Yield Strength Of Metallic Glass Research Articles

Metallic Glasses: Materials for Today and Tomorrow

Abstract Metallic glasses are noncrystalline solid metals that have high strength, excellent corrosion resistance, and impressive hardness compared to their crystalline counterparts. A modern understanding of how glasses are formed along with an appreciation for the high ultimate yield strength of metallic glasses are leading to a boon in applications for these unique materials.

Improving fatigue property of metallic glass by tailoring the microstructure to suppress shear band formation

Metallic glass (MG) usually exhibits poor fatigue property, limiting its application as structural materials. Although fatigue failure happens at stress level much lower than the yield strength of MGs, shear band has been found to be one of main origins of fatigue crack initiation under high stress level. In this work, to enhance the fatigue property of MG, we proposed and verified a new strategy through tailoring the microstructure to suppress the shear band formation and then to impede the fatigue crack initiation, in contrast to the previous way of proliferating shear bands. Heat treatment, which is controllable, nondestructive and easy to conduct in practice, was utilized to adjust the MG microstructure. In order to make shear band difficult to initiate, annealing before glass transition temperature was performed. To determine better microstructure of MG with enhanced fatigue property, we employed the results of simple mechanical tests including uniaxial tension, compression and notch tension as feedbacks. The treated MG materials with improved elastic limit but only slightly decreased notch toughness than the as-cast samples were expected to show enhanced fatigue property, which has been examined by subsequent experiments. The enhanced elastic limit implies the improved resistances for shear band formation and thus fatigue crack initiation, while the still high notch toughness guarantees the large tolerance for fatigue failure. The present strategy provides a promising way for enhancing the fatigue endurance property of MGs without the requirement of intrinsically large plasticity, which may thus be beneficial for the structural application of most MGs with high glass forming ability.

The stochastic transition from size dependent to size independent yield strength in metallic glasses

It has been an enduring and heated debate whether the yield strength of metallic glasses (MGs) is size dependent or size independent. In this work, we first develop a micromechanical model by taking into account the stochasticity for shear band initiation in microcompression. Our modeling is subsequently verified through the extensive in-situ and ex-situ microcompression experiments. Through the efforts of combined experiments and modeling, we show a size-controlled stochastic transition from the size dependent to the size independent yield strength in the MG micropillars. Such a stochastic transition is featured with a strong fluctuation in the measured yield strengths when the micropillar size is near an intrinsic length scale which varies with the chemical composition of MGs. In contrast, such a size-controlled transition appear deterministic with little data scattering in tension. At the fundamental level, our results unfold a size dependent shear band initiation process in MGs, which may be applicable to other amorphous materials of technological importance.

Microstructural heterogeneity perspective on the yield strength of metallic glasses

We report an intrinsic universal correlation between yielding and dynamic activation of flow units in various metallic glasses, analogous to yielding in crystalline materials accompanied by activation of structural defects of dislocation. A transition point of yield strength when test temperature reaches a critical value is observed, and the evolution of the yield strength corresponds well to the evolution of activation energy and fraction of flow units with temperature, indicating the correlation between yield strength and intrinsic structural heterogeneity in glasses. We propose a model based on the flow units to understand the structural origin of yielding phenomenon and the nature of the yield strength.

Correlation between elastic structural behavior and yield strength of metallic glasses

Tremendous research effort has been put into the study of plastic deformation mechanisms of metallic glasses (MGs) in an attempt to elucidate the origin of their high fracture strength. Little attention has, however, been paid to the elastic deformation of MGs. In this paper, a series of MGs with different yield strengths are studied, with a focus on the fine structural evolution (at the atomic level) during elastic deformation. Our results reveal that an atomic reorientation happens in the first nearest-neighbor shell due to elastic deformation. This reorientation subsequently leads to a drop in the local stress, which further results in a cooperative shift of surrounding atoms to counterbalance this change in local stress level. A concordant region is formed as a result. The relation between this concordant region and the yield strength is thoroughly discussed in terms of its size and the stress level in this region. It is proposed that this concordant region could be the missing part that bridges the macroscopic yield strength and the microscopic atomic structure.

Variability in the yield strength of a metallic glass at micron and submicron length scales

This work presents an in-depth investigation of the dispersion in the compressive yield strength of a Zr-based bulk metallic glass as the specimen dimensions decrease from the micron to the submicron length scale. While the investigated alloy belongs to one of the so-called “ductile” glass families, the degree of variation in strength exhibited in the submicron size range approaches that of conventional brittle ceramics. Using a three-parameter Weibull analysis, however, we find that the engineering reliability of the glass is independent of specimen size. The glass exhibits a size-independent failure-free stress of 1825MPa, below which yielding is not expected. The three-parameter Weibull modulus is also size independent. The dispersion in yield strengths above the failure-free stress is due to the change in the sampling volume for specimens 1μm in diameter or larger, while interactions between defects and the surface actually limit the breadth of the strength distribution observed in specimens 200–330nm in diameter relative to what may be expected for that volume. Using an instability criterion for a penny-shaped crack under shear with interfacial friction, a relationship between yield strength and the corresponding defect size is established, and the defect concentration is estimated.

Size-independent strength and deformation mode in compression of a Pd-based metallic glass

We present quasi-static, room temperature compression data for Pd 40Ni 40P 20 metallic glasses, with specimen sizes ranging from the submicron to several millimeters in diameter. We observe no change in deformation mode over this range. At all sizes, plastic flow is localized in shear bands, which are accompanied by sudden strain bursts. This metallic glass shows only a modest increase in strength in going from bulk to micrometer-sized specimens. We show that stress gradients in tapered specimens can complicate measurement of the yield strength of metallic glasses in microcompression. Estimates of yield strength based on the minimum cross-sectional area implicitly assume that yielding is controlled by a maximum effective shear stress criterion. An alternative is the shear plane yield criterion, in which the minimum shear stress on the shear band trajectory determines yield. Application of this criterion in tapered microspecimens reinforces the notion that metallic glasses possess relatively size-independent mechanical properties.