G. N. Yang

Direct experimental evidence of nano-voids formation and coalescence within shear bands

Fracture mechanisms of metallic glasses are fundamentally different from that of crystalline alloys. Nano-voids formation and coalescence inside shear bands were believed to be one of the reasons causing the final failures. Although molecule dynamic simulations have successfully simulated cavitation in shear bands of brittle metallic glasses, direct experimental evidences are still rare. By carefully examining the shear bands of Pd40.5Ni40.5P19, nano-voids and their coalescence have been observed in the center-diffused region of shear bands. It is experimentally confirmed that nano-voids formation and their coalescence into large voids within shear bands is one of fracture mechanisms of metallic glasses.

Unexpected high performance of Fe-based nanocrystallized ribbons for azo dye decomposition

Different from the reported results, multiphase (Fe73.5Si13.5B9Nb3Cu1)91.5Ni8.5 nanocrystalline ribbons (CR-II) show much better degradation capability in Orange II than their metallic glass (MG) counterparts. CR-II ribbons have superior properties, such as high degradation capability, good durability and facile fabrication. In particular, high degradation efficiency could be expected even under an alkaline environment, while most reported zero-valent metals would only be available for acid or neutral environments. Galvanic cells between the α-Fe nanocrystals and intermetallics contribute to the high degradation capability of CR-II by accelerating the α-Fe nanocrystals to lose electrons, leading to the fracture of the azo bond (–NN–). Additionally, 3D nanoporous architectures and 3D flowerlike nanostructure sediments formed during the degradation process are also beneficial to the high degradation capability by promoting mass transport. These findings could provide new clues to design new materials with superior degradation capability and low cost for wastewater treatment.

The shear band controlled deformation in metallic glass: a perspective from fracture

Different from the homogenous deformation in conventional crystalline alloys, metallic glasses and other work-softening materials deform discontinuously by localized plastic strain in shear bands. Here by three-point bending test on a typical ductile Pd-Cu-Si metallic glass, we found that the plastic deformed region during fracture didn’t follow the yielding stress distribution as the conventional material mechanics expected. We speculated that such special behavior was because the shear bands in metallic glasses could propagate easily along local shear stress direction once nucleated. Based on a 3D notch tip stress field simulation, we considered a new fracture process in a framework of multiple shear band deformation mechanism instead of conventional materials mechanics, and successfully reproduced the as-observed complicate shear band morphologies. This work clarifies many common misunderstandings on metallic glasses fracture, and might also provide a new insight to the shear band controlled deformation. It suggests that the deformation of metallic glasses is sensitive to local stress condition, and therefore their mechanical properties would depend on not only the material, but also other external factors on stress condition. We hope that start from this work, new methods, criteria, or definitions could be proposed to further study these work-softening materials, especially for metallic glasses.

Serration behaviours in metallic glasses with different plasticity

Abstract We present a statistical study of serration behaviours in Pd77.5Cu6Si16.5, Ti41Zr25Be26Ag8, Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 and Fe50Ni30P13C7 metallic glasses with different plasticity. The four samples show similar serration patterns in the beginning of yielding, and different patterns during later loading. These results indicate that the shear band initiation process in metallic glasses follow some similar dynamics. And the later serration process follows different dynamics and will lead to different plasticity. Here we interpret these serration behaviours from a perspective of inhomogeneity. The different serration patterns and shear band dynamics could be reasonably understood. The serration pattern of the Fe-based sample suggests that the brittleness of metallic glasses might result from a lower degree of inhomogeneity, and a less tendency of forming shear band intersections. This study might provide new experimental evidences for different micro-structures (or inhomogeneity) and dynamic behaviours in metallic glasses with different plasticity.

The Effect of Purification on the Glass-Forming Ability of a Pd-Cu-Si Alloy

The copper mold casting method is now commonly used for preparing bulk metallic glasses (BMGs). In the present work, it was found that, by employing the copper mold casting method, Pd77.5Cu6Si16.5 (at. pct) glassy rods with 1-mm diameter could be prepared, while the ϕ2-mm Pd77.5Cu6Si16.5 casting rod possesses some crystalline phases embedded within the glass matrix, confirming that the critical size of the glassy alloy is about 1xa0mm. By melt purification with fluxing treatment, the critical size of the glassy rod prepared by copper mold casting is increased to 4xa0mm. Based on thermal property analysis, it was found that melt purification by the fluxing method can greatly enhance the thermal stability and increase the glass forming ability (GFA) of the Pd-Cu-Si alloys. The as-prepared ϕ4-mm Pd-Cu-Si glassy rod exhibits a reduced glass transition temperature (Trg) of 0.599, a supercooled liquid region (ΔT) of 74xa0K (74xa0°C), and a γ parameter of 0.419.

Serration Behavior of a Zr-Based Metallic Glass Under Different Constrained Loading Conditions

To understand the plastic behavior and shear band dynamics of metallic glasses (MGs) being tuned by the external constraint, uniaxial compression tests were performed on Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 MG samples with aspect ratios of 0.5:1, 1:1, 1.5:1, 2:1, 2.5:1, and 3:1. Better plasticity was observed for the samples with smaller aspect ratio (under higher constraint degree). In the beginning of yielding, increasing serration (jerky stress drop) size on the loading curves was noticed for all samples. Statistical analysis of the serration patterns indicated that the small stress-drop serrations and large stress-drop serrations follow self-organized critical and chaotic dynamics, respectively. Under constrained loading, the large stress-drop serrations are depressed, while the small stress-drop serrations are less affected. When changing the external constraint level by varying the sample aspect ratio, the serration pattern, shear band dynamics, and plastic behavior will change accordingly. This study provides a perspective from tuning shear band dynamics to understand the plastic behavior of MGs under different external constraint.

Understanding the effects of Poisson’s ratio on the shear band behavior and plasticity of metallic glasses

In metallic glasses, a high Poisson’s ratio often corresponds to a large plasticity and ductility. Yet the physics underpinning such a connection is still poorly understood. Here through finite element simulations, we reveal that a high Poisson’s ratio could promote the inhomogeneous stress distribution in metallic glasses. The inhomogeneous stress field could cause earlier nucleation and easier arrest of shear bands, and therefore better plasticity. Experimental results also show a trend of decreasing shear limit with Poisson’s ratio. These findings suggest that the stress inhomogeneity might be a key to understand the effects of Poisson’s ratio and loading condition on the plastic deformation behaviors of metallic glasses.

Size effect in Pd77.5Cu6Si16.5 metallic glass micro-wires: More scattered strength with decreasing diameter

A size effect of more scattered strength with decreasing diameter is revealed by tension experiments on melt-spinning fabricated Pd77.5Cu6Si16.5 metallic glass microwires, and is explained from a perspective of structural inhomogeneity and a higher structure sensitivity of metallic glasses under tension condition and in a large aspect ratio. Such a result differs from the understanding of “the smaller the stronger” in the compression behaviors of metallic glasses, but indicates that the shear bands in metallic glasses actually could nucleate in a wide stress range. This finding could provide experimental evidence for the inhomogeneous structure and size effect in metallic glasses at the micro-scale, and could help the further study of their mechanical behaviors and substantial deformation mechanism.

Nanocrystalline Phase Formation inside Shear Bands of Pd-Cu-Si Metallic Glass

Pd77.5Cu6Si16.5 metallic glass was prepared by fluxing treatment and water quenching method. To avoid possible artifacts, shear bands were created by indentation after TEM sample preparation. Bright field image, diffraction pattern, and the dark field image of TEM that covered the shear band region were presented. A few nanocrystalline phases were noticed inside the shear bands, which favored the plastic deformation ability and supported the explanation of mechanical deformation-induced crystallization.

A study of cooling process in bulk metallic glasses fabrication

To study the temperature distribution and evolution during bulk metallic glasses fabrication, finite element method was taken to simulate the cooling process in glassy alloys fabricated by water quenching and copper mold casting. The temperature distribution and evolution in different-sized samples in the two methods were successfully reproduced. The result showed that the temperature distribution in the alloy was strongly affected by fabricating method. Two relations were then proposed to estimate the cooling rate in different-sized samples prepared by these two methods. By comparing the reported data of critical size and critical cooling rate, we showed that the reported critical size and critical cooling rate of metallic glasses didn’t follow a heat transfer relation. Those critical-sized glassy alloys actually experienced cooling rates much larger than the critical cooling rates estimated by the classical nucleation theory or experiments on milligram-scaled samples. It results from the increasing degr...