D.H. Kim

Deformation behavior of Mg-Zn-Y alloys reinforced by icosahedral quasicrystalline particles

Abstract New Mg-rich Mg–Zn–Y alloys, reinforced by quasicrystalline particles, have been developed by thermomechanical processes. The deformation behavior of these alloys at room and elevated temperatures has been investigated. Yield strength of these alloys, which increases with an increase in the volume fraction of quasicrystalline particles, is relatively high due to their strengthening effect. The variation of the flow stress in the alloys is characterized by linking the microstructural evolution during deformation at high temperatures. The flow softening is related to dynamic recrystallization developed under the dislocation climb controlled creep; the flow hardening is related to grain growth that occurs under the grain boundary diffusion controlled creep. Quasicrystalline particles in the Mg–Zn–Y alloys resist coarsening due to their low interfacial energy, thereby forming of stable quasicrystalline particle/matrix interface and also prohibit against microstructural evolution of the α-Mg matrix during deformation at temperatures up to near the eutectic temperature. Stability of both quasicrystalline particles and matrix microstructure in the Mg–Zn–Y alloys provides large elongation to failure with no void formation at the quasicrystalline particle/matrix interface.

Application of quasicrystalline particles as a strengthening phase in Mg-Zn-Y alloys

Abstract Fine-grained magnesium alloys reinforced by quasicrystalline particles were easily developed by thermomechanical processes for as-cast Mg-rich Mg–Zn–Y and Mg–Zn–Y–Zr alloys. The deformation behavior of the alloys at room and high temperatures was investigated and compared with that of commercial AZ31, AZ61 and AZ91 alloys. The yield strength of the Mg–Zn–Y alloys, increasing with an increase of the volume fraction of the quasicrystalline phase, is relatively high due to the strengthening effect of the quasicrystalline particles. At high temperatures, the level of flow stress of the Mg–Zn–Y alloys is lower than that of commercial magnesium alloys due to the softness of the eutectic region, but the alloys exhibit much higher elongation since the large number of quasicrystalline particles in the Mg–Zn–Y alloys can effectively prohibit microstructural evolution of the α-Mg matrix during deformation. Icosahedral particles in the alloy are also stable against coarsening during deformation near the melting temperature of the eutectic due to their low interfacial energy, thereby forming a stable quasicrystalline particle/matrix interface. The stability of both the quasicrystalline particles and the microstructure of the Mg–Zn–Y alloys provides a large elongation with no void opening at the interface between the quasicrystalline particle and the α-Mg matrix.

Fabrication of ternary Mg–Cu–Gd bulk metallic glass with high glass-forming ability under air atmosphere

A new Mg 6 5 Cu 2 5 Gd 1 0 alloy having significantly improved glass-forming ability (GFA) has been developed. In this article, we show that the ternary Mg 6 5 Cu 2 5 Gd 1 0 bulk metallic glass with diameter of at least 8 mm can successfully be fabricated by a conventional Cu-mold casting method in air atmosphere. The critical cooling rate for glass formation was estimated on the order of magnitude of approximately 1 K/s. When compared with the GFA of Mg 6 5 Cu 2 5 Y 1 0 alloy, the significantly improved GFA of Mg 6 5 Cu 2 5 Gd 1 0 alloy cannot be explained by ΔT x and T r g values.

Mechanical behavior of a bulk Cu-Ti-Zr-Ni-Si-Sn metallic glass forming nano-crystal aggregate bands during deformation in the supercooled liquid region

Abstract The deformation behavior of a bulk Cu47Ti33Zr11Ni6Sn2Si1 metallic glass, fabricated by injection casting, has been characterized in the supercooled liquid region. The alloy deforms homogeneously and exhibits large elongation above the glass transition temperature at constant true strain rate below 1×10−2s−1, but it shows a variation of the flow stress during deformation. The flow stress reaches a peak just after yielding and then decreases significantly with increasing strain. After the plateau level of remarkably low flow stress, it rises again and then the alloy finally fails in a brittle manner. DSC data and TEM observations for the tested alloy reveal that the alloy evolves to being crystallized during deformation. Nano-crystals are aggregated and the aggregates are aligned along the load direction. When the volume fraction of the crystalline phase is in the range up to 0.5, the nano-crystal aggregates effectively slide over each other, lowering the apparent stress level. However, as the amount of the crystalline phase further increases, the flow stress continuously increases. This behavior can be explained based on the volume-fraction rule between the crystalline phase and the amorphous phase.

Ni-based bulk amorphous alloys in the Ni–Ti–Zr–(Si, Sn) system

New Ni-based bulk amorphous alloys in the alloy system Ni–Ti–Zr–(Si,Sn) were developed through systematic alloy design based upon the empirical rules for high glass forming alloys. Small additions of Si and/or Sn significantly improved the glass forming ability (GFA) of the alloys Ni 57 Ti 23− x Zr 20 (Si,Sn) x leading to a Ni-based bulk amorphous alloy. The amorphous ribbons of the alloys Ni 57 Ti 23− x Zr 20 (Si,Sn) x exhibited very high glass transition temperatures ( T g > 823 K), crystallization temperatures ( T x > 883 K), and large undercooled liquid regions (δ T x > 50 K) implying the high GFA of the alloys. Fully amorphous rods with the diameter of up to 2 mm can be fabricated by a copper mold casting method. Development of the new Ni-based bulk amorphous alloys having high T g, T x , and δ T x expands the practical applications of amorphous alloys as structural materials.

Design of Bulk metallic glasses with high glass forming ability and enhancement of plasticity in metallic glass matrix composites: A review

To overcome some of the limits of existing metallic alloys, a new alloy design concept has been introduced recently in order to control the crystallinity, i.e. to utilize crystalline, quasicrystalline, and amorphous structures. In particular, bulk metallic glasses (BMGs) receive great attention because of their unique properties due to their different atomic configuration. Recently, significant progress in enhancing glass forming ability (GFA) has led to the fabrication of BMGs having potential for application as structural and functional materials. Moreover, successful design of BMG matrix composite microstructure suggests that the plasticity of BMGs can be controlled properly. In this review article, we introduce recent research results on the design of BMGs with high GFA and on the enhancement of plasticity in metallic glass matrix composites.

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.

The effect of Ag addition on the glass-forming ability of Mg–Cu–Y metallic glass alloys

Abstract The effect of Ag substitution for Cu on the glass forming ability was studied in Mg 65 Cu 25− x Ag x Y 10 ( x =0,5,10,15,20,25) alloys by using thermal analysis and X-ray diffractometry (XRD). With increasing x from 0–25, the glass transition temperature, T g , of melt spun Mg 65 Cu 25− x Ag x Y 10 alloys increases slightly from 426 to 436 K, and the crystallization temperature, T x , decreases from 494 to 459 K, resulting in a decrease in the supercooled liquid region. However, a partial substitution of Cu by Ag in Mg 65 Cu 25 Y 10 promotes the glass formation. The maximum diameter for amorphous phase formation by injection casting increases from 4 mm in Mg 65 Cu 25 Y 10 to 6 mm in Mg 65 Cu 15 Ag 10 Y 10 alloy. Both amorphous Mg 65 Cu 15 Ag 10 Y 10 alloys prepared by melt spinning and injection casting showed similar crystallization process during continuous heating in differential scanning calorimetry (DSC). The relationship between the critical diameter for the formation of an amorphous phase in injection casting and the parameters reflecting glass-forming ability was examined.

Plasticity in Ni59Zr20Ti16Si2Sn3 metallic glass matrix composites containing brass fibers synthesized by warm extrusion of powders

Deformation behavior of centimeter-scale Ni-based metallic glass matrix composites reinforced by brass fibers, synthesized by warm extrusion of gas atomized powders, has been investigated under the uniaxial compression condition at room temperature. Throughout the extrusion process, all blended spherical powders are elongated along the extrusion direction. The brass fibers are well distributed in the metallic glass matrix for the metallic glass matrix composites containing the brass up to 0.4 in volume fraction and no pores are visible. With increasing the brass content, elastic modulus and strength decrease due to the softness of the brass, but enhanced macroscopic plasticity is observed due to the formation of multiple shear bands, initiated from the interface between brass fiber and metallic glass matrix, as well as their confinement between the brass fibers. These behaviors are not observed in the sample synthesized by warm extrusion of only metallic glass powders.

Classification of selective attention to auditory stimuli: Toward vision-free brain-computer interfacing

Brain-computer interface (BCI) is a developing, novel mode of communication for individuals with severe motor impairments or those who have no other options for communication aside from their brain signals. However, the majority of current BCI systems are based on visual stimuli or visual feedback, which may not be applicable for severe locked-in patients that have lost their eyesight or the ability to control their eye movements. In the present study, we investigated the feasibility of using auditory steady-state responses (ASSRs), elicited by selective attention to a specific sound source, as an electroencephalography (EEG)-based BCI paradigm. In our experiment, two pure tone burst trains with different beat frequencies (37 and 43 Hz) were generated simultaneously from two speakers located at different positions (left and right). Six participants were instructed to close their eyes and concentrate their attention on either auditory stimulus according to the instructions provided randomly through the speakers during the inter-stimulus interval. EEG signals were recorded at multiple electrodes mounted over the temporal, occipital, and parietal cortices. We then extracted feature vectors by combining spectral power densities evaluated at the two beat frequencies. Our experimental results showed high classification accuracies (64.67%, 30 commands/min, information transfer rate (ITR) = 1.89 bits/min; 74.00%, 12 commands/min, ITR = 2.08 bits/min; 82.00%, 6 commands/min, ITR = 1.92 bits/min; 84.33%, 3 commands/min, ITR = 1.12 bits/min; without any artifact rejection, inter-trial interval = 6s), enough to be used for a binary decision. Based on the suggested paradigm, we implemented a first online ASSR-based BCI system that demonstrated the possibility of materializing a totally vision-free BCI system.