Kaixin Liu

Atomic-scale bonding of bulk metallic glass to crystalline aluminum

A Ti40Zr25Cu12Ni3Be20 bulk metallic glass (BMG) was welded to a crystalline aluminum by the parallel plate explosive welding method. Experimental evidence and numerical simulation show that atomic-scale bonding between the BMG and the crystalline aluminum can be achieved, and the weldment on the BMG side can retain its amorphous state without any indication of crystallization in the welding process. Nanoindentation tests reveal that the interface of the explosive joints exhibits a significant increase in hardness compared to the matrix on its two sides. The joining of BMG and crystalline materials opens a window to the applications of BMGs in engineering.

Study of the structural damage in the "0001… GaN epilayer processed by laser lift-off techniques

The structural influences of the laser lift-off (LLO) techniques on the created (0001) GaN surface region are characterized by cross-sectional high-resolution transmission electron microscopy and fitted using the model of stress waves caused by a longitudinal impact at the end of a cylindrical bar extending to infinity. The authors study reveals that, in addition to the superficial damage caused by laser absorption, the stress saltation in GaN crystal where the shock waves come into being induces deformation of the lattices and generates a cluster of half loops above the LLO interface. After that, the lattice deformation will be induced every time the partial dissipation of the steady-state shock waves takes place until the shock wave is dissipated to elastic mode.

The failure stress of bulk metallic glasses under very high strain rate

The dynamic uniaxial compressive behavior of Zr-based metallic glasses under a wide high strain rate was studied by a miniaturized split Hopkinson pressure bar, including high strain rate up to 10(4) s(-1). Experimental results indicate that the uniaxial compressive failure stress would decrease suddenly and then tend to hold steady with increasing strain rate. This phenomenon provides a generalized perspective for understanding the effect of local heat generation on the deformation of metallic glasses under dynamic loads.

Micro-nano scale ripples on metallic glass induced by laser pulse

A Zr47.7Cu31Ni9Al12.3 bulk metallic glass was irradiated directly by KrF excimer laser pulses with wavelength 248 nm and duration 10 ns. Scanning electronic microscope photographs indicated that many ripples in micro-nano scale would be generated on the edge of the irradiated area under the action of the higher intensity laser pulse. Detailed observation demonstrated that the ripples exhibited fluidity and became closer and closer out from interior. Theoretical analysis revealed the formation mechanism of the ripples, including melting, subsequent propagation of capillary waves and final solidification.

Mechanical behavior of a Zr-based metallic glass at elevated temperature under high strain rate

In current work, the mechanical behavior of a Zr-based bulk metallic glass at elevated temperatures (from 423 to 683 K) under high strain rate (from 5000 to 104 s−1) is investigated by an improved split Hopkinson pressure bar apparatus. Experimental results reveal that the failure stress goes down along with elevated temperature, while the strain rate dependency is relatively small. Furthermore, three different fracture modes are observed along with elevated temperature by scanning electron microscope (SEM). SEM observation results also indicate that the fractured specimen has been crystallized at temperature near Tg and the embrittlement caused by crystallization is attributed to the fracture mode changing.

Torsional buckling of multi-walled carbon nanotubes under combined axial and radial loadings

This paper investigates the torsional buckling of multi-walled carbon nanotubes under combined axial and radial loadings based on the continuum mechanics model. In particular, an explicit expression is obtained for the torsional buckling of double-walled carbon nanotubes (DWNTs) under combined loadings. Numerical results show that axial tensile stress or internal pressure will make DWNTs resist higher critical shear membrane force, while axial compressive stress or external pressure will lead to a lower critical shear membrane force. Further, for torsional buckling of DWNTs coupling with small axial stress and internal pressure or external pressure, the effect of the axial stress, internal pressure or external pressure on the critical shear membrane force is linear, and the associated buckling wave numbers are unique and the same as that under corresponding pure torque.

Dynamic buckling of double-walled carbon nanotubes under step axial load

An approximate method is presented in this paper for studying the dynamic buckling of double-walled carbon nanotubes (DWNTs) under step axial load. The analysis is based on the continuum mechanics model, which takes into account the van der Waals interaction between the outer and inner nanotubes. A buckling condition is derived, from which the critical buckling load and associated buckling mode can be determined. As examples, numerical results are worked out for DWNTs under fixed boundary conditions. It is shown that, due to the effect of van der Waals forces, the critical buckling load of a DWNT is enhanced when inserting an inner tube into a single-walled one. The paper indicates that the critical buckling load of DWNTs for dynamic buckling is higher than that for static buckling. The effect of the radii is also examined. In addition, some of the results are compared with the previous ones.

Vibration of multi-walled carbon nanotubes with initial axial force and radial pressure

A study is performed for the vibrational characteristics of multi-walled carbon nanotubes (MWCNTs) with initial axial force and radial pressure based on the Donnell model. In particular, explicit expressions are obtained for double-walled carbon nanotubes. Numerical results show that the intertube frequencies are insensitive to the initial axial stress (axial tensile stress or axial compressive stress), internal pressure or external pressure in MWCNTs, while the natural frequency is sensitive to the initial axial stress, internal pressure or external pressure. Further, the natural frequency increases with increasing axial tensile stress and decreases with increasing axial compressive stress. The frequencies increase with increasing internal pressure and decrease with increasing external pressure. The associated modes are insensitive to the initial axial stress, internal pressure or external pressure in MWCNTs.

Dynamic buckling behavior of multi-walled carbon nanotubes subjected to step axial loading

This paper studies the dynamic buckling behavior of multi-walled carbon nanotubes (MWNTs) subjected to step axial loading. A buckling condition is derived, and numerical results are presented for MWNTs under fixed boundary conditions. It is shown that the critical buckling load of MWNTs is of multi-branches and decreases as the time elongates. The associated buckling modes for different layers of MWNTs can be either in-phase or out of phase, which is related to the branch that the critical buckling load belongs to. For MWNTs with the same innermost tube radius, the critical buckling load is decreased when increasing the layers.