Hydrogen storage properties of 2Mg-Fe mixtures processed by hot extrusion at different temperatures

2Mg-Fe mixtures produced by high-energy ball milling were consolidated into bulk form by hot extrusion at different processing temperatures (573 K (300 °C), 623 K (350 °C) and 673 K (400 °C)), aiming to evaluate their influence on the structure and microstructure of bulk materials and their consequent influence on the hydrogen sorption properties. In spite being in the nanosize range, the highest the processing temperature, the larger the grain sizes. However, the nanometric grain size remained after any hot extrusion condition, as estimated by Rietveld refinement. The pinning effect of Fe on Mg grain boundaries explained this effect. In the first absorption (activation), powders showed a hydrogen storage capacity of ～4.53 wt%, while the extruded samples (bulk materials) reached almost the same capacity during the period of hydrogenation (～94% of the maximum hydrogen storage capacity for Mg₂FeH₆ - 5.5 wt%). The smallest crystallite sizes and highest surface area for hydrogenation explain the good performance of powders. However, when comparing only extruded samples, it was observed that the highest capacity and the lowest incubation times were mainly related to grain sizes and to the favorable texture along (002) plane of αMg. The desorption temperature of bulk materials was very similar to that of powders, which is good considering the lower surface area of bulk materials.     

Exothermic reaction characteristics of continuously ball-milled Al/Ni powder compacts

Self-propagating reactions in compacted pellets of continuously low-energy ball-milled aluminium (Al) and nickel (Ni) powders at a composition corresponding to AlNi₃ were investigated. The formation of a bi-modal structure with nanoscale lamellae of Al and Ni surrounding thicker Ni layers was observed. The milled powder sizes decreased for milling durations longer than 4 h, but the pellet green densities remained mostly constant for longer than 2 h of milling. The ignited pellets observed using high-speed optical and infrared imaging revealed that the thermal wave velocity, maximum reaction temperature, ignition initiation duration and ignition temperature decreased with increasing milling times due to solid-state diffusion. X-Ray Diffraction (XRD) analysis after ignition tests showed that the AlNi₃ amount increased with milling time. Thermal analysis using interrupted Differential Scanning Calorimetry (DSC) in combination with XRD revealed that the ball-milled pellets have similarities to nanoscale magnetron sputtered multilayer foils in terms of phase formation sequence and exothermic peak shifts.     

Selective oxidation behavior of an ignition-proof Mg-Y-Ca-Ce alloy

A Mg-Y-Ca-Ce magnesium alloy was optimized for high ignition-proof property, which did not burn in air at 1233 K up to 30 min. Oxidation behavior of the alloy was investigated by X-ray diffraction (XRD...     

Influence of concentration in interstitial elements on mechanical alloying of Fe–C powder mixtures

Abstract

This work concerns the mechanical alloying processing of Fe–C powders likely to be utilised for sintering of tool steel. The influence of synthesis conditions, such as milling time, gas nature in the vial (shown here as reactive) and carbon concentration in the different powder mixtures of iron and graphite, is characterised. In the second stage, the role of thermal treatment, at moderate temperature, on the highly metastable as milled products is studied. It appears that the structure is heterogeneous and shows α-ferrite, α′′′-like cubic disordered phase, hexagonal ?-carbonitride and θ-cementite. The presence of N₂ gas phase appears to favour a precipitation phenomenon of the Fe₃C cementite on the one hand and of epsilon carbonitride on the other hand.     

Evaluating the effect of SiC content on iron-based nanocomposite

Iron has wide application in manufacturing industries and the objective of this study is to investigate the effect of silicon carbide (SiC) particles on iron-based nanocomposites. SiC is effectively reinforced into iron matrix by mechanical alloying process using high-energy ball mill. The Fe–SiC nanocomposites with various compositions of SiC viz., 15%, 20% and 25% are characterised using X-ray diffraction and atomic force microscopy. The nanocomposite powders are compacted and sintered into pellets. Their elastic properties, hardness and Poisson's ratio are determined using the pulse echo technique. Moreover, the densities of the pellets are measured using Archimedes’ principle by the water immersion method.     

Structure and magnetic properties of a multi-principal element Ni–Fe–Cr–Co–Zn–Mn alloy

A nanocrystalline alloy with a nominal composition of Ni₂₀Fe₂₀Cr₂₀Co₂₀Zn₁₅Mn₅ was produced by mechanical alloying and processed using annealing treatments between 450 and 600 °C for lengths from 0.5 to 4 h. Analysis was conducted using x-ray diffraction, transmission electron microscopy, magnetometry, and first-principles calculations. Despite designing the alloy using empirical high-entropy alloy guidelines, it was found to precipitate numerous phases after annealing. These precipitates included a magnetic phase, α-FeCo, which, after the optimal heat treatment conditions of 1 h at 500 °C, resulted in an alloy with reasonably good hard magnetic properties. The effect of annealing temperature and time on the microstructure and magnetic properties are discussed, as well as the likely mechanisms that cause the microstructure development.     

Effects of mechanically alloying Al2O3 and Y2O3 additives on the liquid phase sintering behavior and properties of SiC

In order to improve the sintering of SiC, mixtures of Al₂O₃ and Y₂O₃ powders are commonly included as sintering additives. The aim of this work was to use mechanically alloyed Al₂O₃–Y₂O₃ mixtures as sintering additives to promote liquid phase sintering of SiC using spark plasma sintering. The results showed that milling reduced the particle size of the powders and led to the formation of complex oxide phases (YAP, YAM, and YAG) at low temperatures. As the ball milling time increased, the mass loss of specimens sintered with mechanically alloyed Al₂O₃–Y₂O₃ mixtures decreased, and accordingly the relative density increased. However, the hardness and flexural strength of sintered SiC specimens first increased and then decreased. Because the specimens prepared with oxides milled for a long time contained too much YAG/YAP and accordingly too much liquid at sintering temperature. This negatively affected the mechanical properties of the SiC specimens because of the increased volume of the complex oxide phases, which have inferior mechanical properties to SiC, in the sintered specimens. When the ball milling time was 6 h, the hardness (24.02 GPa) and flexural strength (655.61 MPa) of the SiC specimens reached maximum values.     

Microstructure and properties of CrMnFeCoNi high-entropy alloy prepared by mechanical alloying and spark plasma sintering

The equiatomic ratio CrMnFeCoNi high entropy alloy (HEA) was prepared by mechanical alloying (MA) and spark plasma sintering. This paper reports the behaviour of MA, the phase formation, microstructure and mechanical properties of CrMnFeCoNi HEA. With the increase of milling time, solid solution with single FCC phase was gradually formed. The single FCC phase remained as matrix after SPS at 1373?K and 50?MPa. Ultrafine-grained microstructure and good mechanical properties were obtained: At room temperature, the as-sintered bulks exhibit an excellent combination of high compressive strength (2390?MPa) and high fracture strain (47%).     

Phase-evolution in high entropy alloys: Role of synthesis route

Synthesis route can have a significant influence on the process of phase-choice and -evolution in High Entropy Alloys (HEAs). With the objective of instituting awareness, this communication on the foundation of phase diagrams, adopted using the CALPHAD approach, attempts at describing and deciphering the cause for such gradation.     

Catalytic effect of melt-spun Ni3FeMn alloy on hydrogen desorption properties of nanocrystalline MgH2 synthesized by mechanical alloying

To improve hydrogen desorption properties of magnesium hydride, a composite material with composition of MgH₂-5 at% Ni₃FeMn has been prepared by co-milling MgH₂ powder with Ni₃FeMn alloy either in the form of as-cast (sample A) or melt-spun ribbon (sample B). The study has shown that the addition of Ni₃FeMn alloy to magnesium hydride can yield a finer particle size after mechanical alloying (MA). As a consequence, the desorption temperature of mechanically activated MgH₂ for 30 h has decreased from 319 °C to 307 °C for sample A and to 290 °C for sample B. Furthermore, some favorable effects of Ni₃FeMn alloy on hydrogen desorption kinetics have been observed. Further improvement in the hydrogen desorption of melt-spun containing composite can be related to higher hardness value of the melt-spun powder compared to the as-cast alloy, and probably a more homogeneous distribution of the alloyed elements.