https://dc.engconfintl.org/cgi/viewcontent.cgi?article=1032&context=superalloys_ii

A critical review of high entropy alloys and related concepts

Microstructures and properties of high-entropy alloys

High-entropy alloys (HEAs) are alloys with five or more principal elements. Due to the distinct design concept, these alloys often exhibit unusual properties. Thus, there has been significant interest in these materials, leading to an emerging yet exciting new field. This paper briefly reviews some critical aspects of HEAs, including core effects, phases and crystal structures, mechanical properties, high-temperature properties, structural stabilities, and corrosion behaviors. Current challenges and important future directions are also pointed out.

https://www.tandfonline.com/doi/pdf/10.1080/21663831.2014.912690

High-Entropy Alloys: A Critical Review

The factors of the atomic size difference Delta and the enthalpy of mixing ΔH mιx of the multi-component alloys were summarized from the literatures. The formation zones of solid-solution phases, intermediate phases, and bulk metallic glasses were determined and the validity was verified by experimental results. For forming the solid solution, the alloys should have high entropy of mixing, lower Delta, and not too negative and positive enthalpy of mixing.

Solid‐Solution Phase Formation Rules for Multi‐component Alloys

Alloying has long been used to confer desirable properties to materials. Typically, it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multi-dimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here, we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behaviour has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking. High-entropy alloys have greatly expanded the compositional space for alloy design. In this Review, the authors discuss model high-entropy alloys with interesting properties, the physical mechanisms responsible for their behaviour and fruitful ways to probe and discover new materials in the vast compositional space that remains to be explored.

High-entropy alloys

Alloys with composition of AlCoCrFeNiTix (x: molar ratio; x=0,0.5,1,1.5) were designed by using the strategy of equiatomic ratio and high entropy of mixing. The alloy system is composed mainly of body centered cubic solid solution and possesses excellent room-temperature compressive mechanical properties. Particularly for AlCoCrFeNiTi0.5 alloy, the yield stress, fracture strength, and plastic strain are as high as 2.26GPa, 3.14GPa, and 23.3%, respectively, which are superior to most of the high-strength alloys such as bulk metallic glasses.

Solid solution alloys of AlCoCrFeNiTix with excellent room-temperature mechanical properties

A gate voltage has been used to control quantum oscillations in Bi2Te3 nanoribbons and enhance the surface conduction.

/pdf/manipulating-surface-states-in-topological-insulator-piy6ytb4a5.pdf

Manipulating surface states in topological insulator nanoribbons

We report the first experimental demonstration of electrical spin injection, transport, and detection in bulk germanium (Ge). The nonlocal magnetoresistance (MR) in n-type Ge is observable up to 225 K. Our results indicate that the spin relaxation rate in the n-type Ge is closely related to the momentum scattering rate, which is consistent with the predicted Elliot-Yafet spin relaxation mechanism for Ge. The bias dependence of the nonlocal MR and the spin lifetime in n-type Ge is also investigated.

Electrical spin injection and transport in germanium

With the motivation of realizing an all graphene-based circuit for low power, we present a reliable nonvolatile graphene memory device, single-layer graphene (SLG) ferroelectric field-effect transistor (FFET). We demonstrate that exfoliated single-layer graphene can be optically visible on a ferroelectric lead-zirconate-titanate (PZT) substrate and observe a large memory window that is nearly equivalent to the hysteresis of the PZT at low operating voltages in a graphene FFET. In comparison to exfoliated graphene, FFETs fabricated with chemical vapor deposited (CVD) graphene exhibit enhanced stability through a bi-stable current state operation with long retention time. In addition, we suggest that the trapping/de-trapping of charge carriers in the interface states is responsible for the anti-hysteresis behavior in graphene FFET on PZT. V C 2011 American Institute of Physics. [doi:10.1063/1.3619816] Graphene is considered to be an exceptional material with high potential for future electronics, owing to its excellent electronic properties; 1 linear electron energy dispersion, and high room temperature mobility. If feasible, an all graphene-based circuit, including logic, analog, and memory devices, would be of great interest to further extend the performance of current Si-based electronics. Among various device applications, graphene based memory structures are still in their infancy in comparison to its logic and analog applications. To date, graphene memory has been demonstrated through chemical modification, 2 filament-type memristor, 3 nanomechanical switch, 4 and graphene FFETs. 5‐7 In graphene FFETs, however, the ambipolar conduction leads to undesirable on/off states for memory applications. Moreover, the absence of an electronic bandgap and controlled doping makes it difficult to resolve such issues. Therefore, a systematic study of graphene FFET is beneficial to realize graphene-based memory structures. In this Letter, we investigate graphene/PZT FFET structures using exfoliated- and CVD-SLG and their mechanism of operation. We show that exfoliated SLG can be optically identified on a PZT substrate and exhibit a hysteresis of the Vshaped conductance with a large memory window at low operating gate voltages. We compare exfoliated- with CVDSLG FFETs and show that devices made of CVD-SLG exhibit a robust bi-stable current state with a long retention time. In order to construct the SLG FFET, we first engineered a ferroelectric substrate to identify SLG. Previously, we have demonstrated that SLG is invisible under the optical micro

https://ir.nctu.edu.tw:443/bitstream/11536/21426/1/000293475500037.pdf

Robust bi-stable memory operation in single-layer graphene ferroelectric memory

Due to the strong Fermi-level pinning close to the germanium (Ge) valence band edge, all metal/p-type Ge contacts show Ohmic characteristics, while metal/n-type Ge contacts exhibit rectifying behaviors. In this paper, we report a simple method to alleviate this Fermi-level pinning effect by inserting a thin layer of aluminum oxide (Al2O3), formed by oxidation of aluminum (Al), between the metal/Ge interface. The effective Schottky barrier heights of nickel (Ni)∕n-type Ge, cobalt (Co)∕n-type Ge, and iron (Fe)∕n-type Ge decrease from 0.54, 0.62, and 0.61eV to 0.39, 0.23, and 0.18eV, respectively, with this thin layer of Al2O3. The tunneling oxide significantly suppresses the Fermi-level pinning, and yet does not restrict the current density. This method seems promising to realize low resistance metal contact to n-type Ge, which is essential to realize n-channel Ge complementary metal-oxide-semiconductor field-effect transistor with metal source and drain.

Yi Zhou

Papers

Solid solution alloys of AlCoCrFeNiTix with excellent room-temperature mechanical properties

Manipulating surface states in topological insulator nanoribbons

Electrical spin injection and transport in germanium

Robust bi-stable memory operation in single-layer graphene ferroelectric memory

Alleviation of Fermi-level pinning effect on metal/germanium interface by insertion of an ultrathin aluminum oxide