Structural changes in Zr-based bulk metallic glasses deformed by high pressure torsion

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Abstract

Zr44Ti11Cu10Ni10Be25 and Zr57Ti5Cu20Al10Ni8 bulk metallic glass disks have been subjected to severe plastic deformation by high pressure torsion. Structural changes occurred during torsion straining were detected by high intensity synchrotron X-ray diffraction, modulated thermal analysis and hardness measurements. The samples show no evidence of nanocrystallization, however, they posses atomic bond length change in the amorphous state. Simultaneously, a change in the heat capacity of the deformed state is detected which is preserved above the glass transition in the supercooled liquid range.

Introduction

Bulk metallic glasses (BMGs) have received increased attention from scientific and technological view points in the past couple of decades. BMGs usually posses high strength, large elastic strain limit and excellent wear resistance [1]. The deformation of BMGs is commonly described in terms of the free volume model, which predicts significant increase in atomic mobility and strong dependence of strain rate on slight change of the local free volume [2], [3]. At temperatures well below the glass transition (Tg), the relative volume fraction of the plastically deformed material is very low due to the extreme localization of shear bands [4], therefore, the small contribution of plastic deformation is difficult to be detected by macroscopic methods. Recently, change in the excess free volume content on macro length scale has been studied during in situ thermal cycles by synchrotron X-ray diffraction (XRD) [5]. In these tests the excess free volume is obtained from the small increase (in the order of 10−3 to 10−4) in the average atomic distance presuming an isotropic glass. In order to enhance the ductility of BMGs, several studies have focused on dispersing the applied strain among competitive shear band networks, either by introducing inhomogeneity in the microstructure [6] or by surface constrains techniques [7], [8]. The high pressure torsion (HPT) technique, originally used for producing porosity-free bulk ultrafine-grained specimen [9], can serve as an ideal surface constrain technique to achieve extremely large plastic deformation, γ = 10–100.

Among multicomponent BMGs, Zr-based BMGs with remarkable glass forming ability have been utilized commercially to produce items for industrial applications [10]. In the present study, we demonstrate the effect of room temperature HPT deformation on Zr44Ti11Cu10Ni10Be25 and Zr57Ti5Cu20Al10Ni8 BMGs by applying synchrotron X-ray radiation, modulated calorimetry and microhardness tests.

Section snippets

Experimental

Commercial Zr44Ti11Cu10Ni10Be25 (Vit 1b™, Liquidmetal Technologies Inc.) and Zr57Ti5Cu20Al10Ni8 BMGs were subjected to HPT under constrained conditions [11]. During the HPT processing, disk-shaped samples of height 0.85 mm and radius 4 mm were strained by torsion under 8 GPa pressure N = 2 and 4 whole rotations between two stainless steel anvils. The optical image of a typical disk after torsion straining is seen in Fig. 1.

X-ray diffraction measurements were carried out at the ID-11 beamline of the

Results and discussion

Two-dimensional intensity distribution recorded by the CCD camera along the central line of the Zr44Ti11Cu10Ni10Be25 and Zr57Ti5Cu20Al10Ni8 HPT disks has shown only broad diffuse halo rings (measurement positions are marked by circles in Fig. 1), indicating that the fully amorphous structure was not destroyed during the severe plastic deformation [8], [14]. The integrated diffraction patterns (I(Q)) exhibited two strong halos at around Q = 26 nm−1 and Q = 44 nm−1 (where Q is the absolute value of the

Conclusions

Zr44Ti11Cu10Ni10Be25 and Zr57Ti5Cu20Al10Ni8 bulk metallic glass disks have been subjected to severe plastic deformation by high pressure torsion. Focused synchrotron X-ray diffraction measurements along the diameter of the disks revealed no evidence of nanocrystallization, however, a monotonously increasing shift in the first amorphous halo position reflects structural changes in the glass. TMDSC scans confirmed a minor decrease in the stability of the deformed sample accompanied by a

Acknowledgements

We appreciate the support of the Hungarian Scientific Research Fund under grant No. 67893. Á.R. is indebted for the Bolyai Scholarship of the Hungarian Academy of Sciences. The authors thank the HPT disks for Dr. E. Schafler and the Erich Schmid Institute, Leoben, Austria. We also acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities and we would like to thank Dr. Jonathan Wright for assistance in using beamline ID 11.

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    HPT processing of BMGs typically results in a rejuvenated structure, bringing the material into a higher energy state and increasing the plasticity by the introduction of excess free volume and residual stresses [22,25,46,47]. Rejuvenation is further manifested by a significant reduction in hardness and Young’s modulus as well as an increased relaxation enthalpy measured by differential scanning calorimetry [19,20,25,48,49]. Recently, we could demonstrate a clear direct correlation both between local plastic and elastic softening and between softening and the local mean atomic volume fluctuating considerably more in the rejuvenated structure [25].

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