Plasticity Enhancement of Composite and Nanoscaled Metallic Glasses

Under tensile loading at low temperature metallic glasses generally fail in a brittle manner. So far, significant tensile ductility has been observed if heterogeneities, such as crystalline secondary phases, are introduced into the glassy matrix or when reducing the size of metallic glass samples down the nanoscale regime. Here, we perform molecular dynamics simulations on Cu$_{64}$Zr$_{36}$ composite structures reinforced with B2 CuZr nanowire and investigate how the martensitic phase transformation and the distribution of these precipitates influence the mechanical deformation mechanisms. Additionally, we provide an atomistic understanding of the deformation mechanisms of metallic glass nanowires and differentiate the extrinsic size effects and aspect ratio contribution to plasticity

Deformation behavior of bulk and nanostructured metallic glasses studied via molecular dynamics simulations

In this study, we characterize the mechanical properties of Cu64Zr36 nanoglasses under tensile load by means of large-scale molecular dynamics simulations and compare the deformation behavior to the case of a homogeneous bulk glass. The simulations reveal that interfaces act as precursors for the formation of multiple shear bands. In contrast, a bulk metallic glass under uniaxial tension shows inhomogeneous plastic flow confined in one dominant shear band. The results suggest that controlling the microstructure of a nanoglass can pave the way for tuning the mechanical properties of glassy materials

Metallic glass nanolaminates with shape memory alloys

We model the deformation behavior of metallic amorphous Cu_64Zr_36/crystalline B2 CuZr nanolaminate systems using molecular-dynamics computer simulations. Amorphous-crystalline nanolaminates with shape memory alloys may be a material class which is combining the advantageous properties of metallic glasses with large-strain homogeneous flow at low temperatures and high stresses. We find that the deformation of the glassy and crystalline phases is a coupled process: martensitic transformation leads to shear band formation while the stress at the shear band tip induces martensitic transformation in the shape memory crystal. Moreover, the martensitic transformation changes the shear band morphology, stabilizes the shear flow and avoids a runaway instability. Finally, the critical volume fraction of the B2 layer for which the composite laminate shows a brittle-to-ductile transition is identified. The value of the critical volume fraction can be further decreased when the structure of the metallic glass is rejuvenated. Therefore, tailoring the architecture of metallic glass laminates with shape memory phases may allow the development of materials that exhibit large tensile ductility

From nanoglasses to bulk massive glasses

Molecular dynamics simulations are presented that provide evidence for the existence of diluted interfaces in nanoglasses, which is a class of material that can be synthesized by consolidating glassy nanoparticles. By comparing simulations of a covalently bonded Ge nanoglass and a metallic CuZr nanoglass, we show that the delocalization of the excess free volume initially located within the interfaces depends on the flow strain of the material. Our results suggest that the density distribution within a nanoglass can be controlled by the initial particle size and the annealing conditions. Therefore, nanoglasses represent an alternative route for controlling the properties of glassy materials

Atomic-scale origin of shear band multiplication in heterogeneous metallic glasses

Using molecular dynamics simulations, we provide an atomistic description of the shear band multiplication mechanisms in a heterogeneous metallic glass consisting of two distinct amorphous regions with different amounts of free volume and degrees of short-range order. The structural differences and elastic fluctuations encountered by the developing shear band while crossing the interface between the two regions alter the autocatalytic activation of the shear transformation zones (STZs) and, subsequently, lead to shear band branching. The two-unit STZ-vortex mechanism sheds light on the correlation between the strain distribution during tensile deformation and the directional characteristics of the STZ activation process

Modulating heterogeneity and plasticity in bulk metallic glasses: Role of interfaces on shear banding

Structural heterogeneities in monolithic metallic glasses, i.e. free volume variations or stress/strain fields, are known to be pivotal for their plasticity but the mechanisms how they affect irreversible deformation remain poorly understood. We show that flash-annealing is a capable tool for modifying the free volume content and its distribution, which creates non-affine stress fields in a monolithic CuZr-based bulk metallic glass. Rather than the overall free volume, μm-scale regions of relaxed and rejuvenated glass govern plastic deformation. Complementary molecular dynamics simulations underpin the importance of the interfaces between relaxed and rejuvenated volumes for shear band proliferation. Not only can we reach unmatched states of metastability in metallic glasses with the present approach but also better understand the mechanisms underlying plasticity in monolithic but structurally heterogeneous metallic glasses

Deformation behavior of designed dual-phase CuZr metallic glasses

A nanometer-scale second phase in metallic glass (MG) heterostructures is effective to improve mechanical properties. In this work, molecular dynamics simulations are conducted to investigate the influence of various critical structural aspects such as the size/volume fraction, distribution of a nanoscale secondary phase and different combinations of the matrix and the secondary phase on the deformation behavior of dual-phase MGs. We find an obvious change in deformation mode with varying the size/fraction and the chemical composition of the secondary phase. When the yield stress of the dual-phase MGs is lower than critical shear stresses required for forming a mature shear band (SB), the MGs show homogeneous deformation. Otherwise, those dual-phase MGs with inclusions smaller than width of the SB or that cannot confine plastic zones between the larger inclusions have high tendency for shear instability and brittle failure. By systematically varying the characteristics of the secondary phase one can design MGs with improved mechanical properties

High-resolution transmission electron microscopy investigation of diffusion in metallic glass multilayer films

Lack of plasticity is one of the main disadvantages of metallic glasses. One of the solutions to this problem can be composite materials. Diffusion bonding is promising for composite fabrication. In the present work the diffusion process in glassy multilayer films was investigated. A combination of advanced transmission electron microscopy (TEM) methods and precision sputtering techniques allows visualization and study of diffusion in amorphous metallic layers with high resolution. Multilayered films were obtained by radio frequency sputter deposition of Zr-Cu and Zr-Pd. The multilayers were annealed under a high vacuum (10−5 Pa) for 1 and 5 h at 400 °C, that is, well below the crystallization temperatures but very close to the glass-transition temperatures of both types of the glassy layer. The structural evolution in the deposited films was investigated by high-resolution transmission electron microscopy. It was observed that, despite the big differences in the atomic mass and size, Pd and Cu have similar diffusion coefficients. Surprisingly, 1 h of annealing results in formation of metastable copper nanocrystals in the Zr-Cu layers which, however, disappear after 5 h of annealing. This effect may be connected with nanovoid formation under a complex stress state evolving upon annealing, and is related to the exceptionally slow relaxation of the glassy layers sealed with a Ta overlayer