Microstructural State Research Articles

Hydrogen storage behavior of TiFe alloy activated by different methods

TiFe activation for hydrogen uptake was conducted through different methods and ball milling with ethanol proved to be the most effective one. TiFe alloy after activation could absorb 1.2 wt% hydrogen at room temperature with absorption and desorption plateaus of 0.5 MPa and 0.2 MPa respectively. Investigation on microstructure and chemical state of TiFe sample after milled with ethanol suggested that the well spread metallic Ti and Fe elements helped hydrogen uptake and release. The activation of TiFe alloy by milling with ethanol was achieved at ambient conditions with ease successfully and possibly can be used for large scale production

Investigation of microstructural attributes of steel surfaces through magneto-optical Kerr effect

Magneto-optical Kerr effect (MOKE) microscopy is a versatile technique to image magnetic domains and evaluate the behavior of magnetic materials. However, MOKE has been rarely applied to the research of low magnetic steels with complex microstructure including micron-sized features. This study reveals the potential of implementing MOKE microscopy for analyzing the microstructure and stress state of martensitic steels with high carbide density (example conducted on AISI M2 steel grade). For the first time, the resulting MOKE domain images are used to analyze the size and morphology of martensite lath structures without chemical etching of the material's surface. Furthermore, MOKE microscopy allowed the confirmation and volumetric fraction estimation of retained austenite, despite the small size and volumetric fraction of under 1%. The validity of the MOKE microscopy analysis is cross-checked with scanning electron microscopy of etched samples and X-ray diffraction. Finally, MOKE microscopy in combination with Vickers indentation is applied to determine the residual stress state of the selected steel, which was varied by processing the steel material with conventional heat treatment scheme and heat treatment including deep cryogenic treatment (DCT). The method is proven effective in indicating the residual stress qualitatively. The measurements proved, that the residual stresses are significantly reduced in DCT steel samples, as theoretically anticipated. The conducted pioneering work provides important research towards the application of MOKE microscopy for the research of steel microstructure, properties and residual stresses as well as delivers a new perspective for determining the effectiveness of DCT on the steel properties.

A multivariate grain size and orientation distribution function: derivation from electron backscatter diffraction data and applications.

Two of the microstructural parameters most influential in the properties of polycrystalline materials are grain size and crystallographic texture. Although both properties have been extensively studied and there are a wide range of analysis tools available, they are generally considered independently, without taking into account the possible correlations between them. However, there are reasons to assume that grain size and orientation are correlated microstructural state variables, as they are the result of single microstructural formation mechanisms occurring during material processing. In this work, the grain size distribution and orientation distribution functions are combined in a single multivariate grain size orientation distribution function (GSODF). In addition to the derivation of the function, several examples of practical applications to low carbon steels are presented, in which it is shown how the GSODF can be used in the analysis of 2D and 3D electron backscatter diffraction data, as well as in the generation of representative volume elements for full-field models and as input in simulations using mean-field methods.

Modeling and simulation of backward combined harmonic generation induced by one-way mixing of longitudinal ultrasonic guided waves in a circular pipe

Nonlinear guided wave has been recognized as a potential means to characterize the state of material microstructures in solids. However, nonlinear guided wave approaches based on second harmonic generation or combined harmonic generation induced by counter-directional wave mixing, are applicable only under the condition that transmitters and receivers are placed at two different ends of the tested structures. These approaches are not effective for characterization of the structures with only one accessible end. In this paper, modeling of the backward combined harmonic at difference frequency induced by one-way mixing of two primary co-directional guided waves is investigated in a circular pipe, where the transmitters and receivers are placed at the same end of the pipe. The backward combined harmonic, generated at difference frequency and propagating in the direction opposite to that of two primary co-directional guided waves, is successfully observed numerically. A strong frequency mixing response characterized by a cumulative growth effect of the generated backward combined harmonic is demonstrated. The use of the generated backward combined harmonic for localized material degradation characterization and location is numerically examined in the given pipe. The obtained results indicate that the use of the backward combined harmonic can locate and characterize the localized material degradations in the given pipe by controlling the mixing zone of two primary co-directional guided waves. This study provides a promising means for characterization of localized degradations in pipes.

Synthesis of New Dendritic Titanium Catalysts and Catalytic Ethylene Polymerization.

The 1.0G dendrimer polyamidoamine (PAMAM), 3,5-dichlorosalicylaldehyde, and TiCl4·2THF were used as synthetic materials, and the dendritic salicylaldehyde imide ligand with substituent hindrance and its titanium catalyst were synthesized by the condensation reaction of Schiff base. The structure of the synthesized products was characterized by infrared spectroscopy, nuclear magnetic resonance hydrogen spectroscopy, ultraviolet spectroscopy, electrospray mass spectrometry, and inductively coupled plasma-mass spectrometry. Activated methylaluminoxane (MAO) was used as a catalyst precursor for ethylene polymerization in the process of ethylene catalytic. The effects of ethylene polymerization were studied in terms of the Al/Ti molar ratio, reaction time, reaction temperature, polymerization pressure, and ligand structure of the catalyst. The results show good catalytic performance (70.48 kg PE/mol Ti·h) for ethylene polymerization because of the existence of ortho substituent hindrance on the salicylaldehyde skeleton. Furthermore, high-temperature gel permeation chromatography (GPC)-IR, differential scanning calorimetry (DSC), and torque rheometer were used to characterize the microstructure, thermal properties, and viscoelastic state of the polyethylene samples obtained. The results showed that the product was ultrahigh-molecular-weight polyethylene.

Simultaneous increase in electrical conductivity and fatigue strength of Cu-Ni-Si alloy by utilizing discontinuous precipitates

Cu-6Ni-1.5Si alloy having fully fiber-shaped discontinuous precipitates was fabricated by overaging (DP specimen), and subsequently cold-rolled down to 80% reduction in area (CR-DP specimen). The tensile and fatigue strengths of DP and CR-DP specimens were compared to those of CP specimen with nano-sized continuous precipitates by normal aging. The CR-DP specimen showed the highest tensile and fatigue strengths among the specimens tested, and comparable electrical conductivity to the DP specimen. The reason for such an excellent combination of tensile strength, fatigue strength and electrical conductivity was discussed based on the change in microstructure and surface states during stressing.

Study of the influence of the oscillation frequency on the surface hardening process of the 42CrMo4 alloy using an oscillating laser beam

A study of the surface hardening process of the 42CrMo4 steel alloy by means of an oscillating laser beam has been carried out. For this purpose, a fiber coupled high power diode laser source and a galvanometric mirror scanner have been used. The study is focused on determining the influence of the oscillation frequency parameter on the temperature, geometry, microstructure, microhardness and stress state of the treated region, as well as its relationship with the key parameters of the laser hardening process. The results confirm that the surface hardening process of the 42CrMo4 alloy by means of an oscillating laser beam is an alternative technique to the conventional laser surface hardening methods based on fixed optical guidance systems. On the other hand, the selected frequency of oscillation has a significant influence on the treated region, mainly on the temperature, and, consequently, on the geometry, microstructure and stress state of the same. The selection of a high oscillation frequency, able to maintain the material temperature above that of austenization and below that of melting, is decisive to ensure the stability and uniformity of the treatment.

Superior electrochemical properties of Zirconium and Fluorine co‐doped Li1.20[Mn0.54Ni0.13Co0.13]O2 as cathode material for lithium ion batteries

In the paper, the Zr4+ and F− co-doping was employed to expectedly improve the rate performance and structure stability of Li1.20[Mn0.54Ni0.13Co0.13]O2. The effects of Zr4+ doping and F− doping on the micro-structure, cathode morphology and element chemical state were investigated by XRD, Rietveld refined XRD, SEM, TEM, and XPS. It indicated the Zr4+ and F− were inserted into the cathode crystal structure with ideally designed. Besides, the electrochemical characterization confirmed that the cathode after the Zr4+ or/and F− doping delivered the superior electrochemical properties when compared to the pristine one. Firstly, the Li1.20[Mn0.52Ni0.13Co0.13Zr0.02]O1.95F0.05 could express a high discharge capacity of 132.6 mAhg−1 at 5C rate, which was 40.1 mAhg−1 larger than that of the pristine one (92.5 mAhg−1). In addition, the Li1.20[Mn0.52Ni0.13Co0.13Zr0.02]O1.95F0.05 remained a capacity retention of 90.7% after 100 cycles at 2C rate and with the 148.2 mAh g−1 left. While the bare cathode delivered an initial discharge capacity of 134.1 mAhg−1 at 2C rate, then dramatically attenuated to 108.7 mAhg−1 after 100 cycles with the capacity retention of 81.1%. The enhanced electrochemical properties of Li1.20[Mn0.52Ni0.13Co0.13Zr0.02]O1.95F0.05 was mainly put down to the suppression of charge transfer resistance increase by the Zr4+ and F− doping modification along with cycles.

On the effect of the microstructure on the dynamic behaviour of Ti-6Al-4V

Aerospace components in jet engines need to outstand extreme conditions of high-temperatures and loads. Moreover, these components can sometimes undergo dynamic conditions if impact occurs during the flight. It is critical to understand the behaviour of aerospace alloys under these combined extreme conditions of high-temperature and dynamics loads. One of the most extended alloys used in the compressor and fan stages of commercial jet engines is Ti-6Al4V. The dynamic properties of Ti-6Al-4V are strongly dependent on the microstructure state and the temperature conditions. However, these dependencies are yet not fully understood. Moreover, this alloy can present a wide variety of microstructures depending on the component and manufacturing methods. In this work, we compare the response of five typical Ti-6Al-4V microstructures tested under static and dynamic conditions and different temperatures. The macroscopic response of the alloy is rationalised on the basis of its microstructural state using combined microscopy characterisation and computational modelling. To this end, computational plastic models are constructed and validated against experimental observations. In this way, the relationship between the mechanical properties of each microstructure and the temperature and strain rate conditions can be extracted to optimize the material state under specific dynamics in-service conditions.

Fatigue Analysis of Engineering Structures: State of Development and Achievement

Fatigue failure is a phenomenon that often occurs in mechanical structures, especially in components that receive direct cyclic loading, e.g., marine facilities, automobiles, critical infrastructure, reservoirs, turbines, nuclear reactors, and features that work in extreme conditions. Several factors can affect fatigue resistance, including applied material type, environment temperature, microstructure state, residual stress, corrosion, and crack initiation. Accurately estimating fatigue life is critical. A variety of variables must be taken into the calculation to reduce the risk of dangerous failure. In this paper, a series of development and achievement reviews on the relevant phenomena and advanced research related to fatigue assessment is conducted. Consideration of fatigue assessment methodology, e.g., laboratory experiment and numerical calculation, is discussed to summarize relevant effects to characteristics of stress life, strain life, frequency-based, and fracture mechanic approaches. The review also presents the relationship of previous research and its relevance to the development of the recent study. The purpose of this paper is to provide state-of-the-art investigations as well as demonstrate the challenges of uncharted problems.

Analysis of Phase Transformation and Mechanical Properties of 55CrMo Steel during Induction Hardening

Abstract To optimize the parameters of induction hardening and improve the surface hardness distribution of ball screw, the effects of the induction heating temperature on the microstructure and mechanical properties of 55CrMo steel were investigated. The specimens were heated up to 800°C, 850°C, 900°C, 950°C, 1,000°C, or 1,100°C by the induction heating power supply. Microstructural characterization of specimens was performed using an optical microscope and a high-resolution scanning electron microscope. The volume fraction of the retained austenite was measured using an X-ray diffractometer. The mechanical properties were evaluated by a microhardness tester and an electronic universal testing machine. The results show that the volume fraction of martensite in the hardened specimen increases with the increasing of heating temperature. A mixed microstructure consisting of ferrite + pearlite, bainite, and martensite is produced because of the nonfully and nonuniform austenitic state of original microstructure as the heating temperature is at 800°C. A full martensite can be produced as the heating temperature is more than 850°C. The volume fraction of retained austenite in the hardened specimen is lower when the heating temperature is in the range of 850°C–950°C; nonetheless, the volume fraction of retained austenite increases, and the tensile strength of the specimen reduces with the increasing of the heating temperature when the heating temperature is more than 950°C. The 55CrMo steel has good microstructure and mechanical properties and can meet the requirements of ball screw when the induction heating temperature is in the range of 850°C–950°C.

Topological photonic states in artificial microstructures [Invited

Topological photonics provides a new opportunity for the examination of novel topological properties of matter, in which the energy band theory and ideas in topology are utilized to manipulate the propagation of photons. Since the discovery of topological insulators in condensed matter, researchers have studied similar topological effects in photonics. Topological photonics can lead to materials that support the robust unidirectional propagation of light without back reflections. This ideal transport property is unprecedented in traditional optics and may lead to radical changes in integrated optical devices. In this review, we present the exciting developments of topological photonics and focus on several prominent milestones of topological phases in photonics, such as topological insulators, topological semimetals, and higher-order topological phases. We conclude with the prospect of novel topological effects and their applications in topological photonics.

Mechanochemical synthesis of novel rutile-type high entropy fluorides for electrocatalysis

Novel high entropy ceramics, high entropy fluorides, are introduced. The microstructure and solid solution state of the material are thoroughly characterized. Additionally, promising catalytic activity for oxygen evolution reaction was observed.

Synthesis and ethylene polymerization reaction of dendritic titanium catalysts

ABSTRACT The 1.0 G dendrimer (C22H48N10O4),3,5-di-tert-butylsalicylaldehyde and TiCl4 · 2THF were used as the synthetic materials, and the dendritic salicylaldehyde imide ligand with substituent hindrance and its titanium catalyst were synthesized by the condensation reaction of schiff base. The structure of the synthesized products was characterized by infrared spectroscopy, nuclear magnetic resonance hydrogen spectroscopy, ultraviolet spectroscopy, electrospray mass spectrometry, and inductively coupled plasma mass spectrometry, The actual structure is consistent with the theoretical design structure. Activated methylaluminoxane (MAO) was used as a catalyst precursor for ethylene polymerization in the process of ethylene catalytic. The effects of ethylene polymerization were studied in terms of the Al/Ti molar ratio, reaction time, reaction temperature, polymerization pressure, and ligand structure of the catalyst. The results show at the reaction temperature of 25°C, the reaction time was 30 min, and the ethylene pressure was 1.0 MPa and Al/Ti was 1,000, the catalytic activity can reach 78.56 kg PE/(mol Ti.h). Furthermore, high-temperature GPC-IR, DSC, and torque rheometer were used to characterized the microstructure, thermal properties, and viscoelastic state of polyethylene samples obtained. The results showed that the product was ultra-high-molecular-weight polyethylene.

Online data assimilation of a hybrid flow stress model by particle filtering

Models for the evolution of hidden microstructural states are needed for fast prediction and closed-loop control of workpiece properties. Machine learning allows to obtain models by learning from experimental data, avoiding the limitations of explicitly defined physics-based models. However, the identification of the parameters of deep network structures, reliable extrapolation and fast online assimilation to new measurements are open problems. At the example of titanium forging, a new approach is investigated that combines a hybrid physics-informed microstructure and flow stress model that draws upon a long short-term memory network with a particle filter for online data assimilation to new measurements.

3D Reconstruction of Martensitic Microstructures in Grains of Deformed Nanocrystalline NiTi Wires by TEM

A novel experimental method allowing for 3D reconstruction of martensite variant microstructures evolving during tensile thermomechanical loading test on nanocrystalline NiTi wire is introduced. The method is based on the determination of the location, size and orientation of all martensite variants and interfaces within a selected polycrystal grain using post mortem selected area electron diffraction with dark field image analysis in TEM. It is found that martensitic microstructures in grains of nanocrystalline NiTi wire evolving during tensile thermomechanical loading tests (involve martensite reorientation, stress induced martensitic transformation and plastic deformation of martensite) are rather different than currently assumed in the SMA field. Martensitic microstructures in the NiTi wire deformed up to the end of reorientation (transformation) plateau up to ~7% strain at room temperature (100 °C), respectively, were found to contain single domain (001) compound twinned martensite filling whole grains. This was rationalized by the theoretical treatment of strain accommodation in grains of the fiber textured NiTi wire deformed in tension predicting such singular microstructural state for NiTi wire deformed up to 6.74% strain. Upon reverse martensitic subsequent stress free heating above the Af temperature, this martensitic microstructure retransforms back to the parent austenite yielding recoverable strains typical for NiTi (~6%) accompanied by very small unrecovered strain (~0.6%- 1.5% depending on the test temperature). On further tensile loading up to 15% strain, plastic deformation of oriented martensite starts by coordinated (100) and (201‾) deformation 1 twinning in martensite assisted by [1 0 0](0 0 1) dislocation slip giving rise to characteristic wedge type martensitic microstructure. Upon subsequent stress free heating above the Af temperature, this martensitic microstructure transforms to twinned austenitic microstructure yielding large recoverable strains (~10%) accompanied by large unrecovered strains (~5%). The dislocation slip assisted (100) and (201‾) deformation twinning in martensite renders NiTi excellent combination of strength and deformability and leads to refinement of austenitic microstructure accomplished via introducing {114} austenite twins into it.

Effect of tantalum solid solution additions on the mechanical behavior of ZrB2

Mechanical properties and microstructure were compared for zirconium diboride and two zirconium diboride solid solutions containing 3 and 6 at% tantalum diboride. X-ray diffraction indicated that the ceramics were nearly phase-pure and that tantalum dissolved into the ZrB2 lattice to form (Zr,Ta)B2 solid solutions. Microstructural analysis indicated that samples achieved nearly full relative density with average grain sizes that ranged from 3−5 μm. The three compositions had similar values of Young’s modulus (510−531 GPa), shear modulus (225−228 GPa), Vickers hardness (15.2–16.4 GPa), and flexural strength (391−452 MPa). Fracture toughness ranged from 2.6 to 3.7 MPa m1/2 and with increasing tantalum content, the fracture mode changed from predominantly intergranular to predominantly transgranular. Diboride solid solution materials had comparable properties to the single metal diboride, but differences in microstructure, secondary phases, and strain state among the three ceramics partially obscured the actual effects of the solid solution on fracture behavior.

Automation control system of 3d printing robotic platform with implemented wire + arc welding technology

Purpose. Development of the robotic platform automated control system architecture, development of the software control algorithm.
 Methodology. To implement the algorithm of the control program, computer modeling of thermal regimes in CAE systems is used. The basic parameters of the single layer printing technique were obtained by experimental use of the wire plus arc additive manufacturing (WAAM) technology.
 Findings. Requirements for manufacturability and printing quality of the manufactured parts were defined in the form of geometric dimensions, surface waviness, parameters of the desired microstructure state, residual stresses, maintaining of the optimal manufacturing speed. Based on the requirements of manufacturability analysis, an algorithm for the control program was developed. Robotic platform automated control system architecture with feedback device for the thermal mode control, parameters of the geometrical form of the manufactured part and weld pool were developed. Three -level hierarchical model, which gives an ability to consider in the process of 3D printing each level individually in terms of welding bead, layer and wall, was developed. The input data for the operation of the automated control system of the robotic platform using the technology of electric arc welding are determined. Basic geometrical parameters and the simple welding bead and the methods of overlapping of two or more beads were shown. Critical differences between ideal and real welding overlapping models were considered for necessity of taking into account whilst generating robot control software. Analysis of the possibilities for the CAE simulation of the three-dimensional printing using wire plus arc additive manufacturing technology is performed to determine the influence of the temperature parameters, mechanical loads, toolpath change, and based on the data obtained, it became possible to determine residual stresses and defects in manufactured parts.
 Originality. Robotic platform automated control system architecture with feedback device for the control of thermal mode, parameters of the geometrical form of the manufactured part and weld pool was developed. Three-level hierarchical model for the wire plus arc additive manufacturing (WAAM) technology was created. Software control algorithm which provides an opportunity to improve geometrical and mechanical properties of the manufactured parts was developed.
 Practical value. Development of an automated control system for 3D printing robotic platform with WAAM implemented technology, which will provide an opportunity for increase in the printing accuracy of the manufactured parts and will help to reduce manufacturing time.

Microwave sintered porous CoCrFeNiMo high entropy alloy as an efficient electrocatalyst for alkaline oxygen evolution reaction

Developing efficient, stable and economical electrocatalyst for oxygen evolution reaction (OER) is a significant pathway to produce clean energy from water splitting. Herein, the promising and highly efficient porous CoCrFeNiMo high entropy alloys (HEAs) were prepared by microwave sintering using Mg space holder. Owing to unique properties of high entropy alloys and abundant active surface area, the porous CoCrFeNiMo-20 Mg exhibits excellent catalytic performance and prominent electrochemical stability with a low overpotential of 220 mV to reach current density of 10 mA cm−2, a small Tafel slope of 59.0 mV dec-1 and long-term durability for 24 h in 1.0 M KOH. The results of microstructure and element states indicate that crystal defects, porous structure and villous hydroxides are the reasons for high OER performance of the CoCrFeNiMo. This work not only provides a new way to prepare electrocatalysts, but also proves the important application of high entropy alloys in functional materials.

Fatigue behavior of a Ti-based metallic glass: Effect of sampling location in association with free volume

The large scatter of fatigue property severely hinders the application of metallic glasses (MGs). The sampling location may be one important reason for the fatigue scatter, however the related studies are few. Here we confirmed that, the fatigue property and mechanism of a Ti-based MG were influenced by the sampling location from casting MG plate. The differential scanning calorimetry (DSC) result indicates that the free volume contents decrease from outside to inside along the thickness direction of casting plate, which is probably due to varying cooling rates in this direction during solidification. It is found that the shear-band features after fatigue fracture depend on the sampling location, that is, for the specimens containing smaller near-surface volume, the small amount of the initial free volume contents and increased free volume contents induced by cyclic loading suppress the formation of shear band. The fatigue endurance limit, fatigue ratio and fatigue life of the specimens containing smaller near-surface volume are obviously higher. The superior fatigue property can be well explained by the shear band-mediated fatigue cracking mechanism. These results enhance the understanding on the relation between the free volume, shear band and fatigue property under cyclic loading and provide the instruction how to regulate the fatigue resistance of MG by controlling where the specimens are cut from casting MG plates based on the microstructural state.