Sn Mössbauer studies of the DyAgSn and DyAuSn compounds

X-ray diffraction, magnetic susceptibility measurements and ¹¹⁹Sn Mössbauer spectroscopy were used to study the intermetallic DyAuSn and DyAgSn compounds. It was shown that both compounds order antiferromagnetically at low temperatures. The Dy magnetic moments are normal to the hexagonal c-axis in DyAuSn and are inclined at an angle of 45° to this axis in DyAgSn. The occurrence of a magnetic hyperfine field distribution in DyAgSn suggests that this compound crystallizes in the disordered CaIn₂-type of structure.     

Hyperfine interactions studied by Sn Mössbauer spectroscopy in TbAuSn and TmAuSn compounds

X-ray diffraction, magnetic susceptibility measurements and ¹¹⁹Sn Mössbauer spectroscopy were used to study structure and hyperfine interactions in intermetallic TbAuSn and TmAuSn compounds. It was shown that the first compound orders antiferromagnetically at low temperatures with Tb magnetic moments inclined at an angle of about 25° to the crystallographic c-axis. The second compound stays non-magnetic in the whole measured region. The occurrence of a magnetic hyperfine field distribution in TbAuSn suggests that this compound crystallizes in the disordered CaIn₂-type of structure.     

Electronic structure and momentum density distribution of titanium dioxide

LCAO calculations have been performed for the electronic and structural properties of the rutile TiO₂ under the periodic HF and DFT schemes. The methods have been applied to study Compton profiles and the structure factors. The experimental Compton profile based on Am²⁴¹ Compton spectrometer for polycrystalline TiO₂ has been compared with the calculations. The calculated Compton profile from HF-LCAO has been found to be in good agreement with the measurement compared to the ionic model and DFT-LCAO method. The published experimental X-ray structure factors support the FLAPW method more than the periodic HF-LCAO method. Signatures of charge transfer on compound formation are observed. Partial ionic as well as covalent character of bonding is observed on the basis of structure factor as well as momentum density analysis. The present work enables to examine the DFT and HF approaches in terms of structure factor and the Compton profile studies.     

Solidification study of Al–Co–Cu alloys using the Bridgman method

The solidification of a series of Bridgman-grown Al–Co–Cu alloys with compositions in the vicinity of the quasicrystal was studied by powder X-ray diffraction (XRD), differential thermal analysis (DTA), electron microprobe analysis, and optical microscopy. The phase equilibria and microstructure of solidified alloys are presented; the temperatures of the involved solidification reactions were determined. These experimental data were used to construct a solidification phase diagram as to understand the crystallization path. The decagonal (D) AlCoCu quasicrystals form incongruently, but they can be primarily solidified from off-stoichiometric melts.     

Boron content dependence of crystallization, glass forming ability and magnetic properties in amorphous Fe-Zr-B-Nb alloys

The glass forming ability (GFA) was investigated in Fe_91−xZr₅B_xNb₄ alloys with B contents of 0–36 at.%. The GFA changes with B content, and fully amorphous alloys were prepared by melt spinning for B contents between 5 and 30 at.%. The amorphous alloys crystallize with a primary crystallization mode in the low B content range of 5≤x≤20 at.%, but in the eutectic mode in the high B content range of 20<x<30 at.%. A single new metastable Fe-Zr-B-Nb cubic phase with a lattice constant of 1.0704 nm, a saturation magnetization of 137 emu/g and a coercivity of 7.3 Oe at room temperature is formed when crystallizing in a polymorphous mode at x=30 at.%. The glass transition temperature (T_g), crystallization temperature (T_x), Curie temperature (T_c) and saturation magnetizations (M_s) of the amorphous alloys increase with increasing B content, but the coercivity (H_c) decreases. As the B content exceeds 20 at.%, not only increase the T_g, T_x and GFA sharply, due to the change of crystallization mode, but also the concentration dependence of the T_c and M_s changes. It is concluded that the amorphous alloys have better GFA, thermal stability and soft magnetic properties for the high B contents of 25–30 at.% than for the low B contents of 5–20 at.%.     

Growth of single crystals of Hg(BrxI1−x)2 and their detection capability

Hg(Br_xI_1−x)₂ crystals were grown by the Bridgman method for 0.2 < x < 1.0. They were tested for potential implementation as X- and γ-ray detectors at room temperature. ²⁴¹Am and ⁵⁵Fe were used as radioactive sources. From the corresponding energy spectra, it is evident that crystals with x = 0.2 show enhanced resolution at low energies (below 200 keV), competing those fabricated from HgI₂ and CdTe. Crystals with higher x's were of lower resolution.     

Application of controlled and electrical discharge assisted mechanical alloying for the synthesis of nanocrystalline MgB2 superconducting compound

In this paper we present the results of our efforts to synthesize the nanocrystalline MgB₂ superconducting compound from elemental Mg and B powders by combination of controlled mechanical pre-alloying in a magneto-mill Uni-Ball-Mill 5 under shearing mode followed by electrical discharge (ED) assisted mechanical alloying (MA). There is no conclusive evidence of MgB₂ formation in the Mg-2B mixture using crystalline boron after controlled mechanical alloying (CMA) under protective argon or helium atmosphere as well as subsequent ED assisted alloying. There seems to be some XRD evidence of the strongest (1 0 1) MgB₂ peak presence in the Mg-2B mixture processed using both crystalline and amorphous boron after CMA under hydrogen as well as subsequent ED assisted alloying but this evidence is rather ambiguous. We postulate here that it is highly likely that a certain critical Mg nanograin size must be achieved before a successful reaction to form nanocrystalline MgB₂ is going to be completed. Following recent report by Gümbel et al. [Appl. Phys. Lett. 80 (2002) 2725] this critical value can be roughly estimated at 15 nm or less. Calculations of the Mg nanograin size in the present work show that only three Mg-2B powders ball milled under hydrogen meet this critical nanograin size criterion for the Mg phase. However, a massive formation of the β-MgH₂ hydride in these powders consumes the available Mg in the reaction with hydrogen which may leave inadequate concentration of Mg to form MgB₂ even though the nanograin size of Mg is sufficiently refined, say below 15 nm.     

Stress induced crystallization of amorphous materials and mechanical properties of nanocrystalline materials: a molecular dynamics simulation study

The deformation behavior of amorphous and nanocrystalline pure Ni thin films has been investigated using a molecular dynamics simulation study based on a semi-empirical interatomic potential (MEAM). It was observed that a tensile stress introduced to an amorphous material can enhance crystallization which eventually serves as an important deformation mechanism. After completion of crystallization, grains grow mainly by the rotation and coalescence, and with increasing grain size, the flow stress also increases. It was also found that when the grain size is small (below about 3 nm), the dominant deformation mechanisms are the grain rotation and the grain boundary sliding, the former being more active for smaller grains. The dependence of these observations on the interatomic potential used in the simulation is also discussed.     

Structure and properties of as-cast Ti-Dy alloys

A prediction of the Ti-Dy phase diagram was given, according to which the system is of eutectic type with the eutectic coordinates 1270 °C and 82 at.% Dy. Experimental verification on the two key-alloys confirmed the type of system, as well as the eutectic coordinates. The last were established to be 1280 °C and 82 at.% Dy. The eutectic is fine with a grain size of about 0.2 μm. The hypo-eutectic alloy 95Ti-5Dy showed high plasticity at contraction that is on the level of pure titanium. The Ti-Dy system is proposed as a basis of composite materials of eutectic type.     

Fe基非晶纳米晶的等温退火工艺

通过重力式单辊喷带机制备成分为Fe73.5Si13.5B9Nb3Cu1非晶带材,并对其进行等温退火处理。等温DSC测试结果表明,重力非晶条带的等温晶化过程是受三维扩散控制的、形核率不断减小的晶化过程,以晶化相的稳态长大过程为主。随着退火温度增高,纳米晶α-Fe相的晶格常数不断减小,表明固溶在α-Fe晶格中的Si含量不断增加。与此同时,α-Fe的晶粒尺寸仅仅略有增加。真空退火样品磁性能测试结果表明,该成分样品的最合适的热处理工艺为退火温度552 ℃,保温时间为80 min。     

Features of the HDDR process in R–Fe–B ferromagnetic alloys (R is a mixture of Nd, Pr, Ce, La, Dy and others)

Features of the conventional hydrogenation, disproportionation, desorption, recombination (HDDR) and solid-HDDR processes in some R–Fe–B (R is a mixture of Nd, Pr, Ce, La, Dy) ferromagnetic alloys were studied in the temperature range 20–990 °C and pressure range from 1×10⁻³ Pa to 0.1 MPa. This was carried out by means of differential thermal analysis (DTA), X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) methods. The hydride of the initial phase is formed by heating to 115 °C. The disproportionation of the alloys occurs in the temperature range from 320 to 800 °C. Φ-phase constitutes the base of the initial alloys. Among the disproportionation products, R-hydride, -Fe and two borides (Fe₂B and R_1.1Fe₄B₄) were revealed. The initial phase in all the alloys is recovered after heating in vacuum to a temperature of 990 °C. Full hydrogen desorption occurs in two temperature ranges with the peaks at 200–320 and 630–715 °C.     

A comparative study of magnetron co-sputtered nanocrystalline titanium aluminium and chromium aluminium nitride coatings

A comparative study of magnetron co-sputtered (Ti,Al)N and (Cr,Al)N coatings was made. It was found that while both coatings followed similar development pattern with increasing nitrogen pressure, the (Cr,Al)N coatings achieved much higher deposition rate and hardness, suggesting the coatings had a great potential for many industrial applications.     

Oxidation behavior of amorphous and nanoquasicrystalline Zr–Pd and Zr–Pt alloys

Oxidation behavior of amorphous and nanoquasicrystalline Zr₇₀Pd₃₀ and Zr₈₀Pt₂₀ alloys melt-spun at different wheel speeds has been studied in air by non-isothermal and isothermal techniques. Oxidation resistance of amorphous alloys has been found to be the lowest in comparison to the partially and fully crystallized Zr alloys. It has also been observed that oxidation does not induce crystallization of the amorphous phase. It has been shown that the oxygen diffusion rate increases gradually in the order of crystalline, nanoquasicrystalline, partially nanocrystalline and amorphous states of these alloys. Possible micromechanism of oxidation and the role of different grain/interface boundaries on the oxygen diffusion has been discussed.     

A quantitative analysis of the evolution of texture and stored energy during annealing of cold rolled copper

The experimental evolution of the global texture during recrystallization of cold rolled copper is presented after various rolling reductions. After presentation of the method used for the decomposition of the orientation distribution functions obtained from X-ray diffraction measurements into gaussian peaks, the behavior of each texture component is studied. This quantitative analysis coupled with stored energy measurements also obtained by X-ray diffraction allows the oriented nucleation and growth mechanisms to be linked to stored energy variations. The proposed mechanisms are then confirmed by local observations obtained by EBSD.     

Direct synthesis of MgCNi3 by mechanical alloying

MgCNi₃, an intermetallic compound with superconductivity, was synthesized from the Mg (or Mg₂Ni), Ni and graphite powders by mechanical alloying (MA). It is shown that the preliminary condition for the formation of MgCNi₃ is that Mg₂Ni must form in advance of MgCNi₃ in the MA process or be the starting component.     

Influence of technical parameters on strength and ductility of AlSi9Cu3 alloys in squeeze casting

An orthogonal test was conducted to investigate the influence of technical parameters of squeeze casting on the strength and ductility of AlSi9Cu3 alloys. The experimental results showed that when the forming pressure was higher than 65 MPa, the strength (σ_b) of AlSi9Cu3 alloys decreased with the forming pressure and pouring temperature increasing, whereas σ_b increased with the increase of filling velocity and mould preheating temperature. The ductility (δ) by alloy was improved by increasing the forming pressure and filling velocity, but decreased with pouring temperature increasing. When the mould preheating temperature increased, the ductility increased first, and then decreased. Under the optimized parameters of pouring temperature 730 °C, forming pressure 75 MPa, filling velocity 0.50 m/s, and mould preheating temperature 220 °C, the tensile strength, elongation, and hardness of AlSi9Cu3 alloys obtained in squeeze casting were improved by 16.7%, 9.1%, and 10.1%, respectively, as compared with those of sand castings.     

Effect of Mn addition on the thermal stability and magnetic properties of rapidly-quenched L10 FePt alloys

Nano-composite magnets with L1₀ structure derived from binary FePt alloys and prepared as melt-spun ribbons are of current interest due to their higher operating temperature and the ability to be cast as a two-phase magnet with exchange spring magnetic properties, as both soft and hard magnetic phase may emerge from the same metastable precursor, i.e. the disordered cubic A1 phase. The present paper studies the effect of Mn addition on the thermal stability and phase structure, on the abundance of the hard magnetic phase and relative proportion of the soft ones, on the microstructure of the alloy as a function of temperature and on the overall magnetic properties. The interplay of the various magnetic sublattices in the ordering of the L1₀ phases as a consequence of introducing antiferromagnetically coupled Mn atoms in the alloy composition is discussed and interpreted in terms of microstructural changes induced by this addition as revealed by high resolution transmission electron microscopy and X-ray diffraction. The temperature evolution of the phase composition and structural parameters is monitored using synchrotron radiation powder diffraction, while the compositional aspects are investigated using proton-induced X-ray emission and energy dispersive X-ray spectroscopy. Magnetic measurements reveal the magnetic parameters of interest (coercivity, remanence, Curie temperature, saturation magnetization), as well as the exchange-coupled two-phase nature of these magnets and provide information that hints at possible spin reorientation transitions in the Mn-containing planes of the L1₀ superlattice.     

Crystal structures of R2Pd2Pb (R = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu) compounds

The crystal structures of the R₂Pd₂Pb (R=Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu) compounds were determined using X-ray powder diffraction. The investigated compounds crystallize with Mo₂FeB₂ structure type (space group P4/mbm, Pearson code tP10). The importance of stabilization by polar intermetallic R–Pd bonding is underscored by a bonding analysis derived from electronic band structure calculations.     

Processing of Fe3Al and FeAl alloys by reaction synthesis

The effects of powder particle size, alloy composition, and reaction atmosphere on reaction synthesis of binary Fe---Al alloys were studied. Reactions were observed in an open (air) furnace, under static vacuum (in an evacuated quartz tube) and in a dynamic vacuum furnace. Reactions occurring in the open furnace and in the evacuated quartz tube were recorded using high-speed video equipment. High-speed videotapes of reaction synthesis of compacts formed from 45 μm Fe and 10 μm Al particles reacted in air and under static vacuum revealed that an unusual ‘two-stage’ reaction exists in this system under these conditions. Compacts formed from 9 μm Fe and 3 μm Al powder particles do not exhibit a two-stage reaction under any of the conditions examined in this work. The first stage of the two-stage reaction lasts several seconds and starts at round 650 °C. The second stage begins at about 900 °C, reaching temperatures between 1250 and 1350 °C. The progress of the reaction to the second stage is sensitive to the alloy composition and reaction atmosphere. The reaction behavior is explained in terms of thermodynamics and heat transfer, which control the delicate balance between heat accumulation and heat loss during reaction synthesis.     

Synthesis and hydrogen storage properties of Mg-based alloys

Mg-based alloys have been made by mechanical alloying Mg with some transition and non-transition elements. The thermal stability and hydrogen storage properties have been investigated. It was found that mechanical alloying results in a supersaturated solid solution of some elements in the Mg phase. Thermal annealing and/or hydrogenation cause irreversible decomposition of supersaturated solid solution leading to a composite of Mg or MgH₂ with other phase(s) depending on the composition and contents. Therefore, the plateau pressure or thermodynamic properties of hydrogen absorption/desorption of the supersaturated solid solution are no different from that of the Mg composite. While in some equilibrium systems, the formation of Mg solid solution is reversible upon hydrogenation/dehydrogenation. The plateau pressure of the hydrogenation/dehydrogenation is increased due to the interaction of the alloying elements with the Mg lattice in the solid solution. The Mg–Li system is an exception because of the formation of stable LiH upon hydrogenation of Mg(Li) solid solution. No interaction takes place between Mg or MgH₂ with LiH, therefore, no destabilization of MgH₂ is observed.