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Investigation of shear bands under compressive testing for Zr-base bulk metallic glasses containing nanocrystals

Granular flow of nickel particles down a vertical pipe from a hopper is shown to be retarded by a horizontal ac electric field applied to a local region along the pipe. The particles are released from the hopper by pulling out a stopper in the hopper. Two sequences of experiments with different initial flow conditions are performed. In the first sequence, a dilute flow in the pipe is created after a fixed voltage V (less than or equal to4.8 kV) is applied across two short, vertical copper electrodes. The steady-state flow rate Q remains practically constant for V V-1, the flow becomes dense; Q decreases with a power law, Qsimilar toV(-1). In the second sequence of experiments, V is first set at 4.8 kV; the flow is allowed to start, and soon becomes a dense flow; then, V is reduced to the desired voltage. The new, steady-state Q vs V curve coincides with the previous Q(V) curve of the first sequence, except for V-2 V-2) to a dilute flow (V<V-2). Our results show that a large enough ac electric field can decrease the flow rate of a dilute or dense flow; the critical voltage that can reduce a dense flow, V-2, is less than that for the dilute flow, V-1

Microstructure and ductile–brittle transition of as-cast Zr-based bulk glass alloys under compressive testing

Abstract This paper investigates mechanical properties and fracture mechanisms of Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 alloys with various volume fractions of quenched-in crystalline. The alloys with various volume fractions of quenched-in crystalline were prepared by controlled oxygen content of alloys and overheating of the pouring. The phase structure, particle size and volume fraction of all samples were identified by X-ray diffraction, differential scanning calorimeter (DSC) curves and scanning electron microscopy (SEM) photographs. The mean sizes of crystalline increased from 0.3 to 1.3 μm with increasing volume fraction of crystalline from 4 to 13%. The compressive mechanical tests show a ductile–brittle transition with significant decrease in the fracture stress and ductility. Detailed observations in the flow deformation and fracture surface illustrate the relationship between the quenching-in crystalline and the mechanical behavior. The full bulk amorphous Zr-based alloy exhibits typical ductile deformation and fracture behavior. The torn shear bands form the typical vein patterns on the fracture surface. The effects of quenching-in crystalline on the flow deformation and fracture behavior depend on the nature, size, volume fraction and distribution. The particle size of the crystalline in the sense of the width of shear bands is critical. When the size is larger than the width of the shear bands the particles induce an obvious inhomogeneity of the flow deformation and more microcracks by the separation of the interfaces. Nano-scale particles, on the other hand, may increase the viscosity of the flow but do not form microcracks, resulting in particle strengthening of the metallic glass. Increasing the volume fraction of large-scale particles is favorable to leaking the microcracks and brittle fracture. With increasing particle size and volume fraction up to two times the width of the shear band and 10% vol., respectively, the ductile fracture of bulk amorphous alloy completely transforms to brittle fracture under compressive testing.

Investigation of phases on a Zr-based bulk glass alloy

Zr52.5Ni14.6Al10Cu17.9Ti5 alloy with oxygen content of 0.08 wt.% is cast into bulk sheet samples with different thicknesses. Fully glass and glass with quenched-in crystallites are obtained. With increasing the sheet thickness, the quenched-in crystallites increase in volumetric fraction and tend to form a net-shape phase. Transmission electron microscopy with energy-dispersive X-ray spectroscopy analysis indicates that the quenched-in crystallites, which are enriched in aluminum, have a well-faceted morphology with average size of about 500 nm, and have a fcc structure with cell size a=1.197 nm. In subsequent thermal treatment, the quenched-in crystallites are stable and do not act as nucleation and growth sites. For fully glass samples, different volumetric fraction of nanoscale crystals, with grain size of about 50 nm, form when annealed for 10 min at temperatures from 673 K up to 823 K. The crystallized phases are determined by selected-area diffraction to be ZrAl, Zr2Cu as well as Zr2(Ni0.67O0.33). High-temperatures (200 K above crystallization temperature) and short-time (30 s) annealing induce a nickel and aluminum-enriched fine nanoscale crystalline to form with grain size less than 20 nm.

Kinetics of crystallization process for bulk glass forming Zr-base alloys

Abstract The crystallization kinetics of Zr 65 Ni 10 Cu 17.5 Al 7.5 (alloy I) and Zr 52.5 Ni 14.6 Cu 17.9 Al 10 Ti 5 (alloy II) are investigated. Two-stage crystallization takes place during continuous heating of the glassy alloy I, resulting in the transformation of the glass to the metastable α-Zr-solid solution and Zr-base quasicrystals with an activation energy of 309 KJ/mol in the first-stage and the formation of Zr 2 Cu compound and the stable α-Zr-solid solution with an activation energy of 227 KJ/mol in the second-stage. For alloy II, one-stage crystallization with an activation energy of 333 KJ/mol occurs during continuous heating of the glass, resulting in the formation of Zr 3 Al and α-(Zr,Ti)-solid solution. Based on the DSC data and calculations, both the alloys go through with three stages of crystallization mechanism during isothermal annealing, i.e. (1) surface nucleation and growth, (2) three-dimensional nucleation and growth, and (3) crystal growth. The TEM observation on Zr 52.5 Ni 14.6 Cu 17.9 Al 10 Ti 5 alloy is in good agreement with the calculations.

Structures and properties of a Zr-based bulk glass alloy after annealing

Abstract A bulk glass Zr52.5Ni14.6Al10Cu17.9Ti5 alloy with 6 mm diameter is prepared by pre-melting sponge zirconium together with other pure metal elements and followed by injecting cast. In the samples, the content of oxygen is chemically analyzed in the level of 706 ppm (atomic concentration), which significantly affects the crystallization and the microstructure. When the bulk glass samples are annealed at the temperature far below the crystallization temperature(Tx), the predominant phases of Zr2Ni0.67O0.33 and Zr2Ni compounds crystallize and uniformly distribute on glass matrix. These predominant phases will grow and join together to form net-shape phase when the annealed temperature is in the range of Tx to above Tx. The glass matrix phase separated by the net-shape phase into the size of about 25 μm at 703 K to 15 μm at 823 K almost fully transforms into Zr2Ni and a small amount of Zr2Cu and Zr4Al3. At annealing temperatures far above Tx, Zr2Cu and Zr4Al3 compounds crystallize by phase separation to form nanostructure with nano-scale phases of Zr2Cu and Zr4Al3 compounds distributed on the matrix of Zr2Ni. The micro-compressive tests by Nanoindenter II reveal that the bulk glass phase has a lower elastic modulus and lower microhardness. Increasing the annealing temperature, the modulus and microhardness for the crystallized microstructure increase. With the phase separation taking place, the modulus and microhardness for the nanostructure are improved slightly. But the different deformational mechanism between micro-scale and bulk specimens is unknown.