Dai Lanhong

Serrated Plastic Flow in a Zr-Based Bulk Metallic Glass During Nanoindentation

We investigate plastic deformation of Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass using depth sensing nanoindentation. Numerous serrations in the load-displacement curves during indentation, shear bands and pile-ups around the indent were observed. The results revealed that the serrated plastic flow behaviour in this alloy depends strongly on the indentation strain rate.

Inherent Shear-Dilatation Coexistence In Metallic Glass

Shear deformation can induce normal stress or hydrostatic stress in metallic glasses [ Nature Mater. 2 ( 2003) 449, Intermetallics 14 ( 2006) 1033]. We perform the bulk deformation of three-dimensional Cu46Zr54 metallic glass (MG) and Cu single crystal model systems using molecular dynamics simulation. The results indicate that hydrostatic stress can incur shear stress in MG, but not in crystal. The resultant pronounced asymmetry between tension and compression originates from this inherent shear-dilatation coexistence in MG.

Shear Strength Measurements in LY-12 Aluminium Alloy During Shock Loading

Lateral stress of LY-12 aluminium alloy under plate impact shock loading was measured. Based on the measured data, the Hugoniot relation and shear strength were obtained. The result has demonstrated that the shear strength of the tested material increases remarkably with the increasing longitudinal stress. This means that the assumption of constant shear strength usually adopted in shock stress calculation is not suitable for the present material.

The “ tension transformation zone” model of amorphous alloys

An amorphous alloy is a glassy solid that is formed through the supercooling of a melt. As the melt cools via the glass transition, its atoms freeze into a long-range disordered structure. Amorphous alloys represent a relatively young class of materials, having been first reported in 1960 when Duwez and co-workers produced Au-Si alloys by developing the rapid-quenching technology. The advent of amorphous alloys, especially the bulk samples with their characteristic size in excess of 1 mm, has aroused much interests in the basic science of glass transition, glass structure, and their intriguing properties. For crystalline metals, their structure can be well described by the period lattices and lattice defects including dislocations, twins, stacking faults, grain doundaries, etc. However, these traditional structural defects are not defined in amorphous alloys. Therefore, this type of atomic-disordered alloys manifest a series of excellent mechanical properties, including extraordinary strength, high hardness, large elastic limit and relatively high fracture toughness, making them attractive candidates for many potential applications as structural materials. At temperatures far below the glass transition temperature, the failure of amorphous alloys is generally induced by 10 nm thick shear banding with the single-dominated or multiple mode. It is well known that the shear banding is an instability mode of plastic flow from homogeneous to localized feature. Although the precise mechanism for amorphous plasticity is not well discovered, it is widely accepted that the shear-banding-mediated plasticity originates from a cascade of inelastic shear rearrangements of local atomic groups, called shear transformation zones (STZs). The STZs are thermally activated events with the transient nature, driven by shear stress and giving rise spatially to Eshelby fields. However, many recent works have shown that the failure of amorphous alloys is not always dominated by the shear banding; instead, a brittle failure will take place with a tension mode. The latter is usually accompanied with a new type of fracture surface morphology: fine dimples and/or nanoscale periodic corrugations. In order to understand such a dissipation process of fracture energy, we proposed the “tension transformation zone (TTZ)” model of amorphous alloys in 2008. The TTZ describes the brittle nucleation-controlled cavitation of local atomic groups that can be activated by shear-induced dilatation or direct hydrostatic tension. Here, we review how the TTZ model was developed, including its inherent nature, activation conditions, atomistic simulations and relevant experiments. The difference and relationship between the proposed TTZ and the classical cavitation are extensively discussed. Therefore, the energy dissipation in fracture of amorphous alloys is determined by two competing elementary processes, via. STZs and TTZs ahead of the crack tip. Based on this STZ vs. TTZ picture, the ductile-to-brittle transition of amorphous alloys can be understood as the change in the nature of transformation zones from shear-dominated STZs to dilatation-dominated TTZs. This review ends with the key aspects that deserve further study regarding the TTZ model. These aspects, at least, include (1) the experimental capture of TTZs, (2) the dynamics properties, (3) the spatio-temporal evolution, and (4) the theoretical construction from TTZs to brittle failure in amorphous alloys.

Inherent parameters governing ductile-brittle transition in metallic glasses

The ductile-brittle transition and its mechanism are the hot research topics in the field of material and mechanics. In recent hundred years, the critical conditions for cracking and the governing parameters on ductility have developed for crystalline solids. Metallic glasses (MGs) as an emerging class of structural materials in recent years, show a broad potential applications due to their excellent mechanical properties. However, the low plasticity caused by highly strain localization greatly restrains them from engineering application. Therefore, to clarify the plastic behavior and the ductile-brittle transition in MGs becomes significant. Based on the classical theories, researchers have developed some important conditions to predict the plasticity in MGs, by taking their inherent properties into account. These theories usually include single or two governing parameters. In this paper, a brief review is carried out on these previous work, and further the underlying correlations between these governing parameters and the physics behind the ductile-brittle transition are revealed.

A Modified Free Volume Model for Characterizing of Rate Effect in Bulk Metallic Glasses

We investigate the plastic deformation and constitutive behaviour of bulk metallic glasses (BMGs). A dimensionless Deborah number DeID = tr/ti is proposed to characterize the rate effect in BMGs, where tr is the structural relaxing characteristic time of BMGs under shear load, ti is the macroscopic imposed characteristic time of applied stress or the characteristic time of macroscopic deformation. The results demonstrate that the modified free volume model can characterize the strain rate effect in BMGs effectively.

Release Behaviour of Shock Loaded LY12 Aluminium Alloy

By making use of a light gas gun, a specially designed target is impacted by the LY12 flyer, and the pressure is taken in the range of 0.6–3 GPa. Based on the stress profiles measured in the buffer materials by manganese gauges, the Hugoniot curve and release curves of LY12 aluminium alloy are obtained. Meanwhile, the release curves from different initial shocked states are described in both the pressure-particle velocity plane and the pressure-specific volume plane.