Microstructure investigations and mechanical properties of an Al‐Al2O3 MMC produced by semi‐solid solidification

The influence of the semi‐solid solidification production parameters (shear rate and agitation time) and the concentration of reinforcing particles on the microstructure formation and mechanical properties of a 520 aluminum alloy reinforced with Al₂O₃ particles was investigated. Depending on the content of reinforcing particles and the stirring conditions different rosette structures were formed. The type of wear mechanism (delamination or adhesion) depends on the size of the rosettes and the distribution of Al₂O₃ reinforcements. Best mechanical properties were obtained for metal matrix composites reinforced with 12 wt% of Al₂O₃ stirred at a shear rate of 2100 s^–1 for 1800 s. These samples showed tensile strength and yield stress similar to the commercial A520 alloy. The hardness and wear resistance were improved by the addition of Al₂O₃ particles, meanwhile the elongation to fracture was reduced.     

Kontaktwerkstoffe auf Silberbasis ‐ Gefüge und mechanische Eigenschaften

Silver‐based contact materials – microstructure and mechanical properties Different silver‐based materials have been used in relays and contactors. Silver‐based composite materials in particular have played an important role. To produce such composite materials on an industrial scale, conventional powder mixing and wet‐chemical methods are used. By means of the powder‐metallurgical route, these materials are processed in a second step into wire material. To produce silver‐based composite materials with a comparable microstructure, the usability of alternative production routes was tested. This article shows the potential of the methods high‐energy ball milling (HEM) and intensive mixing compared to the two above‐named conventional methods. The main focus is on the evaluation of the microstructure of the composite powder and the extrusion wires concerning the dispersion of the reinforcement component and the resulting mechanical properties of the wire material.     

Strength recovery of machined Al2O3/SiC composite ceramics by crack healing

Alumina is used in various fields as a machine component. However, it has a low fracture toughness, which is a weakness. Thus, countless cracks may be initiated randomly by machining, and these cracks decrease the component's mechanical properties and reliability. To overcome this problem, a crack‐healing ability could be a very useful technology. In this study, Al₂O₃/SiC composite was sintered. This alumina exhibits excellent crack‐healing ability. Small specimens for a bending test were made from the Al₂O₃/SiC. A semicircular groove was machined using a diamond ball‐drill. The machining reduced the local fracture stress from approximately 820–300 MPa. The machined specimens were crack‐healed under various conditions. The fracture stress of these specimens after crack healing was evaluated systematically from room temperature (RT) to 1573 K. It was found that the local fracture stress of the machined specimen recovered almost completely after crack healing. Therefore, it was concluded that crack healing could be an effective method for improving the structural integrity of machined alumina and reducing machining costs.     

Microstructure of SiCw reinforced Al–12Ti composites prepared by powder metallurgical technique

Abstract

Silicon carbide whisker reinforced Al–12Ti composites were fabricated by a powder metallurgical technique, and the microstructures were characterised by the means of X-ray diffraction, SEM, TEM, and energy dispersion X-ray analysis. It has been shown that secondary phase particles, Al₃ Ti, form in situ during hot pressing after premechanically ball milling, and a small amount of α-Ti is left because the in situ reaction between α-Ti and Al is not complete. High density dislocations including dislocation lines and dislocation loops exist in the coarse Al₃ Ti grains, while, hardly any dislocations can be found using TEM in the very fine (～150 nm) Al₃ Ti grains. In addition, nanometer equiaxed γ-Al₂O₃ and stick shaped Al₄ C₃ dispersoids form in the Al matrix as a result of the addition of a processing control agent. There is no fixed orientation relationships between γ-Al₂O₃ , Al₄ C₃ , and the Al matrix. Dislocations in the Al matrix are too sparse to be found even in the zones around SiC whiskers. Silicon carbide whiskers uniformly scatter in the Al matrix, and no reaction products are formed. A few microzones with nanosized (～20 nm) Al grains exist in the Al matrix, and an amorphous phase is usually found in the zones adjacent to SiC whiskers. The formation mechanism of the amorphous phase is discussed.     

Optimization of solvent-free mechanochemical synthesis of Co/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts using low- and high-energy processes

In this work, alumina-supported cobalt (Co/Al₂O₃) catalysts were prepared using new solvent-free mechanochemical synthesis methods: low-energy vibratory ball milling (Fritsch, Pulverisette 0) and high-energy planetary ball milling (Retsch, PM 100). γ-Al₂O₃ supports and Co/Al₂O₃ catalysts after mechanochemical treatments were characterized using a combination of techniques. The study of solid particles revealed the abrasion and fragmentation phenomena of porous γ-Al₂O₃ particles and pore filling under milling. Functional cobalt particles introduced by the mechanochemical synthesis were observed to be preferentially localized on the outer surface of the alumina supports. High Fischer–Tropsch reaction rates were obtained with the catalysts prepared by optimized mechanochemical synthesis conditions. The enhanced catalytic performance can be attributed to the relatively high dispersion of cobalt and the absence of inert cobalt aluminates which are usually present in the catalysts synthesized by the conventional impregnation.     

Synthesis of LiMn2O4 via high-temperature ball milling process

Spinel LiMn₂O₄ powder was prepared by a novel process of high-temperature ball milling. For comparison, the spinel LiMn₂O₄ powder was also synthesized by the traditional method of solid state reaction. It was found that high-temperature ball milling significantly decreased the synthesis temperature and time. LiMn₂O₄ with pure spinel phase could be successfully synthesized only by 2?h high-temperature ball milling at 500°C and 600°C. However, pure spinel LiMn₂O₄ could not be completely synthesized by 2?h solid state reaction at 800°C. The LiMn₂O₄ particles prepared by high-temperature ball milling are nano-sized (<100?nm) and much smaller than that prepared using solid state reaction. The electrochemical tests results indicated that the as-synthesized LiMn₂O₄ by 2?h high-temperature ball milling at 600°C showed a favorable initial discharge capacity of 124.2 mAh g^?1 at current rate of 0.1 C and still retained a capacity of 119.8 mAh g^?1 at 0.1 C after 80 continuous cycles from 0.1 to 2.0 C.     

Friction stir processing strategies to develop a surface composite layer on AA6061-T6

Friction stir processing (FSP) has been used to produce metal matrix composites by incorporating reinforcement particles in an AA6061-T6 matrix. Two types of particles (Al₂O₃ and SiC) were tested. Powder was placed into a mechanized square section groove on a plate surface and then sealed before FSP. This study investigates the effect of several strategies for reinforcement (number and direction of FSP passes) on the wear resistance behavior of friction stir-processed Al-SiC/Al₂O₃ composites. The distribution and size of the particles in the friction stir-processed zone were studied by optical and scanning electron microscopy. Ball-on-disk test was performed on both base material and surface metal matrix composites (SMMCs), and both friction coefficient and specific wear rate (SWR) were correlated with particle distribution and metallurgical effects on the metallic matrix. For all strategies and for both types of reinforcing particles used in this study, the friction coefficient decreases with respect to the base material. Moreover, the SWR is reduced for the conditions of one single FSP pass and two passes with opposite directions, when SiC are used. However, this positive effect has not been detected with Al₂O₃. Wear mechanisms in base metal and in SMMCs are compared and discussed in detail.     

Crack-healing and mechanical behaviour of Al2O3/SiC composites at elevated temperature

Alumina/silicon carbide (Al₂O₃/SiC) composite ceramics with large self‐crack‐healing ability, high strength and high heat‐resistance limit temperature for strength were developed and subjected to three‐point bending. A semicircular surface crack 100 μm in diameter was made on each sample. Crack‐healing behaviour was systematically studied, as functions of crack‐healing temperature and healing time, and the fatigue strengths of the crack‐healed sample at room temperature and 1373 K were investigated. Four main conclusions were drawn from the present study. (1) Al₂O₃/SiC composite ceramics have the ability to heal after cracking from 1273to 1673 K in air. (2) The heat‐resistance limit temperature for strength of the crack‐healed sample is ?1573 K, and ?68% of the samples fractured from outside the crack‐healed zone in the testing‐temperature range 873–1573 K. (3) The crack‐healed sample exhibited very high fatigue limit at room temperature and also 1373 K. (4) The large self‐crack‐healing ability is a desirable technique for the high structural integrity of ceramic component.     

Ceramic Tool Concepts for the Semi‐Solid Processing of Steel Alloys

Two different ceramic tool concepts for the semi‐solid processing (Thixoforming) of steel alloys are presented. Materials selection is adapted to forming technology (Thixoforging, Thixoextrusion), preset die temperature, and resulting process conditions. Gas‐pressure sintered silicon nitride (Si₃N₄) is chosen as die material in low tool temperature (300...400 °C) thixoforging experiments due to its high strength and outstanding thermal shock resistance. High purity dense alumina (Al₂O₃) is applied as die material for high temperature (1200 °C) thixoextrusion tests. Thixoforging results using Si₃N₄ dies pre‐heated to 300 °C show sufficient thermal shock and corrosion resistance of Si₃N₄ and confirm the applicability of this tool concept. The high temperature tool concept developed at the Institute of Mineral Engineering (GHI) effectively reduced thermal shock impacts on extrusion dies. As expected, corrosion resistance of Al₂O₃ proved to be excellent. Further research will be carried out concerning long‐term behaviour of Si₃N₄ thixoforging dies as well as on the influence of extrusion speed and tool temperature on the quality of products extruded through Al₂O₃ dies at high temperature.     

Preparation of Al2O3–TiB2 nanocomposite powder by mechanochemical reaction between Al,B2O3 and Ti

Mechanochemical processing is a novel technique for the synthesis of nano-sized materials. This research is based on the production of Al₂O₃–TiB₂ nanocomposite powder using mechanochemical processing. For this purpose, a mixture of aluminum, titanium and boron oxide powders was subjected to high energy ball milling. The structural evaluation of powder particles after different milling times was conducted by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results showed that during ball milling the Al/B₂O₃/Ti reacted with a combustion mode producing Al₂O₃–TiB₂ nanocomposite. In the final stage of milling, the crystallite sizes of Al₂O₃ and TiB₂ were estimated to be less than 50 nm.     

Microstructural evolution during high energy ball milling of Fe2O3-SiO2 powders

The reaction of a 25 mol% Fe₂O₃-SiO₂ (hematite-amorphous silica) powder mixture during high energy ball milling in both closed and open containers has been studied by x-ray diffraction and Mössbauer spectroscopy. After around 21 h of milling, the α-Fe₂O₃ powders with an average particle size of 15 nm have formed and no reaction between α-Fe₂O₃ and SiO₂ is found in the two types of milling containers. This demonstrates that the high energy mechanical milling technique is able to prepare a dispersion of ultrafine α-Fe₂O₃ particles. After extended milling in the open container all iron atoms are found in a hematite phase. In the closed container the hematite phase transforms into an iron-rich spinel phase and some of the iron atoms react with the amorphous SiO₂, forming a new Fe(II)-containing silicate compound.     

Effect of SiC‐Reinforcement and Equal‐Channel Angular Pressing on Microstructure and Mechanical Properties of AA2017

This article deals with powder metallurgical production and modification of properties of a composite material based on an age‐hardenable Al–Cu alloy. The main objective is to improve the mechanical properties by particle reinforcement and equal‐channel angular pressing (ECAP). Our approach makes use of four hardening mechanisms: precipitation hardening, particle reinforcement, strain‐hardening, and grain boundary hardening associated with an ultrafine‐grained microstructure produced by ECAP. The main processing steps are high‐energy ball milling, hot‐isostatic pressing, extrusion, heat treatment, and a single ECAP pass. Microstructures are analyzed by optical microscopy, scanning electron microscopy, and scanning transmission electron microscopy. The mechanical properties are characterized by hardness measurements and quasi‐static tensile testing. Our experimental results show that the proposed processing route results in a nearly homogeneous distribution of SiC particles in the matrix. The combination of particle reinforcement and ECAP leads to an improvement of ultimate tensile strength by almost 300 MPa compared to the unreinforced alloy. A subsequent heat treatment leads to a further increase in hardness and strength that can be related to changes in the defect structure. Our study provides detailed information on how processing steps, microstructures, and mechanical behavior are interrelated in this technologically relevant class of materials.     

Fabrication of Al-Zn/α-Al2O3 nanocomposite by mechanical alloying

In this study fabrication and characterization of alumina particles reinforced aluminum-based metal matrix nanocomposite by mechanical alloying were investigated. Aluminum and zinc oxide powders mixture milled by a planetary ball mill in order to produce Al-13.8 wt.% Zn/5 vol.% Al₂O₃ nanocomposite. The structural evaluation milled and annealed powders studied by X-ray diffraction, SEM observation and hardness measurement. The aluminum crystallite size estimated with broadening of XRD peaks by Williamson-Hall formula. The results showed that milling of aluminum and zinc oxide for 60 h led to displacement reaction of the zinc oxide and aluminum to produce Zn and Al₂O₃ phases. The milled powder had a microstructure consisting of nanosized Al₂O₃ particles in an Al-Zn solid solution with a nanoscale grain size of 40 nm. Microhardness of this nanocomposite was found to be about 190 HV.     

Interfacial Engineering Coupled Valence Tuning of MoO3 Cathode for High‐Capacity and High‐Rate Fiber‐Shaped Zinc‐Ion Batteries

Aqueous Zn‐ion batteries (ZIBs) have garnered the researchers' spotlight owing to its high safety, cost effectiveness, and high theoretical capacity of Zn anode. However, the availability of cathode materials for Zn ions storage is limited. With unique layered structure along the [010] direction, α‐MoO₃ holds great promise as a cathode material for ZIBs, but its intrinsically poor conductivity severely restricts the capacity and rate capability. To circumvent this issue, an efficient surface engineering strategy is proposed to significantly improve the electric conductivity, Zn ion diffusion rate, and cycling stability of the MoO₃ cathode for ZIBs, thus drastically promoting its electrochemical properties. With the synergetic effect of Al₂O₃ coating and phosphating process, the constructed Zn//P‐MoO_3?x@Al₂O₃ battery delivers impressive capacity of 257.7 mAh g^?1 at 1 A g^?1 and superior rate capability (57% capacity retention at 20 A g^?1), dramatically surpassing the pristine Zn//MoO₃ battery (115.8 mAh g^?1; 19.7%). More importantly, capitalized on polyvinyl alcohol gel electrolyte, an admirable capacity (19.2 mAh cm^?3) as well as favorable energy density (14.4 mWh cm^?3; 240 Wh kg^?1) are both achieved by the fiber‐shaped quasi‐solid‐state ZIB. This work may be a great motivation for further research on molybdenum or other layered structure materials for high‐performance ZIBs.     

Microstructural characterization of ball-milled metal matrix nanocomposites (Cr,Ni, Ti)-25 wt% (Al2O3np,SiCnp)

The microstructure of high-temperature metals such as Ti, Ni, and Cr can be modified using ceramic nanoparticles to form metal matrix nanocomposites (MMNCs). Such materials are generally prepared via powder metallurgy routes. In this study, 25?wt% SiC_np and Al₂O_3np were separately ball milled as a reinforcement of Ti, Cr, and Ni matrices to investigate their effects on the phase formation and morphology of the MMNCs. The x-ray diffraction (XRD), scanning electron microscopy (SEM), and field emission scanning electron microscopy (FESEM) results indicated that the alumina–metal system could not be thermodynamically stable in a high-energy ball mill, while the SiC reinforcement could be retained and milled with the metals even after 24?h. It was further observed that the distribution of nanoparticles was not affected by the type of metal, ceramic, and milling time. Finally, it was determined that the nanoparticles significantly reduced the average particle size of composite powders.     

Fabrication and characterization of heat and plasma treated SiC/Al2O3-YSZ feedstocks used for plasma spraying

The SiC/Al₂O₃-YSZ (ZrO₂ + 8 wt.% Y₂O₃) powders with different SiC particle sizes were fabricated and treated from spray drying, heat treatment, and plasma spraying. The morphology, phase composition, flowability and density of powders were analyzed. The sphericity and flowability of powders treated by plasma flame are increased greatly, and the particle surface is very smooth. The flowability and density of powder with nano SiC were evident better than those of powder with submicron SiC. The optimum flowability and compactness of powder with submicron SiC is obtained when the critical plasma spray parameter is 341 and 325, respectively. For nano size SiC, the optimum flowability and the maximum compactness of powders are obtained with critical plasma spray parameter of 341. The grain size of powders is increased after heat treatment and plasma spraying. The SiC is oxidized to SiO₂ in the powders after heat treatment and plasma spraying. The Y₂O₃ dissolved from 8YSZ solid solution at higher critical plasma spray parameter. Besides, there is no phase transformation of ZrO₂ for powders. The metastable phase of Al₂O₃ appeared in feedstocks with submicron SiC, but no metastable phase was formed in feedstocks with nano SiC particles, which nano SiC can hinder the formation of Al₂O₃ metastable phase. The densification process and mechanism of reconstituted particles used for plasma spraying were analyzed from surface morphology, cross section and simulation.     

Effect of Powder Processing and Sintering Conditions on the Microstructural,Thermal, and Mechanical Properties of Reticulated Zinc Oxide Ceramic Foams

Ceramic foams are made of zinc oxide using different amounts of Sb₂O₃ and Bi₂O₃ as sintering aids. The effect of a ball milling processing of the starting powders and the sintering temperature on the microstructure and the properties of the ZnO foams is investigated. The focus is set on the evolution of the secondary phases formed within the microstructure of ZnO. A determining effect is identified in the amount of an Al₂O₃ impurity which is introduced by abrasion of the milling vessels during ball milling. Alumina is partially dissolved in a spinel α–Zn₇Sb₂O₁₂ secondary phase which is stabilized by a reduction of the unit cell volume. Remaining Al₂O₃ is incorporated into zinc oxide under formation of a defect wurtzite phase. The phase evolution is a complex function of the content of sintering aids, the Al₂O₃ impurity level and the sintering temperature. The shrinkage during sintering and the porosity evolution are correlated to the phase composition within the ZnO material. The thermal conductivity and the compressive strength of the foams are determined, normalized with respect to their porosity, and correlated to the microstructure and phase composition of the ZnO strut material.     

Synthesis and characterisation of nanostructured Al–Al3V and Al–(Al3V–Al2O3) composites by powder metallurgy

In this study, the formation and characterisation of Aluminium (Al)-based composites by mechanical alloying and hot extrusion were investigated. Initially, the vanadium trialuminide (Al₃V) particles with nanosized structure were successfully produced by mechanical alloying and heat treatment. Al₃V–Al₂O₃ reinforcement was synthesised by mechanochemical reduction during milling of V₂O₅ and Al powder mixture. In order to produce composite powders, reinforcement powders were added to pure Al powders and milled for 5?h. The composite powders were consolidated in an extrusion process. The results showed that nanostructured Al-10?wt-% Al₃V and Al-10?wt-% (Al₃V–Al₂O₃) composites have tensile strengths of 209 and 226?MPa, respectively, at room temperature. In addition, mechanical properties did not drop drastically at temperatures of up to 300°C.     

Constructing Ionic Gradient and Lithiophilic Interphase for High‐Rate Li‐Metal Anode

Li metal is the optimal choice as an anode due to its high theoretical capacity, but it suffers from severe dendrite growth, especially at high current rates. Here, an ionic gradient and lithiophilic inter‐phase film is developed, which promises to produce a durable and high‐rate Li‐metal anode. The film, containing an ionic‐conductive Li_0.33La_0.56TiO₃ nanofiber (NF) layer on the top and a thin lithiophilic Al₂O₃ NF layer on the bottom, is fabricated with a sol–gel electrospinning method followed by sintering. During cycling, the top layer forms a spatially homogenous ionic field distribution over the anode, while the bottom layer reduces the driving force of Li‐dendrite formation by decreasing the nucleation barrier, enabling dendrite‐free plating‐stripping behavior over 1000 h at a high current density of 5 mA cm^?2. Remarkably, full cells of Li//LiNi_0.8Co_0.15Al_0.05O₂ exhibit a high capacity of 133.3 mA h g^?1 at 5 C over 150 cycles, contributing a step forward for high‐rate Li‐metal anodes.     

Pulsed metall inert gas (MIG) welding and its effects on the microstructure and element distribution of an aluminum matrix reinforced with SiC composite material

This study aims to demonstrate the effects of pulsed current on the welding pool and fusion zone microstructures of the aluminum 2014 alloy matrix composite material reinforced with 14 and 20 vol% SiC particles. A programmable synergic controlled MIG welding machine with pulsed power supply was used. One hundred Ampere and 120 Ampere pulsed current values were used to determine the effect of heat input on microstructures. A 1 mm diameter SG‐AlSi₅ wire was used as filler material. The microstructures were studied using a scanning electron microscope (SEM) with energy dispersive X‐ray (EDX) spectroscopy, and the phase analyses were performed via X‐ray diffraction analyzer (XRD). The study showed that increasing the SiC rate has a greater effect on the formation of Al₄C₃ phase than increasing the heat input values. Al₄C₃ formation was not formed as a needle‐like structure.