Effect of Nb on nanoquasicrystalline Al-based alloys

Nanoquasicrystalline Al-based alloys, containing icosahedral particles in an α-Al matrix, exhibit high strength at elevated temperature. The metastability of the quasicrystals can limit the use of these alloys. In the present work, the microstructural evolution of Al₉₃(Fe₃Cr₂)₇ and Al₉₃Fe₃Cr₂Nb₂ (at%) alloys was studied using heat treatments and structural characterization by XRD, TEM and STEM-EDX analysis. It was observed that the Nb is dissolved in the Al–Fe–Cr icosahedral phase. This provides higher thermal stability, retaining the fine nanoquasicrystalline microstructure for longer times at high temperature.     

Inverse deformation-fracture responses between dendrite and matrix in Ti-based nanostructure–dendrite composite

The room-temperature deformation and fracture mechanisms of Ti-based nanostructured alloys are investigated. The monolithic nanostructured alloy goes through a shear banding???kinetic softening vicious cycle and exhibits very limited plasticity. The nanostructure–dendrite composite exhibits large plasticity while retaining a very high strength. Three fracture modes, namely shear fracture of the nanostructured matrix phase, normal ductile fracture of the dendritic phase and the peeling off of the dendrites from the matrix, are clearly observed. With increasing deformation, the nanostructured matrix is kinetically softened while the dendrite phase is work hardened. The inverse deformation responses and the interaction between the nanostructured matrix and the dendrites can effectively retard the inhomogeneous shear deformation of the nanostructured phase and lead to a large plasticity.     

Formation of the icosahedral quasicrystalline phase in Zr70Pd30 binary glassy alloy

It is found that a single icosahedral quasicrystalline phase is formed as a primary precipitation phase in the melt-spun Zr₇₀Pd₃₀ binary glassy alloy with a two-stage crystallization process. The onset temperature of the transformation from the amorphous to the icosahedral phase is 701 K at the heating rate of 0.67 K s-1. The size of the icosahedral particles lies in the diameter range below 10 nm and the particles are distributed homogeneously. The second-stage crystallization reaction results in the formation of a Zr₂Pd phase through a single exothermic reaction. The formation of the nanoscale icosahedral phase indicates the possibility that icosahedral short-range order exists in the Zr-Pd binary glassy state. Comparison with the thermal stability of an icosahedral phase in the Zr-Ni-Pd system shows that the icosahedral phase is stabilized by the addition of Ni. The stabilization is due to the restraint of the long-range rearrangement of the constitutional elements resulting from the strong chemical affinity between Zr and Ni.     

Formation of a 'splitting pattern' associated with L1 2 precipitates in Ir-Nb alloys

A cuboidal L1 2 phase forms in the fcc matrix of an Ir-Nb alloy. Under some conditions, a rod- or plate-like fcc structure can be observed in this phase. This is similar to the 'splitting pattern' observed in some Ni-based alloys. To understand the formation mechanism of this structure, samples were heated under different conditions. Microstructure evolution during ageing was observed by transmission electron microscopy. We observed a splitting pattern only when the L1 2 precipitates were already formed in the as-cast condition after additional heat treatment at the ageing temperature. In this case, the composition of the L1 2 phase was not equilibrated. We conclude that the splitting pattern appears owing to a kinetic phenomenon when the L1 2 phase changes to the equilibrium composition.     

Synthesis,transformation and superconductivity of dual phase In–Sn alloy nanoparticles embedded in an Al matrix

We report the synthesis of nanoembedded biphasic alloy particles of In–Sn near eutectic alloy compositions embedded in an aluminium matrix by rapid solidification processing. Detailed transmission electron microscopy indicates that the two phases present at room temperature in as-synthesized samples are β and γ phases with tetragonal and hexagonal crystal structures, respectively. These co-exist with a small amount of single phase In or Sn particles with sizes less than 10?nm. Low temperature magnetization measurements indicate a superconducting transition temperature of 5?K, suggesting complete decomposition of the β-phase at small sizes and at low temperature. The small particles show type II behavior with a critical field Hc₁≈44?G and two values for Hc₂ of 250 and 1000?G, respectively. These values are considerably lower than those observed in bulk In–Sn alloys.     

Electron spin resonance in superparamagnetic particles dispersed in a non-magnetic matrix

Abstract

The temperature dependence of the electron magnetic resonance line broadening (ΔH _pp) of superparamagnetic particles immersed in an inert matrix is discussed on the basis of the method of moments.     

Crystallization behaviour of Al-based amorphous alloy and nanocomposites by rapid quenching

The microstructure and crystallization behaviour of melt-spun Al₈₈Ni₉Ce₂Fe₁ amorphous alloy and nanophase composites have been studied by means of X-ray diffraction, transmission electron microscopy and scanning and isothermal calorimetry. The diffraction patterns from Al₈₈Ni₉Ce₂Fe₁ amorphous alloys are diffuse, indicating a basically amorphous structure but contain two rings presumed to be associated with quenched-in nuclei. In the cases of Al₈₈Ni₉Ce₂Fe₁ nanophase composites, nanoscale precipitated particles are homogeneously dispersed in an amorphous matrix, and the crystallite diameter and volume fraction are sensitive to quenching conditions. During thermal crystallization, a two-step phase transformation occurs in the amorphous alloy and nanocomposites, which is characterized by a diffusion-controlled precipitation of nanoscale Al particles and the growth of a Al₃(Ni, Fe) nanophase prior to a Al₁₁Ce₃ nanophase. This study gives insight into structure-control for obtaining nanophases dispersed in an amorphous matrix by rapid quenching.     

Thermodynamic analysis of Fe contamination in Cu-Mo alloys processed by mechanical alloying

ABSTRACT

This work aims to verify if Fe atoms coming from balls and jars enter into solid solution of Cu-Mo powders during mechanical alloying. The powders were milled and its structure and composition were characterised by scanning and transmission electron microscopy, and X-ray diffraction. Gibbs free of mixing values for Cu-Mo-Fe system were calculated using the Miedema’s model and showed that Cu-rich corner exhibit smallest energy values (<10?kJ?mol^?1). The observed coherency relation Cu(111) || Mo(101) || Fe(101) confirmed formation of solid solution. Fe atoms have entered in solid solution with Cu and Mo and do not remain as particles of second phase.     

Misfit dislocation loops and critical parameters of quantum dots and wires

Quantum dots and wires, having a mismatch of crystal lattice parameters with respect to the surrounding matrix, are modelled by spherical and cylindrical inclusions, respectively. By considering the energy of a circular prismatic dislocation loop nucleation in the inclusions, the critical radius and critical dilatation for a dot and a wire are calculated. The results are compared with similar critical parameters for a mismatched film on a substrate.     

On diffusion and interfacial enrichment of boron in the Laves phase: an in situ TEM-heat-treatment study on newly developed 3Co–3W–9Cr steel

Using in situ transmission-electron-microscopy (TEM) heating and subsequent energy-filtered TEM (EF-TEM) and high-resolution-TEM (HR-TEM) analysis on a newly developed 3Co–3W–9Cr steel, enrichment of boron at the Fe₂W and matrix interface has been studied for the first time. The heat treatment has been carried out at a temperature of 770°C. From the EF-TEM analysis, it is confirmed that a stacking-fault-assisted diffusion mechanism is responsible for enrichment of boron at the interface. From the HR-TEM study, it is ascertained that the diffusion behaviour of boron in the Laves phase and the extent of the enrichment at the interface is dependent on the amount of stacking faults in the Laves phase.     

Spontaneous atom mixing and unmixing in metallic nanoparticles

The behaviour of atom mixing and unmixing in metallic nanoparticles has been studied in situ by transmission electron microscopy, using particles in the Au₂Pb-Sn system. It is confirmed that not only spontaneous mixing but also spontaneous unmixing take place in Au₂Pb particles. At room temperature, spontaneous mixing of tin atoms into Au₂Pb particles takes place, and this mixing induces unmixing of lead from Au₂Pb, resulting in the formation of AuSn particles accompanied by lead precipitates.     

Phase field modeling of elasto-plastic deformation induced by diffusion controlled growth of a misfitting spherical precipitate

A phase field model accounting for plasticity has been developed using an homogenization scheme for interpolating the constitutive laws within the diffuse interface. The influence of plasticity on the growth of a misfitting spherical precipitate, controlled by solute diffusion has been investigated: plasticity in the matrix slows down the transformation. Moreover, an excellent agreement with the corresponding analytical sharp interface solutions has been achieved.     

On coherent carbonitride precipitation in commercial microalloyed steels

Fine vanadium carbonitride particles nucleated during interphase precipitation have been investigated in the transmission electron microscope under controlled conditions of diffraction contrast. On dark-field micrographs taken with a carbonitride reflection, particle sizes ranged from 5 nm down to below 2 nm in diameter. No matrix strain around these particles could be detected when matrix reflections were used to produce diffraction contrast under two-beam conditions. It was concluded that the vanadium carbonitrides had precipitated in the form of incoherent particles.     

Thermomagnetic study of devitrification in Fe-Si-B-Cu-Nb(-X) alloys

Thermomagnetic measurements have been used to study the magnetic and structural changes occurring at the two steps of the crystallization process of Fe_73.5Si_13.5B₉Cu₁Nb₁X₂ (X = Zr, Nb, Mo and V) alloys. Alloying raises the thermal stability of the amorphous phase against nanocrystallization in the order V < Mo < Nb < Zr and some differences in the final crystalline phases are found. The Curie temperature of the amorphous phase increases (about 15K) during structural relaxation. In the course of nanocrystallization a further increase of about 30K in the Curie temperature of the amorphous intergranular is observed for samples with X = Zr, Nb and Mo, but only of about 15K for samples containing V. The observed increase in the Curie temperature of the Fe-Si phase between the end of the first crystallization process and the end of the second crystallization process is associated with a reduction in the Si content, in agreement with X-ray diffraction results.     

Effect of Si on the formation and stability of the icosahedral quasicrystalline phase in Al–Fe–Cr–Ti alloys

The formation and stability of the quasicrystalline icosahedral (i) phase in melt-spun Al_{93– x} Fe₃Cr₂Ti₂Si _x (x?=?0–5) ribbons are reported. Samples were characterized by X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. Primitive (P-type) ordered i phase particles were found to be homogeneously distributed in an fcc α-Al matrix in the as-melt-spun ribbons. The size of the i phase particles decreased and their thermal stability increased with increasing substitution of Al by Si. The i phase had a decomposition temperature of approximately 480°C in an Al₉₃Fe₃Cr₂Ti₂ ribbon whereas that in an Al₉₂Fe₃Cr₂Ti₂Si₁ ribbon was approximately 500°C. The i phase particles are resistant to coarsening prior to decomposition into crystalline phases. The presence of a small quantity of Si (up to 1.0?at.%) is beneficial to both the thermal stability and the hardness of nanoquasicrystalline Al–Fe–Cr–Ti alloys.     

Precipitation of γ’ during cooling of nickel-base superalloy Haynes 282

ABSTRACT

Cooling-induced precipitation of the strengthening γ’ phase is commonly investigated in Ni-base superalloys with a high γ’ volume fraction, where it is used to control the final microstructure and properties. Less is known about the phase separation in low-volume-fraction alloys during cooling, although the microstructural state after cooling from solution treatment is known to affect subsequent heat-treatment steps. We use atomic-scale characterisation of Ni-base superalloy Haynes 282 (equilibrium γ’ volume fraction around 20%) to show that air cooling after solution or carbide stabilisation results in precipitation of nm-sized γ’ particles, whereas precipitation was suppressed during water quenching. The solution treatment has a significant effect on the hardness and γ’ precipitation during air cooling from the subsequent carbide stabilisation temperature. Also, the carbide-stabilisation treatment itself affects the γ’ precipitation during subsequent air cooling.     

Nanoprecipitates of icosahedral phase in quasicrystal-strengthened Mg-Zn-Y alloys

Extensive microstructural studies have been performed with respect to the formation of the icosahedral quasicrystalline phase and its relationship to other phases in Mg 95 Zn 4.2 Y 0.8 alloy. The icosahedral phase forms as an intergranular eutectic phase as well as precipitates in the matrix. The precipitates are nanosized (typically 50 nm) with a definite orientation relationship with the matrix, sharply faceted on twofold planes which are on the basal and prismatic planes of the matrix. The detailed crystallographic relationship with the matrix is described. The icosahedral phase is occasionally found to coexist with the cubic W-Zn 3 Mg 2 Y 3 phase with a definite crystallographic relationship.     

Attention Bias for Threatening Information in Children Following a Distressing Medical Procedure

Attention bias is common in adults with post‐traumatic stress disorder (PTSD) but is less studied in children. Children (n = 22) who experienced a potentially distressing procedure in an outpatient clinic (removal of K‐wires from orthopaedic fractures) and a group of medically unwell children (illness group; n = 27) were compared with healthy controls (n = 32). Children's baseline level of PTS symptoms were indexed prior to the medical procedure, and again at 1‐week follow‐up. Immediately after the K‐wire removal, children completed a dot probe task using two categories of target words (medical threatening and emotionally threatening). While K‐wire children showed an overall bias away from negative words relative to healthy controls, the illness group did not significantly differ from healthy controls. Attention bias in K‐wire and illness groups was unrelated to later PTS symptoms.     

Tibialis posterior muscle pain effects on hip,knee and ankle gait mechanics

BackgroundTibialis posterior (TP) dysfunction is a common painful complication in patients with rheumatoid arthritis (RA), which can lead to the collapse of the medial longitudinal arch. Different theories have been developed to explain the causality of tibialis posterior dysfunction. In all these theories, pain is a central factor, and yet, it is uncertain to what extent pain causes the observed biomechanical alterations in the patients. The aim of this study was to investigate the effect of experimental tibialis posterior muscle pain on gait mechanics in healthy subjects.MethodsTwelve healthy subjects were recruited for this randomized crossover study. Experimental pain was induced by ultrasound-guided injection of 1 mL hypertonic saline into the upper part of the right tibialis posterior muscle with the use of isotonic saline as non-pain-inducing control. Subsequently, kinematic data during three self-paced over ground walking for each condition were collected. Ground reaction forces and external moments were measured from force plates installed in the floor. Painful areas were evaluated using body charts and pain intensity scoring via a verbal numerical rating scale.FindingsDecreased hip internal rotation was observed during the pain condition at the end of the stance phase. There were no changes in gait velocity and duration of stand phase between the pain and no pain conditions. Reduced external joint moment was found for external knee rotation and for external hip rotation.InterpretationThe study has demonstrated that induced pain in the TP muscle evokes kinematic alteration in the hip and the knee joints, but not in the ankle, which suggest an underlying early stage joint compensatory mechanism. These findings suggest the need to include those joints in current physical evaluations of tibialis posterior dysfunction.     

A study of the fcc (FeCo) 23 B 6 phase in fully crystallized Fe-Co-Nb-B-Cu alloys

Fe-Co-Nb-B-Cu alloys lose their nanocrystalline microstructure at a second crystallization process in which (FeCo) 23 B 6 crystals appear as the main boride phase. In this work the structural characteristics and composition of this phase are studied. The amount and grain size of the (FeCo) 23 B 6 phase increase as the Co content in the alloy increases. After recrystallization, f -FeCo crystals remain at a nanometric size. The lattice parameter and Curie temperature of the (FeCo) 23 B 6 phase are reported.