Alloy Engineering Research Articles

Thermochemical surface hardening of Ti-6Al-4V: On the role of temperature and treatment media

The present work addresses the surface hardening and heat treatment response of Ti-6Al-4V grades G5 and G23 on thermochemical surface treatment in different temperature regimes as well as in different gaseous and plasma-based media. Grades G5 and G23 were subjected to gaseous surface hardening using various gas compositions and temperature regimes. Two different series of gaseous surface treatments were carried out: 1) Carbo-oxidizing of G23 in mill-annealed condition was performed in CO gas at (high) temperatures ranging from 777 to 1027 °C. Treatment at 1027 °C resulted in the deepest case with the formation of a thin surface layer of titanium sub-oxides supported by a thick TiC x O 1-x layer as the dominant phase. Post-nitriding of the carbo-oxidized specimen raised the hardness of the TiC x O 1-x phase by the incorporation of nitrogen, yielding values up to ~2900 HV and multi-layered structures. 2) Carbo-oxidizing of G5 with a bi-modal microstructure in CO/CO 2 gas mixture at (intermediate) temperatures in the range 677–777 °C. Treatment at 777 °C resulted in a surface hardness of ~1900 HV and a ~60 μm-thick diffusion zone. In addition to gaseous surface hardening, also plasma-assisted methods were applied and compared to their gaseous counterparts. Intermediate- and low-temperature plasma treatments were carried out on G23: 3) Plasma (carbo-nitro)oxidizing was performed in a CO 2 and N 2 gas mixture at 750 °C and (carbo-)oxidizing in CO 2 gas at 650 °C. 4) Plasma nitriding was conducted in a N 2 /H 2 gas mixture at 750 °C and 850 °C. The widely different types of treatments applied in this work represent the diversity of the gaseous and plasma-based thermochemical methods that can be applied for surface engineering of titanium alloys . • Surface hardening of titanium is feasible in widely different temperature regimes. • Hard δ-TiC x O 1-x requires a high a C to form at high temperatures. • High p O 2 maintains a high hardness value in the expanded α at lower temperatures. • Plasma nitriding reduces the required temperature for the formation of hard nitrides. • Temperature is the key factor for surface hardening using CO 2 .

Co-tuning composition and channel-rich structure of Ag-Pd alloys toward sensitive electrochemical biosensing

Noble-metal nanostructures are promising for biosensing applications owing to their fantastic physicochemical properties, but the rational design of their easy-accessible and reactive surface is still challenging. This work is devoted to the interfacial engineering of Ag-Pd alloys, including tuning alloy composition and geometric structure, to promote the electrochemical biosensing performance. Through a co-tuning tactic with controlled reduction, channel-rich Ag1Pd3 alloys with optimal electronic configurations and accessible active surface are successfully obtained, which afford high sensitivity, outstanding selectivity and excellent repeatability to detect H2O2 and glucose biomarkers. As experimentally and theoretically evidenced, tailoring the composition of Ag-Pd alloys ameliorates electronic configurations due to ligand effects, which can optimize the binding with the key intermediate of OH*, and thereby boost electrochemical kinetic steps on surface. Meanwhile, tuning geometric structure with abundant channels enables fast transport of analytes toward largely-exposed active sites. Providing an atomic-level interpretation of structure-dependent biosensing, this work is anticipated to offer guidelines for the development of efficient sensing platforms.

Effect of Surface Micro-Hardness Change in Multistep Machining on Friction and Wear Characteristics of Titanium Alloy

The machined surface quality, especially the micro-hardness of machined surface layers, is strongly correlated to the friction and wear characteristics of titanium alloy engineering parts. Therefore, to explore relationship of the local surface micro-hardness change in multistep machining and the surface wear resistance of the machined parts is urgently necessary. The machined surfaces were acquired through two-step (roughing and finishing) and three step (roughing, semi-finishing, and finishing) cylindrical turning experiments. The dry friction and wear tests were carried out by UMT-2 friction and wear tester on the multistep final machined surface along the feed direction. The surface wear microtopography and subsurface microstructure were observed and analyzed by scanning electron microscope. The micro-hardness variation in the local area of the finishing surface will cause the extension of unstable friction time stage while withstanding the cyclic and alternating contact stresses, and the soft–hard alternating area should be the sources of friction and wear defects, for instance cracks, peeling pits, fracture striations and even the wear fracture zone to crack propagation and peeling off. This will be of great significance to accurately control the machined surface quality and adaptively improve the surface wear resistance of titanium alloy components.

Developing a multilateral-based neural network model for engineering of high entropy amorphous alloys

Due to a great number of composition-processing factors, it is very difficult to design high entropy amorphous alloys without performing manifold trial-and-error experimentations. To solve this problem, in this study we developed a machine learning-based approach, namely multilateral-based neural network, which is able to predict new high entropy amorphous compositions through estimating the highest glass forming ability and the critical casting thickness. In this approach, the entropy parameters were individually correlated to each input, which leads to the improvement of predictive model in evaluating the high entropy glassy alloys. As a case study, Ti20Zr20Hf20Be20Co20 high entropy metallic glass (MG) was considered and the effects of added elements such as Y, Ni, Cr and V and Cu on the glass formation and critical casting thickness were investigated. According to the results, it is determined that the Y addition acts as a microalloying process in the base composition, while other elements improve the configurational entropy and the total negative heat of mixing, which lead to the engineering of equi-atomic high entropy MGs.

Twinning engineering of high-entropy alloys: An exercise in process optimization and modeling

In a bid to improve the mechanical properties of high-entropy alloys, particularly, their strain hardening capability, we adapted the time-proven concept of ‘twinning engineering’, developed in the context of TWIP steels, to twinning-assisted high-entropy materials. The strategy chosen involved a two-step thermomechanical processing that consisted of low-temperature pre-straining and subsequent annealing. This approach was trialled on CoCrFeMnNi as an exemplary high-entropy alloy. The annealing conditions selected ensured that the deformation twins generated under low-temperature deformation were retained, whilst the dislocation density was recovered. The viability of this strategy was convincingly confirmed for room temperature deformation of the alloy. A constitutive model accounting for the effect of the pre-straining induced deformation twins was proposed. It was shown to provide a reliable description of the low-temperature and room-temperature deformation behavior of CoCrFeMnNi when deformation twins are involved.

Experimental and Numerical Study of Combined High and Low Cycle Fatigue Performance of Low Alloy Steel and Engineering Application.

The fatigue behaviors of metals are different under different in-service environment and loading conditions. This study was devoted to investigating the combined effects of high and low cycle fatigue loads on the performance of the low alloy steel Q345. Three kinds of experiments were carried out, including the pure high cycle fatigue (HCF) tests, the pure low cycle fatigue (LCF) tests, and the combined high and low cycle fatigue (HLCF) tests. The prediction formulae were proposed for the combined high and low cycle fatigue failure. Scanning electron microscopy (SEM) and stereo microscope were used to analyze the microstructure and fracture morphology due to different fatigue loads. Case study on the combined high and low cycle fatigue damage of a steel arch bridge was carried out based on the FE method and the proposed formula. The results show that the LCF life decreases evidently due to the prior HCF damages. The HLCF fracture surface is relatively flat near the crack initiation side, and rugged at the other half part. The fatigue damages at the bridge joints increase significantly with consideration of the pre-fatigue damages caused by traffic load. In the 100th anniversary of service, the fatigue damage index without considering the HCF pre-damage is only about 50% of the coupled damage value.

Penta carbides: Two-dimensional group-IV semiconductors containing C2 dimers for nanoelectronics and photocatalytic water splitting

The discovery of novel materials with superior tailored properties is highly attractive for nano- and optoelectronics. Inspired by penta graphene, we introduce a novel family of two-dimensional group-IV materials with ${\mathrm{C}}_{2}$ dimers, namely penta carbides. The first-principles calculations reveal that the unitary, binary, and ternary penta carbides have excellent energetic, dynamical, thermal, and mechanical stability with remarkable properties. The unitary penta carbide displays auxetic behavior whereas the binary and ternary penta carbides posses near zero and positive Poisson's ratio. The phonon dispersion curves of penta carbides indicate a remarkable phononic gap, which can be tuned by alloy engineering and hydrogenation. They are semiconductor in nature with band-gap energy ranging from 1.35 and 2.39 eV. Alloying and strain engineering enable the direct modification of atomic bonding and thereby tuning of electronic band gap of penta carbides. Such novel tunable electronic and phononic properties of penta carbides can find applications in the field of nanoelectronics, sensors, and frequency filter applications. The band edge positions of penta carbides (except for Sn-based ones) straddle the redox potentials of water. Remarkably, penta carbides exhibit very high optical absorption in the visible and ultraviolet regions (up to ${10}^{\ensuremath{-}6} {\mathrm{cm}}^{\ensuremath{-}1}$). They have small and anisotropic carrier effective masses, indicating fast carrier transport characteristics and promoting the photo-generated electron-hole separation efficiency. The high specific surface areas, suitable and sizable band gaps, appropriate band edges, small effective carrier masses, and excellent optical absorption capability, all these exotic properties taken together, make penta carbides promising candidates for photocatalytic water splitting.

Large-scale MoS2(1−x)Se2x monolayers synthesized by confined-space CVD

Alloy engineering is efficient in modulating the electronic structure and physical and chemical properties of Transition metal dichalcogenides (TMDs). Here, we develop an efficient and simple confined-space CVD strategy by using a smaller quartz boat nested in a larger quartz boat for the preparation of ternary alloy MoS2(1−x)Se2x monolayers on SiO2/Si substrates with controllable composition. The effect of hydrogen ratio of the mixed carrier gas (Ar/H2) on the resultant flakes are systematically investigated. A hydrogon ratio of 15% is demonstrated to be the most appropriate to synthesize large size (more than 400 μm) single crystalline MoS2(1−x)Se2x alloy monolayers. The composition of the alloy can also be changed in a full range (2x = 0–2) by changing the weight ratio of Se and S powder. The as-grown monolayer MoS2(1−x)Se2x alloys present continuously high crystal quality in terms of Raman and PL measurements. Furthermore, to visible light (532 nm), the MoS2(1−x)Se2x based photodetectors display wonderful photoresponse with a fast response of less than 50 ms. Our work may be usedful in directing the synthesis of TMDs alloys as well as their optoelectronic applications.

Novel two-dimensional Group-IV carbides containing C2 dimers: Sizable direct band gap, high carrier mobility, and anisotropic properties for nanoelectronics

Exploring novel materials with superior properties is of a great significance for potential applications in future electronics and optoelectronics. Based on first-principles calculations, we predict new two-dimensional (2D) group-IV carbides containing isolated C2 dimers rather than individual carbon atoms, offering novel properties for nanoelectronics. The 2D carbides, namely th-XC2 (X = Si, Ge, Sn, and so on), are not only dynamically, mechanically, and thermally stable, but also exhibit exceptional properties such as anisotropic elasticity, ideal strength, and tunable phononic band gap. Moreover, they are all narrow direct band gap semiconductors with band gaps up to ∼0.9 eV, which can be continuously tuned by strain and alloy engineering. They show excellent transport properties including strong anisotropic and small electron/hole effective mass and ultrahigh carrier mobilities (up to ∼105cm2V−1s−1). Excitingly, the anisotropy of the hole effective mass can be rotated by 90∘ simply via alloying X element in tetrahexagonal carbon (th-C). Thus, the intrinsic carrier mobility in th-C is significantly enhanced by not only strain but also alloy engineering. In short, the 2D carbides are unique as composed of C2 dimers with novel properties making them promising for nanoelectronics.

Preface

The Institute of Materials Science and Engineering (Institut für Werkstoffwissenschaft und Werkstofftechnik, IWW) of Chemnitz University of Technology is proud to have hosted the Chemnitz Seminar on Materials Engineering – Werkstofftechnisches Kolloquium – for the 22nd time. Due to the long-lasting COVID-19 pandemic, this year’s conference was staged as a one-day online-live event in late March.Scientists and engineers, members of academia and industry, presented their most recent research results in 28 scientific talks, organized in two parallel webrooms.The topics of our – from now on biennial – meeting ranged from general challenges in materials and surface engineering to specific applications of transmission electron microscopy for analysis of materials’ surfaces. Further key sessions of this year’s event were focused on the engineering of high-entropy alloys, shape memory alloys, composite materials, additive manufacturing as well as material and process simulation.In addition to the scientific talks, interdisciplinary lectures were given on the key topic “Industry 4.0” (“4th industrial revolution”). Prof. Detlev Zühlke, who was the main initiator of the technology initiative SmartFactory KL e. V. and is now retired, spoke about the necessary framework to bring the industrial revolution from vision to reality. Dr. Franziska Bocklisch, commissionary head of the professorship General Psychology and Human Factors of Chemnitz University of Technology, highlighted the importance of transdisciplinary collaboration for successful implementation of human-technology cooperation.Following the encouraging trend of the last years, most of our contributors again decided to publish their work in this international, peer-reviewed journal. Consequently, this issue of IOP Conference Series: Materials Science and Engineering provides an excellent overview of the main topics of the Seminar.Chemnitz, March 2021List of Editors Conference photography are available in this pdf.

Alloy Engineering of a Polar (Si,Ge)2N2O System for Controllable Second Harmonic Performance.

Although silicon oxynitrides are important semiconductors for many practical applications, their second-order nonlinear optical (NLO) properties have never been systemically explored. Using the first-principles calculations, in this article, we discover that the sinoite (e.g., silicon oxynitride Si2N2O) can simultaneously exhibit wide optical band gap, strong second-harmonic generation (SHG) effect, and large birefringence, which are further confirmed by our preliminary experimental measurement. Importantly, we propose that alloying engineering can be further applied to control the balanced NLO properties in the Si2N2O system. Based on first-principles cluster expansion theory, we demonstrate that alloying Ge into Si2N2O can form low formation-energy Si2(1-x)Ge2xN2O alloys, which can in turn achieve controllable phase-matching harmonic output with high SHG efficiency at different energy ranges. Therefore, alloy engineering could provide a unique approach to effectively control the NLO performance of Si2(1-x)Ge2xN2O, making this polar alloy system hold potential applications in tunable laser conversion and controllable optical devices.

Band engineering in nitrogen-rich AlGaNAs quaternary alloys

New alloys from the III-V family are presented. Undoped (0001)-oriented AlGaNAs epitaxial layers with 3%–16% Al and with constant As concentration equals 0.6% were grown by molecular beam epitaxy. Photoelectron spectroscopy, and secondary ion mass spectrometry confirm As incorporation into AlGaN and show that a small amount of As causes valence band (VB) modification. The VB maximum is shifted towards the Fermi level compared to AlGaN. Calculations based on the band anticrossing model shows the band gap of the AlGaNAs is reduced relative to AlGaN but its magnitude is higher than for GaNAs with the same As content. It is also suggested that the VB maximum in (Al)GaNAs alloys in the whole Al concentration in the diluted regime of As concentration is at the same energy on the absolute scale in the first approximation. This study shows the new potential of As in AlGaN-based alloy engineering.

Modulation of electronic and magnetic properties of monolayer chromium trihalides by alloy and strain engineering

Monolayer CrI3 is a rare ferromagnetic semiconductor with intrinsic long-range magnetic order, which makes it a great potential material in spintronic devices [Song et al., Science 360, 1214 (2018)]. To extend the applications of monolayer CrI3 in flexible devices, the modulation of its electronic and magnetic properties is important. Here, we investigated the combined effect of strain and alloy on the properties of monolayer CrI3 by first-principles calculations. Br is chosen as the alloyed element due to the similar atomic configuration and property of CrX3 (X = Br, I), and the strain is applied by simultaneously changing the in-plane lattice constants (a and b). We find that the bandgap of monolayer Cr2I6−xBrx can be tuned greatly, while the magnetic moment of monolayer Cr2I6−xBrx is regulated very little under different strain and Br concentration. This unique property of monolayer Cr2I6−xBrx under strain makes it a good candidate for the flexible spintronic devices.

Atomistic study and continuum modeling of solute strengthening in slip-CTB interaction

The slip-coherent twin boundary (CTB) interaction, which is one of the most important issues determining the strength and ductility of the face-centered cubic metals, is investigated by molecular dynamics (MD) simulations on a bi-crystal model of Ni with interstitial Cr and Fe in CTB. Two interaction types, i.e., absorption and transmission, are observed, with prominent dependence on both the type and fraction of the solute. The threshold shear stress triggering the slip-CTB reaction event is quantitatively evaluated, and based on the local lattice friction, a new interpretation of the mechanism for solute affected slip-CTB interaction is proposed. A continuum model for solution affected slip-CTB interaction is established from a discrete dislocation scenario, including modified formulations in terms of the balanced condition of stress acting on the leading partial and elevated lattice friction, where the solute effect is effectively accounted and incorporated. Accordingly, the criterion to determine the nature of slip-CTB reaction is proposed, yielding good prediction of the critical events observed in MD simulations. This paper provides critical understanding and predictive tools for solute affected slip-CTB interaction in Ni, and offers new insights into alloy and microstructure engineering of Ni based superalloys with improved mechanical properties.

Validating quench stresses in complex aluminium components

This paper presents results from a numerical and experimental investigation motivated by the need to explore the effectiveness of residual stress relief techniques in aluminium alloy engineering components of complex geometry. Quenching is part of the heat treatment to establish mechanical properties. It can create high levels of residual stress in an engineering component. Finite element analysis (FEA) was used to predict the residual stresses generated by quenching and the location of peak residual stresses corresponding to probable in service failure sites. The residual strains and stresses were characterised using neutron diffraction techniques in components with high and low quench stresses to validate a FEA study, and also to appraise the evolution of a commercial stress relief method. An excellent correlation existed between the simulations and measurements.

Highly crystalline Mo1-xRexS2 monolayers by NaCl-assisted and space-confined chemical vapor deposition

Alloy engineering is of great importance in expanding the two-dimensional (2D) material family and modulating the bandgap of atomically thin 2D transition metal dichalcogenides (TMDs). Here, we report the synthesis of large area and high quality 2H Mo1-xRexS2 (x<=0.02) monolayer crystals by NaCl-assisted and space-confined chemical vapor deposition strategy. The combined effects of both common salt and confined space help promote the growth of Mo1-xRexS2 alloy monolayer crystals. The as-grown Mo1-xRexS2 alloy monolayers are uniformly distributed on the whole growth substrates with a domain size of up to 90 μm. Optical images, X-ray spectroscopy, Raman and photoluminescence spectroscopy measurements and corresponding mapping figures reflect the high quality and uniformity of the monolayers. The Mo1-xRexS2 alloy based phototransistor device exhibit good photoresponse to visible light with a response time of less than 0.2 s. Such alloy engineering of atomically thin 2D TMDs is useful for their future applications in optoelectronics.

Atomic arrangement matters: band-gap variation in composition-tunable (Ga1–xZnx)(N1–xOx) nanowires

<h2>Summary</h2> We synthesized single-crystal (Ga_1–xZn_x)(N_1–xO_x) nanowires with fully tunable compositions (0 < x < 1) using a customized chemical vapor deposition strategy. Despite the uniform distributions of component elements at the nanometer scale, X-ray absorption fine structure analysis in combination with <i>ab initio</i> multiple-scattering calculation verified the existence of a strong clustering tendency, i.e., the energetic preference of the valence-matched Ga-N and Zn-O pairs, in the synthesized nanowires. The strong clustering tendency plays a dominant role in determining the electronic band structures of the nanowires, causing a continuous band-gap reduction with increasing ZnO content, which is interpreted via a type II band alignment among the intracrystalline heterojunctions formed between the incorporated clusters and the host material. This, ultimately, makes the sample with the highest ZnO content show the highest water-splitting activity. Atomic arrangement engineering will provide an additional tool for band-gap engineering of semiconductor alloys, greatly benefiting the development of new functional materials for energy conversion applications.

Aqueous Solution Derived Amorphous Indium Doped Gallium Oxide Thin-Film Transistors

In this study, we report high-performance amorphous Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> metal-oxide (AMO) thin film transistor (TFT) using an low-temperature solution-process coupling with In alloy engineering. In doping can lower the activation temperature of gallium oxide and increase the oxygen vacancy concentration to further activate the device. The optical bandgap of IGO film can be changed from 5.3 to 4.25 eV with the In doping concentration (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In</sub> ) increasing from 0 % to 50 %. All TFTs with IGO channels exhibit n-type transistor characteristics and the evolution of their key electrical parameters with the In-dopant is well elucidated by the structural and morphological characterization. With the increase of C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In</sub> , the performance of the device becomes better. Finally, a saturation field-effect mobility of 3.63 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , a current on/off ratio of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> , and a threshold voltage of 2.5 V are achieved by the In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> O (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In</sub> = 50%) based device. The In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> O TFT also demonstrates good bias stress stability. Under the action of 20 V and -20 V gate bias for 3000 s, the ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> TH</sub> is +2.27 V and -1.95 V, respectively.

Alloy engineering in mixed Sn–Ge perovskites for photovoltaic application

The concerted action of alloying on both band edges and defect states leads to the most benign defect properties (SnI and GeI) in CsGe0.5Sn0.5I3, compared to the other components.

Twinning Engineering of High-Entropy Alloys: An Exercise in Process Optimization and Modeling

In a bid to improve the mechanical properties of high-entropy alloys, particularly their strain hardening capability, we adapted the time-proven concept of ‘twinning engineering’, developed in the context of TWIP steels, to this group of materials. The strategy chosen involved a two-step thermomechanical processing that consisted in low-temperature pre-straining and subsequent annealing. This approach was trialled on CoCrFeMnNi as an exemplary high-entropy alloy. The annealing conditions selected ensured that the deformation twins generated under low-temperature deformation were retained, whilst the dislocation density was recovered. The viability of this strategy was convincingly confirmed for room temperature deformation of the alloy. A constitutive model accounting for the effect of the pre-straining induced deformation twins was proposed. It was shown to provide a reliable description of the low-temperature and room-temperature deformation of CoCrFeMnNi.