Investigation of Mechanical Properties and Plastic Deformation Behavior of (Ti45Cu40Zr10Ni5)100−xAlx Metallic Glasses by Nanoindentation

The effect of Al addition onmechanical properties and plastic deformation behavior of (Ti 45 Cu 40 Zr 10 Ni 5 ) 100−xAlx (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.


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][3][4].Compared with their crystalline counterparts, metallic glasses exhibit unique mechanical, physical, and chemical properties [5][6][7][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][14][15].
In this work, mechanical response of a series of (Ti 45 Cu 40 Zr 10 Ni 5 ) 100−x Al 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.

Experimental
Multicomponent (Ti 45 Cu 40 Zr 10 Ni 5 ) 100−x Al 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 assynthesized specimen was examined by X-ray diffraction (XRD) using Cu-K 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).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).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 .

Result and Discussion
The effect of the loading rate on plastic deformation behavior of the as-quenched (Ti 45 Cu 40 Zr 10 Ni 5 ) 100−x Al x ( = 0, 2, 4, 6, and 8) amorphous alloy ribbons has been  and 8) alloy ribbons at a loading rate of 2 mN/s.investigated.As an example, Figure 4 shows the typical loaddisplacement 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][17][18][19][20][21].The similar trend is also observed for other alloys.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 (Ti 45 Cu 40 Zr 10 Ni 5 ) 100−x Al 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.

Conclusions
Nanoindentation investigations of mechanical properties and plastic deformation behavior of (Ti 45 Cu 40 Zr 10 Ni 5 ) 100−x Al 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.

Figure 1
Figure1shows the XRD patterns of the as-quenched (Ti 45 Cu 40 Zr 10 Ni 5 ) 100−x Al x ( = 0, 2, 4, 6, and 8) alloy ribbons, together with the TEM image of the alloy with 2 at.%Al.As shown in Figure1(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 Figure1(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).Figure2indicates the DSC curves of the as-quenched (Ti 45 Cu 40 Zr 10 Ni 5 ) 100−x Al x ( = 0, 2, 4, 6, and 8) alloy ribbons.As shown in Figure2(a), all alloy ribbons exhibit an endothermic characteristic of the glass transition followed

Figure 2 Figure 3 :Figure 4 :
Figure1shows the XRD patterns of the as-quenched (Ti 45 Cu 40 Zr 10 Ni 5 ) 100−x Al x ( = 0, 2, 4, 6, and 8) alloy ribbons, together with the TEM image of the alloy with 2 at.%Al.As shown in Figure1(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 Figure1(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).Figure2indicates the DSC curves of the as-quenched (Ti 45 Cu 40 Zr 10 Ni 5 ) 100−x Al x ( = 0, 2, 4, 6, and 8) alloy ribbons.As shown in Figure2(a), all alloy ribbons exhibit an endothermic characteristic of the glass transition followed

Figure 6 :
Figure 6: Typical surface deformation features and pileup through indentation of the alloy with 2 at.%Al obtained after nanoindentation.