Eun-Seo Park

The effect of Sn addition on the glass-forming ability of Cu-Ti-Zr-Ni-Si metallic glass alloys

Abstract The effect of Sn substitution for Ni on the glass-forming ability was studied in Cu 47 Ti 33 Zr 11 Ni 8− x Sn x Si 1 ( x =0,2,4,6,8) alloys by using thermal analysis and X-ray diffractometry. With increasing x from 0 to 8, the glass transition temperature, T g , of melt-spun Cu 47 Ti 33 Zr 11 Ni 8− x Sn x Si 1 alloys increased gradually from 720 to 737 K. On the other hand, the crystallization temperature, T x , increased from 757 K at x =0 to 765 K at x =2, being nearly same with further increase of x . Partial substitution of Ni by Sn in Cu 47 Ti 33 Zr 11 Ni 8 Si 1 promotes the glass formation. Both amorphous Cu 47 Ti 33 Zr 11 Ni 8− x Sn x Si 1 alloys prepared by melt spinning and injection casting showed similar crystallization process during continuous heating in DSC. Temperature range of undercooled liquid region exhibits good correlation with the critical diameter for the formation of an amorphous phase in injection casting.

Correlation between fragility and glass-forming ability/plasticity in metallic glass-forming alloys

In the present study, the authors draw attention to the relationship among fragility index (m), glass-forming ability (GFA), and plasticity in various metallic glass-forming alloys (MGAs), and show that the m value is closely related to both characteristics. In particular, m can be formulated with ν (Poisson’s ratio): ν=−0.179+0.312logm, which means that high m as well as large ν values might be regarded as indicators of the MGAs ductility. As an example, it can be rationalized that the lowest m value (21.5 at 5K∕min) of Ca65Mg15Zn20 supports both outstanding GFA and extreme brittleness.

Induction of striatal neurogenesis enhances functional recovery in an adult animal model of neonatal hypoxic-ischemic brain injury

While intraventricular administration of epidermal growth factor (EGF) expands the proliferation of neural stem/progenitor cells in the subventricular zone (SVZ), overexpression of brain-derived neurotrophic factor (BDNF) is particularly effective in enhancing striatal neurogenesis. We assessed the induction of striatal neurogenesis and consequent functional recovery after chronic infusion of BDNF and EGF in an adult animal model of neonatal hypoxic-ischemic (HI) brain injury. Permanent brain damage was induced in CD-1 (ICR) mice (P7) by applying the ligation of unilateral carotid artery and hypoxic condition. At 6 weeks of age, the mice were randomly assigned to groups receiving a continuous 2-week infusion of one of the following treatments into the ventricle: BDNF, EGF, BDNF/EGF, or phosphate buffered saline (PBS). Two weeks after treatment, immunohistochemical analysis revealed an increase in the number of BrdU(+) cells in the SVZ and striata of BDNF/EGF-treated mice. The number of new neurons co-stained with BrdU and betaIII-tubulin was also significantly increased in the neostriata of BDNF/EGF-treated mice, compared with PBS group. In addition, the newly generated cells were expressed as migrating neuroblasts labeled with PSA-NCAM or doublecortin in the SVZ and the ventricular side of neostriata. The new striatal neurons were also differentiated as mature neurons co-labeled with BrdU(+)/NeuN(+). When evaluated post-surgical 8 weeks, BDNF/EGF-treated mice exhibited significantly longer rotarod latencies at constant speed (48 rpm) and under accelerating condition (4-80 rpm), relative to PBS and untreated controls. In the forelimb-use asymmetry test, BDNF/EGF-treated mice showed significant improvement in the use of the contralateral forelimb. In contrast, this BDNF/EGF-associated functional recovery was abolished in mice receiving a co-infusion of 2% cytosine-b-d-arabinofuranoside (Ara-C), a mitotic inhibitor. Induction of striatal neurogenesis by the intraventricular administration of BDNF and EGF promoted functional recovery in an adult animal model of neonatal HI brain injury. The effect of Ara-C to completely block functional recovery indicates that the effect may be the result of newly generated neurons. Therefore, this treatment may offer a promising strategy for the restoration of motor function for adults with cerebral palsy (CP).

Parameter for glass forming ability of ternary alloy systems

A parameter (σ parameter) for glass forming ability (GFA) of the ternary bulk metallic glasses is proposed, taking into consideration the relative thermodynamic stability and atomic configuration of the liquid phase. The σ parameter, defined by ΔT*×P′, combines the effects of melting temperature depression (ΔT*) and effective atomic size mismatch between constituting elements (P′). The parameter shows better correlation with GFA than previously proposed parameters. The relationship between σ and GFA can be expressed as Dmax=2.0×10−1exp(20.01σ), where Dmax is the maximum thickness for glass formation. Since σ can be calculated simply using data on melting temperature and atomic size, the GFA can be easily estimated using this parameter.

Correlation between plasticity and fragility in Mg-based bulk metallic glasses with modulated heterogeneity

A correlation between the kinetic fragility index (m) and enhanced plasticity is presented in Mg-based bulk metallic glasses (BMGs) with modulated heterogeneity by both suitable minor addition and optimal cooling rate. As case study, with minor addition of Gd in Mg65Cu7.5Ni7.5Zn5Ag5Y10−xGdx BMG, plasticity as well as glass-forming ability improves in a wide composition range (2.5≤x≤7.5 at. %). Furthermore, the cooling rate can affect the structural heterogeneity and the deformability of BMGs. With increasing diameters for the alloy x=5, the compressive fracture strain increases from 2.97% at d=1 mm up to 3.74% at d=2 mm and then largely decreases. In particular, the variation of heterogeneity, which can directly affect in the property of BMG, can be evaluated by comparing the m values reflecting the inherent structure of BMGs. This concept can be used as a means of guiding alloy design and processing to alleviate brittleness of BMGs.

Correlation between volumetric change and glass-forming ability of metallic glass-forming alloys

We draw attention to the relationship between volumetric change and glass-forming ability (GFA) in various metallic glass-forming alloys. The critical cooling rate can be expressed as Rc=2.5×10−5exp(5.0×102Vlg), where Vlg is defined as (Vl-Vg)∕Vl. We specifically plot the temperature dependence of specific volume for Ca65Mg15Zn20 alloy, and then demonstrate that the larger slope between Vl and Vg, closely related to thermal expansion coefficient, corresponds well to the GFA of the alloy. This insight can provide a clue if the correlations are to be used in the appreciation of glass formation and in the development of an alloy system with enhanced GFA.

Role of minor addition of metallic alloying elements in formation and properties of Cu–Ti-rich bulk metallic glasses

The effect of minor addition (MA) of metallic alloying elements in Cu–Ti-rich Cu–Ti–Zr–Ni–Si bulk metallic glasses (BMGs) has been investigated. MA of elements having a relatively small positive enthalpy of mixing (partial substitution of Zr with Nb) leads to enhancement of compressive plasticity (up to about 5% of fracture strain) when the addition leads to improvement in glass-forming ability (GFA). If the GFA is reduced (partial substitution of Ni with Ag or Co), the plasticity is also reduced. On the one hand, the MA of elements having a relatively large positive enthalpy of mixing (partial substitution of Zr with Y) can lead to the liquid-state phase separation in Cu–Ti–Zr–Ni–Si(–Sn) BMGs, although the addition can lead to drastic deterioration in GFA and plasticity. This concept would be considered to be effective even in design of other BMG systems with tailored properties.

Poisson’s ratio and fragility of bulk metallic glasses

The correlation among apparent global plasticity, Poisson’s ratio, and fragility in monolithic bulk metallic glass (BMG) alloys has been investigated in the present study. The shear and bulk moduli in monolithic Cu-based BMG alloys have been measured by resonant ultrasound spectroscopy (RUS) and ultrasonic technique. The Cu 43 Zr 43 Al 7 Ag 7 BMG alloy showing a large apparent global plasticity (∼8%) exhibits a high Poisson’s ratio when compared with that of Cu 43 Zr 43 Al 7 Be 7 BMG alloy. In addition, the fragility of Cu-based BMG alloys can be obtained by differential scanning calorimetry (DSC). The fragility index m of Cu 43 Zr 43 Al 7 Ag 7 BMG alloy is slightly larger than that of Cu 43 Zr 43 Al 7 Be 7 BMG alloy. The correlation between Poisson’s ratio and fragility in BMG alloys can be presented by a simple relation of m − 17 = 14 ( B ∞ / G ∞ − 1). Poisson’s ratio and fragility might be regarded as an important parameter that controls global plasticity of glass-forming alloys.

Orbit Determination of Spacecraft Using Global Positioning System Single-Frequency Measurement

The dynamic orbit determination of a low Earth orbiter using global positioning system single-frequency measurements has been implemented. Currently two methods are being applied to eliminate or reduce ionospheric path delay in single-frequency measurement. One is a group and phase ionosphere calibration technique using code pseudorange and L1 carrier phase, and the other is application of total electron content values from an ionospheric model using only L1 carrier phase to determine the orbit. A new method based on the latter has been developed, which estimates the scale factors of total electron content values in the location of a low Earth orbiter once per each measurement time. Orbit determination using actual global positioning system measurements of the TOPEX/POSEIDON and the Challenging minisatellite payload was conducted to verify the accuracy of the new method. It is verified that, if the total electron contents scale factor estimation technique were applied, 1-m level position accuracy (1σ) for low Earth orbit below 500-km altitudes could be achieved using precision orbit determination based on the global positioning system double-differencing method.

Quasicrystals and related approximant phases in Mg–Zn–Y

As-cast microstructure of Mg-rich Mg(68)Zn(28)Y(4) has been investigated by a detailed transmission electron microscopy study. The as-cast Mg(68)Zn(28)Y(4) alloy consisted of three different types of phases: 10-20 m size primary solidification phase, dendritic phase grown from the primary phase and a eutectic structure formed at the later stage of solidification. The primary solidification phase has an icosahedral structure with a large degree of phason strain. 1/1 rhombohedral approximant phase with lattice parameters a=27.2 A and =63.43 degrees is first observed in Mg-Zn-Y system. The rhombohedral structure can be obtained by introducing phason strain in the six-dimensional face centered hyper-cubic lattice. The decagonal phase nucleates with orientation relationship with the icosahedral phase, and Mg(4)Zn(7) nucleates with orientation relationship with the decagonal phase, indicating a close structural similarity between the three phases. Gradual depletion of Y during solidification plays an important role in heterogeneous nucleation of decagonal and Mg(4)Zn(7) phases from icosahedral and decagonal phases, respectively.