Propagation of shear bands in metallic glasses and transition from serrated to non-serrated plastic flow at low temperatures

The mechanical behavior and the kinetics of shear banding in bulk metallic glasses was investigated at room and liquid nitrogen temperature using the acoustic emission (AE) technique. It was demonstrated that the intensive AE reflecting the activity of strongly localized shear bands at room temperature vanishes at the transition of plastic flow from serrated to non-serrated with a reduction in temperature. The disappearance of AE clearly suggests that the shear band propagation velocity significantly decreases at low temperature, and sliding along the principle shear band is observed at the machine-driven rate.     

Enhanced mechanical properties due to structural changes induced by devitrification in Fe–Co–B–Si–Nb bulk metallic glass

Fe₃₆Co₃₆B_19.2Si_4.8Nb₄ bulk glassy rods were synthesized by copper mould casting. The effects of annealing treatments on the microstructure, elastic and mechanical properties of this alloy are investigated. Annealing below the glass transition temperature induces the formation of atomic clusters with pseudo-tenfold symmetry with a close relationship to the Fe₂₃B₆ phase. Annealing at sufficiently high temperatures promotes the formation of stable Fe₂B and FeB phases and Fe(Co) solid solution. The as-cast alloy exhibits ultra-high hardness (H > 14 GPa), high reduced Young’s modulus (E_r > 200 GPa) and good wear resistance. These properties are further enhanced after thermal treatments (H > 18 GPa and E_r > 260 GPa are achieved in the fully crystallized sample). The mechanical hardening is accompanied with an increase of the elastic recovery and a decrease of the Poisson’s ratio. The different microstructural mechanisms responsible for these annealing-induced changes in mechanical and elastic properties are discussed.     

Spark plasma sintering behavior of pure aluminum depending on various sintering temperatures

We have successfully fabricated high-density pure aluminum (Al) bulk by means of a spark-plasma-sintering (SPS) process. The relative density of Al was enhanced as the sintering temperature of the SPS process increased. During the SPS process for pure Al power, the Al oxide layer on the surface of the Al particle was partially broken by the microplasma and applied pressure. The microstructures of the spark-plasma-sintered compacts obtained at various temperatures were observed by optical microscopy, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy. We believe that the pinning effect, rapid heating cycle, and applied pressure played an important role in restraining the particle growth despite the increase in sintering temperature. It is feasible that the employed SPS process could be very useful to achieve fully densified Al compact.     

Oxidation of green rust suspensions containing different chromium ion species

The oxidation of hydrosulphate green rust (GR2(SO₄²⁻)) suspension containing different chromium ion species was investigated by X-ray diffraction, X-ray absorption spectroscopy and transmission electron microscopy. The pH, oxidation-reduction potential and amount of dissolved oxygen in aqueous solutions were measured during the reactions. The results show that the addition of Cr(III)₂(SO₄)₃ solution suppresses the transformation of GR2(SO₄²⁻) into iron oxyhydroxides and oxides in aqueous solution, while the addition of Na₂Cr(VI)O₄ solution promotes the transformation of GR2(SO₄²⁻) in which Cr(VI) is reduced to Cr(III); α-FeOOH particles were refined by the addition of the chromium ions.     

Evaluation of hydrogen absorption behaviour during acid etching for surface modification of commercial pure Ti, Ti-6Al-4V and Ni-Ti superelastic alloys

The hydrogen absorption behaviour during acid etching for the surface modification of commercial pure Ti, Ti-6Al-4V and Ni-Ti superelastic alloys has been investigated on the basis of the surface morphology, electrochemical behaviour and hydrogen thermal desorption analysis. To simulate the conventional acid etching for the improvement of the biocompatibility of Ti alloys, the specimens are immersed in 1 M HCl, 1 M H₂SO₄ or 0.5 M HCl + 0.5 M H₂SO₄ aqueous solution at 60 °C. Upon immersion, commercial pure Ti absorbs substantial amounts of hydrogen irrespective of the type of solution. In H₂SO₄ or HCl + H₂SO₄ solutions, the hydrogen absorption occurs for a short time (10 min). For Ti-6Al-4V alloy, no hydrogen absorption is observed in HCl solution, whereas hydrogen absorption occurs in other solutions. For Ni-Ti superelastic alloy, the amount of absorbed hydrogen is large, resulting in the pronounced degradation of the mechanical properties of the alloy even for an immersion time of 10 min, irrespective of the type of solution. The hydrogen absorption behaviour is not necessarily consistent with the morphologies of the surface subjected to corrosion and the shift of the corrosion potential. The hydrogen thermal desorption behaviour of commercial pure Ti and Ni-Ti superelastic alloy are sensitively changed by acid etching conditions. The present results suggest that the evaluation of hydrogen absorption is needed for each condition of acid etching, and that the conventional acid etching often leads to hydrogen embrittlement.     

Transverse Acoustic Impedance Measurements for Surface States of Superfluid 3He A1 and A2 Phases

We measured the complex transverse acoustic impedance in both superfluid ³He A₁ and A₂ phases. This impedance is sensitive to surface states. In our preliminary results, the temperature dependence of the impedance in the A₁ phase is similar to that in A phase, and the imaginary part shows an anomaly in the A₂ phase. These anomalies occurred at the temperature defined as T _k , which is lower as the frequency gets higher. The similar frequency dependence of T _k in each phase suggests that the anomaly is attributed to the same origin. The frequency dependence of the T _k /T _c indicates that the shape of the surface density of each spin pair state did not greatly change in the present experimental temperature range.     

Size-controlled recrystallization of fullerene by gas–antisolvent process

Gas–antisolvent (GAS) process using CO₂ successfully prepared crystals of fullerene (C₆₀) from a toluene solution through a precipitation. Particle sizes of precipitated C₆₀ could be easily controlled by changing the initial pressurization rate of CO₂, the temperature at the initial pressurization, and the quantity of C₆₀. Fullerene particles obtained by GAS method had regular octahedral shapes, whereas the particles obtained by adding liquid ethanol as an antisolvent to C₆₀ solution did not have such shape. Particle sizes could be changed in a range of 1.0–8.5 μm by GAS method; they were wider than the range of particles obtained by ethanol addition (0.7–1.8 μm).     

Growth of ZnO crystal on sapphire and nitridated sapphire substrates at 1000 °C by halide vapor phase epitaxy

Using a halide vapor phase epitaxy (HVPE) technique in which the starting materials are ZnCl₂ generated by the reaction between high purity Zn metal (7 N grade) and Cl₂ gas, and H₂O, ZnO crystals have been grown at a high temperature of 1000 °C on sapphire substrates with and without surface nitridation treatment. It was found that the nitridation treatment resulted in a change of the (112?0) sapphire surface to a (0001) AlN structure, leading to two possible sets of orientations for (0001) ZnO crystals. In addition, the nitridation treatment leads to a smaller average ZnO grain size and a higher density of nuclei.     

Novel agarose and agar fibers: Fabrication and characterization

In this study, natural agarose and agar polysaccharides were successfully developed to novel fibrous form for the first time via wet-spinning. Dimethyl sulfoxide (DMSO)/H₂O = 9:1(v/v) mixture was used as an appropriate solvent which was amenable to the wet-spinning process that produced continuous agarose and agar fibers in ethanol coagulation bath. Results of SEM investigation, swelling ratio and tensile test suggested that the smooth and homogeneous agarose fibers had considerable water swelling capacity (400-500%) and tensile strength (30-50 MPa). The agar fiber showed better water swelling capacity than the agarose fiber; however the existence of agaropectin leads to its flexibility flaws. These results demonstrate that the agarose fiber fabricated in this study is a good candidate material for wound-dressing applications.     

Roles of In-Situ Forming Hard Particles in a Zr-Based Bulk-Metallic Glass during Monotonic and Cyclic Loading

Zr₆₇Ni₂₀Cu₅Al₈ (in at. pct) is a bulk-metallic glass (BMG) with in-situ forming hard particles. Some crystalline particles can be observed in the polished beam samples under optical microscopy (OM). The hardness measurements show that these crystalline particles are harder than the matrix of Zr₆₇Ni₂₀Cu₅Al₈. The energy-dispersive spectroscopy (EDS) analyses demonstrate that there is little difference in the compositions between the crystalline particles and the matrix. The Zr₆₇Ni₂₀Cu₅Al₈ BMG under compressive loading exhibits yield strength of 1580 MPa with an up to 7.7 pct fracture strain. The fatigue behavior of Zr₆₇Ni₂₀Cu₅Al₈ is investigated under four-point-bending loading. The fatigue results show that the fatigue limit of Zr₆₇Ni₂₀Cu₅Al₈ is approximately 361 MPa, based on the stress range. It is generally found that the fatigue cracks initiate from the hard particles. The influence of the hard inclusions on the fracture and fatigue behavior of Zr-based BMGs is discussed.