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Glass forming ability, thermal stability and indentation characteristics of Ce60Cu25Al15 − xGax (0 ≤ x ≤ 4) metallic glasses
Journal of Non-Crystalline Solids 427 (2015) 98–103
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Glass forming ability, thermal stability and indentation characteristics of Ce60Cu25Al15 − xGax (0 ≤ x ≤ 4) metallic glasses Dharmendra Singh a, R.K. Mandal b, O.N. Srivastava a, R.S. Tiwari a,⁎ a b
Department of Physics, Nano-Science and Technology Unit, Banaras Hindu University, Varanasi 221005, India Department of Metallurgical Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India
a r t i c l e
i n f o
Article history: Received 13 May 2015 Received in revised form 1 July 2015 Accepted 7 July 2015 Available online xxxx Keywords: Metallic glasses; Glass forming ability; Indentation characteristics; Melt spinning; Ce–Cu–Al–Ga alloys
a b s t r a c t Glass forming ability, thermal stability and indentation characteristics of Ce60Cu25Al15 − xGax (0 ≤ x ≤ 4) metallic glasses have been investigated. Amorphous phase has been confirmed in all these samples with the help of X-ray diffraction (XRD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) studies. The compositional dependence of the glass forming ability (GFA) and its correlation with related parameters as well as thermal stability have also been discussed. The load dependent hardness behavior of metallic glasses is reported in detail. It has been observed that substitution of 1 at.% Ga improved micro-hardness property of Ce– Cu–Al–Ga alloy. The value of yield strength of the materials is estimated with the help of hardness data and Meyer exponent. It has been found that this exponent decreases with respect to Ga substitution. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Cerium based metallic glasses have been developed for a decade and attracted attention due to their novel physical characteristics. They refer to glass forming ability [1], magnetic [2], mechanical [3,4], super-plastic [5] and thermoplastic properties in super-cooled liquid region [5,6]. The Ce based metallic glass (MG) with an exceptionally low glass transition temperature Tg, similar to or lower than that of many polymers, has generated lot of interest owing to their thermoplastic behavior near room temperature, excellent magnetocaloric effect and potential applications as glassy materials [7–10]. A number of novel rare-earth (RE) based (such as Ce-, Dy-, Er-, Sm-, Y-) metallic glasses, have been developed recently by choosing compositions with addition of other elements having superior glass forming ability (GFA) [11–13]. Recently, a new ternary Ce–Al–Ga glass forming system was found with highly improved glass forming ability [14,15]. Substitution of Ga on Al sites in many alloy systems has shown interesting structural and microstructural characteristics [16,17]. The limitation of thickness of melt spun ribbons has been surmounted by possibility of bulk metallic glasses. The mechanical and electrical properties of metallic glasses are, for some applications, superior to those of polymeric glasses [18,19] and are suited for many applications due to higher hardness and yield strength. Very few studies on substitution and their mechanical behavior of Ce75Al25 ⁎ Corresponding author at: Department of Physics, Banaras Hindu University, Varanasi 221005, India. E-mail address: [email protected] (R.S. Tiwari).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.07.020 0022-3093/© 2015 Elsevier B.V. All rights reserved.
metallic glass have been done so far [1,19,20]. It is important to realize that indentation studies offer opportunities to investigate the mechanical behavior of glasses and their composites [21–23]. Such investigations are necessary to understand the nature of deformation mechanism in metallic glasses. Keeping these in view, we report the effect of partial replacement by Ga on Al site in Ce60Cu25Al25 metallic glass composition. Since Ga and Al are isoelectronic with similar atomic radii, the substitution of Al by Ga does not change the e/a ratio of Ce60Cu25Al15 alloy system. Our earlier investigations on the nature of Ga systems in different alloy compositions are also relevant in this connection [24–27]. The purpose of this work is to understand the compositional dependence of glass forming ability and their mechanical properties in Ce60Cu25Al15 − xGax alloys. Tang et al. have investigated the GFA and its correlation with GFA related parameters of Ce–Al–Cu and Ce–Al–Ni metallic glasses [28]. Zhang et al. have also conducted similar studies for improving the GFA of La–Ce based alloy [29]. In this investigation, Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) amorphous alloys have been synthesized. The aim is to examine the influence of Ga on the glass forming ability (GFA), thermal stability and indentation characteristics of Ce60Cu25Al15 − xGax melt spun glassy ribbons. Some of the important temperatures for GFA refer to glass transition temperature (Tg), onset crystallization temperature (Tx) and supercooled liquid region (ΔTx). They are used for vitrification and characterization of a super-cooled liquid. All these properties of investigated alloys will be reported. It will be shown that 1 at.% of Ga substitution on Al site changes the GFA and indentation behavior significantly.
D. Singh et al. / Journal of Non-Crystalline Solids 427 (2015) 98–103
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Fig. 1. XRD patterns of as synthesized ribbons of Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) alloys.
Fig. 3. DSC curves of the melt-spun Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) alloys at the heating rate of 20 K/min.
2. Experimental details
HMV-2 T microhardness tester at different loads. The indentations were also observed under scanning electron microscope (SEM) (Quanta-200) for understanding nature of deformation during micro-hardness measurements. The tests were conducted up to a load till cracks around the indentation impression were observed.
For the preparation of alloys, poly-crystalline cerium (99.9%) was taken as an ingot of high-purity kept in silicon oil. Copper (99.9%), aluminum (99.98%) and gallium (99.99%) were taken in the forms of foils and tablets respectively. They were weighed in appropriate proportions and melted in a silica crucible with the help of induction furnace. The ingots thus were remelted for 5 to 10 times to improve homogeneity. To convert the ingots into ribbons, a part of them was placed in a silica nozzle with a circular orifice of ~ 1 mm diameter. All glassy Ce60Cu25Al15 − xGax samples with x = 0, 1, 2 and 4 were made by single roller melt spinning onto a copper wheel (~ 14 cm diameter) rotating at a speed of 40 m/s under ~ 60 kPa pressure of high purity Argon. The continuous flow of Argon was done to avoid oxidation of the alloys during making of ribbons. The length and thickness of the ribbons were ~ 1 to 3 cm and ~ 40 μm respectively. The structural characterization of the ribbon was done by X'Pert Pro diffractometer operating at 25 kV with a tube current of 20 mA using Cu target. As-synthesized ribbons were thinned using an electrolyte (70% methanol and 30% nitric acid) at 253 K. The thinned samples were then observed under transmission electron microscopy (TEM) using Technai 20 G2 operating at 200 kV. The thermal analysis of as-synthesized samples has been investigated with the help of SHIMADZU DSC-60 under continuous flow of high purity nitrogen at 20 K/min heating rate. Microhardness measurements of all the as-synthesized samples were done with the help of SHIMADZU
3. Results and discussion 3.1. Structure and microstructural characteristics Representative XRD patterns of melt spun ribbons are shown in Fig. 1. The amorphous nature of the ribbons may be discerned form these patterns. Fig. 2 and the inset show the representative TEM micrograph showing contrast free region and corresponding selected area (SAD) diffraction pattern displaying diffuse halos for Ce60Cu25Al15 − xGax (x = 0 and 2) alloys respectively. We note the absence of residual contrast in the bright field image (shown for x = 0 and 2). Figs. 1 and 2 indicate the formation of a homogenous glassy phase. Other samples also displayed similar features. The values of T g and T x were determined with the help of DSC measurements in Fig. 3. This figure displays the DSC scans for the samples with x = 0–4 recorded at a heating rate of 20 K/min. Table 1 summarizes the thermal stability data for all the investigated samples. A smeared endothermic peak of the glass transition is observed at almost the same temperature for all the samples when DSC traces and thermal properties are considered (Fig. 3). This
Fig. 2. A representative bright field TEM microstructure of Ce60Cu25Al15 − xGax alloys for (a) x = 0 and (b) x = 2. The corresponding selected area diffraction patterns are shown in inset.
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D. Singh et al. / Journal of Non-Crystalline Solids 427 (2015) 98–103
Table 1 Thermal analysis of the melt-spun Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) ribbons. Alloys Tg (at.%) (K)
Tx (K)
Tp (K)
ΔTx = Tx − Tg Tm (K)
Trg = Tg / Tm γ = Tx / (Tg + Tm)
x=0 x=1 x=2 x=4
468 473 471 469
476 481 479 477
82 88 87 86
0.580 0.583 0.585 0.584
386 385 384 383
666 660 656 653
0.445 0.453 0.453 0.453
Tg: glass transition temperature; Tx: crystallization temperature; ΔTx: supercooled liquid region; Tp: exothermic peak; Tm: melting temperature; Trg: reduced glass transition.
confirms the results of Johnson and Peker [30] that T g varies only slightly for an alloy system. In contrast, significant differences in crystallization behavior, with increasing Ga content in the samples, were detected. It can be seen that glass transition temperature Tg and melting temperature Tm decrease simultaneously with Ga substitution, and they decrease from 386 to 383 K and from 666 to 653 K respectively. However, the super-cooled liquid region ΔTx increases gradually from 82 to 88 K with 1 at.% Ga substitution. Following this, there is no significant change with further Ga substitution [5th column of Table 1]. The reduced glass transition temperature (Tg / Tm) [31] and γ parameter [Tx / (Tg + Tm)] [32] when estimated form DSC results, they are found to be in the range of 0.580–0.585 and 0.445–0.453, respectively. For comparison, the GFA parameters of the Ce60Cu25Al11Ga4 melt spun alloys are compared with those of others typical bulk metallic glasses (BMGs) [1,6] and are given in Table 2. It is seen that in comparison to other BMGs, Ce 60Cu 25 Al 11 Ga 4 alloy has higher ΔTx but lower Tg value. The Tg for Ce60Cu25Al11Ga4 alloy is 383 K. which is lower than that of the lowest known value of Tg 402 K for Ce75Cu15Al10 metallic glass [6]. The ΔTx for Ce60Cu25Al11Ga4 alloy is 86 K which is higher than of Ce60Cu25Al10 metallic glass [28]. The lower value of glass transition temperature and relatively large values of ΔTx support high GFA of both these alloys. The GFA shows very weak dependence on the thermal stability data in the present system, which is in conformity with the results reported in literature [1,14]. In addition, it is important to note that every DSC exhibits only one endothermic peak corresponding to the glass transition and an exothermic peak. This means that homogeneous glass is formed in this system. This is being supported by TEM observation of Fig. 2(a) and (b) having uniform contrast. This is unlike our previous observation of Ce–Al(Ga) metallic glasses where microstructural variation depicting phase separation has been found. Such a behavior can be understood in terms of presence of copper in the present investigation. Perhaps chemical pressure owing to Ga addition in different valent states on Al site gets compensated due to copper that also enjoys multi-valent states.
Table 2 Comparison of Tg, Tx and ΔTx values of Ce60Cu25Al15 − xGax (x = 0 and 4 at.%) melt-spun alloy with some other Ce-based bulk metallic glasses (BMGs). Alloys
Tg (K)
Tx (K)
ΔTx (K)
Reference
Ce60Al15Ni15Cu10 Ce70Al10Ni20 Ce70Cu17Al13 Ce65Cu25Al10 Ce75Cu15Al10 Ce60Cu30Al10 Ce60Cu25Al15 Ce60Cu25Al11Ga4
390 410 439 422 402 434 386 383
468 430 483 500 443 489 468 469
78 20 44 79 41 55 82 86
Zhang et al. 2004 [1] Zhang et al. 2006 [6] C. Tang et al. 2014 [28] C. Tang et al. 2014 [28] C. Tang et al. 2014 [28] C. Tang et al. 2014 [28] Present study Present study
was observed at 500 g for alloy with x = 4 (Fig. 6c). The deformation seems to occur by the nucleation and propagation of shear bands [33]. This deformation feature is quite similar to those of bulk metallic glasses [34]. The hardness (H) was computed by the formula in GPa units [35]; H ¼ 1:854 9:8
P 2
d
ð1Þ
where, P is the load (g) and d is the diagonal length in μm. Fig. 4 shows hardness versus load characteristic of Ce60 Cu25 Al15 − x Ga x melt spun alloys for x = 0, 1, 2 and 4 respectively. The load dependence is seen in this figure. The figure shows that the hardness decreases with increase in the load for all the samples due to indentation size effect [36–38]. Table 3 presents hardness values at 100 g of load for all the melt spun alloys. We clearly observe increase in hardness with respect to Ga substitution. All other relevant parameters that could be inferred based on hardness versus load curves are given in the table. In the present case, when we substitute the Ga in place of Al up to 1 at.% then increase in the hardness from 2.54 GPa to 2.60 GPa for 100 g load can be noticed. The increase in the hardness of the metallic glasses may be attributed to the variation of free volume with Ga substitution. The Ga substitution may increase the packing of coordination polyhedral and thus would lead to the decrease in the free volume. When we increase the Ga up to 4 at.% then there is decrease of hardness from 2.60 GPa to 2.10 GPa. At higher Ga concentration, packing of coordination polyhedra may saturate leading to increase in the free volume. This may facilitate flow
3.2. Indentation characteristics In this section, we present the results of indentation behavior of as-synthesized ribbons of Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) alloys. The micro-hardness measurements were carried out by Vickers indenter using a micro-hardness tester. The mean hardness value reported here (Fig. 4) is the average of at least five to seven readings on each sample at each of the loads. Fig. 5(a–d) is the representative optical micrographs of indentation marks for the as-synthesized ribbons with x = 0, 1, 2 and 4 respectively. The wavy like patterns around the indentation periphery reveal the formation of shear bands (marked by arrows in Fig. 5). There is no significant variation observed in the formation of shear bands for the as-synthesized ribbons. Fig. 6 depicts SEM images of the indented samples for the melt spun ribbons with x = 0, 2 and 4 respectively. These micrographs revealed that the indentation impressions that are regular and crack free at load up to 500 g for alloys with x = 0 to 2. However, crack
Fig. 4. Variation of hardness (VHN) with respect to load (g) for the as synthesized ribbons of Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) alloys.
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Fig. 5. Nature of indentation at different loads for the as-synthesized ribbons of Ce60Cu25Al15 − xGax alloys (a) x = 0, (b) x = 1, (c) x = 2 and (c) x = 4 showing the formation of shear bands around the indentation periphery (marked by arrows). Indentation impressions from various regions of the sample are superimposed.
of materials under load and thereby decreasing the hardness values. Such an observation pertaining to the decrease in the hardness of metallic glasses with alloying addition has been reported earlier [39,40]. The hardness values permit us to calculate the 0.2% offset yield strength. This can be determined by employing methods mentioned in many texts [41]. This will require determination of Meyer exponent (n), which can be obtained from log P versus log d curves. The slope gives n, whereas the intercept K relates to a material constant pertaining to the resistance against penetration by the indenter.
Fig. 7 shows log P versus log d curves for the melt spun alloys of x = 0, 1, 2 and 4. The values of exponent ‘n’ are given in Table 3. The change in the values of n and Log K with increasing value of x has been observed. The values of n are less than 2, as observed for intermetallics [42]. The value of n decreases with increase in Ga concentration and are found to be minimum for x = 4 and value of k first increases for x = 1 form 1.68 to 1.78 then decreases for higher Ga concentration up to 1.58. The 0.2% offset Yield Strength first decreases from 1.56 GPa (for x = 0) to 1.47 GPa (for x = 1) and then increases to 1.68 GPa (for x = 2). The strength of metallic glasses is
Fig. 6. SEM micrographs for as-synthesized ribbons of Ce60Cu25Al15 − xGax alloys for (a) x = 0 and (b) x = 2 displaying closely packed shear bands (100 g load) (c) x = 4 showing cracks for 500 g load (marked by arrows).
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D. Singh et al. / Journal of Non-Crystalline Solids 427 (2015) 98–103
Table 3 Values of hardness (VHN), Meyer's exponent (n), material constant (K) and yield strength (σ0) of as-synthesized ribbons of Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) alloys. x (in at.%)
VHN (GPa) at 100 g load (±0.10)
n
Log K
σ0 (GPa) at 100 g load (±0.10)
VHN / σ0
0 1 2 4
2.54 2.60 2.49 2.10
1.73 1.77 1.71 1.68
1.69 1.78 1.67 1.58
1.57 1.47 1.61 1.46
1.62 1.77 1.54 1.43
closely related to the chemical and physical properties of the various elements present in the alloy [43]. Such a change may be attributed to alteration of packing of coordination polyhedra owing to Ga addition. However, we are not in a position to delineate this aspect further.
4. Conclusions i. The glass transition temperature (Tg) and melting temperature (Tm) decrease simultaneously with Ga substitution in Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4). In contrast, onset crystallization Tx and supercooled liquid region ΔTx do not seem to be sensitive to Ga substitution. ii. The hardness value at 100 g load of metallic glass increases with 1 at.% of Ga and has been found maximum (~ 2.60 GPa) for this alloy at 100 g load, and decrease rapidly with Ga substitution N 1 at.%. The absence of cracks around the indented area up to 300 g of load suggests better fracture toughness of the glassy alloys. The deformation seems to occur by the evolution of shear bands.
Acknowledgment One of the authors (DS) acknowledges the financial support by the University Grant Commission (UGC), New Delhi, India under the scheme of Rajiv Gandhi National Fellowship (RGNF) with grant number 033995/2013–14.
Fig. 7. Log P vs. Log d plots for the as synthesized ribbons of Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) alloys.
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Glass forming ability, thermal stability and indentation characteristics of Ce60Cu25Al15 − xGax (0 ≤ x ≤ 4) metallic glasses Dharmendra Singh a, R.K. Mandal b, O.N. Srivastava a, R.S. Tiwari a,⁎ a b
Department of Physics, Nano-Science and Technology Unit, Banaras Hindu University, Varanasi 221005, India Department of Metallurgical Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India
a r t i c l e
i n f o
Article history: Received 13 May 2015 Received in revised form 1 July 2015 Accepted 7 July 2015 Available online xxxx Keywords: Metallic glasses; Glass forming ability; Indentation characteristics; Melt spinning; Ce–Cu–Al–Ga alloys
a b s t r a c t Glass forming ability, thermal stability and indentation characteristics of Ce60Cu25Al15 − xGax (0 ≤ x ≤ 4) metallic glasses have been investigated. Amorphous phase has been confirmed in all these samples with the help of X-ray diffraction (XRD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) studies. The compositional dependence of the glass forming ability (GFA) and its correlation with related parameters as well as thermal stability have also been discussed. The load dependent hardness behavior of metallic glasses is reported in detail. It has been observed that substitution of 1 at.% Ga improved micro-hardness property of Ce– Cu–Al–Ga alloy. The value of yield strength of the materials is estimated with the help of hardness data and Meyer exponent. It has been found that this exponent decreases with respect to Ga substitution. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Cerium based metallic glasses have been developed for a decade and attracted attention due to their novel physical characteristics. They refer to glass forming ability [1], magnetic [2], mechanical [3,4], super-plastic [5] and thermoplastic properties in super-cooled liquid region [5,6]. The Ce based metallic glass (MG) with an exceptionally low glass transition temperature Tg, similar to or lower than that of many polymers, has generated lot of interest owing to their thermoplastic behavior near room temperature, excellent magnetocaloric effect and potential applications as glassy materials [7–10]. A number of novel rare-earth (RE) based (such as Ce-, Dy-, Er-, Sm-, Y-) metallic glasses, have been developed recently by choosing compositions with addition of other elements having superior glass forming ability (GFA) [11–13]. Recently, a new ternary Ce–Al–Ga glass forming system was found with highly improved glass forming ability [14,15]. Substitution of Ga on Al sites in many alloy systems has shown interesting structural and microstructural characteristics [16,17]. The limitation of thickness of melt spun ribbons has been surmounted by possibility of bulk metallic glasses. The mechanical and electrical properties of metallic glasses are, for some applications, superior to those of polymeric glasses [18,19] and are suited for many applications due to higher hardness and yield strength. Very few studies on substitution and their mechanical behavior of Ce75Al25 ⁎ Corresponding author at: Department of Physics, Banaras Hindu University, Varanasi 221005, India. E-mail address: [email protected] (R.S. Tiwari).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.07.020 0022-3093/© 2015 Elsevier B.V. All rights reserved.
metallic glass have been done so far [1,19,20]. It is important to realize that indentation studies offer opportunities to investigate the mechanical behavior of glasses and their composites [21–23]. Such investigations are necessary to understand the nature of deformation mechanism in metallic glasses. Keeping these in view, we report the effect of partial replacement by Ga on Al site in Ce60Cu25Al25 metallic glass composition. Since Ga and Al are isoelectronic with similar atomic radii, the substitution of Al by Ga does not change the e/a ratio of Ce60Cu25Al15 alloy system. Our earlier investigations on the nature of Ga systems in different alloy compositions are also relevant in this connection [24–27]. The purpose of this work is to understand the compositional dependence of glass forming ability and their mechanical properties in Ce60Cu25Al15 − xGax alloys. Tang et al. have investigated the GFA and its correlation with GFA related parameters of Ce–Al–Cu and Ce–Al–Ni metallic glasses [28]. Zhang et al. have also conducted similar studies for improving the GFA of La–Ce based alloy [29]. In this investigation, Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) amorphous alloys have been synthesized. The aim is to examine the influence of Ga on the glass forming ability (GFA), thermal stability and indentation characteristics of Ce60Cu25Al15 − xGax melt spun glassy ribbons. Some of the important temperatures for GFA refer to glass transition temperature (Tg), onset crystallization temperature (Tx) and supercooled liquid region (ΔTx). They are used for vitrification and characterization of a super-cooled liquid. All these properties of investigated alloys will be reported. It will be shown that 1 at.% of Ga substitution on Al site changes the GFA and indentation behavior significantly.
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Fig. 1. XRD patterns of as synthesized ribbons of Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) alloys.
Fig. 3. DSC curves of the melt-spun Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) alloys at the heating rate of 20 K/min.
2. Experimental details
HMV-2 T microhardness tester at different loads. The indentations were also observed under scanning electron microscope (SEM) (Quanta-200) for understanding nature of deformation during micro-hardness measurements. The tests were conducted up to a load till cracks around the indentation impression were observed.
For the preparation of alloys, poly-crystalline cerium (99.9%) was taken as an ingot of high-purity kept in silicon oil. Copper (99.9%), aluminum (99.98%) and gallium (99.99%) were taken in the forms of foils and tablets respectively. They were weighed in appropriate proportions and melted in a silica crucible with the help of induction furnace. The ingots thus were remelted for 5 to 10 times to improve homogeneity. To convert the ingots into ribbons, a part of them was placed in a silica nozzle with a circular orifice of ~ 1 mm diameter. All glassy Ce60Cu25Al15 − xGax samples with x = 0, 1, 2 and 4 were made by single roller melt spinning onto a copper wheel (~ 14 cm diameter) rotating at a speed of 40 m/s under ~ 60 kPa pressure of high purity Argon. The continuous flow of Argon was done to avoid oxidation of the alloys during making of ribbons. The length and thickness of the ribbons were ~ 1 to 3 cm and ~ 40 μm respectively. The structural characterization of the ribbon was done by X'Pert Pro diffractometer operating at 25 kV with a tube current of 20 mA using Cu target. As-synthesized ribbons were thinned using an electrolyte (70% methanol and 30% nitric acid) at 253 K. The thinned samples were then observed under transmission electron microscopy (TEM) using Technai 20 G2 operating at 200 kV. The thermal analysis of as-synthesized samples has been investigated with the help of SHIMADZU DSC-60 under continuous flow of high purity nitrogen at 20 K/min heating rate. Microhardness measurements of all the as-synthesized samples were done with the help of SHIMADZU
3. Results and discussion 3.1. Structure and microstructural characteristics Representative XRD patterns of melt spun ribbons are shown in Fig. 1. The amorphous nature of the ribbons may be discerned form these patterns. Fig. 2 and the inset show the representative TEM micrograph showing contrast free region and corresponding selected area (SAD) diffraction pattern displaying diffuse halos for Ce60Cu25Al15 − xGax (x = 0 and 2) alloys respectively. We note the absence of residual contrast in the bright field image (shown for x = 0 and 2). Figs. 1 and 2 indicate the formation of a homogenous glassy phase. Other samples also displayed similar features. The values of T g and T x were determined with the help of DSC measurements in Fig. 3. This figure displays the DSC scans for the samples with x = 0–4 recorded at a heating rate of 20 K/min. Table 1 summarizes the thermal stability data for all the investigated samples. A smeared endothermic peak of the glass transition is observed at almost the same temperature for all the samples when DSC traces and thermal properties are considered (Fig. 3). This
Fig. 2. A representative bright field TEM microstructure of Ce60Cu25Al15 − xGax alloys for (a) x = 0 and (b) x = 2. The corresponding selected area diffraction patterns are shown in inset.
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D. Singh et al. / Journal of Non-Crystalline Solids 427 (2015) 98–103
Table 1 Thermal analysis of the melt-spun Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) ribbons. Alloys Tg (at.%) (K)
Tx (K)
Tp (K)
ΔTx = Tx − Tg Tm (K)
Trg = Tg / Tm γ = Tx / (Tg + Tm)
x=0 x=1 x=2 x=4
468 473 471 469
476 481 479 477
82 88 87 86
0.580 0.583 0.585 0.584
386 385 384 383
666 660 656 653
0.445 0.453 0.453 0.453
Tg: glass transition temperature; Tx: crystallization temperature; ΔTx: supercooled liquid region; Tp: exothermic peak; Tm: melting temperature; Trg: reduced glass transition.
confirms the results of Johnson and Peker [30] that T g varies only slightly for an alloy system. In contrast, significant differences in crystallization behavior, with increasing Ga content in the samples, were detected. It can be seen that glass transition temperature Tg and melting temperature Tm decrease simultaneously with Ga substitution, and they decrease from 386 to 383 K and from 666 to 653 K respectively. However, the super-cooled liquid region ΔTx increases gradually from 82 to 88 K with 1 at.% Ga substitution. Following this, there is no significant change with further Ga substitution [5th column of Table 1]. The reduced glass transition temperature (Tg / Tm) [31] and γ parameter [Tx / (Tg + Tm)] [32] when estimated form DSC results, they are found to be in the range of 0.580–0.585 and 0.445–0.453, respectively. For comparison, the GFA parameters of the Ce60Cu25Al11Ga4 melt spun alloys are compared with those of others typical bulk metallic glasses (BMGs) [1,6] and are given in Table 2. It is seen that in comparison to other BMGs, Ce 60Cu 25 Al 11 Ga 4 alloy has higher ΔTx but lower Tg value. The Tg for Ce60Cu25Al11Ga4 alloy is 383 K. which is lower than that of the lowest known value of Tg 402 K for Ce75Cu15Al10 metallic glass [6]. The ΔTx for Ce60Cu25Al11Ga4 alloy is 86 K which is higher than of Ce60Cu25Al10 metallic glass [28]. The lower value of glass transition temperature and relatively large values of ΔTx support high GFA of both these alloys. The GFA shows very weak dependence on the thermal stability data in the present system, which is in conformity with the results reported in literature [1,14]. In addition, it is important to note that every DSC exhibits only one endothermic peak corresponding to the glass transition and an exothermic peak. This means that homogeneous glass is formed in this system. This is being supported by TEM observation of Fig. 2(a) and (b) having uniform contrast. This is unlike our previous observation of Ce–Al(Ga) metallic glasses where microstructural variation depicting phase separation has been found. Such a behavior can be understood in terms of presence of copper in the present investigation. Perhaps chemical pressure owing to Ga addition in different valent states on Al site gets compensated due to copper that also enjoys multi-valent states.
Table 2 Comparison of Tg, Tx and ΔTx values of Ce60Cu25Al15 − xGax (x = 0 and 4 at.%) melt-spun alloy with some other Ce-based bulk metallic glasses (BMGs). Alloys
Tg (K)
Tx (K)
ΔTx (K)
Reference
Ce60Al15Ni15Cu10 Ce70Al10Ni20 Ce70Cu17Al13 Ce65Cu25Al10 Ce75Cu15Al10 Ce60Cu30Al10 Ce60Cu25Al15 Ce60Cu25Al11Ga4
390 410 439 422 402 434 386 383
468 430 483 500 443 489 468 469
78 20 44 79 41 55 82 86
Zhang et al. 2004 [1] Zhang et al. 2006 [6] C. Tang et al. 2014 [28] C. Tang et al. 2014 [28] C. Tang et al. 2014 [28] C. Tang et al. 2014 [28] Present study Present study
was observed at 500 g for alloy with x = 4 (Fig. 6c). The deformation seems to occur by the nucleation and propagation of shear bands [33]. This deformation feature is quite similar to those of bulk metallic glasses [34]. The hardness (H) was computed by the formula in GPa units [35]; H ¼ 1:854 9:8
P 2
d
ð1Þ
where, P is the load (g) and d is the diagonal length in μm. Fig. 4 shows hardness versus load characteristic of Ce60 Cu25 Al15 − x Ga x melt spun alloys for x = 0, 1, 2 and 4 respectively. The load dependence is seen in this figure. The figure shows that the hardness decreases with increase in the load for all the samples due to indentation size effect [36–38]. Table 3 presents hardness values at 100 g of load for all the melt spun alloys. We clearly observe increase in hardness with respect to Ga substitution. All other relevant parameters that could be inferred based on hardness versus load curves are given in the table. In the present case, when we substitute the Ga in place of Al up to 1 at.% then increase in the hardness from 2.54 GPa to 2.60 GPa for 100 g load can be noticed. The increase in the hardness of the metallic glasses may be attributed to the variation of free volume with Ga substitution. The Ga substitution may increase the packing of coordination polyhedral and thus would lead to the decrease in the free volume. When we increase the Ga up to 4 at.% then there is decrease of hardness from 2.60 GPa to 2.10 GPa. At higher Ga concentration, packing of coordination polyhedra may saturate leading to increase in the free volume. This may facilitate flow
3.2. Indentation characteristics In this section, we present the results of indentation behavior of as-synthesized ribbons of Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) alloys. The micro-hardness measurements were carried out by Vickers indenter using a micro-hardness tester. The mean hardness value reported here (Fig. 4) is the average of at least five to seven readings on each sample at each of the loads. Fig. 5(a–d) is the representative optical micrographs of indentation marks for the as-synthesized ribbons with x = 0, 1, 2 and 4 respectively. The wavy like patterns around the indentation periphery reveal the formation of shear bands (marked by arrows in Fig. 5). There is no significant variation observed in the formation of shear bands for the as-synthesized ribbons. Fig. 6 depicts SEM images of the indented samples for the melt spun ribbons with x = 0, 2 and 4 respectively. These micrographs revealed that the indentation impressions that are regular and crack free at load up to 500 g for alloys with x = 0 to 2. However, crack
Fig. 4. Variation of hardness (VHN) with respect to load (g) for the as synthesized ribbons of Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) alloys.
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Fig. 5. Nature of indentation at different loads for the as-synthesized ribbons of Ce60Cu25Al15 − xGax alloys (a) x = 0, (b) x = 1, (c) x = 2 and (c) x = 4 showing the formation of shear bands around the indentation periphery (marked by arrows). Indentation impressions from various regions of the sample are superimposed.
of materials under load and thereby decreasing the hardness values. Such an observation pertaining to the decrease in the hardness of metallic glasses with alloying addition has been reported earlier [39,40]. The hardness values permit us to calculate the 0.2% offset yield strength. This can be determined by employing methods mentioned in many texts [41]. This will require determination of Meyer exponent (n), which can be obtained from log P versus log d curves. The slope gives n, whereas the intercept K relates to a material constant pertaining to the resistance against penetration by the indenter.
Fig. 7 shows log P versus log d curves for the melt spun alloys of x = 0, 1, 2 and 4. The values of exponent ‘n’ are given in Table 3. The change in the values of n and Log K with increasing value of x has been observed. The values of n are less than 2, as observed for intermetallics [42]. The value of n decreases with increase in Ga concentration and are found to be minimum for x = 4 and value of k first increases for x = 1 form 1.68 to 1.78 then decreases for higher Ga concentration up to 1.58. The 0.2% offset Yield Strength first decreases from 1.56 GPa (for x = 0) to 1.47 GPa (for x = 1) and then increases to 1.68 GPa (for x = 2). The strength of metallic glasses is
Fig. 6. SEM micrographs for as-synthesized ribbons of Ce60Cu25Al15 − xGax alloys for (a) x = 0 and (b) x = 2 displaying closely packed shear bands (100 g load) (c) x = 4 showing cracks for 500 g load (marked by arrows).
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Table 3 Values of hardness (VHN), Meyer's exponent (n), material constant (K) and yield strength (σ0) of as-synthesized ribbons of Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) alloys. x (in at.%)
VHN (GPa) at 100 g load (±0.10)
n
Log K
σ0 (GPa) at 100 g load (±0.10)
VHN / σ0
0 1 2 4
2.54 2.60 2.49 2.10
1.73 1.77 1.71 1.68
1.69 1.78 1.67 1.58
1.57 1.47 1.61 1.46
1.62 1.77 1.54 1.43
closely related to the chemical and physical properties of the various elements present in the alloy [43]. Such a change may be attributed to alteration of packing of coordination polyhedra owing to Ga addition. However, we are not in a position to delineate this aspect further.
4. Conclusions i. The glass transition temperature (Tg) and melting temperature (Tm) decrease simultaneously with Ga substitution in Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4). In contrast, onset crystallization Tx and supercooled liquid region ΔTx do not seem to be sensitive to Ga substitution. ii. The hardness value at 100 g load of metallic glass increases with 1 at.% of Ga and has been found maximum (~ 2.60 GPa) for this alloy at 100 g load, and decrease rapidly with Ga substitution N 1 at.%. The absence of cracks around the indented area up to 300 g of load suggests better fracture toughness of the glassy alloys. The deformation seems to occur by the evolution of shear bands.
Acknowledgment One of the authors (DS) acknowledges the financial support by the University Grant Commission (UGC), New Delhi, India under the scheme of Rajiv Gandhi National Fellowship (RGNF) with grant number 033995/2013–14.
Fig. 7. Log P vs. Log d plots for the as synthesized ribbons of Ce60Cu25Al15 − xGax (x = 0, 1, 2 and 4) alloys.
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