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Print version ISSN 1516-1439
Mat. Res. vol.15 no.5 São Carlos Sept./Oct. 2012 Epub May 22, 2012
Effect of ion irradiation on mechanical behaviors of Ti40Zr25Be30Cr5 bulk metallic glass
Zheng HuI; Ziqiang ZhaoII; Yanping HuI; Jianshuo XingI; Tong LuI; Bingchen WeiI,*
IKey Laboratory of Microgravity, National Microgravity Laboratory, Institute of Mechanics, Chinese Academy of Sciences, 100190, Beijing, China
IIKey Laboratory of Heavy Ion Physics, Ministry of Education, Institute of Heavy Ion Physics, Peking University, 100871, Beijing, China
In this work, the effect of C4+ and Cl4+ ion irradiation with 25 MeV energy on the hardness and shear banding feature of Ti40Zr25Be30Cr5 bulk metallic glass was studied. Depth-sensing nanoindetation and microindentation were applied for characterizing the multiple-scaled hardness and shear band patterns during plastic deformation. It is shown that the Cl4+ ion irradiation leads to an obvious softening of the sample surface, and distinctly affects the serrated flow feature during plastic deformation. In contrast, C4+ ions have little effect on the hardness and shear band patterns. Besides, the mechanism for the change of the mechanical properties and plastic deformation behavior after ion irradiation was also discussed.
Keywords: bulk metallic glass, ion irradiation, indentation, shear bands, hardness
Nowadays, the development of ion irradiation technologies has ascended to a new level. The interest in both research and industrial areas is aroused by the possibility of modifying physical and chemical properties of materials, even on the nanometer scale, either through the energy deposition by the ions and/or by the implanted ion species themselves. Incident ions lose their energy in solids predominantly via two mechanisms. First, a direct transfer of kinetics energy to target atoms by elastic collisions between a projectile nucleus and target nuclei is commonly denoted as nuclear energy loss Sn. Energy is also transferred to target electrons by the generation of electronic excited or ionized target atoms, which is called electronic energy loss Se that contributes the most to the energy loss of ions with energy above 0.1 MeV.u1. In crystalline metals and alloys, it is well known that ion irradiation leads to severe modifications of the structure through the formation of self‑interstitials, vacancy loops and transmutation. Due to their inherent disordered structure, metallic glasses are supposed to be resistant against displacive irradiation. For this reason, these materials are potential candidates for applications in irradiation environments, such as fusion, spallation sources, etc. Many works on the thin film metallic glasses have also been carry out, where nanocrystallization and macroscopic anisotropic growth effect etc. have been reported2-7. Furthermore, in metallic glasses, target atoms will be displaced from their initial sites upon energetic ion irradiation, which may create vacancy-like defects and even could modify the local atomic structure. This provides a possible route for improving the ductility of metallic glasses. Therefore, the effect of ion irradiation on the mechanical properties in bulk metallic glasses (BMGs) has also been reported. Significant reduction in strength has been found in a Ni ion irradiated Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMG, which was explained with the enhancement in the free-volume content8. However, in other works, distinct surface hardening in Zr55Cu30Ni5Al10 irradiated by Co or Ar ions was demonstrated, which was attributed to the denser packing structures formed under ion irradiation. Therefore, the irradiation effects in BMGs and their underlying mechanism are still unclear. In this work, Ti40Zr25Be30Cr5 BMG was irradiated by C4+ and Cl4+ ions, respectively. Hardness and deformation behavior of the BMG were studied by using depth-sensing nanoindentation. Furthermore, subsurface deformation morphology of as-cast and irradiated samples was also investigated by using the bonded interface technique11.
2. Experimental Methods
Cylinders of Ti40Zr25Be30Cr5 alloy with 5 mm diameter were prepared by arc melting and suction casting under pure argon atmosphere. The amorphous structure of the samples was confirmed by X-ray diffraction (XRD) using Cu Kα radiation. Before irradiation, some 3 × 3 × 10 mm rectangular blocks were cut from the cylindrical samples by using a linear cutting machine and polished with standard metallographic techniques to an exact desired finish for the sake of ion irradiation. The irradiation experiments were performed by 25 MeV C4+ and Cl4+ ions to the polished surface using a 2 × 6 MV tandem accelerator. During ion irradiation the sample holders were cooled by a liquid nitrogen cooling system. The irradiation fluence is 9.60 × 1014 cm2 for C4+ ions and 7.63 × 1015 cm2 for Cl4+ ions, respectively.
Nanoindentation measurements were performed in a MTS Nano Indenter® XP with a Berkovich tip and a cube-corner tip, respectively. The former tip is employed for standard hardness and modulus measurements using continuous stiffness method, while the latter is used for highlighting the serration flow phenomenon12,13. The subsurface deformation morphology was checked through microindentation using the bonded interface technique, wherein the irradiated surface and un-irradiated surface were bonded together for comparison. Themorphology of the plastic deformed region after indentations was observed by a JSM-6460 scanning electron microscope (SEM).
3. Results and Discussion
XRD patterns obtained from the as-cast and irradiated Ti40Zr25Be30Cr5 BMGs are shown in Figure 1. The as-cast specimen consists of only one broad diffraction peak without any sharp Bragg peaks, indicating its amorphous structure. For the ion irradiated samples, the XRD spectrums also show one broad diffraction peak, which indicates that the BMGs still maintain an amorphous structure after irradiation.
The projected range and displacement damage (displacement per atom, dpa) are calculated using SRIM code14. C4+ and Cl4+ ion irradiated samples on the present BMG produces the maximum displacement damage value of 0.1 and 5.7 dpa, respectively. Furthermore, the region of the most severe radiation damage is well correlated to the mean projected range. For C4+ and Cl4+ ion irradiation, the mean projected ranges are approximately 15.8 and 5.26 µm, respectively, below the samples surface. It should be noted that the displacement damage induced by Cl4+ ion irradiation is more severe than that of C4+ ions, while the mean projected range is lower, because of its high atomic number.
From the calculated results by SRIM, we know that ion irradiation with high ion fluence can cause damage on the surface layer of samples. Nanoindentation was used to study the effect of irradiation on hardness and modulus of the BMGs. The profiles of hardness and modulus as a function of probing depth for as-cast, C4+ ion and Cl4+ ion irradiated samples are shown in Figure 2. It is obvious that the hardness of the Cl4+ ion irradiated sample is lower than that of the as-cast sample, while the C4+ ion irradiated sample exhibits a similar hardness with the as-cast one throughout the whole indentation depth range (Figure 2a). The average hardness value of the as-cast, C4+ ion irradiated and Cl4+ ion irradiated sample is 7.18, 7.16 and 6.74 GPa, respectively. This indicates a significant softening by severe Cl4+ ion irradiation damage (5.7 dpa), while the relative weak damage (0.1 dpa) caused by C4+ ion irradiation does not change the hardness of the sample. The modulus values of the as-cast, C4+ ion irradiated and Cl4+ ion irradiated samples are almost the same throughout the penetration process of nanoindentation (Figure 2b). The average modulus values for all these three samples are about 118 GPa. This indicates that irradiation damage in the present study does not change the modulus of the BMG.
In order to explore the effect of ion irradiation on the plastic deformation behavior of the BMGs, nanoindentation measurements with cube-corner tip were made on the as‑cast and irradiated samples. Compared with the Berkovich tip at the same load level, the sharper cube‑corner tip can produce a larger peak-load displacement and a greater proportion of permanent plastic deformation after unloading. Typical load‑displacement (P-h) curves at the loading rate of 16.7 mN/s are presented in Figure 3a. In this figure the origin of each curve has been displaced so that multiple curves can be seen clearly on the graph, and only the loading portions of the P-h curves are shown for clarity. Again, the significant softening can be found in the Cl4+ ion irradiated sample, while the C4+ ion irradiated sample shows comparable hardness with the as-cast sample. Furthermore, pronounced serrated flow during the loading process of nanoindenation can be found in all these three samples. In order to demonstrate the characteristics of this serrated flow clearly, these P-h curves were fitted with smooth power law function15, 16: Pfit = K×hm, where K and m are the loading rate-dependent fitting constants. For a given penetration depth h, the corresponding loads Pfit can be determined using this function, whereas the corresponding experimental values of loads, Pexp, can be obtained from the curves illustrated in Figure 3a. Obviously, the amount ∆P = PexpPfit can show the deviation of the load during loading process. The ∆P-h curves of the three samples during nanoindentation are shown in Figure 3b. It can be seen that the oscillation magnitude of the load deviation ∆P of the as-cast sample is higher than that of the two irradiated samples. It is also remarkable that the oscillation magnitude of irradiated samples are more irregular and the number of small values of ∆P increases, especially in the Cl4+ irradiated sample. It demonstrates that ion irradiation changes the serrated flow feature from a regular serration in the as-cast sample to irregular behavior in the Cl4+ and C4+ ion irradiated samples. Typical morphology of the plastic deformation region around the indent after nanoindentation is shown in Figure 3c. Though numbers of shear bands can be found around the indents, they could not provide enough information on the effect of irradiation on the operation feature of shear band, due to the fact that the evere damage layer just occupies a small portion of the whole plastic deformation region.
For further clarifying the shear band patterns affected by the ion irradiation, the subsurface deformation morphology was checked through microindentation using the bonded interface technique, wherein the irradiated surface and un-irradiated surface were bonded together for a good comparison. In this case, the whole deformation region on the interface has undergone ion irradiation. Typical SEM images for the plastic deformation region underneath the indents are shown in Figure 4. For the as-cast sample, there are principally two sets of shear bands, semicircular and radical, and the former type is dominating (Figure 4a). The radical shear bands intersect the semicircular shear bands and leave small shear-offsets behind (Figure 4b). The shear band patterns of the C4+ ion irradiated sample (Figure 4c) are quite similar to that of the as-cast sample. These two sets of shear bands can also be identified in the deformed zone and the number of the semi-circular shear bands is larger than that of the radical shear bands. The shear band patterns of Cl4+ irradiated sample are shown in Figure 4d. In this figure we could also see a hemispherical deformation zone. However, the shear band patterns are more complex and disordered than those in as-cast sample. The characteristics of semi-circular shear bands are branching and discontinuous. Besides, the spacing between shear bands is larger in the area just blow the indenter tip. The above results demonstrate that the shear band patterns would change when suffering severe irradiation damage by heavy ions.
Under MeV ion irradiation, the density of electronic excitations and ionization becomes so high that new unexpected and collective effects arise. Violent nucleus‑nucleus collisions can create displacement cascade and thermal spikes, which leaves some defects like vacancy rich, nanocrystal, melting and resolidification in solids17. It is possible that amorphous alloys, being metastable, may crystallize on irradiation. However, distinct crystallization upon irradiation does not take place within the detect limit of XRD in this work. Therefore, the possible mechanism of damage process is that when ions are implanted into the surface of BMG, target atoms will be displaced from their initial sites, especially at high energy transfer. This creates vacancy-like defects and increases excessive free volume in the system. Through the results of nanoindentation experiments, it can be found that there is obvious softening on the ion irradiated BMG, especially for the Cl4+ion irradiated sample, with the hardness decreased by about 6%. The sofening is related to the increase in free volume or defects in atomic scale, which is also in accordance with the free volume theory18,19.
As shown in Figure 3 and Figure 4, the serrated flow phenomenon during the plastic deformation become more irregular in ion irradiated samples, and the shear band patterns beneath the indents of irradiated samples also change from a relatively regular distribution in the as-cast sample to a disordered feature in the Cl4+ ion irradiated sample. These phenomena further verify that the formation and propagation of shear bands are indeed disrupted by ion irradiation, due to the introduction of excess randomly distributed defects or local atomic structural changes by ion irradiation. This also changes the stress distribution at the shear band tip during its propagation, giving rise to a deviation of shear band from their original direction during the plastic deformation of the BMG. Therefore, shear band patterns in the irradiated samples tends to be a disordered and branching feature, and the serrated flow feature becomes more irregular. The above results also suggest that ion irradiation could be a possible technique to improve the plasticity of BMG, due to the introduction of excess atomic-scale defects and the change of the mode of shear band propagation.
The effect of C4+ and Cl4+ion irradiation on mechanical behaviors of Ti40Zr25Be30Cr5 BMG was studied. Irradiation damage to the samples is about 5.7 dpa, corresponding damage peak depth of about 5.26 µm by Cl4+ ions, while 0.1 dpa damage, 15.8 µm depth by C4+ ions, respectively. It is found that BMG still maintain an amorphous structure after high energy heavy ions irradiation. C4+ ion irradiation, producing low irradiation damage, does not change the mechanical behavior of the BMG. However, Cl4+ irradiation decreases the hardness of the surface layer of BMG, and distinctly alters the serrated flow feature and shear band patterns during plastic deformation. The change of the deformation behavior is attributed to the introduction of excess atomic-scale defects by ion irradiation.
This work is financially supported by the National Nature Science Foundation of China (Grant Nos. 51071166 and 10675010) and National Basic Research Program of China (973 Program, No. 2010CB832902 and 2010CB832904).
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Received: December 7, 2011
Revised: March 15, 2012