The Effects of Fatigue on the Atomic Structure with Cyclic Loading in Zr50Cu40Al10 and Zr60Cu30Al10 Glasses
- First Online:
- Cite this article as:
- Tong, P., Louca, D., Wang, G. et al. Metall and Mat Trans A (2012) 43: 2676. doi:10.1007/s11661-011-0887-5
The potential damage effect from fatigue on Zr bulk metallic glass alloys of Zr50Cu40Al10 at the eutectic point and Zr60Cu30Al10 away from the eutectic point (in atomic percent) is examined via the local atomic structure, which was obtained from the pair density function analysis of the synchrotron X-ray radiation and neutron data. Samples cut from the same rods were subjected to 104, 105, and 106 compression cycles ex situ, and the evidence for fatigue damage was investigated by comparing alloys before and after cyclic loading. Bond orientation was observed particularly in Zr50Cu40Al10, suggesting that fatigue damage occurs even in the elastic range, below the yield point, and during cyclic loading. The initiation of fatigue changes is observed first within small localized atomic regions.
Bulk metallic glass (BMG) alloys with good glass-forming abilities and slow cooling rates can be potentially suitable in a wide range of engineering applications.[1–5] BMGs have superb physical properties, such as high corrosion and wear resistance, high yield strength, soft magnetic properties, and even superconducting properties.[6–8] Despite their good mechanical properties, their use is limited primarily because BMGs tend to be brittle as they cannot plastically elongate during a uniaxial tensile stress. However, some BMGs can plastically deform under compression, rolling, and bending at ambient temperatures. The deformation process occurs through the formation of localized shear bands.[10–12] The same shear bands may become a site for further plastic flows, resulting in low ductility. Improving their ductility and tolerance to damage is an important step toward allowing BMGs to be used as industrial materials.
BMGs are particularly vulnerable to fatigue damage,[14–18] even under low applied stresses below the global yield limits, often with no visible effects, such as the presence of shear bands, until failure occurs. The elastic-to-plastic transition appears suddenly. Under fatigue-loading conditions, a wide range of fatigue endurance limits[19,20] is usually observed. To date, the physical mechanism that leads to catastrophic failure under fatigue-loading conditions, in which localized damage accumulates and eventually leads to failure, is not well understood.[7,14–18] It is presumed that irreversible changes must be taking place under fatigue, but their nature and the mechanism leading to such localized damage have not been identified. To search for evidence of initial changes that may occur in the atomic structure under high-frequency fatigue loading, the local atomic structure of ternary Zr-based bulk metallic glasses subjected to compression-compression cyclic loading tests is investigated via neutron and X-ray diffraction. The response to cyclic fatigue loading is investigated in two compositions, Zr50Cu40Al10 and Zr60Cu30Al10, which differ only by 10 pct in the Cu content.
Our results indicate that microstructural changes observed in a small sample volume are most likely the effects from cyclic fatigue during compression, where the same atomic regions may act as nucleation sites for subsequent deformation. The local atomic structures of the Zr50Cu40Al10 and Zr60Cu30Al10 were examined after subjecting both alloys to various fatigue cycles ex situ. A structural change, albeit small, is observed in both compositions regardless of the number of cycles, which may serve as evidence for damage caused by fatigue despite no visible external changes in the alloys after testing. The atomic changes are observed in the short-range pair correlations up to ~3.5 Å and occur viscoelastically, in response to the applied stress in both alloys but more so in Zr50Cu40Al10. At distances greater than 4 Å, the atomic structure shows no differences with the number of compression cycles. The results indicate that the effects caused by cyclic-fatigue loading are not elastic and bring about subtle and irreversible atomic rearrangements within small volume pockets. Zr50Cu40Al10 is close to the eutectic composition and shows significant embrittlement by structural relaxation, whereas Zr60Cu30Al10 exhibits high resistivity against the structural relaxation embrittlement. Furthermore, fracture toughness is different between them. The fracture toughness value of the former is 51 and of the latter is 110 MPa m1/2, respectively.
3 Results and discussion
The increase in the intensity of the pair correlation peaks at room temperature must be a consequence of the progressive and localized structural hardening that occurs in the material, when subjected to cyclic loading. With fatigue, the atomic bonds are changed permanently as clearly the atomic structure undergoes irreversible changes. At the same time, the damage is cumulative with increased fatigue cycles for specimens that do not break. The observed changes in the atomic structure may reflect the onset of an elastic to plastic transformation. They must have an impact on the macroscopic yield point and the formation of the first shear band. Note that at longer distances, beyond 3.5 Å, the local structure shows no visible changes under the cyclic loading conditions of this experiment, and neither alloy exhibited any observed changes within the resolution of this experiment. It may be that extending the cyclic loading tests on the same samples, the structural changes may expand to longer distances leading to failure. Such experiments are planned.
What is the origin of the difference in the response observed in the two alloys? The answer must be in the nature of the Zr-Cu correlations. The difference in the atomic percent of Zr and Cu in the two alloys is reflected in the shape of the first PDF peak.[25,26] A higher Cu content shifts the peak’s center of mass to lower r values, whereas a higher Zr content shifts the peak’s center of mass to higher r values, in response to the size of their nominal ionic radii. In Zr50Cu40Al10, one can expect a higher number of Cu-Cu and Zr-Cu bond pair correlations and a lower number of Zr-Zr bond pair correlations in comparison with Zr60Cu30Al10. The reverse is true for Zr60Cu30Al10. To understand the difference in the response to the fatigue damage of the first PDF peak shift in the two alloys requires detailed modeling. To summarize, these results provide evidence for irreversible structural changes with cyclic fatigue similar to the nanoindentation measurements reported in Reference 18.
The authors would like to acknowledge valuable discussions with S.J. Poon and M. Widom. Support for this work was provided by the National Science Foundation, Nos: 130398GA10715 and DMR 0231320. The use of the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.