Crystallization and high-temperature deformation behavior of Cu49Zr45Al6 bulk metallic glass within supercooled liquid region

Journal of Non-Crystalline Solids 376 (2013) 145–151

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Crystallization and high-temperature deformation behavior of Cu49Zr45Al6 bulk metallic glass within supercooled liquid region Kwang Seok Lee ⁎, Yu Mi Jo, Young-Seon Lee Korea Institute of Materials Science, Materials Deformation Department, 797 Changwondaero, Changwon, Gyeongnam 642-831, South Korea

a r t i c l e

i n f o

Article history: Received 1 February 2013 Received in revised form 18 May 2013 Available online 19 June 2013 Keywords: Bulk metallic glasses; Crystallization; High-temperature deformation; Process map; Warm extrusion

a b s t r a c t Crystallization and plastic deformation of a Cu49Zr45Al6 ternary bulk metallic glass were investigated within a supercooled liquid region. In isochronal annealing processes, effective activation energies for the primary crystallization of Cu49Zr45Al6 ternary bulk metallic glass were calculated to be over 367 kJ/mol, which is quite high compared with other Cu–Zr-based bulk metallic glasses. Based on isothermal transformation kinetics described by the Johnson–Mehl–Avrami model, Avrami exponent (n) was calculated to be between 2.52 and 3.50, indicating that crystallization mechanism showed a diffusion-controlled growth with increasing nucleation rate. The appropriate working temperature-strain rate combination for feasible solid-to-solid forming without in situ crystallization was deduced by constructing a process map superimposed on a time–temperature transformation curve, thereby indicating phase crystallization. Furthermore, the actual solid-to-solid formability around this processing window was confirmed by overlapping the laboratory-scale extrusion ability. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Recently developed Cu–Zr-based metallic glass systems have attracted great attention for their unique structural, physical, mechanical and thermal properties together with their excellent glass-forming ability [1–6]. In particular, when the deformation temperature is elevated within the supercooled liquid region (SLR), Cu–Zr-based bulk metallic glasses (BMGs) show unique rheological properties, namely a homogeneous flow through the entire sample at relatively low temperatures within the SLR [7,8]. To utilize this unique viscous flow characteristic, thermoplastic forming process has become important for broadening the application fields of BMGs [9–12]. This means that the deformation mechanism of BMGs (i.e., Newtonian and/or non-Newtonian viscous flow) has to be carefully understood in terms of process conditions, including its deformation rate and temperature [13–16]. Carefully controlled Newtonian viscous flow has a high value of strain-rate sensitivity (ca. 1) during warm deformation, typically depicting a superplastic-like deformation behavior [17]. This enables various near-net shape material fabrication such as rolling [18] and extrusion [19], among others [20–22]. During the solid-to-solid warm deformation of BMGs, the most important issue is that monolithic BMGs are composed of a metastable amorphous phase that is prone to crystallization within their SLR by following preferential pathways [23]. The crystallization-induced embrittlement during “warm” deformation is certain [24], although the influence of an external force upon the crystallization retardation ⁎ Corresponding author. Tel.: +82 55 280 3380; fax: +82 55 280 3499. E-mail address: [email protected] (K.S. Lee). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.05.036

is still being debated [25,26]. Furthermore, severe crystallization can play an important role in increasing the overall flow stress within an SLR [27]. Therefore, one of the main objectives for designing a different process to deform monolithic BMGs is to maintain the amorphous nature of the warm deformed workpiece without compromising its structural features through crystallization. In order to achieve this, it is also essential to understand the crystallization kinetics for obtaining a feasible solid-to-solid processing window. The first part of this paper deals with the practical data on the crystallization kinetics of a monolithic Cu49Zr45Al6 (at.%) BMG exposed at elevated temperatures within an SLR. Then, its high-temperature deformation behavior was investigated under a wide range of strain rates in order to construct an experimental deformation map, as well as to verify the transition of the deformation behavior. By constructing a process map based on the dynamic materials model (DMM), feasible solidto-solid forming conditions without crystallization were also deduced. The full realization of both warm deformation behavior and crystallization kinetics will be vital to further study the superplastic-like deformation; therefore, the probable application of this dislocation-free metallic alloy was also suggested by laboratory-scale extrusion. 2. Experimental procedures An alloy system with a chemical composition of Cu49Zr45Al6 (at.%) was selected because of its large glass-forming ability and its wide SLR span of approximately 60 K, which implies that there is a possibility of maintaining the monolithic glassy state without crystallization above the glass transition temperature at the onset of crystallization [28]. A button-shaped master alloy was first prepared by arc melting

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the mixtures of pure Cu, Al (>99.9 wt.% purities), and Zr (>99.7 wt.% purity) elements under an Ar atmosphere. Then, BMG rods (2 mm in diameter and 60 mm in length) were injection casted into a copper mold. Thermal and structural properties of the fabricated amorphous rods were first examined by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Fig. 1(a) displaces a continuous DSC trace of the Cu49Zr45Al6 BMG injection case at a heating rate of 20 K/min, exhibiting a wide SLR (60 K) between the temperature onsets for glass transition (Tg,onset = 435 °C) and crystallization (Tx,onset = 495 °C). The XRD pattern of as-cast rods is shown in Fig. 1(b), in which no traces of crystalline peaks could be detected within the broad amorphous diffraction maximum around 2θ ~ 39°. In order to characterize the crystallization behavior of Cu49Zr45Al6 BMGs within an SLR, two different types of thermal analyses were carried out using a TA Instruments 2920 DSC under a purified nitrogen atmosphere. Continuous-heating DSC tests were first conducted using various heating rates (2.5, 5, 10 and 20 K/min) to interpret the isochronal crystallization behavior. In order to obtain isothermal heat-flow curves as a function of time, isothermal annealing was performed at seven different target temperatures (425, 440, 455, 470, 480, 490, and 500 °C). All of the isothermal DSC tests started when temperature reached each target after a constant-rate heating from 50 °C at an initial scan rate of 50 K/min. For a series of compression tests within an SLR, cylindrical samples with a diameter of 2 mm and a height of 4 mm were cut from the center of the as-cast Cu49Zr45Al6 rods and then carefully ground and polished in order to maintain both sides perpendicular to the compression parallel axis. Uniaxial compression tests were then carried out using an Instron-type electromechanical testing machine (Instron model 8862) equipped with a halogen furnace capable of fast heating up to the target temperature with a heating rate of greater than 200 K/min. These tests were performed at temperatures between 435 and 495 °C under the

initial strain rates (ε· ) ranging from 10−5 to 0.3 s−1. For evaluating the material macroscopic formability, laboratory-scale extrusion tests on specimens with a diameter of 2 mm and a height of 6 mm were conducted under various punch jig speeds between 2.52 × 10− 2 and 5.04 × 10− 1 mm/min at 450, 470 and 490 °C. The schematic illustration of the laboratory-scale extrusion has been described elsewhere [29]. The punch jig and the bottom part of the extrusion die were made of SKD61 tool steel. The extrusion ratio and the die angle were fixed at 4 and 45°, respectively. During warm extrusion, boron nitride spray was used for lubrication. 3. Results and discussion 3.1. Crystallization behavior The crystallization kinetics of the amorphous materials is typically expressed with physicochemical parameters such as the crystallization activation energy as determined by isochronal annealing and the Avrami exponent deduced by isothermal annealing. First, a continuous heating-based thermal analysis under different scan rates was performed, and then DSC thermal scans of a Cu49Zr45Al6 BMG obtained at selected heating rates were obtained. The scans are shown in Fig. 2, in which all of the DSC traces have an apparent exothermic peak. As observed, both the peak temperature and height of the main exothermic event increases with the heating rate. The onset and peak temperatures for the exothermic event can be plotted as a function of heating rate. The thermal stability of Cu49Zr45Al6 BMGs can be estimated in terms of the effective activation energy Ex, by the Kissinger and Ozawa methods. When the Kissinger method was used, the Ex value for crystallization can be determined by Eq. (1) [30]: ln

a exo.

Heat flow [W/g]

Tg,onset

Tx,onset 400

ð1Þ

where β denotes the heating rate and R is the gas constant. Because the Kissinger plots of ln(β/T2) versus 1000/T should be a straight line according to the model, the value of Ex can be estimated from its fitted slope y. Based on the Kissinger plot (Fig. 3(a)), the effective activation energies of both the onset and peak of crystallization were determined as Exo = (402.84 ± 20.04) kJ/mol and Exp = (367.82 ± 15.46) kJ/mol, respectively. By adopting another widely used non-isothermal method based on Ozawa analysis [31], Ex could be also determined by Eq. (2):

0.2 W/g

300

β E ¼ − x þ Const; RT T2

lnβ ¼ −

AEx þ Const; RT

ð2Þ

500

Temperature [oC]

where A is a constant (=1.0516). The Ozawa plot (lnβ vs. 1000/T) is shown in Fig. 3(b). Similar to the previous method, Exo and Exp values

b

2.5 K/min 5 K/min 10 K/min 20 K/min

0.25

Heat flow (W/g)

Intensity (a.u.)

exo.

20

30

40

50

60

70

80

90

2θ (degree)

200

300

400

500

600

Temperature (oC) Fig. 1. (a) Continuous heating DSC plot and (b) X-ray diffraction pattern of Cu49Zr45Al6 bulk metallic glass.

Fig. 2. Continuous heating DSC traces of Cu49Zr45Al6 BMG under different heating rates.

K.S. Lee et al. / Journal of Non-Crystalline Solids 376 (2013) 145–151


n x ¼ 1− exp −½kðt−τÞ ;

ð3Þ

or alternately by taking the double logarithm of Eq. (3): ln½− lnð1−xÞ ¼ n lnðt−τ Þ þ n lnk;

ð4Þ

0.2

Exp=367.82+15.46 kJ/mol 11

Exo=402.84+20.04 kJ/mol

1

0

2

4

6

8

time (min)

50

100

150

200

time (min) Fig. 4. Isothermal DSC traces of Cu49Zr45Al6 BMG at various annealing temperatures within SLR.

where τ is the transition time or the incubation period, k is a temperature-dependent reaction rate constant related to the temperature and apparent activation energy for crystallization, and n is the Avrami exponent that reflects the mode of crystallization in terms of nucleation and growth. In this study, the incubation time is defined as the time required for the volume fraction of crystallization x to become 0.01 (1%) by plotting ln[−ln(1 − x)] against ln(t − τ), one can obtain the JMA plots for different temperatures (Fig. 5(b)). By linear fitting each (0.15 b x b 0.75), the Avrami exponent n can be deduced from the slope (Table 1). According to the diffusion-controlled growth theory, an Avrami exponent range of 2.5 b n b 3.5 indicates that the

10

1.0

0.8

0.6 o

440 C o 455 C o 470 C o 480 C o 490 C

0.4

0.2

0.0 0 1.30

1.32

1.34

1000

1.36

2000

6000

9000

time (s)

1000/T (K-1)

b

b

0.0

Onset, experimental Peak, experimental Onset, Ozawa Peak, Ozawa

ln[-ln(1-x)]

3

lnβ

2

0

0

Crystallized volume fraction

ln (T2/β)

12

Onset, experimental Peak, experimental Onset, Kissinger Peak, Kissinger

0.1

3

0.0

a

a

o

425 C o 440 C o 455 C o 470 C o 480 C o 490 C o 500 C

Heat flow (W/g)

Heat flow (W/g)

were determined as (415.18 ± 20.17) and (380.38 ± 15.41) kJ/mol, respectively. Interestingly, these Ex values are considerably higher than those of other Cu–Zr-based BMG alloy systems such as Cu47.5Zr47.5Al5 (Ex = 285 kJ/mol) [32], Cu43Zr43Al7Be7 (Ex = 239 kJ/mol) [33], Zr55 Cu30Al10Ni5 (Ex = 230 kJ/mol) [34], Cu36Zr48Al8Ag8 (Ex =303 kJ/mol) [35] and Cu46Zr45Al7Y2 (Ex = 361 kJ/mol) [36]. This means that the adopted ternary Cu49Zr45Al6 bulk amorphous alloy system has an excellent thermal stability against crystallization. The isothermal DSC curves obtained at different annealing temperatures are given in Fig. 4, in which there is no detectable exothermic peak during the 240 min of annealing at 425 °C, i.e. below Tg. Above Tg, all DSC curves exhibit a single exothermic peak. It can also be found here that the incubation time is drastically decreased with increasing isothermal annealing temperature, presumably because an atomic mobility increase within an SLR leads to a critical fluctuation that is fast enough to generate long-range atomic ordering such as crystallization [37]. Further, it is demonstrated that the higher the annealing temperature, the narrower the peak widths will be. The evolution of crystalline volume fraction x versus the annealing time t during isothermal annealing (Fig. 5(a)) could be obtained through the partial integration of each crystallization peak (Fig. 4). Typical sigmoid curves were obtained for x(t), which becomes steeper upon increasing the annealing temperature. The crystallization kinetics can be modeled by the so-called Johnson–Mehl–Avrami (JMA) equation [38,39]:

147

2

-0.5

o

440 C o 455 C o 470 C o 480 C o 490 C

-1.0

Exp=380.38 +15.41 kJ/mol 1

-1.5

Exo=415.18+20.17 kJ/mol 2 1.30

1.32

1.34

1.36

1000/T (K-1) Fig. 3. (a) Kissinger and (b) Ozawa plots for Cu49Zr45Al6 BMG.

4

6

8

ln (t-τ) Fig. 5. (a) Crystallization volume fraction and (b) Johnson–Mehl–Avrami plots for Cu49Zr45Al6 BMG.

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crystallization mechanism is interface-controlled by a three-dimensional isotropic growth with an early-stage saturation of nucleation [40,41]. Because Avrami exponents vary from 2.52 to 3.50, it is apparent that Cu49Zr45Al6 exhibits the same crystallization mechanism during isothermal annealing within an SLR as demonstrated by other Cu–Zr-based bulk amorphous alloys [35,42].

a

True stress [MPa]

-5

-4

1200

Fig. 6 shows the high-temperature deformation behavior of Cu49Zr45Al6 BMG under various initial strain rates. Mainly, a higher peak stress and steady-state flow stress are observed for higher strain rates. In addition, three different types of true stress–strain curves can be identified under a compressive loading within an SLR, depending on the strain rates and temperatures. The first one is a brittle failure without plastic flow caused by localized shear events at higher strain rates. The second one is a steady-state viscous flow accompanied with stress overshoot within the mid-strain rate region, while the last one is a viscoplastic-like flow without stress overshoot. Notably, the flow stress reaches a very low level (below 17 MPa) at the initial stage of compressive deformation at a true strain (ε ~ 0.2), followed by a drastic increase upon further compression under the initial strain rate of 10−4 s−1 at 495 °C (Fig. 6(c)). This phenomenon is presumably caused by crystallization during the slow rate of deformation. As manifested by the compressive true stress–strain curves (Fig. 6), the deformation behavior of Cu49Zr45Al6 BMGs can be described in two separate modes: a homogeneous flow with appreciable plastic flow and an inhomogeneous flow represented by brittle fracture. The homogeneous flow can also be divided by two different flow characteristics such as Newtonian and non-Newtonian viscous flows. Fig. 7 exhibits the double log steady-state flow stress–strain rate data. In the low strain-rate regime at 435 and 450 °C, the values of the strain rate sensitivity exponent m (=Δlogσ/Δlogε· ) were 0.82 and 0.80, respectively, implying that in both cases deformation modes deviate from the ideal Newtonian viscous flows (m ~ 1). However, when the test temperature was increased above 465 °C, m appears nearly equal to unity, indicating that this supercooled liquid has an ideal Newtonian viscous flow behavior under these particular compressive deformation conditions. Within the test condition regime for a Newtonian viscous flow, the relationship between flow stress (σf) and strain rate (ε· ) under a uniaxial compressive loading is given by:

ε =10

ε =3x10

1000

-4

-3

ε =10

ε =3x10

-3

800 600 400 200 0 0.0

0.1

0.2

0.3

0.4

0.5

True strain

b ε =3x10

1600

True stress [MPa]

-5

-4

ε =10

ε =3x10

-4

-3

ε =10

1200

ε =3x10

-3

-2

ε =10

800

400

0 0.0

0.1

0.2

0.3

0.4

0.5

True strain

c

1600 -4

ε =10

1400

True Stress [MPa]

·

ε =10

ε =3x10

3.2. High-temperature deformation behavior

σ f ¼ 3ηε

-5

1400

ð5Þ

ε =3

-4

x 10 -3 ε =10 -3 ε =3 x 10 -2 ε =10 -2 ε =3 x 10

1200 1000 800 600 400 200

where the proportionality constant η is a viscosity parameter. The dotted lines in Fig. 7 represent the linear fittings for Newtonian viscous flow within the low strain-rate regime. As observed, the value of m significantly decreases in the higher stress–strain rate regime. This transition from the Newtonian flow to the transition state is attributed to a subtle rearrangement of free volume, originally suggested by F. Spaepen as a constitutive flow law for metallic glasses based on the competition between annealing-induced free-volume

0 0.0

0.1

0.2

0.3

0.4

0.5

True Strain Fig. 6. Representative true stress–strain curves obtained by uniaxial compression tests for Cu49Zr45Al6 BMG under various initial strain rates at (a) 435 °C, (b) 465 °C and (c) 495 °C.

annihilation and deformation-induced free-volume creation [43]. The constitutive flow relationship in the transition state theory is given by: Table 1 Isothermal crystallization parameters of Cu49Zr45Al6 BMG. Peak time (sec)

τ50% (sec)

Annealing temp. (°C)

Incubation time (τ1%, sec)

425 440 455 470 480 490 500

No crystallization during 4 h 5179 6416 6314 1006 1307 1305 220 305 299 92 116 115 14 39 39 Unable to detect onset crystallization time

·

ε ¼ 2cf kf Avrami exp. (n) (0.15 b x b 0.75) – 2.86 2.52 3.01 2.82 3.50 –

± ± ± ± ±

0.01 0.01 0.03 0.02 0.06

    γ 0 Ωf σγ 0 Ωf σ γ 0 Ωf · sinh ¼ ε 0 sinh : Ω MkB T MkB T

ð6Þ

Here, cf is the defect concentration, kf is a frequency factor related to and unit volume the flow event, γ0Ωf denotes the product of localpstrain ffiffiffi of a defect, Ω is the mean atomic volume, M ¼ 3 under uniaxial compressive condition, and kB is Boltzmann's constant (1.38 × 10−23 J/K). The solid lines in Fig. 7 represent the result predicted by Eq. (6) upon fitting the experimental (ε· , σ) data for obtaining the two fitting parameters ε· 0 and γ0Ωf (Table 2). For the non-Newtonian flow, the rate

K.S. Lee et al. / Journal of Non-Crystalline Solids 376 (2013) 145–151

3.0

149

-1

Inhomogeneous flow -2

Non-Newtonian flow

-3 o

435 C 450oC 465oC 480oC 495oC

2.0

1.5

logε

logσflow (MPa)

2.5

-4

-5

Newtonian Transition state

-6

1.0 -5

-4

-3

No stress overshoot Stress overshoot Brittle fracture Transition between Newtonian and non-Newtonian flow

Newtonian flow

440

-2

450

460

470

480

490

Temperature (oC)

logε Fig. 7. Effective flow stress–strain rate relationship for Cu49Zr45Al6 BMG.

Fig. 8. Empirical deformation map of Cu49Zr45Al6 BMG showing the regions of inhomogeneous, non-Newtonian and Newtonian viscous flows.

dependence of the flow stress can be quantitatively expressed by the two fitting parameters ε· 0 and γ0Ωf in Eq. (6) as summarized in Table 2. An interesting result is that γ0Ωf, representing the curvature of the solid line in Fig. 7, is nearly temperature-independent. Taking the mean atomic volume of Cu49Zr45Al6 BMGs (Ω = 0.012 nm3) into account, it can be interpreted that the activation volume for generating a unit shear event requires at least 13–17 dense random-packed atoms within an SLR following a non-Newtonian flow regime. The predicted curves obtained by Eqs. (5) and (6) overlapped onto the double log flow stress–strain rate data, showing a fairly good agreement within an entire SLR, which implies that the transition state theory and Newtonian viscosity equation can be used separately, depending upon two major variables such as initial strain rate and temperature. This could be verified again by constructing a schematic empirical deformation mechanism map (Fig. 8), prescribing the characteristic process conditions for each deformation mode. The solid line indicates the boundary above which the inhomogeneous flow is dominant. The dotted line denotes the boundary between the Newtonian and non-Newtonian viscous flows within a homogeneous flow regime, which can be determined by connecting the intersection points of the two predicted curves at each temperature (Fig. 7). As observed, the dotted line is in good agreement with the appearance of stress overshoot phenomena. The aforementioned characterization of the deformation behavior can be directly linked to the material formability expectation. Based on the DMM proposed by Prasad et al. [44], a process map can be constructed in terms of power-dissipation efficiency. According to DMM, the total amount of power consumption during a hot forming process consists of functions with G content and J co-content, which are related to heat dissipation and structural rearrangements, respectively. Power-dissipation efficiency (η) can be defined as:

high power-dissipation-efficiency region is strongly preferred in order to provide an enhanced material flow without severe thermal dissipation. In Fig. 9, the Cu49Zr45Al6 BMG process map is exhibited with several iso-efficiency contours, where the white region of the iso-efficiency contour (η ≥ 0.7) can be suggested as the feasible BMG solid-to-solid forming regime. Concurrent nanocrystallization should also be considered in both deformation behavior and process map [45], because the crystallization onset could be the upper limit above which the unique viscous flow from monolithic amorphous phase cannot be fully utilized. After calculating the lower limit of strain rate below which in situ crystallization occurs during hot deformation within an SLR from the experimentally determined crystallization times (Table 1), crystallization curves as a function of temperature can be obtained and superimposed onto the process map, as shown in the two lines of Fig. 9. One can infer here that the present BMG alloy has an extremely narrow SLR region that does not show either severe thermal dissipation or in situ crystallization during warm forming (see the diagonal ellipse in Fig. 9). This implies that the BMG process conditions have to be carefully controlled to perform a warm deformation in order to fully utilize the contribution of viscous flow to the monolithic glassy phase. The protruded samples obtained after the actual warm extrusion tests for the present BMG alloy are shown in Fig. 10(a). Specimens were found to extrude well under the conditions of high deformation rate and low temperature. Fig. 10(b) shows the extrusion ability determined from the macroscopic failure superimposed on the process

η ¼ J=J m ¼ 2m=ðm þ 1Þ

ð7Þ

where m c-rate sensitivity. During hot forming processes, it is expected that a higher forming efficiency can be achieved for larger J values because thermal dissipation is more detrimental than a structural rearrangement. Therefore, the solid-to-solid forming within the

Table 2 Fitting parameters derived from transition state equation. Temp. (°C) 435 450 465 480 495

0.5ε· 0 (s−1) 5.46 1.73 2.62 3.45 6.00

× × × × ×

−7

10 10−6 10−6 10−6 10−5

γ0Ωf (nm3)

Number of atoms per unit shear

0.173 0.165 0.180 0.195 0.198

14.44 13.72 15.03 16.25 16.48

Fig. 9. Processing map of Cu49Zr45Al6 BMG superimposed on crystallization curves, with the line presented as a function of isothermal temperature.

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a

b

metallic glasses [32–36], it was revealed that the Cu49Zr45Al6 BMG has excellent thermal stability in terms of effective activation energies for crystallization onset (415.18 ± 20.17) and (402.84 ± 20.04) kJ/mol, as determined by Ozawa and Kissinger methods, respectively. The Avrami exponent (n) values were calculated to be between 2.52 and 3.50 from isothermal annealing, which indicates that the crystallization mechanism is interface-controlled three-dimensional isotropic growth with an early-stage saturation of nucleation. Moreover, the outstanding viscous flow of a Cu49Zr45Al6 BMG was confirmed by various warm deformation techniques. By applying transition state theory to the non-Newtonian flow regime, we can easily calculate that at least 13 dense random-packed atoms are necessary to generate a unit local flow event. Both empirical deformation and process maps were also constructed, in which very narrow feasible solid-to-solid forming region can be envisaged for this monolithic BMG by taking warm deformation-induced crystallization into consideration. Optimum extrusion condition for the present alloy can be suggested as T ~ 450 °C and ε· ~ 7 × 10−4 s−1. Acknowledgment The authors acknowledge the financial support from a grant (M-2009-01-0014) provided by the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, South Korea. References [1] [2] [3] [4] [5] [6] [7]

[8] [9] Fig. 10. (a) Extrusion test results under various punch jug speeds at three different temperatures within SLR. (b) Results of laboratory-scale extrusion ability superimposed on the process map. Open circle, triangle, cross, and cross circle (○, Δ, x, ⊗) represent full extrusion, partial extrusion, fracture during extrusion, and breakage of the extrudate during separation from extrusion die, respectively.

map. The fully extruded condition has a very good accordance with the white region of high power-dissipation efficiency (η ≥ 0.7) together with escaping crystallization (Fig. 9). Partial rupture during the extrusion midstage was observed within the high temperature– low punch speed condition, marked as a dot box in Fig. 10(b). This result is in accordance with the crystallization-induced embrittlement during warm extrusion within an SLR for a long exposure duration, which adversely influences the extrusion ability. On the other hand, partial extrusion is possible under the condition denoted as a triangle in Fig. 10(b) under the higher punch speed of 5.04 × 10−1 mm/min at temperatures below 470 °C, although these results correspond to low power-dissipation efficiency of below 0.3. Therefore, we can conclude that the region of escaping crystallization during warm deformation of BMGs within an SLR is apparently more crucial to deduce the optimum forming condition, although a reduced thermal dissipation is also essential. 4. Conclusions In this work, we have investigated the crystallization and hightemperature deformation behavior of a Cu49Zr45Al6 BMG at elevated temperatures within an SLR. Compared with other Cu–Zr-based

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