Evolution Of Bulk Metallic Glass Research Articles

Microstructure evolution of bulk metallic glass during thermal oxidation under plastic strain

The microstructure evolution behavior of a Zr-based bulk metallic glass during thermal oxidation under plastic strain has been investigated. In the early stage of deformation, preexisting scale fractures in a brittle and intergranular way. Due to the concentrated strain field formed in front of crack tip, t→m martensitic transformation of t-ZrO2 takes place. With tensile strain increasing, multiple oxide regions consisting of t-ZrO2 dendrites form around fractures due to the combined action of oxidation, deformation and fracture. The formation and fracture of scales under large strains seriously deteriorate the surface finish and dimension accuracy of thermoplastic forming parts.

Nanoindentation study of the creep behavior in a Fe-based bulk metallic glass

In this paper, the creep behavior of a Fe-based bulk metallic glass (BMG) was studied by using the nanoindentation testing. According to the microstructural evolution in BMGs, the generalized Kelvin model was employed to describe the creep behavior of this BMG, and a method of separating the viscoelastic and viscoplastic deformation from the indentation creep was proposed. The creep behaviors at various loading rates were systematically analyzed. The obtained results indicated that the viscoelastic or viscoplastic characteristics of Fe-based BMG exhibited a strong dependency on the loading rate. Moreover, the volume of shear transformation zone (STZ) during the creep of Fe-based BMG has also been calculated to further explore its creep mechanism.

Effect of testing conditions on the compressive plasticity of bulk metallic glasses

Abstract

Numerical study of shear banding evolution in bulk metallic glass composites

In this article, a systematic simulation was performed to demonstrate the detailed shear banding evolution in bulk metallic glass (BMG) composites subjected to the tension, and the relation between microstructures and tensile ductility was therefore elucidated. Free volume was adopted as an internal state variable to characterize the shear banding nucleation, growth and coalescence in BMG matrix with the help of free volume theory, which was incorporated into ABAQUS finite element method (FEM) code as a user material subroutine. The present numerical method was firstly verified by comparing with the corresponding experimental results, and then parameter analyses were conducted to discuss the impacts of particle volume fraction, particle shape, particle orientation and particle yielding strength on the enhancement efficiency of the tensile ductility of BMG composites.

Structural evolution in bulk metallic glass under high-temperature tension

The tensile behavior of a Cu46Zr46Al8 bulk metallic glass (BMG) at elevated temperatures has been studied using in situ x-ray diffraction and molecular dynamics simulation. It is demonstrated that excess open volume is generated during elastic deformation and accumulated enough before plastic flow starts. The open volume almost keeps constant during homogeneous deformation, suggesting that a high content of open volume is a key point for developing BMGs with pronounced tensile plasticity.

Atomic structure evolution in bulk metallic glass under compressive stress

The structural behavior of Cu64.5Zr35.5 bulk metallic glass under compressive stress was investigated by means of in situ high energy x-ray synchrotron diffraction. The topological and chemical short-range order of the glass changes reversible upon loading within the elastic range. The number density of Cu–(Zr,Cu) and Zr–Zr nearest neighbor atomic pairs becomes oriented along the loading direction. The macroscopic stress state is reflected by the medium-range order. The determination of the components of the strain tensor from the shift of the positions of the nearest neighbor distances is not possible due to the structure changes.

Shear bands evolution in bulk metallic glass with extended plasticity

Shear band development and evolution of their spacing under bending in a Zr-based bulk metallic glass with extended plasticity has been monitored as a function of bending angle from the onset of plastic deformation to fracture. We find that sliding of existing shear bands is an important mechanism accounting for the plastic deformation of the plastic bulk metallic glass. The results are beneficial to understanding deformation in metallic glasses.

Oxidation induced phase instability of Cu 54Zr 22Ti 18Ni 6 bulk metallic glass

Effects of isothermal oxidation on the chemical instability which triggers crystallization of Cu 54Zr 22Ti 18Ni 6 (numbers indicate at.%) bulk metallic glass were investigated. Comparing the isothermal heat flux of Cu 54Zr 22Ti 18Ni 6 bulk metallic glass within the supercooled liquid region in differential scanning calorimetry, it could be observed that the isothermal oxidation did decrease the incubation time for crystallization. What is further, this amorphous phase instability was enhanced with the decrease of particle size. Through this study, chemical instability due to preferential oxidation of alloying element and resulting deviation of chemical composition in intact metallic region needs to be considered in phase evolution of bulk metallic glass during material processing.

Free Volume Evolution in Bulk Metallic Glass during High Temperature Creep

ABSTRACTMetallic glasses lack long-range translational symmetry and have excess volume trapped within their amorphous structure, which has a direct bearing on their physical properties including deformation characteristics. Moreover, the trapped excess free volume is directly correlated to the defect concentration facilitating the possibility to model the temperature and time dependence of the free volume changes during creep as a trade off between defect generation and annihilation. Using differential scanning calorimetry (DSC) analysis the residual free volume of a metallic glass can be characterised based on the glass transition peak height (Δcp). In the present work constant strain rate tests were carried out at the ‘onset’ (Tgon = 685 K) and ‘point of inflection’ (Tgp = 705 K) of the calorimetric glass transition to study the time dependent flow behaviour in Zr55Cu30Al10Ni5 bulk metallic glass. Modelling based on DSC analysis and positron lifetime spectroscopy on samples creep deformed to different plastic strain values corroborate the stress decrease after the peak stress (‘stress overshoot’) occurring in bulk metallic glasses with increasing plastic strain to be associated with a small increase in free volume.