Short-range order clustering in BCC Fe–Mn alloys induced by severe plastic deformation

The effect of severe plastic deformation, namely, high-pressure torsion (HPT) at different temperatures and ball milling (BM) at different time intervals, has been investigated by means of Mössbauer spectroscopy in Fe_100–xMn_x (x = 4.1, 6.8, 9) alloys. Deformation affects the short-range clustering (SRC) in BCC lattice. Two processes occur: destruction of SRC by moving dislocations and enhancement of the SRC by migration of non-equilibrium defects. Destruction of SRC prevails during HPT at 80–293 K; whereas enhancement of SRC dominates at 473–573 K. BM starts enhancing the SRC formation at as low as 293 K due to local heating at impacts. The efficiency of HPT in terms of enhancing SRC increases with increasing temperature. The authors suppose that at low temperatures, a significant fraction of vacancies are excluded from enhancing SRC because of formation of mobile bi- and tri-vacancies having low efficiency of enhancing SRC as compared to that of mono vacancies. Milling of BCC Fe_100–xMn_x alloys stabilises the BCC phase with respect to α → γ transition at subsequent isothermal annealing because of a high degree of work hardening and formation of composition inhomogeneity.     

Ferromagnetism caused by severe plastic deformation of the Fe – 35% Al alloy

Results of investigations into the structure and temperature dependence of magnetization of the Fe – 35% Al alloy subjected to severe plastic deformation are presented. It is demonstrated that after plastic deformation, this alloy becomes ferromagnetic, though before deformation it was non-ferromagnetic. The temperature interval of the ferromagnetic order stability arising due to plastic deformation is established. The revealed effect is explained.     

Mössbauer spectroscopy of grain boundaries in ultrafine-grained materials produced by severe plastic deformation

The applicability of Mössbauer spectroscopy in studying grain boundaries in ultrafine-grained materials produced by severe plastic deformation (SPD) is analyzed. It is shown that grain boundaries after SPD are in a nonequilibrium state that is characterized by excess free volume.     

Allotropic forms of carbon in the Invar Fe–Ni–C alloy before and after plastic deformation by upsetting

The effect of cold plastic deformation by upsetting (e = 1.13) on structure and hybridised bonds of carbon in the fcc Invar Fe-30.9%Ni-1.23% C alloy was studied by means of X-ray phase analysis and X-ray photoelectron spectroscopy. Carbon precipitates along grain boundaries and inside of grains in the alloy after annealing and plastic deformation were revealed. The presence of mainly sp²- and sp³-hybridised C–C bonds attributing to graphite and amorphous carbon as well as the carbon bonds with impurity atoms and metallic Fe and Ni atoms in austenitic phase were revealed in the annealed and deformed alloy. It was shown for the first time that plastic deformation of the alloy results in partial destruction of the graphite crystal structure, increasing the relative part of amorphous carbon, and redistribution of carbon between structural elements as well as in a solid solution of austenitic phase.     

An X-ray absorption spectroscopy investigation of the local atomic structure in Cu–Ni–Si alloy after severe plastic deformation and ageing

The local atomic structure of Cu–Ni–Si alloy after severe plastic deformation (SPD) processing and the decomposition of supersaturated solid solution upon annealing were investigated by means of X-ray absorption spectroscopy. The coordination number and interatomic distances were obtained by analyzing experimental extend X-ray absorption fine structure data collected at the Ni K-edge. Results indicate that the environment of Ni atoms in Cu–Ni–Si alloy is strongly influenced by the deformation process. Moreover, ageing at 973 K affects strongly the atomic structure around the Ni atoms in Cu–Ni–Si deformed by equal channel angular pressing and high pressure torsion. This influence is discussed in terms of changes and decomposition features of the Cu–Ni–Si solid solution.     

Properties of Fe–Ga based powders prepared by mechanical alloying

Alloys of Fe–Ga with starting compositions of 17, 19, 21, 23, and 25 at% Ga and Fe₈₁Ga₁₇Z₂ (Z=Si, Sn) have been prepared by mechanical alloying. Samples were milled in a SPEX Model 8000 mill with a ball to sample weight ratio of about 4:1. Phase formation as a function of milling time has been investigated for the 19 at% Ga sample and suggests that milling times of 12 h produce fully alloyed samples. Alloys have been studied by electron microprobe, X-ray diffraction, vibrating sample magnetometery and ⁵⁷Fe Mössbauer effect spectroscopy. Fully milled powders have measured compositions of Fe_100−xGa_x with x=15.7, 17.0, 19.0, 22.4, and 24.0 and Fe_83.1Ga_15.2Z_1.7 (for both Z=Si and Sn). X-ray diffraction showed the presence of a disordered bcc phase with no indication of an ordered D0₃ phase. However, the latter is difficult to observe with X-ray diffraction because of the low intensity of the fcc superlattice peaks. A bimodal Fe hyperfine field distribution as obtained from Mössbauer effect spectra indicated the presence of two discrete Fe environments. The results suggested a lower degree of Ga clustering than has been previously observed in Fe–Ga alloys, of similar composition, prepared by melt spinning. The microstructure is similar to that of Fe–Ga thin films prepared by combinatorial sputtering. Some samples have also been studied after annealing at 800 °C for 8 h. No changes were observed in X-ray diffraction patterns after annealing. However, Mössbauer effect studies show the formation of D0₃ and L1₂ order in annealed samples analogous to the phases observed in melt spun ribbons of similar composition.     

Magnetic properties of Fe–Pt thick-film magnets prepared by RF sputtering

Thick films of L1₀ ordered Fe–Pt alloy magnet with a high maximum energy product were prepared by using a three-dimensional sputtering apparatus. With decreasing the Ar pressure from 3 to 0.6 Pa, the films annealed at 600 °C underwent a gradual phase transformation from the disordered FCC phase to the ordered FCT one. With further decreasing the pressure to 0.43 Pa, the disordered phase appeared again. The values of _Hc$H_{\mathrm{c}}$ and (_BH)max$({\mathrm{BH})}_{\max }$ were maximized to be approximately 399 kA/m and 90 kJ/m³ at 0.6 Pa of Ar pressure, respectively. While varying the input power at a stable pressure of 0.6 Pa, the as-deposited samples were dominated by the disordered phase at the applied power of 100 W RF, and the heat treatment resulted in a change to such L1₀ ordered phase. At input power higher than 120 W, both the as-deposited and annealing samples were ordered to the hard L1₀ FCT phase, and high _Hc$H_{\mathrm{c}}$ and (_BH)max$({\mathrm{BH})}_{\max }$ values of about 446 kA/m and 124 kJ/m³, respectively, were obtained on the sample deposited at the input power of 180 W.     

The role of internal stresses on the plastic deformation of the Al–Mg–Si–Cu alloy AA6111

In this work, we have investigated the internal stress contribution to the flow stress for a commercial 6xxx aluminium alloy (AA6111). In contrast to stresses from forest and precipitation hardening, the internal stress cannot be assessed properly with a uniaxial tensile test. Instead, tension–compression tests have been used to measure the Bauschinger stress and produce a comprehensive study which examines its evolution with (i) the precipitation structure, and (ii) a wide range of applied strain. A large set of ageing conditions was investigated to explore the effect of the precipitation state on the development of internal stress within the material. It is shown that the Bauschinger stress generally increases with the applied strain and critically depends on the average radius of the precipitate and is thus linked to the shearable/non-shearable transition. Further work in the case of non-shearable particles shows that higher strain eventually leads to particle fracture and the Bauschinger stress then rapidly decreases. Following the seminal work of Brown et al. a physically based approach including plastic relaxation and particle fracture is developed to predict the evolution of the internal stress as a function of the applied strain. Knowing the main characteristics of the precipitation structure–such as the average precipitate radius, length and volume fraction–allows one to estimate accurately the internal stress contribution to the flow stress with this model.     

Synthesis and magnetic properties of nanocrystalline Zn1–x Fe x O solid solutions

Bulletin of the Russian Academy of Sciences: Physics - Zn1–x Fe x O nanocrystalline solid solutions (x = 0, 0.025, 0.05) with wurtzite structure and ferromagnetic properties at room...     

Fe–Si core/Si-shell clusters prepared by double glow discharge sources

Journal of Nanoparticle Research - Using an improved plasma gas condensation cluster deposition system, Fe and Si clusters are prepared by a single glow discharge source and Fe/Si hybrid clusters...     

High-field magnetization process of granular Co–Cu alloys prepared by melt spinning

High solid-solubility Co₁₅Cu₈₅ alloys have been prepared by melt spinning and submitted to isothermal and anisothermal annealing to obtain granular alloys. The X-ray diffraction (XRD) patterns were measured to investigate the decomposition of supersaturated solid solution induced by annealing. The atomic diffusion and structural evolution during the heat treatment were investigated. In view of the problems limiting their application, the high-field magnetization curves were measured. By a fit to the high-field magnetization curves, the granular alloys are found hard to be magnetically saturated at the early stage of nucleation and growth. The magnetization behavior was correlated to the annealed-induced structural evolution and also to the magnetoresistance effect.     

Defects and localized states in MBE-grown GaAs1−x N x solid solutions prepared by molecular-beam epitaxy

Defects and localized states have been studied for molecular-beam-epitaxy (MBE)-grown high-resistivity and undoped GaAs_1?x N _x films with a N concentration not exceeding approximately 1.0 at.%. The crystalline quality of the films and hence the defects and localized states were determined by high-resolution X-ray diffraction, photoluminescence spectra, capacitance versus voltage measurements and photoinduced current transient spectra of GaAs and GaAs_1?x N _x layers. It was concluded that incorporation of low concentrations of N into MBE-grown GaAs_1?x N _x films promotes the formation of high densities of deep centres similar to EL2 donors, leading to heavy compensation of the films by some unidentified acceptors. GaAs antisite acceptors were believed to be responsible for the said compensation. A prominent defect band near 1.33-1.38 eV also appeared to be associated with these defects. The most prominent centres in dilute GaAs_1?x N _x films with N content less than 0.35 at.% seem to be the EL2 donors and the hole traps located near E _v +0.3 eV.     

Electrical contact properties of Cu2S nanowires grown vertically on Cu foil by gas–solid reaction

We grew Cu₂S nanowires vertically on Cu foil by gas–solid reaction with a gas mixture of O₂ and H₂S. The electrical contact properties between the Cu₂S nanowires and Cu foil were investigated using a modified current–voltage–temperature plot. The Cu/Cu₂S layer exhibited the characteristics of a Schottky barrier with a barrier height of ∼0.72 eV, which was closer to the value for Cu/Cu₂O than to Cu/Cu₂S. Energy dispersive spectroscopy results showed the presence of Cu-oxide between the Cu₂S nanowires and Cu foil. The overall structure was Cu/Cu-oxide/Cu₂S and the electrical properties were controlled by the Cu/Cu-oxide.     

Effects of Cu doping on nickel oxide thin film prepared by sol–gel solution process

We prepared nickel oxide (NiO) thin films with p-type Cu dopants (5 at%) using a sol–gel solution process and investigated their structural, optical, and electrical characteristics by X-ray diffraction (XRD), atomic force microscopy (AFM), optical transmittance and current–voltage (I–V) characteristics. The crystallinity of the NiO films improved with the addition of Cu dopants, and the grain size increased from 38 nm (non-doped) to 50 nm (Cu-doped). The transmission of the Cu-doped NiO film decreased slightly in the visible wavelength region, and the absorption edge of the film red-shifted with the addition of the Cu dopant. Therefore, the width of the optical band gap of the Cu-doped NiO film decreased as compared to that of the non-doped NiO film. The resistivity of the Cu-doped NiO film was 23 Ω m, which was significantly less than that of the non-doped NiO film (320 Ω m). Thus, the case of Cu dopants on NiO films could be a plausible method for controlling the properties of the films.     

Mössbauer studies of nanosized ferrites prepared by the combustion of metal nitrates–oxalyl dihydrazide solutions

Alkaline earth metal ferrites have been prepared by combustion of redox mixtures containing metal nitrates and oxalyl dihydrazide (ODH) at 673 K. On rapid heating, ODH–metal nitrate mixture undergoes an abrupt exothermic redox chemical reaction that facilitates atomic scale mixing of cations. This leads to the formation of stoichiometrically pure and single-phase nanoparticle ferrites at comparatively reduced temperature (400°C) and in less time than possible by the conventional ceramic method. Because of their different cationic size, polarizability etc., alkaline earth metal cations yield different type of ferrites i.e. MFe₂O₄ (M = Mg, Ba), Ca₂Fe₂O₅ and SrFe₁₂O₁₉.     

Diffuse interface model for compressible fluid – Compressible elastic–plastic solid interaction

An Eulerian hyperbolic diffuse interface model for elastic–plastic solid–fluid interaction is constructed. The system of governing equations couples Euler equations of compressible fluids and a visco-plastic model of Maxwell type materials (the deviatoric part of the stress tensor decreases during plastic deformations) in the same manner as models of multicomponent fluids. In particular, the model is able to create interfaces which were not present initially.The model is thermodynamically compatible: it verifies the entropy inequality. However, a numerical treatment of the model is particularly challenging. Indeed, the model is non-conservative, so a special numerical splitting is proposed to overcome this difficulty. The numerical algorithm contains two relaxation procedures. One of them is physical and is related to the plastic relaxation mechanism (relaxation toward the yield surface). The second one is numerical. It consists in replacing the algebraic equation expressing a mechanical equilibrium between components by a partial differential equation with a short relaxation time. The numerical method was tested in 1D case (Wilkins’ flying plate problem), 2D plane case (impact of a projectile on a plate) and axisymmetrical case (Taylor test problem, impact with penetration effects, etc.). Numerical examples show the ability of the model to deal with real physical phenomena.     

Phonon- and phason-dynamics in Al–Cu–Fe quasicrystals

The lattice dynamics of quasicrystals includes local phason jumps as well as phonons. Phason dynamics is important for the understanding of both the structure and atomic motion in quasicrystals, leading to short-ranged atomic motion not involving vacancies in addition to diffusion. We have studied the phason and phonon dynamics of icosahedral i-Al₆₂Cu_25.5Fe_12.5. Quasielastic Mössbauer spectroscopy (QMS) was used to probe the iron phason dynamics. Inelastic nuclear-resonant absorption (INA) of synchrotron radiation and inelastic neutron scattering (INS) were used to study the iron-partial as well as the total vibrational DOS (VDOS). We find from preliminary QMS studies that iron atoms jump on a time scale about two orders of magnitude slower than that found for copper. The EFG shows an abrupt change in slope at ca. 825 K which may be related to a transition from simple (isolated) to more complicated (co-operative) phason jumps. From INA we find that the iron-partial VDOS differs radically from that of the total (neutron-weighted) generalised VDOS measured by INS. Both these properties are related to the specific local environments of Fe and Cu in i-AlCuFe.