Cold rolling improves the fracture toughness of a Zr-based bulk metallic glass

Journal of Alloys and Compounds 694 (2017) 1109e1120

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Cold rolling improves the fracture toughness of a Zr-based bulk metallic glass Shenghui Xie a, *, Jamie J. Kruzic b, ** a

College of Materials Science and Engineering, Shenzhen University, Shenzhen Engineering Laboratory for Advanced Technology of Ceramics and Shenzhen Key Laboratory of Special Functional Materials, Shenzhen, 518060, China b School of Mechanical and Manufacturing Engineering, UNSW, Sydney, NSW, 2052, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 July 2016 Received in revised form 14 October 2016 Accepted 16 October 2016 Available online 17 October 2016

Mode I fracture tests were conducted on both as-cast and cold-rolled Zr-Cu-Ni-Al-Nb bulk metallic glass (BMG) single edge notch bend samples. The results show a 54% improved fracture toughness (KJ ¼ 55.2 vs. 85.0 MPa√m) is obtained by cold rolling to ~2.5% true strain. The precrack patterns and fractography were also analyzed to evaluate the influence of cold rolling on the fracture process and also the validity of the mode I fracture toughness results. Cold rolling was found to promote numerous shear bands along the main crack, crack bifurcation, and a tortuous crack path, all of which are thought to contribute to the improved toughness. The influence of various precrack morphologies on the fracture toughness results are discussed. It was found that it is difficult to produce perfect standardized precracks in BMGs with heterogeneous glassy structures induced by cold rolling and this issue needs further evaluation if fracture toughness testing standards for BMGs are to be established. © 2016 Elsevier B.V. All rights reserved.

Keywords: Bulk metallic glasses Fracture toughness Fatigue precracks Cold rolling Shear bands Crack propagation

1. Introduction As a relatively new family of metallic materials, bulk metallic glasses (BMGs) have been widely studied since their discovery in the 1990s [1]. Due to their amorphous structure, which is absent of long-range order, BMGs are endowed with high specific strength, high specific hardness, a large elastic strain limit, high resistance to abrasion and chemical corrosion, etc [1e7]. BMGs have great potential for applications as precision structural parts for microelectronic components and electronic products, such as cell phones; tablet PCs and intelligent wearable devices [1]. In recent years, many multi component systems with large super-cooled liquid regions have been found and advances in predictive modeling and melt processing technologies have enabled the formation of BMGs with dimensions large enough to fulfill the requirements for structural components in electronic products [6]. However, the limited ductility of BMGs under some loading conditions is seen as an obstacle to their widespread usage [2e8]. The concept of ductility for BMGs is more complex than for

* Corresponding author. ** Corresponding author. E-mail address: [email protected] (S. Xie). http://dx.doi.org/10.1016/j.jallcom.2016.10.134 0925-8388/© 2016 Elsevier B.V. All rights reserved.

crystalline materials [7]. BMGs derive their high specific strength from their lack of long range order and crystalline defects such as dislocations and twins. As a consequence, BMGs are unable to undergo homogeneous plastic deformation by dislocation-mediated crystallographic slip. Rather, the plastic flow in BMGs at ambient temperatures is highly localized in shear bands. While the local strain in those shear bands may be high (e.g., >100%) [9], if the macroscopic deformation is not well distributed among many shear bands then cracks will initiate and rapidly propagate along them causing fracture [1]. Thus, the macroscopic ductility of BMGs relies on the formation of multiple shear bands that distribute the strain in the material. Accordingly, significant research has been conducted to create BMG based materials with an inherent ability to form multiple shear bands that can provide improved macroscopic ductility while maintaining most of the inherent strength [10e18]. Crystalline phases have often been shown to promote ductility by blocking individual shear band propagation, which in turn promotes their bifurcation and/or the formation of new shear bands. Different methods of incorporating crystalline phases have included: 1) adding solid crystalline phases to BMG melts [10], 2) precipitating crystalline phases in-situ from the liquid state [11e14], or 3) precipitating nano-scale crystallites during deformation in the solid state [15e17]. All of these approaches, when

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carefully applied, can result in improved BMG ductility. Other researchers have exploited a fact that a multitude of glass states can exist for a single BMG forming alloy that are differentiated by their chemical composition and/or their glassy structure (free volume, short range order (SRO), medium range order (MRO), etc.). For example, researchers have shown that the addition of Gd or Y to Cu-Zr-Al or Cu-Hf-Al based bulk metallic glasses can cause phase separation in the liquid state that leads to a compositionally heterogeneous glass with improved compressive ductility after casting [18]. Alternatively, for a compositionally homogeneous BMG mechanical deformation can be used to rejuvenate the glass and also induce structural heterogeneities that manifest as locally hard and soft regions that improve the macroscopic ductility and fracture toughness [19e22]. In mechanically deformed BMGs the hard regions act to block shear band propagation while the soft regions act as nucleation sites. Of the various potential types of mechanical deformation, cold rolling had been the subject of extensive research [22e32] because rolling equipment is readily available and cold rolling is commonly used in the traditional manufacturing of crystalline metal components. While studies to date have focused on the effect of percent thickness reduction and rolling direction on ductility, hardness, structure, free volume, and crystallization kinetics [22e32], no studies have focused on the effect of cold rolling on fracture toughness. Fracture toughness is an important measure of the flaw tolerance of a material and is commonly used by engineers in the design of mechanical and structural components. Furthermore, because of the very high strengths of BMGs, fracture toughness is much more likely to be the limiting design parameter for such applications [7,33]. Accordingly, in this paper a designed directional cold-rolling procedure was applied to a Zr-based BMG to improve the fracture toughness. Additionally, the effects of cold rolling on the fatigue precracks and fracture surfaces are reported. 2. Experiments A BMG with composition Zr63.78Cu14.72Ni10Al10Nb1.5, which has a high strength of 1756 MPa and good compressive ductility [34], was chosen for the present experiments. The BMG samples with dimensions 2  4  55 mm3 were cast by injection molding into a copper mold using 99.999% pure argon gas according to a previous reported method [34]. The cast ingots were then cut into ~2  4  22 mm3 beams for fracture toughness tests. All the beams were annealed at 473 K for 10 min in a flowing ultrahigh-purity nitrogen environment in order to relieve the residual stresses during quenching. This annealing temperature is far below the glass transition temperature for this Zr-based BMG which is Tg ¼ 645 K for a 20 K min1 heating rate [34]. The amorphous structure of the BMG samples was confirmed using X-ray diffraction (XRD) with Cu-Ka radiation at a scan rate of 12 per min (D8advance, BRUKER, Germany). The glass transition and the crystallization behavior were determined using differential scanning calorimetry (DSC, Netzsch DSC 200 F3, Germany) under a continuous argon flow at the heating rate of 0.33 K s1. Typical XRD and DSC results can be found in Ref. [34]. The elastic constants of BMGs were measured using the pulse echo overlap method [35] (MATEC 6600, Matec Instrument Inc. USA). The designed directional cold rolling was carried out at room temperature as schematically shown in Fig. 1. The beams were placed into a rectangular cavity of 4.1  1.4  28 mm3 in a steel holder. The rolling direction was selected as 45 from the longitudinal direction of the sample. After each pass of rolling, the holder was rotated 90 clockwise. The stepwise thickness decrease was 0.01 mm for each pass and rolling was continued until a true strain, e ¼ ln(1þε), of ~2.5% was achieved.

Fig. 1. Schematic illustration of the designed directional cold rolling.

Single edge notch bend beams, SEN(B), were used for fracture toughness tests. A straight, through-thickness notch was cut by a slow speed diamond saw (IsoMet® 1000, Buehler, Lake Bluff, IL, USA). Then the notch was extended by repeatedly sliding a razor blade across the notch in the presence of 1 mm polycrystalline diamond paste using a custom built razor notching machine. The final root radii, r, of the notch tips for the samples was between 5 and 10 mm. Fatigue precracking was then performed by cycling the samples with a sine wave (frequency: f ¼ 25 Hz; load ratio: R ¼ Pmin/Pmax ¼ 0.1) using a computer controlled electromechanical test machine (ElectroForce 3200, Bose Corporation, Eden Prairie, MN, USA). The required applied stress intensity range (DK ¼ Kmax  Kmin) to initiate precracks was consistently between 6.5 MPa√m and 8.5 MPa√m. The precrack patterns were observed using an optical microscope (DMRM, Leica Microsystems, Germany) and scanning electron microscope (QUANTA 600F, FEI, Hillsboro, OR, USA). The crack length, a, of each sample was between 0.47W and 0.69W in accordance with ASTM standard E1820 for J-integral based testing. According to ASTM standard E1820, the precrack extension from the starter notch should be not less than 0.05B and not less than 1.3 mm. While the former requirement was easily met by all of the precracks in this study, the latter requirement was not met due to the small width dimension of the samples, a common problem with the testing of small dimension BMG samples, e.g. Refs. [21,36e38]. Instead, to avoid the notch stress field from affecting the crack tip stress field it was ensured all precracks extended several notch root radii from the starter notch [39]. All precracked samples were gradually ground to a smooth surface and finally polished to 0.05 mm surface finish using alumina powders. In general accordance with ASTM standard E1820, threepoint bending fracture toughness tests were conducted using a 15.6 mm loading span on a computer controlled servo-hydraulic testing machine (Model 8501, Instron Corporation, Norwood, MA, USA) with a 5 kN load cell and a constant displacement rate of 0.83 mm s1. The fracture surfaces were observed using a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) and also XRD analysis was conducted to determine if any crystallization occurred during fracture. After testing, the conditional fracture toughness, KQ, was calculated. Checks of the sample thickness (B), ligament width (b ¼ Wa) and crack length (a) revealed that only two samples with the lowest measured toughness values had large enough sample dimensions:


B; a; b  2:5

KQ

sys

2 (1)

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to meet the plane strain KIC fracture toughness requirements. In order to make relevant comparisons between as-cast and coldrolled samples, both KQ values and KJ values are reported, with the latter calculated based on the J-integral using equations (2)e(4) [40].

KJ ¼

Jel ¼

Jpl ¼

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi u  uE J þ J t el pl 1  n2   KQ2 1  n2 E

hpl Apl Bb

(2)

(3)

(4)

In Eqs. (2)e(4), JeI and JpI are the elastic and plastic components of the J-integral, respectively, while hpl ¼ 1.9 and Apl is the area under force versus displacement curve. Based on Ref [36], it is assumed all toughness results are sample size dependent and specific to our chosen sample dimensions. However, by keeping the SEN(B) specimen dimensions constant within this study the effect of cold rolling can be determined. Statistical comparisons were done using a student's t-test with p < 0.05 considered statistically significant. 3. Results 3.1. Observed precrack patterns Precracks did not always follow the mode I path from the starter notch. For some samples there was difficulty getting a well formed precrack emanating from the notch tip and this issue was more common for the cold-rolled samples. For the purpose of later discussion, the precracks were categorized as follows. Type 1 precracks: These precracks met three conditions: (1) they initiated at the tips of the starter notches, (2) they ended on the mode I crack path within the tolerance allowed by ASTM standard E1820 after some initial minor deflections, (3) a single crack tip was identified not visible through optical microscope observation and thus any crack branching was restricted to the nanometer scale. With regard to the latter criterion, it should be noted that crack tips for BMGs are often accompanied by crack branching at different size scales, which is quite different from the fatigue precracks in most crystalline materials. These precracks were only seen for 43% of the as-cast samples and none of the cold-rolled samples (Table 1). Examples of type 1 precracks are shown in Fig. 2(a and b). The load-displacement curves for all type 1 precrack samples were linear elastic type III. Type 2 precracks: Examples of type 2 precracks are shown in Fig. 3. These samples were considered distinct from the type 1 precracks because multiple precracks initiated (Fig. 3a and b), because of frequent crack bifurcations (Fig. 3a, c and d), or because the deflection angle with respect to the mode I crack was slightly outside of what is allowed by ASTM standard E1820 (Fig. 3c). The criteria for classifying precracks as type 2 were: (1) they all initiated at the tips of the starter notches, (2) they ended on the mode I crack path within the tolerance allowed by ASTM standard E1820 despite more significant tortuosity, more noticeable crack branching, wider crack

1111

width, and/or a more mixed mode path relative to type 1 precracks, (3) while crack branching was micrometer scale and obvious through optical observation, only one crack grew during the fracture toughness experiment. Type 2 precracks were observed for 43% of the as-cast and coldrolled samples (Tables 1 and 2). The load-displacement curves associated with type 2 precracks were linear elastic (type III) for all as-cast samples while the cold-rolled samples had a mixture of linear-elastic and elastic-plastic (type I) load-displacement curves. Overall, type 2 precracks were considered to slightly violate ASTM standard E1820; however, for the as-cast samples there was no significant difference in the measured fracture toughness between type 1 and type 2 precracks (Table 1). Furthermore, since no type 1 precracks could be obtained for the cold-rolled samples, type 2 precracks were included in the results for both the as-cast and coldrolled groups. Type 3 precracks: These precracks were considered to grossly violate ASTM standard E1820. The main features for these precracks were: (1) they were not always initiated at the tips of the starter notches, (2) they had large deflections from the mode I crack path and/or multiple large cracks initiating from the notch (Fig. 4), (3) and above all, the precracks seen on each side of the sample did not match in morphology indicating a tortuous crack front through the thickness of the sample (Fig. 4), which was also verified from the fractured surfaces (Fig. 9). All of these samples with tortuous precrack fronts demonstrated elastic-plastic (type I) load displacement curves and elevated toughness values (Table 2). The one as-cast sample with a type 3 precrack had a measured KJ ¼ 96.9 MPa√m while the three coldrolled samples with type 3 precracks had an average fracture toughness of KJ ¼ 156.0 MPa√m. However, it is important to note that the KQ values for all type 3 precrack samples was similar to the typical cold-rolled samples and demonstrated a very small standard deviation (Table 2). Thus, the elevated toughness values of these samples originate from the tortuous crack front instead of the intrinsic crack propagation resistance. Type 4 precracks: These samples had obvious preexisting defects (e.g., pores (Fig. 5a) or cracking (Fig. 5b)) that interacted with the fatigue precracks. Samples categorized with type 4 precracks contained obvious: (1) preexisting pores (Fig. 5a) larger on the micrometer scale that were easily observable by optical microscopy, or (2) preexisting cracks (Fig. 5b) which affected the initiation and/ or propagation of the fatigue precracks. This kind of precrack morphology was only observed for coldrolled samples and thus it is thought that cold rolling initiated cracking or damage at preexisting defects such as pores. The average fracture toughness for these cold-rolled samples was low with KJ ¼ 49.6 ± 17.1 MPa√m (Table 2). Since these samples reflect a damaged state and do not conform to ASTM standard E1820, they are not thought to reflect the intrinsic properties of the BMG. Accordingly, samples with type 4 precrack morphologies are ignored in the following sections. 3.2. Fracture toughness results Young's

modulus

(E)

and

Poisson's

ratio

(n)

for

the

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Table 1 Summary of fracture toughness results for samples with acceptable precracks. Sample state Type 1 Precrack as-cast as-cast as-cast Type 2 Precrack as-cast as-cast as-cast Average KQ, KIC, KJ Type 2 Precrack Cold-rolled Cold-rolled Cold-rolled Cold-rolled Cold-rolled Cold-rolled Average KQ, KIC, KJ a

KQ (MPa m1/2)

KIC, KJ (MPa m1/2)

Load-disp. curve

0.61 0.53 0.58

51.5 32.7a 73.7

51.5 32.7a 73.7

III III III

0.54 0.52 0.48

54.7 49.2 69.1

54.7 49.2 69.1 55.2 ± 14.8

III III III

69.9 72.2 88.5 64.9 50.9 82.9

69.9 72.2 114.5 64.9 92.1 96.5 85.0 ± 19.2

III III I III I I

B (mm)

W (mm)

a/W

2.10 2.10 2.10

4.21 4.26 4.18

2.09 2.11 2.11

4.24 4.22 4.24

e (%)

55.2 ± 14.8 2.01 2.02 2.05 2.02 2.04 2.03

4.14 4.29 4.36 4.28 4.29 4.29

0.54 0.47 0.56 0.64 0.57 0.51

2.51 2.69 2.54 2.82 2.64 2.44 71.6 ± 13.3

Sample meets the KIC testing requirements in Eqs. (2)e(4).

Fig. 2. Optical images of typical type 1 precracks in as-cast samples with fracture toughness, KJ, of (a) 32.7 MPa√m, (b) 73.7 MPa√m.

Fig. 3. Optical images of typical type 2 precracks with fracture toughness, KJ, of 54.7 MPa√m (a, as-cast), 49.2 MPa√m (b, as-cast), 69.9 MPa√m (c, cold-rolled), 72.2 MPa√m (d, cold-rolled).

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Table 2 Summary of fracture toughness results for samples with rejected precracks. Sample state Type 3 Precrack as-cast Type 3 Precrack Cold-rolled Cold-rolled Cold-rolled Average KQ, KIC, KJ Type 4 Precrack Cold-rolled Cold-rolled Cold-rolled Cold-rolled Cold-rolled Average KQ, KIC, KJ a

B (mm)

W (mm)

a/W

2.10

4.26

0.50

2.01 2.05 2.06

4.24 4.31 4.32

0.60 0.59 0.48

2.04 2.04 2.01 2.00 2.03

4.33 4.34 4.35 4.11 4.30

0.48 0.69 0.54 0.57 0.49

KQ (MPa m1/2)

KIC, KJ (MPa m1/2)

Load-disp. curve

73.8

96.9

I

3.13 2.49 2.37 72.7 ± 2.1

71.7 75.1 71.4

153.5 167.1 147.5 156.0 ± 10.0

I I I

2.69 2.54 2.81 3.13 2.48 47.4 ± 17.4

76.0 41.1 51.1 33.6 35.1a

76.0 52.2 51.1 33.6 35.1a 49.6 ± 17.1

III I III III III

e (%)

Sample which meets the KIC testing dimensions in Eqs. (2)e(4) but severely violated the precrack requirements.

Fig. 4. Optical images of typical type 3 precracks on both sides of cold-rolled samples with measured fracture toughness, KJ, 153.5 MPa√m (a, b) and 167.1 MPa√m (c, d).

Fig. 5. Optical images of typical type 4 precracks of cold-rolled samples with KJ of 35.1 MPa√m (a), 51.1 MPa√m (b).

Zr63.78Cu14.72Ni10Al10Nb1.5 BMG were measured to be 82.1 GPa and 0.39, respectively, and those values were used in Eqs. (2)e(4) to calculate the KJ values from the load-displacement curves. Table 1

shows a summary of the fracture toughness results for samples with precracks deemed acceptable (Type 1 and 2 precracks). Samples with poorly formed precracks (type 3 and 4) are reported

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in Table 2 and are excluded from the calculation of the average fracture toughness. The measured load-displacement curves for as-cast samples with well-formed precracks were linear elastic to fracture which is called type III according to ASTM E1820. In contrast, the loaddisplacement curves for half of cold-rolled samples showed plasticity before fracture, i.e., type I according to ASTM E1820. The observed load-displacement curve type for each sample is given in Tables 1 and 2. The average values of KQ for cold-rolled and as-cast samples with acceptable precracks are 71.6 and 55.2 MPa√m, indicating a 30% improved result after cold rolling. However, this difference was not quite statistically significant (p ¼ 0.07) since the plastic work of the toughest samples was not taken into account. Taking the plastic work during the fracture process into account (i.e., calculating KJ), the cold-rolled samples had a 54% higher toughness than the as-cast samples, 85.0 versus 55.2 MPa√m, respectively, and this difference was statistically significant (p ¼ 0.01). 3.3. Fracture surfaces The typical fracture surface morphology for as-cast samples with type 1 precracks is shown in Fig. 6. There were four regions of distinct morphology observed on the fracture surfaces, denoted here as Region I, Region II, Region III and Region IV. Region I is relatively smooth and featureless, which is formed by crack blunting with multiple shear bands flowing upon loading at the initial stage of crack propagation. It is should be noted that this region extends no more than 10 mm and was not even detectable for some samples. Region II is a transition to a typical vein pattern, which has referred to as the Taylor meniscus instability zone [21]. Region I and Region II indicate a stopping of shear band sliding and beginning of fracture. Region III, which occupies most of the fracture surface, had a dimple pattern corresponding to ductile fracture via microvoid coalescence. The size scale of the dimples is in the range of ~10e20 þ micrometers. Large dimples often contained several smaller ones confirming that microvoids initiate and coalesce into larger ones. At the edge of the fracture surface, the stress state is changed from plane strain to plane stress or complicated stress states and the fracture morphology is also changed from dimples into a mixed pattern (including dimples, river-like or core pattern), as indicated in Fig. 6b, Fig. 7b and Fig. 8d. For samples with type 1 precracks, the crack trajectory was relatively straight, as indicated in Fig. 6(d). The typical fracture morphology for as-cast samples with type 2 precracks is shown in Fig. 7. The same fracture surface patterns denoted as Region I, II, III and IV were also found on the fracture surfaces similar to the type 1 precrack samples. The dimples in Region III are of similar size scale to the type 1 precrack samples, which is in agreement with the similar fracture toughness results for both types of sample (Fig. 6c vs Fig. 7c). Some stair-like ridges perpendicular to the crack propagation direction were found in Region III, which suggests that shear bands deflected into multiple planes leaving a stair step behind the crack front (Fig. 7d). A relatively straight crack trajectory was observed for these samples (Fig. 7e). Particle-like features with dimensions on the order of 5e17 mm can be found in the center of the dimples (indicated by arrows in Fig. 7c). These features often appeared to contain microcracks, suggesting these regions are locally more brittle than the surrounding matrix. No apparent difference in composition between these features and the surrounding matrix could be found through EDS analysis (the results are not shown here). Moreover, no crystallization peak was found on the fracture surface by XRD (Fig. 7f). These particle features are uniformly distributed on the Region III

area, which indicates they are formed during shear banding process. Considering that they were plentiful on the fracture surface, based on the XRD results it is doubtful that they are crystalline. The typical fracture surface morphology for cold-rolled samples with type 2 precracks is shown in Fig. 8. A rather tortuous precrack (Fig. 8a) was typical for these samples, which is distinct from the ascast samples. The tortuosity of the precracks suggests that multiple shear bands formed on different planes during precracking. The same typical fracture surface patterns discussed above were also identified for these samples, as shown in Fig. 8(bed). The dimple size in Region III was larger than that of as-cast samples with type 1 and 2 precracks (Fig. 8c, e) which is in agreement with the higher fracture toughness, KJ. Particle-like features about 3e15 mm in size were also found in the center of dimples (Fig. 8e). Additionally, some secondary cracks oriented perpendicular to the crack propagation direction were found in Region III, as shown in Fig. 8(f). Compared to the as-cast samples, a more tortuous crack trajectory was found as shown in Fig. 8g. The typical fracture morphology for cold-rolled samples with type 3 precracks is shown in Fig. 9. Because the observed precracks on the sides of the samples were on different planes, a tortuous precrack front is seen on the fracture surface. Steps in the precrack plane were tens to hundreds of micrometers in size. The same general fracture surface patterns were observed as for the other precrack types, as shown in Fig. 9(b and c). However, the spatial distribution of typical morphologies was less well defined due to the tortuous precrack fronts and mixed mode stress state. Stair-like ridges perpendicular to the crack propagation direction can also be found on the fracture surface, which indicates frequent crack bifurcations and more energy consumption. In addition to the typical morphologies discussed above, for these samples some core patterns were also found on the fracture surface (Fig. 9d), which further confirms a different stress state driving fracture for coldrolled samples with type 3 precracks relative to those with types 1 or 2 precracks. Further evidence of crack bifurcations and a tortuous crack trajectory were found on the side surfaces of the samples (Fig. 9e). Since type 3 precracks grossly violated ASTM standard E1820 and the measured toughness values were invariably high due to the tortuous crack path and mixed mode stress state, the results from samples with type 3 precracks were excluded from calculating the reported mean toughness for each group (ascast and cold-rolled). 4. Discussion 4.1. Influence of precrack patterns on fracture toughness testing Although a recent study indicated a small scatter in notch fracture toughness for a Zr-based BMG using identical samples made through thermoplastic replication in a Si mold [41], precracked BMG samples commonly have scattered fracture toughness values, especially for the tougher BMG systems [4,36,42]. A major difficulty is that repeatable, identical precracks are sometimes impossible for BMGs through traditional metal fatigue cracking procedures. Indeed, in this study it was difficult to make perfect mode I precracks meeting ASTM standard E1820, and furthermore they were impossible for the cold-rolled samples. Another contributing factor to the observed scatter is that most precracked BMGs samples do not meet the dimensional requirements of ASTM standard E1820 for plane strain KIC testing due to limitations in the glass forming ability that restrict the possible sample dimensions. This likely amplifies the scatter since the tougher individual samples will deviate farther from plane strain conditions than the lower toughness samples. In crystalline metals, dislocations move on planes of the

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Fig. 6. SEM images of the fracture morphology for as-cast samples with type 1 precracks indicating precrack, Region I, Region II, Region III (a) and Region IV (b). (c) A magnified morphology of Region III indicating dimple patterns. (d) Example crack trajectory for as-cast samples with type 1 precracks. The arrows indicate the crack propagation direction in each panel.

maximum resolved shear stress, which leads to repeatable fatigue precrack propagation perpendicular to the maximum tensile stress direction. However, this is not the case in BMGs that are structurally inhomogeneous at micrometer and/or nanometer scales [1]. The initiation and propagation of precracks in BMGs appear to be strongly affected by both the applied stress state and the local BMG structural state. As a consequence, some precracks may not initiate or propagate along the direction of the maximum crack driving force (i.e., the mode I path) and instead follow a structurally weak path. Moreover, it has been well documented that cold rolling introduces structural heterogeneities into BMGs not only by the introduction of shear bands but also nanocrystallization [43,44], nanovoids [45,46], and/or free volume fluctuations [29,47]. Overall, the results of this study suggest that the heterogeneous cold-rolled structure promotes a more tortuous fatigue crack growth path; thus, it is difficult, if not impossible, to get perfect type 1 precracks for the cold-rolled samples. This difficulty in precracking appears to be concomitant with the higher toughness of the cold-rolled samples, i.e., cracks tend to propagate in a more tortuous path under both fatigue and monotonic fracture conditions. As-cast samples also have some degree of structural heterogeneity that may be caused by local variations in composition, cooling rate, and/or fluid flow during the casting process [48e53]. However, the intrinsic heterogeneity associated with casting will certainly be less than samples that have been cast and cold rolled. Thus, it was found that some as-cast samples exhibited perfect type 1 precracks and a much smaller fraction had type 2 or 3 precracks than with the cold-rolled samples. It should be noted that the notch tip radius may also influence the precrack patterns. The notch tip radii in this paper are ranged from 5 to 10 mm as a result of the razor blade micronotching

procedure. In future research, the use of extremely sharp and well controlled notch tips, for example as can be produced by thermoplastic molding [37], may allow for the systematic evaluation of the effect of notch radius on precrack patterns and toughness results. Furthermore, a thorough understanding of the factors controlling precrack patterns may aid in establishing a fracture toughness testing standard that is more appropriate for BMGs. Type 2 precracks were the best that could be achieved for the cold-rolled samples; however, such samples do not strictly conform to the ASTM E1820 fracture toughness testing standard. However, for the as-cast samples there no significant difference in toughness results between type 1 and 2 precracks (52.6 verses 57.7 MPa√m, respectively). Furthermore, it has been shown that the fracture surface dimple size correlates to fracture toughness [54] and there was no apparent difference in the size of the dimples observed on the fracture surfaces (Figs. 6c and 7c, respectively), which explains the similar toughness values. Taking all of these into account, the type 1 and 2 precrack samples were grouped to compare the as-cast to cold-rolled states and a 54% increase in toughness (KJ) with cold rolling was found (Fig. 10). Furthermore, samples with type 3 precracks appear to be especially heterogeneous and tough based on the difficulty getting good fatigue precracks and extensive plasticity during fracture. These types of precracks were much more common for cold-rolled samples, further confirming the beneficial toughening effect of cold rolling. For samples with type 3 precracks, an important feature was the noncoplanarity of the precracks seen on each side of each sample. When cracks propagate on different mode I planes, mode III tearing occurs between the planes which contributes significantly to the fracture energy. The tortuous crack trajectory seen on the side faces (Fig. 9e) and the elastic-plastic type I load-displacement curves also

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Fig. 7. SEM images of the fracture morphology for as-cast samples with type 2 precracks indicating precrack, Region I, Region II, Region III (a) and Region IV (b). (c, d) A magnified morphology of Region III indicating dimple patterns and stair-like ridges. (e) Example crack trajectory for as-cast samples with type 2 precracks. (f) Typical XRD pattern of fracture surface embedded with particle like features. The arrows indicate the crack propagation direction in each panel or particle like features.

give direct evidence for high energy dissipation. While it is concluded that these samples were quite tough, it is not fair to compare the results as being an intrinsic fracture toughness of the material as several extrinsic factors related to the precrack morphology and subsequent crack trajectory likely played a role in the high apparent toughness. 4.2. Influence of designed cold rolling on fracture toughness of BMGs The fracture toughness of the cold rolled samples with acceptable precracks (types 1 & 2) was found to be 54% higher than for ascast samples (Fig. 10). Furthermore, as discussed above, those samples rejected with type 3 precracks were potentially even tougher. The cold rolling procedure was designed to produce shear bands at 45 from the mode I cracking direction [55]. Thus, the propagating crack was forced to cross preexisting shear bands which in turn is expected to cause crack deflection and/or new

shear bands or cracks to initiate at an angle to the mode I fracture path. Yi et al. have shown the effectiveness of preexisting shear bands in guiding and deflecting cracks [56]. It should be noted the preexisting shear bands resulted from directional cold rolling may not always obey the designed rolling direction (45 from the mode I cracking direction) due to the structurally inhomogeneity. So no other directional rolling experiments were used here. In addition to producing individual shear bands, cold rolling also introduces local regions of strain-induced softening due to the production of free volume [29,47]. It is thought that the resulting soft regions are locations that promote shear band initiation while locally harder regions serve to block shear band propagation, as has been seen in other studies [20,21]. Overall, forcing the crack path to cross preexisting shear bands and an array of hard and soft regions should promote significant shear band branching and crack bifurcation, as was found along the fatigue precracks for the cold-rolled samples (Fig. 3c and d). Crack bifurcations were also observed on the fracture surfaces of cold-rolled samples (Fig. 8f), and this effect can also

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Fig. 8. SEM images of the fracture morphology for cold-rolled samples with type 2 precracks indicating precrack (a), Region I, Region II, Region III (b, c) and Region IV(d). (e) A magnified image of the morphology of Region III showing the dimple patterns. (f) Secondary cracks in Region III, indicating the propensity for crack bifurcation. (g) Example crack trajectory for cold-rolled samples with type 2 precracks. The arrows indicate the crack propagation direction in each panel. In (c) arrows are also used to point out particle like features.

be seen in the resulting tortuous crack paths (Fig. 8g). In summary, cold-rolling promotes numerous shear bands along the main crack, crack bifurcation, and a tortuous crack path, all of which are thought to contribute to an improved toughness. Some particle-like features appear in bottoms of the dimples on

the fracture surfaces of type 2 precracked samples (Figs. 7c and 8e). The diameter of these features is ~5e17 mm for the as-cast samples and about ~3e15 mm for the cold-rolled samples. After carefully analysis, these particle-like features appear to be in an amorphous state and have similar composition with the surrounding matrix.

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Fig. 9. SEM images of the fracture morphology for cold-rolled samples with type 3 precracks showing the tortuous precrack (a), Region I, Region II, mixed Region III and Region IV zone (b, c). Core patterns formed in tensile stress are found on the fracture surface (d). Example crack trajectory for cold-rolled samples with type 3 precracks (e). The arrows indicate the crack propagation direction in each panel. In (e) arrows are also used to point out core patterns in the fracture morphology.

Additionally, there appears to be microcracking inside these regions. No similar features were found for type 1 precracked samples which are assumed to be intrinsically less heterogeneous than the samples that formed type 2 precracks. Based on these observations, it is suspected that these features are formed at locally harder heterogeneous regions. As surrounding softer regions initiate shear flow during crack propagation, the locally harder zones may crack and initiate microvoid formation. Thus, these features appear at the bottom of the microvoids similar to inclusions or second phases in a steel alloy. However, more detailed studies are needed to fully understand the nature of these observed features.

Fracture Toughness MPa*m

1/2

120

100

Cold-rolled As-cast

80

60

40

20

0

KJ 54% Improved after cold-rolled As-cast

Cold-rolled

Fig. 10. The improved fracture toughness (KJ) of Zr63.78Cu14.72Ni10Al10Nb1.5 BMG after cold rolling. Only data for type 1 and 2 precracks are included in the plot.

5. Conclusions A designed cold rolling procedure was applied to a Zr-Cu-Ni-AlNb BMG, which creates preexisting shear bands and alternating soft and hard regions intended to promote shear band formation during cracking. The cold-rolled BMG samples (~2.5% true strain) had an average fracture toughness of KJ ¼ 85 MPa√m, which was 54% higher than that of as-cast samples. The precrack patterns and

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fractography analysis indicated that cold rolling promoted a more tortuous crack path with shear banding and crack bifurcations along the main crack that contribute to the improved fracture toughness. Furthermore, the difficulty in getting perfect standard precracks in BMG samples was discussed, a problem that was much worse for the cold-rolled samples due to the more heterogeneous glassy structure. It was found that small deviations from the standard precrack requirements did not appear to affect the measured fracture toughness for as-cast samples; however, precracks with large deviations from the mode I path and those that were not coplanar through the thickness gave a much higher apparent measured toughness. Accordingly, it was concluded that such samples should not be included in comparisons of the intrinsic BMG fracture toughness. Because of the difficulty in getting standard precracks in BMG samples, the influence of fatigue precrack patterns on mode I fracture tests needs to be further evaluated and there is a need to establish a special toughness testing standard to account for the complexities of testing BMG samples relative to crystalline metals. Acknowledgements This work was supported by Shenzhen Science and Technology Research Grant under contract No. JCYJ20160422104921235, JCYJ20160422143659258, JCYJ20160422144751573 and Program of Introducing Innovative Research Team in Dongguan under contract No. 2014607109. References [1] W.H. Wang, C. Dong, C.H. Shek, Bulk metallic glasses, Mater. Sci. Eng. R44 (2004) 45e89. [2] C.A. Schuh, T.C. Hufnagel, U. Ramamurty, Mechanical behavior of amorphous alloys, Acta Mater. 55 (2007) 4067e4109. [3] T. Egami, T. Iwashita, W. Dmowski, Mechanical properties of metallic glasses, Metals 3 (2013) 77e113. [4] B.A. Sun, W.H. Wang, The fracture of bulk metallic glasses, Prog. Mater. Sci. 74 (2015) 211e307. [5] S.V. Madge, Toughness of bulk metallic glasses, Metals 5 (2015) 1279e1305. [6] M.M. Trexler, N.N. Thadhani, Mechanical properties of bulk metallic glasses, Prog. Mater. Sci. 55 (2010) 759e839. [7] J.J. Kruzic, Bulk metallic glasses as structural materials: a review, Adv. Eng. Mater. (2016), http://dx.doi.org/10.1002/adem.201600066 in press. [8] R. Narasimhan, T. Parag, I. Singh, R.L. Narayan, U. Ramamurty, Fracture in metallic glasses: mechanics and mechanisms, Int. J. Fract. 191 (2015) 53e75. [9] C. Suryanarayana, A. Inoue, Bulk Metallic Glasses, CRC Press, Boca Raton, 2011. [10] H. Choi-Yim, R. Busch, U. Koster, W. Johnson, Synthesis and characterization of particulate reinforced Zr57Nb5Al10Cu15.4Ni12.6 bulk metallic glass composites, Acta Mater. 47 (1999) 2455e2462. [11] C. Hays, C. Kim, W. Johnson, Microstructure controlled shear band pattern formation and enhanced plasticity of bulk metallic glasses containing in situ formed ductile phase dendrite dispersions, Phys. Rev. Lett. 84 (2000) 2901e2904. [12] D.C. Hofmann, J.Y. Suh, A. Wiest, G. Duan, M.L. Lind, M.D. Demetriou, W.L. Johnson, Designing metallic glass matrix composites with high toughness and tensile ductility, Nature 451 (2008) 1085e1089. [13] Z.W. Zhu, S.J. Zheng, H.F. Zhang, B.Z. Ding, Z.Q. Hu, P.K. Liaw, Y.D. Wang, R.Y. Ren, Plasticity of bulk metallic glasses improved by controlling the solidification condition, J. Mater. Res. 23 (2008) 941e948. [14] K. Mondal, T. Ohkubo, T. Toyama, Y. Nagai, M. Hasegawa, K. Hono, The effect of nanocrystallization and free volume on the room temperature plasticity of Zrbased bulk metallic glasses, Acta Mater. 56 (2008) 5329e5339. [15] S.W. Lee, M.Y. Huh, S.W. Chae, J.C. Lee, Mechanism of the deformationinduced nanocrystallization in a Cu-based bulk amorphous alloy under uniaxial compression, Scr. Mater. 54 (2006) 1439e1444. [16] S. Pauly, S. Gorantla, G. Wang, U. Kühn, J. Eckert, Transformation-mediated ductility in CuZr-based bulk metallic glasses, Nat. Mater. 9 (2010) 473e477. [17] M.H. Lee, J.K. Lee, K.T. Kim, J. Thomas, J. Das, U. Kühn, J. Eckert, Deformationinduced microstructural heterogeneity in monolithic Zr44Ti11Cu9.8Ni10.2Be25 bulk metallic glass, Phys. Status Solidi-R 3 (2009) 46e48. [18] E.S. Park, J.S. Kyeong, D.H. Kim, Phase separation and improved plasticity by modulated heterogeneity in Cu-(Zr,Hf)-(Gd,Y)-Al metallic glasses, Scr. Mater. 57 (2007) 49e52. [19] Y. Sun, A. Concustell, A.L. Greer, Thermomechanical processing of metallic glasses: extending the range of the glassy state, Nat. Rev. Mater. (2016) 16039.

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