Single Crystalline Alloys Research Articles

Growth of single crystalline GeSn alloy epilayer on Gd2O3/Si (111) engineered insulating substrate using RF sputtering and solid phase epitaxy

The article showcases a low-cost, low-temperature deposition andHVMtechnique to develop single crystalline GeSn alloy epilayers on Gd2O3/Si (111) substrate. First, GeSn alloy amorphous layer is deposited on the insulating substrates using an Radio Frequency (RF) sputtering apparatus. Subsequently, an inductively coupled plasma-assisted chemical vapor deposition (ICP-CVD) process is used to deposit a SiO2 capping layer to protect against Sn out-diffusion during heat treatment. The samples are then subjected to solid phase epitaxy (SPE) at 450 °C, 550 °C, and 650 °C. Sample processed for SPE at 450 °C has weak crystallinity and only shows Type-A stacking. Those processed for SPE at 550 °C and 650 °C, on the other hand, have revealed formation of the single-crystalline GeSn alloy epilayer with Type-A and Type-B stacking. However, SPE at 650 °C revealed tin out-diffusion and segregation effects. This work is significant for enabling the preparation of high-Sn-content GeSn alloy epilayers on insulating Gd2O3/Si (111) substrates, as it requires the initial deposition of a GeSn amorphous alloy epilayer using RF sputtering. This advancement promises benefits which includes advantages such as lower operating voltage, reduced leakage current, and minimized parasitic and short-channel effects, making it ideal for advancing RF technology.

Roles of lattice and grain boundary on hydrogen diffusion and trap behaviors in single-and poly-crystalline CrCoNi medium-entropy alloy

In this study, single-crystalline and poly-crystalline CrCoNi alloys are utilized as model systems to analyze the distinct roles of each GB and interstitial lattice sites. To effectively reveal hydrogen behavior, both electrochemical and gaseous hydrogen pre-charging methods are applied. Hydrogen content, diffusivity, and trap behaviors are quantified using thermal desorption analysis and hydrogen permeation tests, which determines (1) changes in hydrogen behavior depending on the presence of GB and (2) alterations in hydrogen behavior depending on lattice crystallographic orientation. The results indicate that GB and interstitial lattice sites exhibit comparable binding energies for hydrogen trapping. However, the introduction of GB alters the primary trapping sites from interstitial lattice sites to GB. In this case, the hydrogen content in the poly-crystalline alloy is determined by the trap site density of the primary trapping site. On the other hand, in the single-crystalline alloy, where only interstitial lattice sites exist, the crystallographic orientation of the hydrogen-charged plane is an important variable that determines hydrogen content and hydrogen diffusivity. Such insights contribute to a deeper understanding of hydrogen behavior within a more intricate microstructure, suggesting the alloy design approach to enhance resistance to HE.

Anomalies in the short-range local environment and atomic diffusion in single crystalline equiatomic CrMnFeCoNi high-entropy alloy

Multi-edge extended X-ray absorption fine structure (EXAFS) spectroscopy combined with reverse Monte Carlo (RMC) simulations was used to probe the details of element-specific local coordinations and component-dependent structure relaxations in single crystalline equiatomic CrMnFeCoNi high-entropy alloy as a function of the annealing temperature. Two representative states, namely a high-temperature state, created by annealing at 1373 K, and a low-temperature state, produced by long-term annealing at 993 K, were compared in detail. Specific features identified in atomic configurations of particular principal components indicate variations in the local environment distortions connected to different degrees of compositional disorder at the chosen representative temperatures. The detected changes provide new atomistic insights and correlate with the existence of kinks previously observed in the Arrhenius dependencies of component diffusion rates in the CrMnFeCoNi high-entropy alloy.

Thickness-Controlled Synthesis of Uniform Hexagonal Boron Nitride Films on Metal Alloys by Chemical Vapor Deposition

Two-dimensional (2D) multilayer hexagonal boron nitride (hBN) is a superior substrate for other 2D materials due to its insulating property, atomic-scale smoothness, and absence of dangling bonds, enhancing electrical or optical properties in high-performance 2D material-based devices. In most fabrication processes for 2D heterostructure devices, thick hBN flakes exfoliated from bulk have been used because of their high quality and simplicity in the process. However, exfoliated hBN flakes are limited in size and thickness control, impeding scale-up production for industrialization. Therefore, new synthesis strategies are required to overcome the drawback of the exfoliation method. In this work, we investigated the synthesis of large-area, multilayer hBN via atmospheric pressure chemical vapor deposition using single crystalline metal and single crystal alloy substrates. The synthesized results are characterized by optical microscopy, scanning electron microscopy, Raman spectroscopy, atomic force microscopy, etc. These studies provide a more comprehensive understanding of the growth mechanism of hexagonal boron nitride.

The importance of grain boundaries on the reactive elements effect in β-NiAlHf alloy

Cyclic oxidation of single crystalline and polycrystalline β-NiAlHf alloy was investigated at 1200 °C. The effect of grain boundary on the role of reactive elements was discussed. The results indicate that Hf effectively reduced the oxide scale growth rate and enhanced the scale adhesion in polycrystalline NiAl. But the single crystalline NiAlHf exhibits a higher oxide scale growth rate, severer internal oxidation and catastrophic scale spallation. The absence of the grain boundary leads to the negative effects of Hf.

Molecular Dynamics Simulation of NiTi Shape Memory Alloys Produced by Laser Powder Bed Fusion: Laser Parameters on Phase Transformation Behavior.

In this study, the deposition, powder spreading, and laser fusion processes during the laser powder bed fusion (L-PBF) process were studied using molecular dynamics (MD) simulation. The effect of Ni content on the characteristic phase transformation temperatures was also investigated. Shape memory effect and superelasticity of NiTi alloys with Ni content ranged from 48.0% to 51.0% were analyzed. By employing MEAM potentials, the effects of the laser power, spot diameter, and scanning speed on the molten pool size and element evaporation were studied. Simulation results showed that a larger spot diameter renders a higher Ni content in the molten pool, also a larger molten pool. A faster scanning speed leads to a higher Ni content in the molten pool, and a smaller molten pool. The element is difficult to evaporate using small laser power and a large spot diameter. The element in the molten pool expresses a great evaporation effect when the Es is larger than 0.4 eV/ų. According to Ni content within the molten pool during laser fusion, characteristic phase transition temperatures in single crystalline NiTi alloys with variant Ni content were investigated by employing a 2NN-MEAM potential. Characteristic phase transition temperature changes as the Ni content increases from 48.0% to 51.0%. Austenite boundaries and Ni content in the boundary were found to be the keys for controlling the characteristic phase transformation temperature.

Massive reorientations of bulk single and oligocrystals via solid state processing

Single crystalline materials have the potential to exhibit superior performance because they exclude grain boundaries, which increase susceptibility to creep, oxidation, and corrosion, and make thermal and electronic transport inefficient. However, single crystal properties vary significantly with crystallographic orientation, making the ability to control the orientation critical for their use in applications. The complex nature of crystal nucleation and growth processes makes such control challenging. Here we report a new crystal reorientation mechanism that results in abrupt and massive orientation changes in bulk single crystalline and oligocrystalline alloys via solid state thermal processing. We demonstrated this method in two alloy systems, FeMnAlNi and CuAlMn, and achieved repeated, massive orientation changes in the solid state. These findings offer a new strategy for manipulating the orientation of large single crystals on demand in order to take advantage of their superior and highly anisotropic properties.

Non-Hookean large elastic deformation in bulk crystalline metals

Crystalline metals can have large theoretical elastic strain limits. However, a macroscopic block of conventional crystalline metals practically suffers a very limited elastic deformation of <0.5% with a linear stress–strain relationship obeying Hooke’s law. Here, we report on the experimental observation of a large tensile elastic deformation with an elastic strain of >4.3% in a Cu-based single crystalline alloy at its bulk scale at room temperature. The large macroscopic elastic strain that originates from the reversible lattice strain of a single phase is demonstrated by in situ microstructure and neutron diffraction observations. Furthermore, the elastic reversible deformation, which is nonhysteretic and quasilinear, is associated with a pronounced elastic softening phenomenon. The increase in the stress gives rise to a reduced Young’s modulus, unlike the traditional Hooke’s law behaviour. The experimental discovery of a non-Hookean large elastic deformation offers the potential for the development of bulk crystalline metals as high-performance mechanical springs or for new applications via “elastic strain engineering.”

Improving the Effectiveness of the Solid-Solution-Strengthening Elements Mo, Re, Ru and W in Single-Crystalline Nickel-Based Superalloys

Tailoring the partitioning behavior of solid solution strengtheners is a crucial design strategy for advanced Ni-based superalloys. The goal of this study was to maximize the enrichment of Mo, Re, Ru and W in the γ-matrix phase. To determine the composition dependency of the partitioning behavior, a set of polycrystalline superalloys with varying contents of these solid solution-strengthening elements and a W-containing single-crystalline alloy series with varying concentrations of the γ′-forming elements Ta and Ti was produced. Assessed properties include phase compositions by electron-probe micro-analysis, phase transformation temperatures by differential scanning calorimetry and creep behavior. Re exhibits the most pronounced enrichment in the γ-matrix, followed by Mo, Ru and W. Due to the preference of the Al-sites in the γ′-phase in the order Ta &gt; Ti &gt; W &gt; Mo &gt; Re, the solid solution strengthening elements Mo, Re and W are displaced from the γ′-phase by increasing Ti and Ta contents. The investigated solutes do not directly influence the partitioning behavior of Ru as it prefers Ni-sites in the γ′-phase. Compressive creep experiments reveal a correlation between the content of solid solution strengtheners in the γ-phase and creep performance.

Polycrystalline Superalloy Membranes Produced by Load-Free Coarsening of Incoherent γ'-Precipitates: Microstructure Evolution and Mechanical Properties.

Nanoporous superalloy membranes are a functional extension of the use of nickel-based alloys. The material, which is usually used for high-temperature applications, consists mainly of the two phases γ and γ′. Through coarsening of the precipitates and thus forming of a bicontinuous γ/γ′ network, membranes can be produced by removing either of these phases. From the single-crystalline alloy CMSX-4, the bicontinuous network can be formed either thermo-mechanically by directional coarsening of coherent precipitates or by load-free coalescence of incoherent precipitates. Recent investigations have shown that membranes also can be produced from polycrystalline starting material in both ways. In this article, the process route for membranes by load-free coarsening of incoherent γ′ precipitates from a carbon-free version of the polycrystalline alloy Nimonic 115 is presented. This manufacturing method has the advantage of its simplicity and in comparison to single-crystalline membranes it can be realized in larger scales. We discuss the microstructure and show the mechanical properties by means of tensile tests. Despite the grain boundaries as a mechanical weak link, polycrystalline membranes show promising mechanical properties. Their strength even exceeds that of the single-crystalline membranes despite the significantly higher pore volume content.

The mechanisms of controlling FCT life and failure mode of Ni-based EBPVD YSZ thermal barrier coatings via the effects of both substrate elements and reactive element doping

Electron-beam physical vapor deposition (EBPVD) of NiCoCrAlY- and Hf-modified bond coats on (1) selected polycrystalline, directionally solidified, (2) single crystalline substrate alloys and (3) an uncoated NiCrAl bond-coat surrogate substrate, all of them covered with standard EBPVD YSZ topcoats were subjected to cyclic furnace testing (FCT) at 1100 °C. The lifetime and spallation failure upon FCT were evaluated. A typical mixed layer zone (MZ) of alumina and zirconia has formed during topcoat processing above the thermally growing oxide layer. The MZ was investigated by energy-dispersive X-ray spectroscopy after intermediate lifetimes and at the end of life. Chemical composition of the MZ and lifespan data were related to each other thus accounting for rate-determining reactions which could be assigned to either cation- or anion-controlled transport mechanisms. These provide a new approach to address FCT life and failure mode of even complex TBC systems containing reactive elements (e.g. Y and Hf). The cation-controlled processes are accelerated according to their concentration by tetravalent elements of the substrates, while the anion-controlled processes are unaffected by this and only adopt a cation-dominated mode when alloying elements of a low valence (e.g. Ti+) reach a supercritical concentration.

Single-crystalline GePb alloys formed by rapid thermal annealing-induced epitaxy

Single-crystalline GePb alloys have been successfully achieved by implanting Pb into Ge, followed by rapid thermal annealing under N2 atmosphere. The high crystallinity and the thickness of around 20 nm of the GePb alloys was determined by high-resolution x-ray diffraction and high-resolution transmission electron microscopy. The root-mean-square value of the as-implanted GePb sample is evaluated to be 1.25 nm in the 5 × 5 um scan area, which indicates a rather smooth surface. After being annealed at 400 °C, the result of a selected area electron diffraction pattern suggested that a single-crystalline alloy film was formed for the first time. The Pb composition of this sample is approximately 0.23% according to the x-ray photoelectron spectroscopy results. This value corresponds with the measurement result of the secondary ion mass spectroscopy. In addition, mobility enhancement in GePb/Ge samples has been identified by temperature-dependent Hall measurement, which indicates GePb alloys have great potential to be a promising high-mobility channel material for future monolithic optoelectronics integration application.

Preparation and Growth Mechanism of Pt/Cu Alloy Nanoparticles by Sputter Deposition onto a Liquid Polymer.

We use a green sputtering technique to deposit a Pt/Cu alloy target on liquid polyethylene glycol (PEG) to obtain well-dispersed and stable Pt29Cu71 alloy nanoparticles (NPs). The effects of sputtering current, rotation speed of the stirrer, sputtering time, sputtering period, and temperature of PEG on the particle size are studied systematically. Our key results demonstrate that the aggregation and growth of Pt/Cu alloy NPs occurred at the surface as well as inside the liquid polymer after the particles landed on the liquid surface. According to particle size analysis, a low sputtering current, high rotation speed for the stirrer, short sputtering period, and short sputtering time are found to be favorable for producing small-sized single crystalline alloy NPs. On the other hand, varying the temperature of the liquid PEG does not have any significant impact on the particle size. Thus, our findings shed light on controlling NP growth using the newly developed green sputtering deposition technique.

Microstructure and Mechanical Properties of CMSX-4 Single Crystals Prepared by Additive Manufacturing

Currently, additive manufacturing (AM) experiences significant attention in nearly all industrial sectors. AM is already well established in fields such as medicine or spare part production. Nevertheless, processing of high-performance nickel-based superalloys and especially single crystalline alloys such as CMSX-4® is challenging due to the difficulty of intense crack formation. Selective electron beam melting (SEBM) takes place at high process temperatures (~ 1000 °C) and under vacuum conditions. Current work has demonstrated processing of CMSX-4® without crack formation. In addition, by using appropriate AM scan strategies, even single crystals (SX SEBM CMSX-4®) develop directly from the powder bed. In this contribution, we investigate the mechanical properties of SX SEBM CMSX-4® prepared by SEBM in the as-built condition and after heat treatment. The focus is on hardness, strength, low cycle fatigue, and creep properties. These properties are compared with conventional cast and heat-treated material.

Mechanical behavior and solid solution strengthening model for face-centered cubic single crystalline and polycrystalline high-entropy alloys

In the present work, a solid solution strengthening effect in high-entropy alloys (HEAs) was studied by investigating mechanical characteristics of single crystalline CoCrFeMnNi HEA and a corresponding model was developed. (100) and (110) oriented single crystals of the CoCrFeMnNi alloy were grown and their single crystallinity was identified using X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD) analyses. The mechanical testing of the single crystalline CoCrFeMnNi alloy was performed and the critical resolved shear stress (CRSS) was obtained. A solid solution strengthening modeling based on the lattice friction stress and an intrinsic residual strain of HEAs was performed. The solid solution strengthening effect of HEAs was verified using the model proposed.

Effect of long processing time on microstructure and transport properties of Fe1.1Se alloy produced by chemical vapor transport (CVT) technique

In this work we present physical, electrical and magnetic properties of β-Fe1.1Se single crystalline alloys produced by chemical vapor transport technique using short (120 h) and long (700 h) heating processes. The best Tc was obtained from the long time processed samples. Magnetization results showed that samples are typical type-II superconductor. Calculated critical current density values, at zero field, were found to be 1.1 × 106 Acm−2 for long time processed samples at 2 K and indicates strong pinning mechanism and bulk superconducting nature. The zero temperature value of higher upper critical field was calculated to be Hc2(0) = 244 kOe. Calculated values of coherence length were found between 3.65 and 3.67 nm. Pauli paramagnetic limit, H P (0) = 1.84T c , is exceeded for both series of samples. Results obtained suggested that the spin-paramagnetic effect possibly the dominant pair-breaking mechanism for both series of samples.

Ni-Mn-Ga high temperature shape memory alloys: Function stability in β and β + γ regions

Record-breaking values of the tensile superelastic strain (about 12%) have been found previously in Ni-Mn-Ga single crystalline alloys at 400 °C which placed such materials ahead of known high temperature shape memory alloys (HTSMAs) promising in the automotive or aerospace industries operating in the range of 400–500 °C and above. This paper addresses two main issues that commonly affect Ni-Mn-Ga HTSMAs and limit their application, namely cycling stability of the transformation temperatures and thermomechanical actuation. These issues have been studied systematically by using six different Ni-Mn-Ga HTSMAs. The results show initial transformation temperatures up to 500 °C, which evolve, together with transformation strains, during more than 300 thermal cycles with and without mechanical loading. The specific evolution of a given sample depends on the microstructure, heat treatment prior to the cycling and whether the initial state of austenite is a single (β) or a dual phase (β + γ). The cycling protocol employed can be considered as an innovative training procedure to achieve the stabilization of the functionality and longer lifetime of these materials.

Proposal of fractographic analysis method coupled with EBSD and ECCI

Fracture surface contains key information to analyze the crack propagation behavior and identify the causes of fracture in post-mortem specimens/structural parts. For instance, fatigue crack propagation rate and the associated ΔK can be estimated from a fractographic feature, i.e., the striation spacings. However, the current fractography-based methods for the estimation of fatigue crack propagation rate and ΔK require the presence of striations. This requirement limits the capacity for the quantitative analysis of the fracture surface. Therefore, further advancement of fatigue fractography is required to facilitate the quantitative assessment of fracture, using post-mortem specimens/structural parts. In this study, we propose fractography coupled with microstructural evolution underneath the fracture surface. Microstructural characterization was performed, using electron backscattering diffraction (EBSD) and electron channeling contrast imaging (ECCI). In this study, we used a Fe-3Al bcc single crystalline alloy. EBSD-based grain reference orientation deviation analysis showed discrete plastic zones appearing along the crack propagation direction, with spacings corresponding to the crack propagation rate. Furthermore, it was confirmed via ECCI that underneath the fracture surface low- and high-ΔK regions showed vein-like and labyrinth structures, respectively. This information is expected to be useful for microstructure-based estimation of fatigue crack propagation rate and ΔK.

High damping capacity of single crystalline iron-palladium alloy exhibiting a weak first-order martensitic transformation

We have investigated the influence of twin boundary properties on the damping behavior of single crystalline Fe-31.2Pd (at%) alloy, which exhibits a weak first-order martensitic transformation. Different static tensile stress was applied by dynamic mechanical analyzer to adjust the twin boundary density and hydrogen was doped to modify the twin boundary mobility. A high damping capacity (tanδ > 0.1) in a wide temperature range from about 140 K to 230 K was observed under a low static stress of about 0.3 MPa. Hydrogen doping further increased the maximum value of the damping capacity by introducing a relaxation tanδ peak. The damping capacity decreased with increasing static stress probably due to the decrease in twin boundary density.

Topotactic reduction of layered double hydroxides for atomically thick two-dimensional non-noble-metal alloy

Layered double hydroxides (LDHs) have been widely used as catalysts owing to their tunable structure and atomic dispersion of high-valence metal ions; however, limited tunability of electronic structure and valence states have hindered further improvement in their catalytic performance. Herein, we reduced ultrathin LDH precursors in situ and topotactically converted them to atomically thick (~2 nm) two-dimensional (2D) multi-metallic, single crystalline alloy nanosheets with highly tunable metallic compositions. The as-obtained alloy nanosheets not only maintained the vertically aligned ultrathin 2D structure, but also inherited the atomic dispersion of the minor metallic compositions of the LDH precursors, even though the atomic percentage was higher than 20%, which is far beyond the reported percentages for single-atom dispersions (usually less than 0.1%). Besides, surface engineering of the alloy nanosheets can finely tune the surface electronic structure for catalytic applications. Such in situ topotactic conversion strategy has introduced a novel approach for atomically dispersed alloy nanostructures and reinforced the synthetic methodology for ultrathin 2D metal-based catalysts.