Bulk Metallic Glasses

Electrodeposition is a simple and economical method that can be employed for fabricating nanostructured surface coatings and bulk materials. This review presents applications of electrodeposition in the area of bulk metallic glasses (BMGs). BMGs are a key group of materials that have potential for diversified industrial applications; however, they suffer from poor plasticity. It has been shown that geometric confinement of shear bands through electrodeposited surface coatings enhances overall plasticity. The present review frames its objective in this direction. The review starts with a detailed introduction on BMGs and electrodeposition, and then presents various strategies employed to enhance plasticity. Reported works on electrodeposited surface coatings on BMGs for plasticity improvement are covered in Section 2 (reports on electroless coatings are also included). In Section 3 we deal with electrodeposition of BMGs. Potentials for future development are discussed in Section 4. In addition, recent interesting works reported at the interface of electrochemistry and BMGs are presented in Section 5.

The influence of Ag addition on the microstructure of rapidly quenched (Cu0.5Zr0.5)100xAgx melts was
investigated (x = 0–40 at.%). Fully glassy alloys were obtained for 0 6 x 6 20 at.% Ag, which are characterized
by a homogeneous microstructure without any indication of phase separation. For 30 6 x 6 40 at.%
Ag a composite structure is formed consisting of fcc-Ag nano-crystallites 5 nm in size and an amorphous
matrix phase Cu40Zr40Ag20. With higher Ag-content the volume fraction of the fcc-Ag phase becomes
increased mainly due to crytal growth during quenching. The primary formation of fcc-Ag for
30 6 x 6 40 at.% Ag is confirmed by the analysis of the microstructure of mold cast bulk samples which
were fully crystalline. From the experimental results we conclude that the miscibility gap of the liquid
phase of the ternary Ag–Cu–Zr system may occur only for x > 40 at.% Ag. For the bulk glass forming quaternary Cu40Zr40Al10Ag10 alloy a homogeneous element distribution is observed in accordance with the microstructure of ternary (Cu0.5Zr0.5)100xAgx glasses (x = 10, 20 at.%).

The influence of Ag addition on the microstructure of rapidly quenched (Cu0.5Zr0.5)100ÿxAgx melts was investigated (x = 0–40 at.%). Fully glassy alloys were obtained for 0 6 x 6 20 at.% Ag, which are characterized by a homogeneous microstructure without any indication of phase separation. For 30 6 x 6 40 at.%
Ag a composite structure is formed consisting of fcc-Ag nano-crystallites 5 nm in size and an amorphous matrix phase Cu40Zr40Ag20. With higher Ag-content the volume fraction of the fcc-Ag phase becomes increased mainly due to crytal growth during quenching. The primary formation of fcc-Ag for
30 6 x 6 40 at.% Ag is confirmed by the analysis of the microstructure of mold cast bulk samples which were fully crystalline. From the experimental results we conclude that the miscibility gap of the liquid phase of the ternary Ag–Cu–Zr system may occur only for x > 40 at.% Ag. For the bulk glass forming quaternary Cu40Zr40Al10Ag10 alloy a homogeneous element distribution is observed in accordance with the
microstructure of ternary (Cu0.5Zr0.5)100ÿxAgx glasses (x = 10, 20 at.%).

A Ti-based bulk metallic glass matrix composite (BMGMC) with a homogeneous distribution of dendrites and the composition of Ti46Zr20V12Cu5Be17 is characterized by a high tensile strength of ∼1640MPa and a large tensile strain of ∼15.5% at room temperature. The present BMGMC exhibits the largest tensile ductility and highest fracture absorption energy under the stress–strain curve of all dendrite-reinforced BMGMCs developed to date. Tensile deformation micromechanisms are explored through experimental visualization and theoretical analyses. After tension, fragmentation of the dendrites, rather than crystallization within the glass matrix and/or atom debonding near the interface of dual-phase composites, is responsible for the high tensile ductility. The subdivisions within the interior of dendrites are separated by shear bands and dense dislocation walls, and local separation of dendrites under modes I and II prevails. The multiplication of dislocations, severe lattice distortions, and even local amorphization dominate within the dendrites. Good structural coherency of the interface is demonstrated, despite being subjected to significant plastic deformation. Theoretical analyses reveal that the constitutive relations elastic–elastic, elastic–plastic, and plastic–plastic of dual-phase BMGMC generally correspond to the (1) elastic, (2) work-hardening, and (3) softening deformation stages, respectively. The capacity for work-hardening is highly dependent on the large plastic deformation of the dendrites and the high yield strength of the glass matrix. The present study provides a fundamental basis for designing work-hardening dual-phase BMGMCs exhibiting remarkably homogeneous deformation.

Quantitatively correlating the amorphous structure in metallic glasses (MGs) with their physical properties has been a long-sought goal. Here we introduce 'flexibility volume' as a universal indicator, to bridge the structural state the MG is in with its properties, on both atomic and macroscopic levels. The flexibility volume combines static atomic volume with dynamics information via atomic vibrations that probe local configurational space and interaction between neighbouring atoms. We demonstrate that flexibility volume is a physically appropriate parameter that can quantitatively predict the shear modulus, which is at the heart of many key properties of MGs. Moreover, the new parameter correlates strongly with atomic packing topology, and also with the activation energy for thermally activated relaxation and the propensity for stress-driven shear transformations. These correlations are expected to be robust across a very wide range of MG compositions, processing conditio...

The influence of alternating shear orientation and strain amplitude of cyclic loading on yielding in amorphous solids is investigated using molecular dynamics simulations. The model glass is represented via a binary mixture that was rapidly cooled well below the glass transition temperature and then subjected to oscillatory shear deformation. It was shown that periodic loading at strain amplitudes above the critical value first induces structural relaxation via irreversible displacements of clusters of atoms during a number of transient cycles, followed by an increase in potential energy due to the formation of a system-spanning shear band. Upon approaching the critical strain amplitude from above, the number of transient cycles required to reach the yielding transition increases. Interestingly, it was found that when the shear orientation is periodically alternated in two or three dimensions, the number of transient cycles is reduced but the critical strain amplitude remains the same as in the case of periodic shear along a single plane. After the yielding transition, the material outside the shear band continues strain-induced relaxation, except when the shear orientation is alternated in three dimensions and the glass is deformed along the shear band with the imposed strain amplitude every third cycle.

Micro-electrical discharge machining (micro-EDM) and grinding of Zr-based metallic glass (MG) were performed by using sintered polycrystalline diamond (PCD) as a hybrid tool. The microstructural changes of the workpiece surface layer were investigated. X-ray diffraction (XRD) patterns indicated that the ZrC phase and a few other unknown crystalline phases existed in the surface layer after micro-EDM. After shallow grinding, other new crystalline phases were detected on the surfaces. After deep grinding, however, the crystalline phases disappeared and an amorphous surface was obtained, the XRD and micro-Raman characteristics of which were similar to those of the as-cast MG surface. These results suggested that crystallization of MG in micro-EDM was hierarchical along the depth direction with different mechanisms. The results of surface morphology showed that grinding after micro-EDM not only effectively removed the crystallization layers to obtain an amorphous MG surface, but also improved the surface quality. The hybrid process provides a promising method to create three-dimensional micro features on a MG surface.

Quantitatively correlating the amorphous structure in metallic glasses (MGs) with their physical properties has been a long-sought goal. Here we introduce 'flexibility volume' as a universal indicator, to bridge the structural state the MG is in with its properties, on both atomic and macroscopic levels. The flexibility volume combines static atomic volume with dynamics information via atomic vibrations that probe local configurational space and interaction between neighbouring atoms. We demonstrate that flexibility volume is a physically appropriate parameter that can quantitatively predict the shear modulus, which is at the heart of many key properties of MGs. Moreover, the new parameter correlates strongly with atomic packing topology, and also with the activation energy for thermally activated relaxation and the propensity for stress-driven shear transformations. These correlations are expected to be robust across a very wide range of MG compositions, processing conditions and length scales.