Recent Developments in Ultrafine Shape Memory Alloys Using Amorphous Precursors.

In this review, we systematically reviewed the recent advances in the development of ultrafine shape memory alloys with unique shape memory effects and superelastic behavior using amorphous metallic materials. Its scientific contribution involves defining and expanding the range of fabrication methods for single-phase ultrafine/nanocrystalline alloys with multicomponent systems. In multicomponent amorphous alloys, the crystallization mechanism depends on the alloy composition and is a selectable factor in the alloy designing method, considering the thermodynamic and physical parameters of constituent elements. The crystallization kinetics can be controlled by modulating the annealing condition in a supercooled liquid state with consideration of the crystalline temperature of the amorphous alloys. The phase stability of austenite and martensite phases in ultrafine shape memory alloys developed from amorphous precursors is determined according to alloy composition and grain size, which strongly influence the shape memory effect and superelastic behavior. A methodological framework is subsequently suggested to develop the ultrafine shape memory alloys based on the systematic alloy designing method, which can be considered an important strategy for developing novel ultrafine/nanocrystalline shape memory alloys with excellent shape memory and superelastic effects.

Role of Cr Element in Highly Dense Passivation of Fe-Based Amorphous Alloy.

The effect of the Cr element on the corrosion behavior of as-spun Fe72-xCrxB19.2Si4.8Nb4 ribbons with x = 0, 7.2, 21.6, and 36 in 3.5% NaCl solution were investigated in this work. The results show that the glass formability of the alloys can be increased as Cr content (cCr) is added up to 21.6 at.%. When cCr reaches 36 at.%, some nanocrystals appear in the as-spun ribbon. With increasing cCr content, the corrosion resistances of as-spun Fe-based ribbons are continually improved as well as their hardness properties; during the polarization test, their passive film shows an increase first and then a decrease, with the highest pitting potential as cCr = 7.2 at.%, which is confirmed by an XPS test. The dense passivation film, composed of Cr2O3 and [CrOx(OH)3-2x, nH2O], can reduce the number of corrosion pits on the sample surface due to chloride corrosion and possibly be deteriorated by the overdosed CrFeB phase. This work can help us to design and prepare the highly corrosion-resistant Fe-based alloys.

Development of Precipitation-Strengthened Al0.8NbTiVM (M = Co, Ni) Light-Weight Refractory High-Entropy Alloys.

Single-phase solid-solution refractory high-entropy alloys (RHEAs) have been receiving significant attention due to their excellent mechanical properties and phase stability at elevated temperatures. Recently, many studies have been reported regarding the precipitation-enhanced alloy design strategy to further improve the mechanical properties of RHEAs at elevated temperatures. In this study, we attempted to develop precipitation-hardened light-weight RHEAs via addition of Ni or Co into Al0.8NbTiV HEA. The added elements were selected due to their smaller atomic radius and larger mixing enthalpy, which is known to stimulate the formation of precipitates. The addition of the Ni or Co leads to the formation of the sigma precipitates with homogeneous distribution. The formation and homogeneous distribution of sigma particles plays a critical role in improvement of yield strength. Furthermore, the Al0.8NbTiVM0.2 (M = Co, Ni) HEAs show excellent specific yield strength compared to single-phase AlNbTiV and NbTiVZr RHEA alloys and conventional Ni-based superalloy (Inconel 718) at elevated temperatures.

Thermally-triggered Dual In-situ Self-healing Metallic Materials.

The microstructural evolution and crack filling phenomena of (Al81Cu13Si6)100-x(Sn57Bi43) x (x = 0, 1, and 3 at.%) composites was investigated. The Sn and Bi elements were selected by considering the ability for liquid phase separation when combined with Al, Cu, and Si. Because of liquid phase separation, both Al-Cu-Si-rich L1 and Sn-Bi-rich L2 phases separately solidified at different temperatures yielding a trimodal eutectic structure in the cast alloys. The Sn and Bi elements have high mobilities due to the large interface of the eutectic microstructure and tend to strongly diffuse towards higher strained region during heat treatment. Furthermore, the mobile Sn and Bi elements in the Al-Cu-Si-based bimodal eutectic structure evidently fill cracks during warm rolling at 423 K. These results reveal that the developed alloy system has simultaneously dual self-healing characteristics, derived from the both precipitated Sn-Bi-rich particles and low melting agent, and the proposed alloy design based on liquid phase separation provides a novel strategy for creating self-crack filling metallic materials.

Experimental data of inorganic gel based smart window using silica sol-gel process.

In this article experimental data are presented for inorganic gel based smart window fabricated using silica sol-gel process. Parallel beam transmittances were measured as functions of voltages for samples fabricated with different concentrations of nitric acid. Spectroscopic transmittance data at different driving voltages for samples fabricated with different LC concentrations are shown. Transmittance spectra of the Si-Ti based gel-based-liquid-crystal (GDLC) device measured as different driving voltages were compared with those of PDLC. GDLC showed much lower operating voltages, 10-15 V, for on-state. Formation of the LC droplet in gelation process is illustrated. The methyl organic group surrounds LC droplets. Demonstration of GDLC based smart window showed the successful operation with low driving voltages. GDLC window shows clear color, even at off-state, compared with PDLC.

Characterization of Graphene-based FET Fabricated using a Shadow Mask.

To pattern electrical metal contacts, electron beam lithography or photolithography are commonly utilized, and these processes require polymer resists with solvents. During the patterning process the graphene surface is exposed to chemicals, and the residue on the graphene surface was unable to be completely removed by any method, causing the graphene layer to be contaminated. A lithography free method can overcome these residue problems. In this study, we use a micro-grid as a shadow mask to fabricate a graphene based field-effect-transistor (FET). Electrical measurements of the graphene based FET samples are carried out in air and vacuum. It is found that the Dirac peaks of the graphene devices on SiO2 or on hexagonal boron nitride (hBN) shift from a positive gate voltage region to a negative region as air pressure decreases. In particular, the Dirac peaks shift very rapidly when the pressure decreases from ~2 × 10(-3) Torr to ~5 × 10(-5) Torr within 5 minutes. These Dirac peak shifts are known as adsorption and desorption of environmental gases, but the shift amounts are considerably different depending on the fabrication process. The high gas sensitivity of the device fabricated by shadow mask is attributed to adsorption on the clean graphene surface.

Study of Graphene-based 2D-Heterostructure Device Fabricated by All-Dry Transfer Process.

We developed a technique for transferring graphene and hexagonal boron nitride (hBN) in dry conditions for fabrication of van der Waals heterostructures. The graphene layer was encapsulated between two hBN layers so that it was kept intact during fabrication of the device. For comparison, we also fabricated the devices containing graphene on SiO2/Si wafer and graphene on hBN. Electrical properties of the devices were investigated at room temperature. The mobility of the graphene on SiO2 devices and graphene on hBN devices were 15,000 and 37,000 cm(2) V(-1) s(-1), respectively, while the mobility of the sandwich structure device reached the highest value of ∼100,000 cm(2) V(-1) s(-1), at room temperature. The electrical measurements of the samples were carried out in air and vacuum environments. We found that the electrical properties of the encapsulated graphene devices remained at a similar level both in a vacuum and in air, whereas the properties of the graphene without encapsulation were influenced by the external environment.

Micro-to-nano-scale deformation mechanisms of a bimodal ultrafine eutectic composite.

The outstading mechanical properties of bimodal ultrafine eutectic composites (BUECs) containing length scale hierarchy in eutectic structure were demonstrated by using AFM observation of surface topography with quantitative height measurements and were interpreted in light of the details of the deformation mechanisms by three different interface modes. It is possible to develop a novel strain accommodated eutectic structure for triggering three different interface-controlled deformation modes; (I) rotational boundary mode, (II) accumulated interface mode and (III) individual interface mode. A strain accommodated microstructure characterized by the surface topology gives a hint to design a novel ultrafine eutectic alloys with excellent mechanical properties.

"Work-Hardenable" ductile bulk metallic glass.

Usually, monolithic bulk metallic glasses undergo inhomogeneous plastic deformation and exhibit poor ductility (< 1%) at room temperature. We present a new class of bulk metallic glass, which exhibits high strength of up to 2265 MPa together with extensive "work hardening" and large ductility of 18%. Significant increase in the flow stress was observed during deformation. The "work-hardening" capability and ductility of this class of metallic glass is attributed to a unique structure correlated with atomic-scale inhomogeneity, leading to an inherent capability of extensive shear band formation, interactions, and multiplication of shear bands.