Investigation of Mechanical Properties and Plastic Deformation Behavior of (Ti₄₅Cu₄₀Zr₁₀Ni₅)_100−xAl_x Metallic Glasses by Nanoindentation

Abstract

The effect of Al addition on mechanical properties and plastic deformation behavior of (Ti₄₅Cu₄₀Zr₁₀Ni₅)_100−xAl_x (x = 0, 2, 4, 6 and 8) amorphous alloy ribbons have been investigated by nanoindentation. The hardness and elastic modulus do not simply increase with the increase of Al content. The alloy with 8 at.% Al exhibits the highest hardness and elastic modulus. The serrations or pop-in events are strongly dependent on the loading rate and alloy composition.

1. Introduction

Ever since the first report of Au-Si amorphous alloy obtained by rapid solidification in 1960 [1], metallic glass formation has been found in a variety of alloy systems by this technique [2–4]. Compared with their crystalline counterparts, metallic glasses exhibit unique mechanical, physical, and chemical properties [5–8]. However, the lack of any significant plastic deformation at room temperature limits their potential applications [9, 10]. Shear localization is considered to be the primary plastic deformation mechanism in metallic glasses [11, 12]. Therefore, mechanical properties and deformation of metallic glasses have been given more and more attention. As an important tool to study nanomechanical properties of various materials, nanoindentation has been widely used for exploring the mechanical response such as hardness and elastic modulus of metallic glasses because it allows considerably larger plastic deformation to be accumulated in quasi-brittle materials in a localized area around the indented regions [13–15].

In this work, mechanical response of a series of (Ti₄₅Cu₄₀Zr₁₀Ni₅)_100−xAl_x ( = 0, 2, 4, 6, and 8) amorphous alloys subjected to nanoindentation tests has been systematically investigated based on the change of alloy composition and the applied loading rate. It is expected that our work could provide insight into better understanding of the mechanical properties and deformation behavior of metallic glasses during nanoindentation.

2. Experimental

Multicomponent (Ti₄₅Cu₄₀Zr₁₀Ni₅)_100−xAl_x ( = 0, 2, 4, 6, and 8) (all compositions in atomic percent) alloys were prepared by high purity raw materials by arc melting under Ti-gettered argon atmosphere. The alloy ribbons were fabricated using a single-roller melt spinning apparatus at a speed of 40 m/s. The amorphous nature of the as-synthesized specimen was examined by X-ray diffraction (XRD) using Cu-Kα radiation and transmission electron microscopy (TEM). Thermal properties were investigated by a differential scanning calorimeter (DSC) at a heating rate of 0.17 K/s. Nanoindentation tests were conducted using an Ultra Nanoindentation tester with a Berkovich diamond tip. The indentations were performed in the load-control mode with maximum load of 30 mN at various loading rates of 0.5, 1, 2, 4, and 10 mN/s and a constant unloading of 0.33 mN/s. At least 5 indents were measured to verify the accuracy and scatter of the indentation data. The morphologies of the indents were characterized using atomic force microscopy (AFM).

3. Result and Discussion

Figure 1 shows the XRD patterns of the as-quenched (Ti₄₅Cu₄₀Zr₁₀Ni₅)_100−xAl_x ( = 0, 2, 4, 6, and 8) alloy ribbons, together with the TEM image of the alloy with 2 at.% Al. As shown in Figure 1(a), only broad diffraction maxima can be seen without distinct sharp peak corresponding to crystalline phases, indicating the formation of a glassy phase in all ribbons. The TEM micrograph and corresponding selected area diffraction (SEAD) displaying diffuse halos for the alloy with 2 at.% Al are shown in Figure 1(b). It can be seen that there is no discernible contrast in the TEM bright field micrograph. This further confirms the amorphous nature of the alloy system and similar features are also observed for other alloy ribbons (not shown here).

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Figure 1 

(a) XRD patterns of the as-quenched (Ti45Cu40Zr10Ni5)100−xAlx (𝑥 = 0, 2, 4, 6, and 8) alloy ribbons; (b) TEM micrograph and corresponding selected area diffraction (SEAD).

Figure 2 indicates the DSC curves of the as-quenched (Ti₄₅Cu₄₀Zr₁₀Ni₅)_100−xAl_x ( = 0, 2, 4, 6, and 8) alloy ribbons. As shown in Figure 2(a), all alloy ribbons exhibit an endothermic characteristic of the glass transition followed by three exothermic events indicating the successive stepwise transformations from the super-cooled liquid state to crystalline phases. The glass transition temperature () and onset crystallization temperature () steadily increase with the increase of Al content, while thermal stability is not obviously improved by Al addition. The melting behaviors of the (Ti₄₅Cu₄₀Zr₁₀Ni₅)_100−xAl_x ( = 0, 2, 4, 6, and 8) alloy ribbons are presented in Figure 2(b), where the melting temperature () and liquidus temperature () are marked by arrows. The and also increase, and the temperature range for the melting process becomes wider with increasing Al content from 0 to 8 at.%. This means that the composition of the alloys moves away from the pseudoeutectic composition.

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Figure 2 

DSC curves of the as-quenched (Ti45Cu40Zr10Ni5)100−xAlx (𝑥 = 0, 2, 4, 6, and 8) alloy ribbons.

Nanoindentation measurement was used to investigate the mechanical properties and plastic deformation behavior of the as-quenched (Ti₄₅Cu₄₀Zr₁₀Ni₅)_100−xAl_x ( = 0, 2, 4, 6, and 8) amorphous alloy ribbons. The hardness (), elastic modulus (), and Vickers hardness (HV) of the as-quenched (Ti₄₅Cu₄₀Zr₁₀Ni₅)_100−xAl_x ( = 0, 2, 4, 6, and 8) alloy ribbons, obtained by Oliver-Pharr method, are shown in Figure 3. The increases from 59 to 159 GPa for the increase of from 0 to 8, but it does not exhibit a simply increasing trend with the increase of Al content. For and HV, the minor addition of Al ( = 2) induces mechanical softening, manifested in a little decrease of and HV shown in Figure 3. For other alloys with higher Al content, the and HV exhibit a similar trend as .

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Figure 3 

Hardness (𝐻), elastic modulus (𝐸), and Vickers hardness (HV) of the as-quenched (Ti45Cu40Zr10Ni5)100−xAlx (𝑥 = 0, 2, 4, 6, and 8) alloy ribbons.

The effect of the loading rate on plastic deformation behavior of the as-quenched (Ti₄₅Cu₄₀Zr₁₀Ni₅)_100−xAl_x ( = 0, 2, 4, 6, and 8) amorphous alloy ribbons has been investigated. As an example, Figure 4 shows the typical load-displacement curves for the alloy with 2 at.% Al at the loading rates of 0.5, 1, 2, 4, and 10 mN/s. For clarity, each successive curve is plotted with its displacement origin offset by 100 nm. As shown in Figure 4, the serration size increases with decreasing loading rate, and the largest serrated flow occurs at the lowest loading rate (0.5 mN/s), which is in agreement with the previous results [16–21]. The similar trend is also observed for other alloys. Figure 5 shows the typical load-displacement curves of the as-quenched (Ti₄₅Cu₄₀Zr₁₀Ni₅)_100−xAl_x ( = 0, 2, 4, 6, and 8) alloy ribbons at a loading rate of 2 mN/s. It can be found from Figure 5 that the higher Al content alloy exhibits a higher slope indicating a higher hardness except for the alloy with 6 at.% Al, and the serrated flow is most pronounced in the load-displacement curve for the low Al content and Al-free alloys. The load-displacement curves gradually become smoother with the increase of Al content. At the highest Al content ( = 8), there is no obviously serrated flow. This suggests that the Al addition obviously influences the nucleation and propagation of shear bands. According to the previous work [22, 23], the increase of Al content promotes continuous formation and propagation of shear bands in (Ti₄₅Cu₄₀Zr₁₀Ni₅)_100−xAl_x ( = 0, 2, 4, 6, and 8) amorphous alloys, which decreases serration size and the interval between operations of two consecutive shear bands because Al may decrease the microyield stress of the amorphous alloy. Therefore, with the increase of Al content the serrations or pop-in events gradually disappear. To further characterize the localized plastic deformation behavior, AFM observation around indents has been performed. Figure 6 shows the typical surface deformation features and pileup through indentation of the alloy with 2 at.% Al obtained after nanoindentation. A number of partial circular patterned shear bands can be seen in the pileup region, and the pileup is discontinuous. This reveals that the plastic deformation occurs during nanoindentation.

Figure 4 

Load-displacement curves for the alloy with 2 at.% Al at the loading rates of 0.5, 1, 2, 4, and 10 mN/s.

Figure 5 

Load-displacement curves of the as-quenched (Ti45Cu40Zr10Ni5)100−xAlx (𝑥 = 0, 2, 4, 6, and 8) alloy ribbons at a loading rate of 2 mN/s.

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Figure 6 

Typical surface deformation features and pileup through indentation of the alloy with 2 at.% Al obtained after nanoindentation.

4. Conclusions

Nanoindentation investigations of mechanical properties and plastic deformation behavior of (Ti₄₅Cu₄₀Zr₁₀Ni₅)_100−xAl_x ( = 0, 2, 4, 6, and 8) amorphous alloy ribbons have been conducted. The alloy with 8 at.% Al exhibits the highest hardness and elastic modulus, but the hardness and elastic modulus do not simply increase with the increase of Al content. The currently studied metallic glasses exhibit typical localized plastic deformation during nanoindentation such as serrations or pop-in events. The increase of Al content retards the occurrence of the serrations obviously.

Conflict of Interests

The authors state that there is not conflict of interests regarding the publication of this paper.

Acknowledgments

The work was supported by Open Project Program of Shenzhen Key Laboratory of Special Functional Materials of China (Grant no. T0907) and Open-End Fund for the Valuable and Precision Instruments of Central South University (Grant no. CSUZC2013024 and Grant no. CSUZC2014032).