Fatigue Behavior Of Metallic Glass Research Articles

Shear Band Evolution under Cyclic Loading and Fatigue Property in Metallic Glasses: A Brief Review

The fatigue damage and fracture of metallic glasses (MGs) were reported to be dominated by shear band. While there exist several reviews about the fatigue behavior of MGs, an overview that mainly focuses on shear bands under cyclic loading is urgent, and is of great importance for the understanding of fatigue mechanisms and properties. In this review paper, based on the previous research results, the shear band evolution under cyclic loading including shear band formation, propagation and cracking, was summarized and elucidated. Furthermore, one strategy of enhancing the fatigue property through manipulating the microstructure to suppress the shear band formation was proposed. Additionally, the applications of the effect of annealing treatment and processing condition on fatigue behaviors were utilized to verify the strategy. Finally, several future directions of fatigue research in MG were presented.

Fatigue behavior of Zr-based metallic glass micropillars

Metallic glass with thermoplastic properties in the supercooled liquid region is a potential type of material for fabricating micro molds. Fatigue behavior of metallic glass is the crucial property that enables it to serve as a mold material. Here, fatigue and fracture behaviors of the micro-sized Zr41·2Ti13·8Cu12·5Ni10Be22.5 (at.%, Vit1) metallic glass micropillars were investigated by uniaxial compression-compression fatigue tests. It was shown that the micropillars have fatigue–endurance limit of ~1000 MPa and a fatigue ratio of ~0.56. The effects of sample size and heat treatment on the fatigue limit were also studied. After annealing at 450 °C, the fatigue limit decreased to ~600 MPa and the fatigue ratio decreased to ~0.43. The fracture angles between the stress axis and the shear plane of unannealed metallic glass were found to be ~42°, while the annealed Vit1 micropillars failed in break or splitting modes. These findings provided the experimental evidence for the inhomogeneous structure and the size effect in metallic glass failure modes at the micro-scale size. Our results can serve as a guideline for further studies to investigate the mechanical behavior and deformation mechanisms of the metallic glass.

Fatigue damage and fracture behavior of metallic glass under cyclic compression

A comprehensive understanding on the fatigue behavior of metallic glasses (MGs) is of vital importance for their structural applications. Here we present a systematic study on the fatigue damage and fracture behavior of a Zr52.5Cu17.9Ni14.6Al10Ti5 MG under cyclic compression of different stress levels, with the assistance of 3D X-ray tomography (XRT) and 2D scanning electron microscope to examine the origins of fatigue cracks. The results indicate that the fatigue fracture mode of the MG depends strongly on the stress level in such a manner similar to that of high-strength steels. The MG exhibits shear fracture along shear band under high cyclic stress level, just like the case for monotonic compression; under low cyclic stress level, in contrast, the fracture is dominated by splitting where the fatigue cracks originate from extrinsic defects. The increased propensity for cavitation and cracking under low stress level, as revealed by XRT, suggests a reduced resistance of the MG to cleavage failure which eventually leads to the transition of the fatigue fracture mode. The results support the idea that the fatigue performance of MGs can be improved by reducing the extrinsic defects and properly controlling the shear banding behavior.

Atomistic origin of size effects in fatigue behavior of metallic glasses

While many experiments and simulations on metallic glasses (MGs) have focused on their tensile ductility under monotonic loading, the fatigue mechanisms of MGs under cyclic loading still remain largely elusive. Here we perform molecular dynamics (MD) and finite element simulations of tension-compression fatigue tests in MGs to elucidate their fatigue mechanisms with focus on the sample size effect. Shear band (SB) thickening is found to be the inherent fatigue mechanism for nanoscale MGs. The difference in fatigue mechanisms between macroscopic and nanoscale MGs originates from whether the SB forms partially or fully through the cross-section of the specimen. Furthermore, a qualitative investigation of the sample size effect suggests that small sample size increases the fatigue life while large sample size promotes cyclic softening and necking. Our observations on the size-dependent fatigue behavior can be rationalized by the Gurson model and the concept of surface tension of the nanovoids. The present study sheds light on the fatigue mechanisms of MGs and can be useful in interpreting previous experimental results.

Cyclic Deformation in Metallic Glasses.

Despite the utmost importance and decades of experimental studies on fatigue in metallic glasses (MGs), there has been so far little or no atomic-level understanding of the mechanisms involved. Here we perform molecular dynamics simulations of tension-compression fatigue in Cu50Zr50 MGs under strain-controlled cyclic loading. It is shown that the shear band (SB) initiation under cyclic loading is distinctly different from that under monotonic loading. Under cyclic loading, SB initiation takes place when aggregates of shear transformation zones (STZs) accumulating at the MG surface reach a critical size comparable to the SB width, and the accumulation of STZs follows a power law with rate depending on the applied strain. It is further shown that almost the entire fatigue life of nanoscale MGs under low cycle fatigue is spent in the SB-initiation stage, similar to that of crystalline materials. Furthermore, a qualitative investigation of the effect of cycling frequency on the fatigue behavior of MGs suggests that higher cycling frequency leads to more cycles to failure. The present study sheds light on the fundamental fatigue mechanisms of MGs that could be useful in developing strategies for their engineering applications.

Modeling size effects on fatigue life of a zirconium-based bulk metallic glass under bending

A size effect on the fatigue-life cycles of a Zr50Cu30Al10Ni10 (at.%) bulk metallic glass has been observed in the four-point-bending fatigue experiment. Under the same bending-stress condition, large-sized samples tend to exhibit longer fatigue lives than small-sized samples. This size effect on the fatigue life cannot be satisfactorily explained by the flaw-based Weibull theories. Based on the experimental results, this study explores possible approaches to modeling the size effects on the bending-fatigue life of bulk metallic glasses, and proposes two fatigue-life models based on the Weibull distribution. The first model assumes, empirically, log-linear effects of the sample thickness on the Weibull parameters. The second model incorporates the mechanistic knowledge of the fatigue behavior of metallic glasses, and assumes that the shear-band density, instead of the flaw density, has significant influence on the bending fatigue-life cycles. Promising predictive results provide evidence of the potential validity of the models and their assumptions.