Mechanical properties of as-synthesized and annealed ribbons of Zr69.5Al7.5-
In the present chapter, results of our recent investigations on the role of gallium (Ga) on the aluminum (Al) site in Zr69.5Al7.5-xGaxCu12Ni11 metallic glass (MG) composition have been discussed. The material tailoring and cooling rate effects on the mechanical behavior of Zr-based metallic glasses and their nanocomposites have been studied. The substitution of Ga on the Al site in Zr–Al–Cu–Ni alloy affects the nucleation and growth characteristics of quasicrystals (QCs) and consequently changes the morphology of nanoquasicrystals. The Zr69.5Al7.5-xGaxCu12Ni11 system displayed metallic glass formation in the range of x = 0–7.5. In this process, we have come out with a new glass composition; Zr-Ga-Cu-Ni with glass transition temperature (Tg)—614 K. The effect of cooling rate on the glass forming ability (GFA) and mechanical properties for this new metallic glass composition has been discussed and compared with some other Zr-based metallic glasses. The various indentation parameters such as microhardness, yield strength, strain hardening constant, nature of shear band formation, and so on for the alloys have been analyzed. The study is focused on investigations of these materials to understand the structure (microstructure) property correlations.
- metallic glasses
- mechanical properties
- cooling rate
Metallic materials are traditionally considered as crystalline in nature, possessing translational as well as orientation symmetry, i.e., their constituent atoms are arranged in a regular and periodic manner in three dimensions. However, a revolution in the concept of metals was brought, when metallic glasses (MGs) and quasicrystals (QCs) have been discovered. MGs are amorphous in nature possessing short-range ordering while QCs possess aperiodic long-range order associated with crystallographically forbidden rotational symmetries. Both quasicrystal-forming alloys and MGs giving rise to nanoquasicrystalline phase on annealing have attracted attention owing to their promise to qualify for many potential applications.
1.1. Metallic glasses
Ever since the formation of first metallic glass in the Au–Si system by rapid solidification, numerous investigations have been carried out over the past 15 years due to their attractive properties and technological potential. In initial period of metallic glass study, high cooling rates of the order of 105 to 106 K/s were the usual requirement for the formation of glassy phase. However, in the recent years, a new class of metallic glass known as bulk metallic glass (BMG) has been synthesized using very slow cooling rates. These newly developed BMGs have generated immense research activity driven by both a fundamental interest in the structure and properties of disordered materials and their unique promise for structural and functional applications. MGs have very high-yield strength and very high elastic limit compared to crystalline steel and Ti alloys (Figure 1(a)). They have very high fracture strength coupled with 2–3% of elastic strain. Conventional aluminum, titanium alloys and steels can sustain 1–2% of elastic strain. The glasses have tensile yield strength (
MGs possess a number of very attractive properties, and in many cases, these properties are enhanced by suitable heat treatment. The ability to store a high amount of elastic energy has made this material to use as a potential spring material. This has led to its first and most visible use in the heads of golf clubs. The addition of ceramic second-phase particles into the material improves its ductility. This composite can be used in aircraft frames and automobiles as armor penetrator material and medical implants. Due to large super-cooled liquid regions, the workability of these materials is very high. This property has been applied in friction welding of Pd-based bulk MGs . The high strength, hardness, fracture toughness, and fatigue strength of MGs make them ideal for the use as optical, die, tool, and cutting materials [4, 5].
Among the large number of multicomponent glassy alloy systems, Zr-based MGs have outstanding glass forming ability (GFA). The exceptionally high-yield strength, close to the theoretical limit, high hardness, and elastic modulus of these MGs offer them potential for structural applications. However, plastic deformation at room temperature occurs in a highly localized manner by the formation of shear bands. In these MGs, the definitive correlations between mechanical behavior and atomic structures have not been clearly understood.
Another important class of material is QCs. The breakthrough experiments by Shechtman et al. on rapidly solidified Al-14% Mn alloys have created a new concept of nonperiodic atomic arrangements with only orientational order, which exhibit sharp diffraction peaks with five-fold symmetry . This new form of ordered structures having orientational order and lacking strict translational periodicity was designated as “quasicrystal” by Levine and Steinhardt . It may be noted that in contrast to both crystal and QCs, amorphous solids possess neither orientational nor translational order. Most familiar quasicrystalline systems are Al-, Ti-, and Mg-based binary and ternary alloys, though there have been a few reports in other systems such as Cd-Mg-Yb, Ag-In(Cd), Al-Zn-Ce, and Cu-Ga-Mg-Sc, etc. The discovery of the quasicrystalline phases has also generated a great deal of interest in regard to complex crystalline structures known as approximant phases, which have remarkable similarities with their parent quasicrystalline structures. These often coexist with QCs and have similar chemical compositions and similar electron diffraction patterns (Figure 2). Quasicrystal approximants have similar local atomic structures to QCs [8–12]. Because of these structural similarities, the search for other possible phases as well as intensive investigations of their phase transformation has become quite pertinent in connection with the determination of the phase stability of quasicrystalline system.
The successful applications of QCs are very limited. QCs are corrosion resistant and have low coefficients of friction, and thus, they can be used as a surface coating for frying pans. They can also be used in wear resistant coatings. Al-based quasicrystalline alloys, e.g., Al-Mn-Ce containing nanoicosahedral particles may be used in surgical blades. Ti- and Zr-based QCs could be incorporated into hydrogen storage materials.
Quasicrystal forming alloys and MGs promise to qualify for many potential applications. However, bulk QCs are mostly brittle and this problem can be surmounted by producing glass-nanocrystal (nc)/nanoquasicrystal (nqc) composites (Zr69.5Al7.5Cu12Ni11) through controlled crystallization of MGs. Quasicrystal evolution from metallic glass systems may provide a way to produce nanostructured quasicrystalline alloys with attractive mechanical properties. The advantage of formation of quasicrystalline phase through devitrification of MGs is also due to the fact that the microstructure can be precisely controlled. The control of microstructure is very important as the optimum property design is related to the microstructure. It has been pointed out that Ti- and Zr-rich alloys have significantly higher hardness in the nanoquasicrystalline state (755 and 610 VHN, respectively) compared to the amorphous state in melt-spun condition. The hardness values of Ti- and Zr-rich alloys increase further by nanoquasicrystallization of the amorphous phase to 810 and 620 VHN, respectively. Misra et al.  have studied the plastic deformation in nanostructured bulk glass composites during nanoindentation. The structural changes are accompanied by decrease in specific volume, bulk modulus and Poisson’s ratio. Small specific changes upon primary devitrification suggest a close relationship between the glassy structure and the icosahedral structure.
2. Effect of material tailoring on the mechanical properties
Elemental substitution is widely used to find new MGs and QCs with improved properties. In this section, the role of Ga on the Al site in Zr69.5Al7.5-
2.1. Microstructural and structural features
The composition of the alloys (in at.%) based on electron probe microanalysis (EPMA) has been found to be Zr69.6Al7.6Cu12.5Ni10.3 (for
2.2. Mechanical properties
In this section, we present the results of micro-/nanoindentation behavior of the three glassy compositions and their respective composites. Figure 5 depicts the images of microindent for the as-synthesized and annealed ribbons of
|As-synthesized ribbons||Annealed ribbons|
|Microhardness (GPa) at 100 g load (±0.1)||Nanohardness (GPa) at 5000 μN (±0.2)||Reduced Modulus (GPa) at 5000 μN (±5.0)||Microhardness (GPa) at 100 g load (±0.1)||Nanohardness (GPa) at 8000 μN (±0.2)||Reduced Modulus (GPa) at 8000 μN (±5.0)|
To study the nature of indentation at submicroscopic scale as well as the plastic deformation of composites containing nc/nqc phases, we now present the results of nanoindentation. The indentation impressions of Berkovich indenter at 5000 μN for
Figures 7(a) and (b) depict the load (
We have observed pop-ins during loading (marked by the arrow in Figures 7(a) and (b)) for the melt-spun ribbons. These pop-ins indicate displacement bursts that signify the formation of shear bands . The pop-ins are prominent in amorphous alloys while the presence of these is either less prominent or even completely suppressed in case of annealed alloys. Our observation is in agreement with the results reported by others [29, 38, 39]. The pop-ins are absent in composites and this may be attributed to the presence of nc/nqc grains in the glassy matrix. The nature of deformation can be influenced by structural features while the chemistry affects such behavior in a quantitative way. This is the reason why we observe similar kind of
The change in the mechanical behavior of MGs and their composites can be understood on the basis of free volume model. In the present case, the variation of free volume with Ga substitution may increase the hardness of the MGs. The atomic radius of Ga (0.141 nm) is intermediate between the atomic radius of Al (0.143 nm) and Ni (0.125 nm) and the atomic radius of Zr (0.160 nm) and Cu (0.128 nm). Thus, the substitution of Ga may increase the packing density of the alloy and this would lead to the decrease in the free volume . The high resistance to plastic deformation under applied stress may be attributed to a low free volume . The increase in the hardness of MGs with alloying addition has been reported recently . The shear transformation zones (STZs) are the primary carriers of plasticity in amorphous materials [43, 44]. The formation of STZs depends upon the availability of free volume. The precipitation of nc/nqc phases in the case of composites decreases the free volume and this causes densification of the metallic glass . This results in an increased resistance to plastic deformation and therefore enhancement of hardness of the metallic glass upon structural relaxation and nanocrystallization. This observation is consistent with the results reported earlier [46, 47]. In the case of glass-nc/nqc composites, the hardness increases with increase in Ga addition. This may be due to the grain refinement of nanocrystals/nanoquasicrystals that produces many interfaces leading to the strengthening phenomenon.
3. Effect of cooling rate on the mechanical properties
The absence of grain boundaries and dislocations in MGs contributes to its exceptional properties [48–53]. MGs lack long-range order and thus, they can be considered as solids with frozen-liquid structures composed of tightly bonded atomic clusters and free volume zones [54–56]. The frozen-in excess volume is often interpreted as an increase of free volume content in the MGs [57, 58]. The functional and mechanical properties of a metallic glass are determined by its internal atomic configuration [59, 60]. The different variables such as the cooling rate and composition affect the structure of MGs [14, 61, 62]. Among these, the critical cooling rate is a very important factor that plays a crucial role in determining the atomic structure and hence deformation behavior of MGs [63–65]. The limited macroscopic plastic strain before fracture of MGs constrains their applications . Plastic deformation of MGs is localized within relatively thin regions called shear bands, resulting in a very low macroscopic plastic flow limit [13, 43, 66]. Recent investigations show that the plastic strain of some monolithic MGs can be improved by enhancing the homogeneity in microstructure through the high cooling rate . The high cooling rate may result in the configurationally looser atomic packing and thus more free volume zones, which therefore contribute to larger plasticity. Chen et al.  suggested that the plasticity for MGs can be tailored by applying different cooling rates during solidification. Jiang et al.  found that the Cu-based bulk metallic glass (BMG) is having higher hardness as compared to its ribbon counterpart of the same composition but synthesized at a much higher cooling rate. It has been observed that decreasing the cooling rate of glass forming promoted the formation of denser atomic configuration in the resultant alloy . The study of cooling rate effect on the nanomechanical response for a Ti-based BMG reveals that the hardness increases while the plastic deformation gradually decreases from the edge to the center of the sample . Recently, Huang et al.  reported the effect of cooling rate on the local atomic ordering and the wear behavior of Zr-Cu-Al-Ag BMG. These results indicate that the cooling rate used during glass formation is a processing parameter that may be tuned to change the mechanical properties of MGs.
The effect of cooling rate on the mechanical behavior of Zr69.5Ga7.5Cu12Ni11 metallic glass has been studied using microindentation technique. The ribbons of alloy have been synthesized at three cooling rates, corresponding to wheel speeds of 30, 40, and 50 m/s. The different properties such as glass forming indicators, structural relaxation heat, microhardness, yield strength, strain-hardening constant, material constant related to the resistance of the metal to penetration and pileup parameter pertaining to nature of shear band are expected to throw light on the internal structure of glass. They are compared and discussed with respect to the rate of cooling. This study provides some insights to understand the correlation between the cooling rate and the mechanical behavior of Zr-Ga-Cu-Ni metallic glass.
3.1. Microstructural and structural features
Figure 9 shows the X-ray diffraction (XRD) patterns of as-synthesized Zr69.5Ga7.5Cu12Ni11 melt-spun ribbons synthesized at different wheel speeds. It has been observed that all the patterns of the alloys consist of only broad diffraction maxima (at the position 2θ ≈ 36°) without a detectable sharp Bragg peak. This shows formation of a glassy phase. The XRD pattern of the ribbon synthesized at 40 m/s revealing the presence of a glassy phase exhibits greater peak broadening and lower XRD intensity as compared to the ribbons synthesized at 30 m/s. These effects are more pronounced by further increasing the wheel speed to 50 m/s. The full width at half maximum (FWHM) was found to be 5.5°, 4.7°, and 3.9° for the ribbons synthesized at 50, 40, and 30 m/s, respectively. These results indicate that the ribbon synthesized at 30 m/s has higher degree of short-range ordering. The formation of a glassy phase in these samples was further investigated by TEM. Figure 10 and the insets therein show the TEM micrographs and the corresponding selected area electron diffraction (SAED) patterns for Zr69.5Ga7.5Cu12Ni11 melt-spun alloys synthesized at 50, 40, and 30 m/s, respectively. We note that all TEM micrographs depict no discernible contrast and the corresponding SAED patterns displaying diffuse halos. This confirms to the XRD results presented above.
Figure 11 shows differential scanning calorimeter (DSC) scans taken at a heating rate of 20 K/min for the glassy alloys prepared under different cooling conditions. All DSC curves exhibited one clear endothermic heat event, characteristic of the glass transition to a supercooled liquid state, followed by only one single exothermic peak between about 670 and 820 K. It can be seen that the DSC curves of all the samples are very similar with glass transition temperature (
|Cooling rate (m/s)||Tg (K)||Tx (K)||Tp (K)||ΔTx (K)|
As evident from Figure 11 and Table 2, there is a slight increase in the value of
|Cooling rate (m/s)||Hardness (VHN) (GPa) (±0.10)||n||Log K||α = A/As||Crystallization enthalpy (ΔH) (J/g)||Structural relaxation enthalpy (J/g)|
3.2. Mechanical properties
In this section, we present the results of cooling rate effect on the mechanical behavior of Zr69.5Ga7.5Cu12Ni11 MGs synthesized at different wheel speeds. The microhardness measurements were carried out by Vickers microhardness tester. The mean hardness reported here is the average of at least five points on each sample. Figure 12 shows the representative optical micrographs of indents at different loads in Zr69.5Ga7.5Cu12Ni11 MGs synthesized at 50, 40, and 30 m/s, respectively. These micrographs reveal that the indents are crack free up to the load of 300 g for all the samples. The wavy patterns around the indent reveal the generation and formation of shear bands (marked by arrows in Figure 12). It can be clearly seen that the number of visible shear bands for the ribbons synthesized at 50 m/s is higher than those observed for the ribbons synthesized at 40 and 30 m/s. To further confirm this, we have calculated the pileup parameter. The characteristic spiral pattern around indents constitutes pileup and is related to the formation of shear bands. Pileups at which shear band reaches the surface are extensively observed around indents in amorphous alloys [19, 20]. The pileup parameter (
The hardness (H) was calculated in GPa units by employing the following relationship :
|Alloys||Tg (K)||Tx (K)||ΔTx (K)||Hardness (GPa)||Reference|
|Zr41.2Ti13.8Cu12.5Ni10Be22.5||623||705||82||5.34||Raghavan etal. |
|Zr55Pd10Cu20Ni5Al10||696||775||79||5.28||Liu et al. |
|Zr55Cu17.5Al7.5Ni10Si10||700||748||48||7.20||Jang et al. |
|(Zr69.5Al7.5Cu12Ni11)88Ti12||628||686||58||6.00||Singh et al. |
|Zr51.9Cu23.3Ni10.5Al4.3||705||801||96||5.50||Sun et al. |
|Zr51Ti5Ni10Cu25Al9||675||729||54||5.42||Sun et al. |
|Zr46Cu37.6Ag8.4Al8||706||796||90||5.54||Sun et al. |
|Zr57Cu27Al11Ni5||682||745||63||5.85||Jana et al. |
|Zr69.5Al7.5Cu12Ni11||624||702||78||4.70||Singh et al. |
|Zr65Al7.5Cu17.5Ni10||656||735||79||5.50||Jang et al. |
|Zr69.5Ga7.5Cu12Ni11||616||676||60||6.43||Singh et al. |
The load independent hardness values permits us to compute the 0.2% offset yield strength (
In this chapter, the recent progress in the development of metallic glasses, quasicrystals and their nanocomposites are discussed. The Zr69.5Al7.5-
In addition to, the cooling rate effect on the glass forming ability, crystallization and mechanical behavior of Zr69.5Ga7.5Cu12Ni11 metallic glass composition is presented. A slower cooling rate leads to higher degree of structural relaxation, less free volume content and therefore better short-range ordering. Such relatively ordered atomic configuration and less free volume content result in a higher hardness and yield strength for the samples synthesized at slower cooling rate than those synthesized at faster cooling rate. The ribbons synthesized at faster cooling rate contain the large free volume and have the highest shear band density. The glass forming ability parameters and hardness values of Zr69.5Ga7.5Cu12Ni11 alloy have shown significant improvement in comparison to some other known Zr-based alloys.
The authors are thankful to Dr. M.A. Shaz and Dr. T.P. Yadav for many stimulating discussions. One of the authors (Devinder Singh) gratefully acknowledges the financial support by Department of Science and Technology (DST), New Delhi, India in the form of INSPIRE Faculty Award [IFA12-PH-39]. Sections 2 and 3 of this chapter are reproduced with permission from References  and  (©2015, 2011 Elsevier).
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