Influence of the Ag concentration on the medium-range order in a CuZrAlAg bulk metallic glass

Microstructure and Wear Properties of HVAF Sprayed Cu-Zr-Al-Ag-Co Amorphous Coatings at Different Spray Temperatures

Wear-resistant Cu-Zr-Al-Ag-Co amorphous coatings were fabricated by high-velocity air-fuel spray technology using (Cu43Zr47Al7Ag3)99.5Co0.5 powder at different temperatures (i.e., 645, 725, and 805 K). The feedstock powders (98.6 wt.% amorphous phase) were produced by a gas atomization method. Thermal properties and microstructure of the powders and the coatings were comparably investigated by differential scanning calorimeter, scanning electron microscope, and transmission electron microscopy techniques. Wear properties were studied by a dry sliding wear tester under the linear reciprocating sliding in a ball-on-plate mode using a GCr15 ball as the counterpart at room temperature in air. A large fraction of amorphous phase (~67.5 wt.%) and crystalline phases (ZrO2, Al2.5Cu0.5Zr, and AlZr3) are found in the coating fabricated at a temperature (725 K) between the glass transition temperature (Tg) and the onset crystallization temperature (Tx). In addition, the coating also exhibits the highest Vickers hardness (554 HV0.1), bonding strength (59.3 MPa), a relatively low porosity (1.65%), and superior wear resistance. The wear mechanism of the coating is primarily abrasive wear and slight adhesive wear.

Ag Surface and Bulk Segregations in Sputtered ZrCuAlNi Metallic Glass Thin Films

We report on the formation of Ag-containing ZrCuAlNi thin film metallic glass (nano)composites by a hybrid direct-current magnetron sputtering and high-power pulsed magnetron sputtering process. The effects of Ag content, substrate temperature and substrate bias potential on the phase formation and morphology of the nanocomposites were investigated. While applying a substrate bias potential did not strongly affect the morphological evolution of the films, the Ag content dictated the size and distribution of Ag surface segregations. The films deposited at low temperatures were characterized by strong surface segregations, formed by coalescence and Ostwald ripening, while the volume of the films remained featureless. At higher deposition temperature, elongated Ag segregations were observed in the bulk and a continuous Ag layer was formed at the surface as a result of thermally enhanced surface diffusion. While microstructural observations have allowed identifying both surface and bulk segregations, an indirect method for detecting the presence of Ag segregations is proposed, by measuring the electrical resistivity of the films.

Direct measurement of nanostructural change during in situ deformation of a bulk metallic glass

To date, there has not yet been a direct observation of the initiation and propagation of individual defects in metallic glasses during deformation at the nanoscale. Here, we show through a combination of in situ nanobeam electron diffraction and large-scale molecular dynamics simulations that we can directly observe changes to the local short to medium range atomic ordering during the formation of a shear band. We observe experimentally a spatially resolved reduction of order prior to shear banding due to increased strain. We compare this to molecular dynamics simulations, in which a similar reduction in local order is seen, and caused by shear transformation zone activation, providing direct experimental evidence for this proposed nucleation mechanism for shear bands in amorphous solids. Our observation serves as a link between the atomistic molecular dynamics simulation and the bulk mechanical properties, providing insight into how one could increase ductility in glassy materials.

Observing defect formation during bulk metallic glass deformation remains challenging. Here, the authors combine in situ nanobeam electron diffraction and large-scale molecular dynamics simulations to directly link changes to the local atomic ordering with shear band formation in a metallic glass.

Origin of large plasticity and multiscale effects in iron-based metallic glasses

The large plasticity observed in newly developed monolithic bulk metallic glasses under quasi-static compression raises a question about the contribution of atomic scale effects. Here, nanocrystals on the order of 1–1.5 nm in size are observed within an Fe-based bulk metallic glass using aberration-corrected high-resolution transmission electron microscopy (HRTEM). The accumulation of nanocrystals is linked to the presence of hard and soft zones, which is connected to the micro-scale hardness and elastic modulus confirmed by nanoindentation. Furthermore, we performed systematic simulations of HRTEM images at varying sample thicknesses, and established a theoretical model for the estimation of the shear transformation zone size. The findings suggest that the main mechanism behind the formation of softer regions are the homogenously dispersed nanocrystals, which are responsible for the start and stop mechanism of shear transformation zones and hence, play a key role in the enhancement of mechanical properties.

Iron-based bulk metallic glasses are remarkably plastic, but the origin of their plasticity remains challenging to isolate. Here, the authors use high resolution microscopy to show that nanocrystals are dispersed within the glass and form hard and soft zones that are responsible for enhancing ductility.

Elastostatic reversibility in thermally formed bulk metallic glasses: nanobeam diffraction fluctuation electron microscopy

The unparalleled shaping ability of bulk metallic glasses can revolutionize commercial products having multi-length scale features with a processing time of several minutes. Despite the widespread shaping ability of these polymer-like multicomponent alloys, thermoplastic forming (TPF) can severely degrade the intrinsic properties, particularly when complex stress states are activated. The present work emphasizes the importance of elastostatic loading (ESL) which not only fully reverses deteriorated room temperature plasticity originating under TPF or post-cryostatic conditions, but also activates a rejuvenation mechanism by rendering an extended resistance against strain softening. Furthermore, the reduction in the supercooled liquid region and crystallization enthalpy measured by differential scanning calorimetry are found to be temporary, and can be fully reversed to the initial condition. HRTEM imaging of the samples are performed with an imaging spherical aberration corrector. Individual nanobeam diffraction patterns obtained by the fluctuation electron microscopy (FEM) measurements are acquired using a scanning transmission electron microscope with a probe size of 1.2 nm from a 10 × 10 raster, yielding 100 diffraction patterns. The normalized variance of a series of nanodiffraction patterns of the post-elastostatically loaded sample reveals a height decrease in the first broad peak of normalized intensity variance V(k) suggesting modifications in the medium-range structural order which in turn dramatically restores the mechanical and thermal properties. Overall, the combination of TPF and post-ESL treatment in advanced glassy metals can open a new avenue for ultra-high mechanical and thermal performance micro- and nanomechanical devices for biosensors, MOSFETs and robotics.