To gain insight into the large toughness variability observed between metallic glasses (MGs), we examine the origin of fracture toughness through bending experiments and molecular dynamics (MD) ngineering ceramics are strong, with high yield strength, but suffer from brittleness. In contrast, crystalline metals tend to have high fracture toughness because dislocation motion promotes plastic deformations that suppress cracks propagations, but concomitantly this dislocation motion reduces yield strength. Metallic glasses (MGs) tend to have high strength, and for some compositions, the high strength is accompanied by a high fracture toughness, making MGs promising engineering materials (1). The fracture toughness in MGs, which is accommodated by shear banding and limited by cavitation, is thought to arise from initiation of a crack opening at the core of an extending shear band (2-4). Then, new high-strength and high-toughness MGs may be designed by identifying compositions capable of suppressing cavitation during shear band extension. However, the complex physics of cavitation in MGs has obscured the development of models to illustrate cavitation's origin.For MGs, cavitation leads to the crack opening process that controls directly the fracture toughness, a fundamental property for material design and applications. Since the first amorphous alloy (Au 75 Si 25 ) reported at the California Institute of Technology in 1960 (5), tremendous effort has been dedicated to understand why the amorphous structure leads to such excellent mechanical properties as high elastic limit, yield strength, and hardness (2, 6). However, toughness, which varies dramatically between MG compositions, ranging from values typical of brittle ceramics to those typical of engineering metals (2, 6, 7), is still poorly understood. More recently, improved alloys have been developed that demonstrate very high toughness, including a bulk Pd-rich, Si-bearing glass, Pd 79 Ag 3.5 P 6 Si 9.5 Ge 2 (7), and a bulk Zr-rich, Cu/Al-bearing glass, Zr 61 Ti 2 Cu 25 Al 12 (8), in which shear band plasticity suppresses crack opening.The fracture resistance of MGs is understood to arise from a competition between two processes: shear band plasticity and void nucleation. Currently, the process of shear band plasticity is widely recognized to be accommodated by the cooperative shearing of local atomic clusters [shear transformation zones (STZs)] (9, 10). However, to describe the fracture process, a condition for cavitation is needed coupled with the description of shear band plasticity to account for a crack opening along an operating shear band. Recently, Rycroft and Bouchbinder (11) coupled a continuum STZ model with a condition for cavitation to describe the fracture of MGs. The authors found that cavitation plays an essential role in the initiation of fracture, where they found a crack to evolve by successive void nucleation events along an operating shear band. In the context of molecular dynamics (MD) simulations, we and others proposed that cavitation prec...

Metastable carbonates play important roles in geochemistry, biomineralization, and serve as model systems for non-classical theories of nucleation and growth. Balcite (Ca0.5Ba0.5CO3) is a remarkable high-temperature carbonate phase with barium concentrations far greater than the equilibrium solubility limit of calcite, but that can be synthesized in a metastable form at ambient conditions. Here, we investigate crystallization pathways in the Ba-Ca-CO3-H2O system, with a focus on the transformation of amorphous calcium barium carbonate (ACBC) to balcite over a range of barium concentrations, and subsequent decomposition into the equilibrium calcite (CaCO3) and witherite (BaCO3) phases. Density functional theory calculations show that balcite is metastable but accessible through the amorphous ACBC precursor for x ≲ 0.5, and predict its decomposition into calcite and witherite. We confirm this pathway experimentally, but found demixing to proceed slowly and remain incomplete even after nine months. Nucleation kinetics of balcite from ACBC were assessed using a microfluidic assay, where increasing barium content led to a surprising increase in nucleation rate, despite decreasing thermodynamic driving force. We attribute crystallization rates that dramatically accelerate with time to changes in interfacial structure and composition during coarsening of the amorphous precipitate. By carefully quantifying the thermodynamic and kinetic contributions in the multistep crystallization of a metastable carbonate, we produce insights that allow us to better interpret the formation and persistence of metastable minerals in natural and synthetic environments. ACBC and balcite (R3 & m) synthesisWe prepared amorphous calcium barium carbonate (ACBC, Ca1-xBaxCO3•nH2O where n ≅ 1.2 and 0 < x < 0.5) as described elsewhere.( 7) Bulk crystallization of ACBC was initiated by aging at ambient conditions for 24 hours in its mother liquor. The precipitate was then collected by vacuum filtration and dried at 110°C for 1 hour. To establish whether balcite is stable against decomposition into calcite and witherite, 1 g powdered balcite (x = 0.48) was equilibrated with 10 mL Nanopure water and continuously agitated in a rotating mixer at 30 rpm. Samples were removed after three, seven, and nine months, washed with water, and dried in a convection oven at 120ºC.

An accelerator beam can generate low energy electrons in the beam-pipe, generally called electron cloud, that can produce instabilities in a positively charged beam. One method of measuring the electron cloud density is by coupling microwaves into and out of the beam-pipe and observing the response of the microwaves to the presence of the electron cloud. In the original technique, microwaves are transmitted through a section of beam-pipe and a change in EC density produces a change in the phase of the transmitted signal. This paper describes a variation on this technique in which the beam-pipe is resonantly excited with microwaves and the electron cloud density calculated from the change that it produces in the resonant frequency of the beam-pipe. The resonant technique has the advantage that measurements can be localized to sections of beam-pipe that are a meter or less in length with a greatly improved signal to noise ratio.

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