol).

The activation energy of crystallization process (Ea) was calcu-lated using the Kissinger method and is illustrated in Figure 4d.

The fully amorphous alloys (6÷14 at. % of silver) exhibited rela-tively low Ea (below 220 kJ/mol). Table 1 summarizes all measured and calculated thermal values.

Comparing Figure 4c to 4d, one can notice similar shapes of the curves. It is visible that silver influence on the thermal properties is not as strong as aluminium: 2 at. % of Al content (Zr48Cu36Al2Ag14) is enough to achieve critical diameter of at least 3 mm. On the other side, it is needed more than 4 (probably at least 6) at. % of Ag to achieve comparable GFA.

Figure 5 shows microhardness dependence on the silver content. The samples with some crystalline phase content (x = 0, 2, 4 and 16 at. %) exhibited much higher microhardness (650÷800 HV0.05) than the fully amorphous ones (550÷650 HV0.05). It is presumably connected with the hard metastable intermetallic phases presence within these alloys volume. Moreover, when the silver amount in-creases the microhardness decreases indicating that silver softens the system.

The representative engineering stress–strain curves for each al-loy are juxtaposed in Figure 6. Partially crystalline samples always exhibited brittle behaviour and fractured catastrophically under relatively low stress. On the other hand, the fully amorphous sam-ples failed under significantly higher load and usually exhibited plastic strain. It is worth noticing that when the silver content was increased, the yield strength decreased slightly what corresponds with the microhardness tendency.

Fig. 4. Thermal properties of Zr48Cu36Al16 – xAgx alloys: a) characteristic temperatures, b) supercooled liquid region width (ΔTx), c) enthalpy of crystallization, d) activation energy of crystallizationRys. 4. Właściwości cieplne stopów Zr48Cu36Al16 – xAgx: a) temperatury charakterystyczne, b) szerokość zakresu cieczy przechłodzonej (ΔTx), c) ental-pia krystalizacji, d) energia aktywacji krystalizacji

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The Zr48Cu36Al4Ag12 alloy exhibited the lowest microhard-ness (554 HV0.05) yet it was the most plastic one (0.4% plastic strain). The stress–strain curve for this alloy is presented in Fig-ure 7 together with the enlargement of the plastic strain area. The “zig-zag” effect is commonly known for crystalline metallic alloys as the Portevin Le Chatelier effect (PLC). However, this phenom-enon in the amorphous metals has a different nature. In the case of

BMGs, the slip bands that propagate throughout the loaded sample are responsible for accommodating the strain [20]. Every time the slip band passes through the entire sample a slight stress decrease is observed.

The compression strength of the alloys is presented in Figure 8. It can be noticed that the partially crystalline samples fractured under diversified loads, while fully amorphous ones fractured un-der relatively similar loads. It is intuitive since fragile materials are more sensitive to any internal defects. Figure 9 shows elastic and plastic strain in all samples. The Zr48Cu36Al10Ag6 alloy exhibited the highest σc value (1881 MPa). The yield strength of other fully amorphous samples was close to 1.8 GPa which is in good correla-tion with literature reports [10].

SEM images were taken for partially crystalline (Fig. 10a) and fully amorphous (Fig. 10b) scrap samples after compression tests. Figure 10a shows the fracture surface of the Zr48Cu36Al16 alloy. There are small crystallites (of submicron size) visible on the sur-face submerged in the amorphous matrix. This result confirms for-mer XRD investigations. Figure 10b presents typical fracture sur-face for fully amorphous sample Zr48Cu36Al10Ag6. A characteristic vein-like pattern is visible on the entire image what is commonly observed on the fracture surfaces of compressed samples [21].

Table 1. Summary of thermal properties of Zr48Cu36Al16 – xAgx alloysTabela 1. Zestawienie zbiorcze właściwości cieplnych stopów Zr48Cu36Al16 – xAgx

X Tg, K Tx, K Tp, K ΔTx, K HCr, J/g Ea, kJ/mol

0 740 782 785 42 –5.6 290

2 735 783 787 48 –10.6 272

4 732 789 797 57 –16.8 231

6 729 800 804 71 –35.0 212

8 726 811 815 85 –34.0 193

10 719 810 813 91 –53.5 172

12 714 797 800 83 –63.1 175

14 697 771 775 74 –39.9 206

Fig. 5. Vickers microhardness of Zr48Cu36Al16 – xAgx alloysRys. 5. Mikrotwardość Vickersa stopów Zr48Cu36Al16 – xAgx

Fig. 6. Stress–strain curves of Zr48Cu36Al16 – xAgx alloysRys. 6. Krzywe naprężenie–odkształcenie uzyskane dla stopów Zr48Cu36Al16 – xAgx

Fig. 7. Stress–strain curve of the Zr48Cu36Al4Ag12 alloyRys. 7. Krzywa naprężenie–odkształcenie uzyskana dla stopu Zr48Cu36Al4Ag12

Fig. 8. Compression strength comparison of Zr48Cu36Al16 – xAgx alloysRys. 8. Porównanie wytrzymałości na ściskanie stopów Zr48Cu36Al16 – xAgx

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Low Ea tells us about relative ease in transforming the atomic struc-ture and switching their bonds among each other. Easy atomic rear-rangement results in high efficiency of the elastic energy dissipation during a mechanical strain. The shear transformation zones (STZs) connects to form a shear band (SB) with no significant heat emis-sion. SB operates cold and causes no catastrophic failure. On the other hand, when the Ea is high, the rearrangement is more difficult and occurs not until the accumulated elastic energy is high enough. Its rapid release forms a hot and quickly expanding SB what results in a quasi-brittle fracture [22].

The second idea is connected with the crystallization of a de-formed glassy structure in the area of SB operation. Lower activa-tion energy of crystallization allows the structure to transform with greater efficiency. Crystallites are stronger than the glass and are usually able to stop the propagating shear band. This mechanism can result in the proliferation of SBs and give the macroscopic plas-ticity. On the other hand, high Ea makes the crystallization more difficult and the multiple shear banding is not observed [23]. Either explanation may be correct or even these two mechanisms may oc-cur at the same time. Nevertheless, these considerations have been confirmed by our results.

The Zr48Cu36Al4Ag12 alloy exhibited high ΔTx value of 83 K and was the most plastic one (εpl = 0.4%). This alloy also possessed rel-atively high compression strength (about 1.8 GPa) and may be con-sidered as the most optimum one of this series. However, chemical composition change step in this work was roughly set for 2 at. %. Further and more precise studies may be required in order to find even better glass forming alloy.

5. CONCLUSIONS

The silver influence on the GFA and the mechanical properties of the Zr48Cu36Al16 – xAgx alloys were investigated and determined: – Ag content below 6 and over 14 at. % resulted in poor GFA and

rods with 3 mm in diameter were partially crystalline, whereas the samples with 6÷14 at. % of silver were fully amorphous and exhibited ΔTx over 70 K,

– the Zr48Cu36Al6Ag10 alloy exhibited the widest supercooled liq-uid region (91 K) and the lowest Ea value (172 kJ/mol), although the Zr48Cu36Al4Ag12 alloy possessed the largest enthalpy of crys-tallization (–63.1 J/g),

– microhardness of fully amorphous alloys varied from 565 to 637 HV,

– the highest compression strength of 1881 MPa was achieved for Zr48Cu36Al10Ag6 alloy, although the Zr48Cu36Al4Ag12 alloy was the most plastic one,

– the silver content increase resulted in the microhardness and the compression strength decrease.High microhardness, compression strength and slight plasticity

of the fully amorphous alloys examined at this work seems to be promising set of properties for structural applications. However, the Zr48Cu36Al4Ag12 is the most promising one since it optimally com-bines good GFA with good mechanical performance.

ACKNOWLEDGMENTS

Authors are grateful to Warsaw University of Technology for providing a possibility to conduct all the research.

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Fig. 9. Elastic and plastic strain of Zr48Cu36Al16 – xAgx alloysRys. 9. Odkształcenie sprężyste i plastyczne stopów Zr48Cu36Al16 – xAgx

Fig. 10. SEM images taken on the main fracture surface of compressed samples: a) partially crystalline Zr48Cu36Al16, b) fully amorphous Zr48Cu36Al10Ag6 after compression testsRys. 10. Fotografie SEM wykonane na przełomach ściskanych próbek: a) częściowo krystalicznych Zr48Cu36Al16, b) w pełni amorficznych Zr-48Cu36Al10Ag6 po próbie ściskania

4. DISCUSSION

The influence of silver content on the glass forming ability and me-chanical properties of the Zr48Cu36Al16 – xAgx alloys (x = 0, 2, 4, 16 at. %) has been successfully examined. Glassy rods with a diameter of 3 mm were manufactured by the copper mould casting. The al-loys with the silver content of: 0, 2, 4 and 16 at. % contained some crystallinity due to poor GFA. These alloys were brittle and hard, what was probably caused by metastable intermetallic phases pres-ence within these alloys volume. On the other hand, fully amor-phous alloys (x = 6, 8, 10, 12 and 14 at. %) were slightly plastic and of relatively lower hardness.

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