Serrated flow is one important characteristic of shear bands through which metallic glasses (MGs) accommodate plastic deformation. Serrated flow can be affected by intrinsic properties such as elastic modulus or extrinsic variables such as strain rate. However, the influences of pre-deformation and interfaces on serrated flow are less well understood. In this study, by using in situ micropillar compression inside a scanning electron microscope, we show that pre-deformation (consisting of cyclic loading/unloading below the nominal elastic limit) suppresses serrated flows in amorphous-CuNb but enhances serrated flows in amorphous-CuZr at both high and low strain rates. Moreover, layer interfaces in Cu/amorphous-CuNb multilayers mitigate serrated flows, and the average stress drop and strain duration associated with shear banding process can be tailored. Strain accommodation and energy dissipation via shear banding have clear impact on serrated flows. This study provides new perspectives on tailoring serrated flows and enhancing plastic deformation of MGs.

A versatile method to fabricate taper-free micro-/nanopillars of large aspect ratio was developed with focused ion beam (FIB) cutting. The key features of the fabrication are a FIB with an incident angle of 90° to the long axis of the pillar that enables milling of the pillar sideways avoiding tapering and the FIB current can be reduced step by step so as to reduce possible radiation damage of the milled surface by Ga ions. A procedure to accurately determine the cross-section of each pillar was developed.

Great research efforts to investigate the glass-forming ability (GFA) in alloys have been made, leading to an observation that a pinpoint composition produces the best glass-forming characteristics. The reason for this observation is still unknown, limiting the development of bulk metallic glasses (MGs) with a relatively large size. In this work, systematic experimental measurements coupled with calculations were performed to address this issue using the NiNb binary alloy system. It is found that the atomic-level packing efficiency and the clusters-level regularity parameters strongly contribute to their GFA. In particular, the best glass former found in a pinpoint composition possesses the local maximum of the atomic-packing efficiency and the highest degree of the cluster regularity. This work provides an understanding of GFA from atomic and cluster levels and will shed light on the development of more MGs with relatively large critical casting sizes.

Decoupled growth often occurs in the nonfacetted–facetted eutectic systems. And it is generally considered that the nonfacetted solid solution acts as the leading phase in the decoupled growth. In this work, Fe40Ni40P14B6 eutectic alloys were systematically studied via solidification of undercooled melts and crystallization of amorphous alloys. Upon solidification of melts subjected to different undercoolings, as the undercooling increases, the growth mechanism develops from cooperative growth to decoupled growth. Upon crystallization of amorphous alloys, the partially crystallized sample consists only of strongly faulted intermetallic (Fe,Ni)3(P,B) with chemical composition deviating from stoichiometry. Formation of supersaturated solid solution γ(Fe, Ni) in the solidification and supersaturated intermetallic (Fe,Ni)3(P,B) in the amorphous crystallization indicates that decoupled growth results from solute trapping and disorder trapping in rapid growth of solid solution and intermetallic, respectively. Further application of rapidly quenched experiments and theoretical analysis declare that the decoupled growth results from a competition between the growth of γ(Fe, Ni) and (Fe,Ni)3(P,B), which are controlled by solute trapping and disorder trapping, respectively.

Bulk metallic glasses foams are under consideration as highly functional material, such as energy absorbers or ultra-lightweight parts, due to their porous structure combined with desirable strengths at relatively low densities. Also, the characteristics of high surface area of foams synthesized from metallic glasses make them viable candidates as emerging materials such as high-sensitivity sensors, catalysts or hydrogen storage media. In current study, bulk metallic glass foams were fabricated by extruding powder mixtures comprised of metallic glasses blended with various kinds of fugitive phases followed by dissolution of the fugitive phases in an aqueous chemical solution, for example HNO3, to yield the final porous structure. As a result of dissolution process, porous metallic glass having uniformly distributed micro-scale porosity was fabricated using a sequential synthesis technique beginning with composite particles comprised of composites of MG and a fugitive phase followed by consolidation of the composite powders via warm extrusion.

Development of metallic glasses is hindered by the difficulties in manufacturing bulk parts. We report on the preparation of glassy alloys using the spark plasma sintering of gas atomized powders. The Zr57Cu20Al10Ni8Ti5 alloy processed has a high glass forming ability. Densification is obtained at the glass transition temperature (390 °C) under high pressure (≈500 MPa). Systematic study of the effect of powder particles size and sintering time is performed through structural and thermal analyses. Local and partial devitrification of the amorphous alloy is detected, resulting from local temperature overshoots. From these analyses, an approach of the spark plasma sintering mechanism of metallic glasses is proposed.

We report variable resolution fluctuation electron microscopy (VRFEM) measurements on Cu64.5Zr35.5 metallic glass acquired using scanning transmission electron microscopy nanodiffraction using coherent probes 0.8 to 11 nm in diameter. The VRFEM results show that medium range atomic order structure of Cu64.5Zr35.5 bulk metallic glass at the ∼1 nm scale has large fluctuations, but the structure becomes almost completely homogeneous at the 11 nm scale. We show that our experimental VRFEM data are consistent with two different models, the pair persistent model and the amorphous/nanocrystal composite model. We also report a new way to filter VRFEM data to eliminate the effect of specimen thickness gradient using high-angle annular dark field images as references.

Mechanical properties and micro-plastic deformation behavior of five bulk metallic glasses (BMGs) were studied by instrumented indentation. These materials included La60Al10Ni10Cu20, Mg65Cu25Gd10, Zr52.5Al10Ni10Cu15Be12.5, Cu60Zr20Hf10Ti10, and Ni60Nb37Sn3 alloys. Remarkable difference in deformation behavior was found in the load–displacement curves of nanoindentation and pileup morphologies around the indents. Serrated plastic deformation depended on the loading rate was found in Mg-, Zr-, and Cu-based BMGs. The subsurface plastic deformation zone of typical alloys was investigated through bonded interface technique using depth-sensing microindentation. Large and widely spaced shear bands were observed in Mg-based BMG. The effect of loading rate on the indentation deformation behaviors in different BMGs was elucidated by the change of shear band pattern.

The addition of Ag to Cu–Zr alloys is very effective for the increase in the stability of supercooled liquid as well as the glass-forming ability (GFA). The large supercooled liquid region (ΔTx) exceeding 60 K in Cu–Zr–Ag ternary system was obtained in a wide range of 25–55 at.% Cu, 40–65 at.% Zr, and 5–25 at.% Ag. The best GFA was obtained around Cu45Zr45Ag10, and glassy alloy rods with diameters up to 6.0 mm were formed by copper mold casting. The bulk glassy alloys exhibit good mechanical properties, i.e., compressive fracture strength of 1780–1940 MPa, Young's modulus of 106–112 GPa, compressive plastic elongation of 0.2–2.9%, and Vickers hardness of 534–599. The finding of the new Cu–Zr–Ag ternary glassy alloy system with high GFA and good mechanical properties is important for development and scientific studies of bulk glassy alloys.

In this study, the rate-dependent mechanical behavior of a Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass was studied using quasi-static and dynamic shear-punch testing at room temperature. The results demonstrate that the shear strength of this alloy is insensitive to the applied strain rate. However, the formation of shear bands and the serrated flow exhibits a significant strain rate effect. The shear banding-induced fracture patterns and the fracture-melting phenomenon were analyzed based on the free volume theory and the energy transfer model.

Using depth-sensing indentation with sub-nanometer displacement resolution, the plastic deformation of a range of materials, including a metallic glass, amorphous selenium, Ni3Al, pure Nb, Al, Cu, and Zn metals, and an Al-Mg alloy, has been investigated at room temperature. In amorphous selenium, even the sub-nanometer displacement resolution of the nanoindentation technique cannot reveal any strain burst during deformation at room temperature. In all other metals studied, what may appear to be smooth load-displacement curves at macroscopic scale during indentation deformation in fact turn out to consist of a continuous series of random bursts of the nanometer scale. The occurrence probability of the bursts is found to decrease at increasing burst size. In all of the crystalline metals and alloys studied, the size distribution of the strain bursts seems to follow an exponential law with a characteristic length scale. The absence of the self-organized critical behavior is likely a result of the small size of the strained volume in the nanoindentation situation, which gives rise to a constraint of a characteristic strain. In the metallic glass sample, due to the limited range of the burst sizes encountered, whether the deformation bursts follow an exponential or a power-law behavior corresponding to self-organized criticality is inconclusive. From a theoretical viewpoint based on the Shannon entropy, the exponential distribution is the most likely distribution at a given mean burst size, and this is thought to be the reason for its occurrence in different materials.

The crystallization behavior of melt-spun amorphous Al92−xNi8Lax (x = 4 to 6) alloys was investigated by means of differential scanning calorimetry, x-ray diffractometry, and transmission electron microscopy. Crystallization kinetics were analyzed by Kissinger and Johnson–Mehl–Avrami approaches. Microhardness of all the ribbons was examined at different temperatures and correlated with the corresponding structural evolution. The results show that the variation of La content from Al88Ni8La4 to Al86Ni8La6 has significant influence on the crystallization pathways from amorphous to stable crystalline phases and on the evolution of microhardness with temperature. The two stages of crystallization in Al88Ni8La4 and Al87Ni8La5 alloys correspond to formation of fcc-Al and Al11La3, Al3Ni, Al3La. In Al86Ni8La6, three stages of crystallization are observed which correspond to formation of a metastable phase, fcc-Al, Al11La3, Al3Ni, and Al11La3, Al3Ni, Al3La, and decomposition of a metastable phases to stable crystalline phases.

The oxidation kinetics and the effects of alloying on the oxidation behaviors of copper-based bulk metallic glasses were studied. The oxidation kinetics, oxide compositions, and structures were investigated by thermogravimetric analysis (TGA), x-ray diffraction, x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. Both the TGA results and the XPS depth profile measurements showed that the oxidation resistance of Cu60Zr30Ti10 bulk metallic glass was improved by adding Hf, but it deteriorated when it was alloyed with Y. The oxide phases were found to be ZrO2, Cu2O, and CuO in samples heated at 573 K while an additional metallic Cu phase was detected in the ones heated at 773 K. A porous oxide structure was observed in the (Cu0.6Zr0.3Ti0.1)98Y2 metallic glass oxidized at 673 K, and the poor oxidation resistance of the alloy is attributed to the porous structure.

Nanoindentation tests of Pd40Cu30Ni10P20 bulk metallic glass were performed over a wide range of indentation rates (from 0.04 up to 6.4 mN s−1) under the standard load control mode. New results using the feedback displacement control mode are also presented. The dependence of the pop-in formation on the loading rate is investigated. Variations in hardness and reduced elastic modulus as a function of the indentation rate are observed. A softening effect occurs when increasing the loading rate. This is explained by the differences in plastic deformation achieved at different indentation rates. The displacement control mode was used to avoid the shear localization of the free volume, leading to the almost complete absence of pop-ins along the loading curve. The obtained results suggest that plastic flow in bulk metallic glasses is governed by the rate of creation of free volume, which depends on the strain rate and its localization into shear bands.

The compressive properties and the Vickers hardness of Cu-, Fe-, Mg-, and Zr-based monolithic bulk metallic glasses (BMGs) as well as Ti-based nanostructure-dendrite composites were investigated and compared. The monolithic BMGs exhibit nearly the same yield strength σ y and fracture strength σf but poor plasticity. The Vickers hardness HV of the monolithic BMGs follows the empirical relationship HV/3 ≈σy ≈σf. The Ti-based composites yield at a relatively low stress level (less than 850 MPa) but fail at a very high fracture stress (∼ 2 GPa) and exhibit a large strain hardening ability. Accordingly, the Vickers hardness HV of the Ti-based nanostructure-dendrite composites obeys the relationship σy <HV/3 <σf. Based on these results, the relationship between the Vickers hardness and the compressive properties of the investigated materials will be discussed by taking the yield and fracture strength (σ y and σf), the strain hardening exponent n, and the elastic and plastic energy stored upon deformation (δΕ and δP) into account.

Mg−Cu−Y alloys with optimal glass forming ability have been found at off-eutectic compositions. The critical size for bulk metallic glass formation at the pinpointed compositions more than doubles that of the previously discovered eutectic Mg65Cu25Y10 alloy, leading to fully glassy rods with near-centimeter diameters in the ternary system upon copper mold casting. The result is a striking demonstration of the strong composition dependence of the glass forming ability, as well as of the need to scrutinize off-eutectic compositions. The implications of the discovery are discussed.

W-rich particle-reinforced Ti-based bulk metallic glass (BMG) matrix composites with a compressive strength approaching 3 GPa and a fracture strain of approximately 12% were developed. In contrast to most existing BMG matrix composites, in which the improved ductility was obtained only at the expense of the strength, the composites developed in this study exhibited a significant enhancement in their strength, as well as an improvement in the plasticity. This improvement in the plasticity was attributed to the blocking and circumscription of the shear band propagation, leading to the formation of a large number of shear bands. Using a classical elasticity theory of inclusions, the improvement of the strength was interpreted as resulting from the generation of tensile residual stresses in the matrix due to the difference in the coefficient of thermal expansion between the W-rich particles and the BMG matrix.

The effect of Ag substitution for Cu on the glass forming ability (GFA) and mechanical properties of Mg65Cu25−xAgxGd10 (x = 0, 5, 10, 15, 20, 25) alloyswere investigated using x-ray diffractometry and differential scanning calorimetry.The partial substitution of Cu by Ag in Mg65Cu25Gd10 promoted the GFA. Mg65Cu20Ag5Gd10 bulk metallic glass (BMG) with a diameter of 11 mm could be fabricated by conventional copper-mold casting method in air atmosphere. The Mg65Cu20Ag5Gd10 BMG exhibits yielding and plastic deformation during compressive loading. The compressive fracture strength, total strain to failure, and plastic strain to failure of the Mg65Cu20Ag5Gd10 BMG were 909 MPa, 2.21% and 0.5%, respectively.

Compositional and microstructural differences from bottom to top along a water-quenched Zr41Ti14Cu12.5Ni10Be22.5 alloy rod, 90 mm in length and 22 mm in diameter, were investigated experimentally by x-ray diffraction measurement, differential scanning calorimetry, and composition analysis. The results show that the upper part of the rod contains more beryllium atoms and is amorphous. The lower part with less beryllium atoms contains crystalline phases. The composition gradient is possibly due to the gravity-driven transport of Be-rich clusters and un-melted tiny solid pieces in the alloy melt.

Interest in finding binary alloys that can form bulk metallic glasses has stimulated recent work on the Cu–Zr system, which is known to show glass formation over a wide composition range. This work focuses on copper mold casting of Cu50Zr50 (at.%), and it is shown that fully amorphous rods up to 2-mm diameter can be obtained. The primary intermetallic phase competing with glass formation on cooling is identified, and the glass-forming ability is interpreted in terms of a metastable eutectic involving this phase. Minor additions of aluminum increase the glass-forming ability: with addition of 4 at.% Al to Cu50Zr50, rods of at least 5-mm diameter can be cast fully amorphous. The improvement of glass-forming ability is related to suppression of the primary intermetallic phase.