Effect of temperature on solid-state formation of bulk nanograined intermetallic Al3Ni during high-pressure torsion

A bulk form of nanograined intermetallic Al₃Ni was produced by severe plastic deformation using high-pressure torsion (HPT). Powder mixtures of 75?mol% Al and 25?mol% Ni were processed by HPT at a selected temperature in the range of room temperature (RT) to 573?K under a pressure of 6?GPa. X-ray diffraction analysis revealed that the Al₃Ni intermetallic formed after processing for 50 revolutions at RT but, as the processing temperature increased, less revolutions (i.e. lower imposed strain) were required for the formation of Al₃Ni. Observations by transmission electron microscopy showed that the microstructure consists of ultrafine grains having a size of 300–2000?nm after 3 and 10 revolutions. Once the Al₃Ni formed after a higher number of revolutions, equiaxed nanograins with a size of ~30?nm prevailed with a significant increase in hardness. The increase in hardness was more significant when processed at higher temperatures because of increasing the fraction of Al₃Ni. It was shown that the solid-state formation of Al₃Ni occurred due to enhanced diffusion (i.e. decreased activation energy for diffusion) through the presence of high density of lattice defects.     

Significance of stacking fault energy on microstructural evolution in Cu and Cu–Al alloys processed by high-pressure torsion

Disks of pure Cu and several Cu–Al alloys were processed by high-pressure torsion (HPT) at room temperature through different numbers of turns to systematically investigate the influence of the stacking fault energy (SFE) on the evolution of microstructural homogeneity. The results show there is initially an inhomogeneous microhardness distribution but this inhomogneity decreases with increasing numbers of turns and the saturation microhardness increases with increasing Al concentration. Uniform microstructures are more readily achieved in materials with high or low SFE than in materials with medium SFE, because there are different mechanisms governing the microstructural evolution. Specifically, recovery processes are dominant in high or medium SFE materials, whereas twin fragmentation is dominant in materials having low SFE. The limiting minimum grain size (d _min) of metals processed by HPT decreases with decreasing SFE and there is additional evidence suggesting that the dependence of d _min on the SFE decreases when the severity of the external loading conditions is increased.     

Texture of bismuth telluride-based thermoelectric semiconductors processed by high-pressure torsion

P-type Bi₂Te₃-based thermoelectric semiconductors were prepared, having a grain-refined microstructure and a preferred orientation of anisotropic crystallographic structure. Disks with a nominal composition of Bi_0.5Sb_1.5Te_3.0 were cut from an ingot grown by the vertical Bridgman method (VBM) and deformed at 473 K under a pressure of 6.0 GPa by high-pressure torsion (HPT). The crystal orientation was characterized by X-ray diffraction. The microstructures were characterized using optical microscopy and scanning electron microscopy (SEM). It was found that the HPT disks had a fine and preferentially oriented grain compared to that of the VBM disks. Further, the power factor of the HPT disks was about twice as large as that of the VBM disks. These results indicate that HPT is effective in improving the thermoelectric properties of Bi₂Te₃-based thermoelectric semiconductors.     

Mechanical response of proton beam irradiated nitinol

The present investigation deals with the study of mechanical behavior of proton beam irradiated nitinol at room temperature. The specimens in austenitic phase were irradiated over periods of 15, 30, 45 and 60 min at room temperature using 2 MeV proton beam obtained from Pelletron accelerator. The stress-strain curves of both unirradiated and irradiated specimens were obtained using a universal testing machine at room temperature. The results of the experiment show that an intermediate rhombohedral (R) phase has been introduced between austenite and martensite phase, which resulted in the suppression of direct transformation from austenite to martensite (A-M). Stresses required to start R-phase (σ_RS) and martensitic phase (σ_MS) were observed to decrease with increase in exposure time. The hardness tests of samples before and after irradiation were also carried out using Vickers hardness tester. The comparison reveals that the hardness is higher in irradiated specimens than that of the unirradiated one. The increase in hardness is quite sharp in specimens irradiated for 15 min, which then increases linearly as the exposure time is increased up to 60 min. The generation of R-phase, variations in the transformation stresses σ_RS and σ_MS and increase in hardness of irradiated nitinol may be attributed to lattice disorder and associated changes in crystal structure induced by proton beam irradiation.     

Microstructure and thermostability of a W–Cu nanocomposite produced via high-pressure torsion

A coarse-grained W–25% Cu alloy is subjected to high-pressure torsion (HPT) at room temperature to different strains. Evolution of the microstructure during HPT processing is studied using X-ray diffraction analysis, scanning and transmission electron microscopy. It is demonstrated that HPT processing results in fragmentation of the tungsten particles and the formation of a 5–15?nm grain size nanostructure at equivalent strains of ≥256 (saturation). It is shown that the nanostructured W–25% Cu is thermostable up to 500°C, with grain growth up to 50?nm at 720°C. During HPT processing, the lattice parameter of the copper and tungsten was found to increase and decrease, respectively, with increased level of equivalent strain. This is proposed to occur through the interdiffusion of copper atoms into tungsten grains and tungsten atoms into copper grains, as suggested by energy-dispersive X-ray analysis of the individual grains. The formation of a limited solid solution is considered and possible mechanisms for this effect discussed.     

Phase Transformations in Nd–Fe–B-Based Alloys under High Pressure Torsion at Different Temperatures

In this work, we studied the behavior of the Nd–Dy–Fe–Co–Cu–B alloy for permanent magnets under high pressure torsion (HPT). In the initial state of the studied alloy, it mainly contained the crystalline phase τ₁ (Nd, Dy)₂(Fe, Co, Cu) ₁₄B. After HPT at room temperature (T_HPT = 30°C), a mixture of an amorphous phase with nanocrystalline inclusions of the τ₁ phase is observed in the alloy. In the equilibrium phase diagram, this state is equivalent to a mixture of the τ₁ phase with the melt at the temperature T_eff= ∼1100°C. The thus determined T_eff value is called the effective temperature. When the T_HPT temperature of the HPT treatment increases to 300 and 400°C, the amorphous phase disappears, and the Fe₂B and γ-Fe phases appear instead. In the equilibrium phase diagram, this state is equivalent to a mixture of phases τ₁+ Fe₂B + γ-Fe, which is observed in the temperature range from ∼950 to ∼1050°C. We explain this phenomenon by the fact that with an increase in the HPT temperature T_HPT, the rate of formation of defects during deformation remains constant, but the rate of their thermal relaxation (annihilation) increases. This is equivalent to decrease in the effective temperature T_eff in the equilibrium phase diagram. The previously predicted decrease in T_eff with an increase in T_HPT is observed for the first time.

     

Spin-order and magnetic properties of BaZn2Fe16O27 (Zn2-W) hexagonal ferrite

M?ssbauer and magnetic measurements have been carried out both on single crystals and polycrystalline samples of BaZn₂Fe₁₆O₂₇ (Zn₂-W) hexagonal ferrite. The saturation magnetization at 0 K and at room temperature turns out to be very high, that is, 123 and 79 Gauss·cm³/g, respectively. The results have been interpreted by assuming a local reversal or a weakening of the Fe³⁺ magnetic moments due to the perturbing action of Zn²⁺ ions. The magnetic anisotropy is confirmed to be uniaxial with an anisotropy field at room temperature of 12.500 Oe.     

Study on the effect of Ar9+ ion irradiation of Zr–2.5 wt.% Nb alloy pressure tube

Response of Zr–2.5 wt.% Nb alloy pressure tube, used in PHWR nuclear reactors, to 315 keV Ar⁹⁺ ion irradiation at room temperature was investigated in the fluence range of 3.1?×?10¹⁵–4.17?×?10¹⁶ Ar⁹⁺?cm^?2. Changes in microstructural parameters, viz., the size of coherently scattering domains, microstrain and dislocation density, upon irradiation were ascertained through grazing incidence X-ray diffraction. In general, a decrease in domain size was observed with fluence with a corresponding increase in microstrain and dislocation density. Residual stress measurement showed the development of compressive stresses in place of tensile after irradiation. Transmission electron microscopy showed the formation of dislocation loops of ?a?-type and ?c?-type during irradiation. The hardness of irradiated samples, probed through nanoindentation technique, was found to be higher in comparison with unirradiated samples. The above findings have been rationalised on the basis of the defects generated during the Ar⁹⁺ ion irradiation.     

Enhanced multiferroic characteristics in Fe-doped Bi4Ti3O12 ceramics

Modification of Bi₄Ti₃O₁₂ multiferroic ceramics prepared by a conventional solid state reaction method were investigated by substituting Ti partly with Fe. The introduction of Fe does not change the layered perovskite structure of Bi₄Ti₃O₁₂. Upon increasing Fe content, the remnant polarization of the samples is enhanced. The magnetism of the ceramics at room temperature develops from diamagnetism to weak ferromagnetism with increasing Fe doping. The largest variations of 15% and 6% in remnant polarization and magnetization, achieved in a Bi₄Ti₁Fe₂O_12?δ sample after poling it in a magnetic field at 1 T and a DC electric field at 30 kV/cm for 10 min, are evidence of magnetoelectric coupling between the electric dipoles and magnetic dipoles at room temperature. The present results suggest a new candidate for a room temperature multiferroic material with enhanced properties.     

Ultra-thin a-SiNx protective overcoats for hard disks and read/write heads

This paper reports that amorphous silicon nitride (a-SiNx) overcoats were deposited at room temperature by microwave ECR plasma enhanced unbalanced magnetron sputtering. The 2 nm a-SiNx overcoat has better anti-corrosion properties than that of reference a-CNx overcoats (2-4.5 nm). The superior anti-corrosion performance is attributed to its stoichiometric bond structure, where 94.8% Si atoms form Si-N asymmetric stretching vibration bonds. The N/Si ratio is 1.33 as in the stoichiometry of Si3N4 and corresponds to the highest hardness of 25.0 GPa. The surface is atomically smooth with RMS < 0.2 nm. The ultra-thin a-SiNx overcoats are promising for hard disks and read/write heads protective coatings.     

Ferroelectricity in SrTiO3 nanocrystalline disks

SrTiO₃ and CaTiO₃ conventional bulk materials are incipient ferroelectrics. In this note, we report for the first time that ferroelectricity could occur in SrTiO₃ nanocrystalline disks even at room temperature. The peak in the temperature dependence of permittivity for a CaTiO₃ nanocrystalline disk at a low temperature is also observed. The observed ferroelectricity (or permittivity peak) in SrTiO₃ (or CaTiO₃) nanocrystalline disks could be attributed to the strain effect.     

Structural,electrical and dielectric studies of nanocrystalline LiMnPO4 particles

Olivine-structured LiMnPO₄ nanoparticles were prepared by microwave-assisted solvothermal method. The as obtained LiMnPO₄ sample was characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and impedance spectroscopy techniques. The XRD pattern confirms the formation of LiMnPO₄ phase with an orthorhombic structure. The electrical conductivity of the sample at room temperature is found to be 1.2654?×?10^?7 S cm^?1. Dielectric spectra show an increase in dielectric constant with increase of temperature. The dielectric loss spectra reveal the predomination of DC conduction in the sample. The modulus studies indicate the non-Debye nature of the sample which corresponds to the distribution of elements in the sample. Galvanostatic battery testing showed that LiMnPO₄ nanoparticles displayed good cycleability in 30 cycles.     

The structure and mechanical properties of Fe2AlMn single crystals

The microstructure and mechanical properties of off-stoichiometric single-crystal Fe₂AlMn have been investigated. After casting, the alloy contained two sets of thermal antiphase boundaries, (a/4)?111? and (a/2)?100?, which are attributed to the fact that the compound solidifies into a bcc structure and subsequently orders to a B2 structure and then a L2₁ structure respectively upon further cooling. Crystals strained under tension at room temperature in air at 1s^?1 showed 6% elongation, whereas specimens strained at 1?×?10^?5?s^?1 showed no elongation, indicating that the compound is sensitive to the testing environment. Fracture occurred on {100} in both cases. Compression tests showed that a yield anomaly was present at intermediate temperatures, with the peak yield strength occurring at about 800?K, which is slightly below the L2₁–B2 transition temperature of 898?K. The slip systems were found to be ?111?{110} at room temperature and 800?K. Transmission electron microscopy observations showed fourfold-dissociated ?111? dislocations in specimens strained at room temperature but only paired ?111? dislocations in specimens strained at the peak temperature. The room-temperature yield strength of quenched specimens increased with increasing quench temperatures from 700 to 1100?K.     

Synthesis and characterization of in situ nanocrystalline intermetallic phase reinforced AlTiSi amorphous matrix composite

Composites with partially amorphous matrix were synthesized by mechanical alloying of an Al₅₀Ti₄₀Si₁₀ elemental powder blend in a high energy planetary ball-mill, followed by high pressure (8 GPa) low temperature (350–450°C) sintering. Microstructural studies and compositional micro-analysis were carried out using scanning and transmission electron microscopy, and energy dispersive spectroscopy, respectively. Phase evolution as a function of milling time and isothermal temperature and their thermal stability was determined by X-ray diffraction at room or elevated temperature and differential scanning calorimetry, respectively. The microstructure of composites sintered between room temperature and 450°C showed nano-size (≈50 nm) crystalline precipitates of Al₃Ti dispersed in an amorphous matrix. The composites sintered at 400°C with 8 GPa pressure exhibited the highest density (3.58 Mg/m³), nanoindentation hardness (8.8 GPa), Young's modulus (158 GPa) and compressive strength (1940 MPa). A lower hardness and modulus on sintering at 450°C is attributed to additional amorphous to nanocrystalline phase transformation and partial coarsening of Al₃Ti.     

Synthesis of copper alloys with extended solid solubility and nano Al2O3 dispersion by mechanical alloying and equal channel angular pressing

Cu–4.5 wt % Cr and Cu–4.5 wt % Cr–3 wt % Ag alloys, with and without nanocrystalline Al₂O₃ dispersions (particle size <10 nm), were synthesized by mechanical alloying/milling and consolidated by equal-channel angular pressing (ECAP) at ambient temperature. Microstructural characterization and phase analysis by X-ray diffraction, as well as scanning and transmission electron microscopy, provided evidence for the formation of a Cu-rich extended solid solution with nanometric (<30 nm) crystallite size after 25 h of milling, with uniformly dispersed alumina nanoparticles embedded in it. Consolidation of Cu–4.5 wt % Cr–3 wt % Ag alloy with 10 wt % nanocrystalline Al₂O₃ by eight ECAP passes was shown to result in a composite with an exceptionally large hardness of 390 VHN and enhanced wear resistance. The electrical conductivity of the pellets of the latter alloy without Al₂O₃ is about 30% IACS (international annealing copper standard), whereas pellets with 5 or 10 wt % Al₂O₃ dispersion exhibit a conductivity of about 20–25% IACS. Thus, the present room temperature synthesis and consolidation route appear to offer a promising avenue for developing high-strength, wear/erosion-resistant Cu-based electrical contacts with nano-ceramic dispersion.     

The effect of gamma radiation of amino acid derivatives in the solid state: an EPR study

The spectroscopic features of free radicals produced in gamma irradiation of the N-methyl-dl – alanine, N,N – dimethyl glycine hydrochloride and d – (+) – galactosamine hydrochloride were investigated at room temperature at the dose 15?kGy using EPR technique. The paramagnetic species observed in these compounds were identified as CH₃?HCOOH, ?HOH(CHOH)₃NH₂(CH)₂O, and (CH₃)₂N?HCOOH.HCl, respectively. A comparison with a new paramagnetic species recently observed and characterized in irradiated amino acids indicates that the new radical presented here has a similar structure. In this paper, we have also studied the stability of these compounds at room temperature after irradiation.     

Transport properties of quinolinium (TCNQ)2 under pressure

We have measured the electrical conductivity and thermoelectric power of quinolinium (TCNQ)₂ in the pressure range from atmospheric to 20 kilobars at room temperature and for 15 kilobars from 300 K to 30 K. The room temperature conductivity is enhanced by a factor of seven at 20 kilobars in a manner similar to TTF-TCNQ, whereas the thermopower is only slightly reduced (～ 30%). At 15 kilobars the low temperature behavior remains dominated by disorder and Coulomb correlations.     

An investigation of the stored energy and thermal stability in a Cu–Ni–Si alloy processed by high-pressure torsion

ABSTRACT

The stored energy and activation energy for recrystallization were investigated for a Cu-Ni-Si alloy after high-pressure torsion processing for N?=?½, 1, 5 and 10 turns at room temperature. The contributions of geometrically necessary dislocations (GNDs), statistically stored dislocations (SSDs) and vacancies to the stored energy were calculated through the Vickers microhardness measurements, kernel average misorientation (KAM) measurements and an analysis by differential scanning calorimetry (DSC). The results show that the total stored energy decreases rapidly after equivalent strain of ε_eq?～?9 and then saturates through ε_eq?～?86 at ～70 J/mol. Concurrently, the local stored energy in GNDs and SSDs was found to depend strongly on the radial distance from the centre of the disc and increase with increasing equivalent strain at ε_eq?～?16 and saturate with further straining. Accordingly, the results indicate that the GNDs and vacancies are responsible for the high stored energy in the initial stage of deformation at equivalent strain range of ε_eq?=?8.6–16 and thereafter their contribution decreases slightly due to the occurrence of dynamic recrystallization and the formation of fine grains. At the same time, the contribution of the SSDs is similar to that of the GNDs only in high strain deformation as at ε_eq?=?49.3 to accommodate the deformation process. An activation energy for recrystallization was estimated in the range of ～?89.7–98.7 kJ/mol, thereby suggesting poor thermal stability.     

Laser isotope separation of tritium from deuterium: CO2-laser-induced multiphoton dissociation of C2TF5 in C2DF5

Isotope separation of tritium from deuterium in heavy water was attempted by CO₂-laser-induced, highly-selective multiphoton dissociation of C₂TF₅ present in C₂DF₅. Single-step T/D separation factors exceeding 3000, 1000, and 500 were attained, respectively, for the first time with CO₂ laser 10P(34) 931.0 cm^?1 at 10, 20, and 30 Torr pentafluoroethane pressures at ?78 °C (i.e., equivalent to 15, 30, and 45 Torr at room temperature).     

On the phenomena occurring at the interface between iron and iron–platinum thin films

FePt films were rf sputtered at room temperature and 550?C on MgO monocrystalline substrates. Room temperature films were post-annealed at 550?C. The high temperature growth induces a tetragonal distortion of the structure with the formation of L1₀-FePt phase. Soft iron layers, with different thickness, were e-beam deposited on these hard films. The phenomena occurring at the interface have been analysed and connected with the final magnetic behaviour of the system. It has been found that a strong exchange coupling between soft and hard layers has been established through the formation of an interfacial layer constituted by FePt small particles.