Formation and properties of the Zr75−xAlxNi10Cu10Ti5 bulk metallic glasses

Zr-based bulk metallic glasses (BMGs) exhibit interesting mechanical properties since they combine high fracture stress, elastic strain (up to 2%), significant fracture toughness and good corrosion resistance. Quaternary systems with general composition Zr–Ni–Cu–Ti show wide composition ranges in which BMG can be obtained. The addition of the another element to the quaternary alloys often increases the glass forming ability (GFA). The aim of this work was to study the influence of aluminium content on the GFA and on the mechanical properties of the Zr–Ni–Cu–Ti alloys. Multicomponent Zr_75−xAl_xNi₁₀Cu₁₀Ti₅ (x = 15, 20 at%) alloys were produced by melt spinning method obtaining ribbons, and by casting technique into a copper mould, manufacturing rod shape samples with maximum diameter of 2 mm. Supercooled liquid region depends on chemical composition and exceeds 45 °C. Vickers microhardness of studied alloys is comparable to the highest ones for other Zr-based BMG.     

Quantifying the strain-induced dissolution of precipitates in Al alloy microstructures using nuclear magnetic resonance

Nuclear magnetic resonance (NMR) has been used for the first time to directly monitor the dynamic partitioning of Cu atoms from shearable precipitates into the solid solution as a function of straining at room temperature in two Al–Cu-based alloys. Al–3Cu–0.05Sn (wt.%) and Al–2.5Mg–1.5Cu (wt.%) alloys were heat-treated to provide a fine distribution of 5 nm Guinier–Preston (GP) zones and <1 nm Guinier–Preston–Bagaryatsky (GPB) zones, respectively, and were then subjected to rolling strains up to 100%. It is shown that in the Al–Cu–0.05Sn alloy, strains up to 40% can pump solute from the 5 nm GP zones back into solid solution for the temperature and strain-rate of deformation employed here. In the case of the Al–Cu–Mg alloy, no dissolution of the GPB zones is observed. A simple model for the strain-induced dissolution of the shearable precipitates is given and compared with the experimental results. The dependence of the Cu repartitioning process on the precipitate size is emphasized. These observations and modeling give guidelines for the design of Al–Cu-based alloys to exploit the dynamic interplay of strain-induced Cu partitioning between metastable states, e.g. solid solution and GP (or GPB) zones, for tailoring ultimate mechanical properties. It is proposed that this strain-induced phase transformation is a form of dynamically responding microstructure that can be employed to obtain aluminum alloys with well-designed microstructures.     

Amorphous and nanocrystalline sputtered Mg–Cu thin films

Binary Mg–Cu amorphous alloys were first fabricated in 1980s via liquid quenching. In this study, the Mg_1−xCu_x (x varying from 38 at.% to 82 at.%) partially amorphous thin films are prepared via co-sputtering. Upon thermal annealing, the Mg₂Cu or MgCu₂ nanocrystalline phases are induced in the Mg-rich or Cu-rich thin films, respectively. Due to the presence of fine nanocrystalline Mg₂Cu or MgCu₂ particles in the Mg–Cu amorphous matrix, the as-sputtered thin films show satisfactory Young's modulus 100 GPa and hardness 4 GPa.     

Thermoelectric properties of novel skutterudites with didymium: DDy(Fe1−xCox)4Sb12 and DDy(Fe1−xNix)4Sb12

In order to improve the thermoelectric properties via efficient phonon scattering Didymium (DD), a mixture of Pr and Nd, was used as a new filler in ternary skutterudites (Fe_1−xCo_x)₄Sb₁₂ and (Fe_1−xNi_x)₄Sb₁₂. DD-filling levels have been determined from combined data of X-ray powder diffraction and electron microprobe analyses (EMPA). Thermoelectric properties have been characterized by measurements of electrical resistivity, thermopower and thermal conductivity in the temperature range from 4.3 to 800 K. The effect of nanostructuring in DD_0.4Fe₂Co₂Sb₁₂ was elucidated from a comparison of both micro-powder (ground in a WC-mortar, 10 μm) and nano-powder (ball-milled, 150 nm), both hot pressed under identical conditions. The figure of merit ZT depends on the Fe/Co and Ni/Co-contents, respectively, reaching ZT > 1. At low temperatures the nanostructured material exhibits a higher thermoelectric figure of merit. The Vickers hardness was measured for all samples being higher for the nanostructured material.     

The precipitation process in Mg–Ca–(Zn) alloys investigated by positron annihilation spectroscopy

Coincidence doppler broadening (CDB) spectroscopy has been applied to study the precipitation process induced by aging in Mg–1.0 wt.% Ca and Mg–1.0 wt.% Ca–1.0 wt.% Zn alloys. In addition positron lifetime experiments and microhardness measurements have been performed. A peak centered at 11.5 × 10⁻³m₀c is found in the CDB ratio spectra of the alloys aged at 473 K. It is attributed to annihilations with the core electrons of Ca. The results indicate the formation of a particle dispersion that hardens the alloys. This dispersion is correlated with the appearance of the peak attributed to Ca atoms. Zn atoms in the Mg matrix inhibit the formation of quenched-in vacancies bound to Ca atoms in the aged ternary alloy producing the dispersion refinement.     

Electrochemical behaviour and corrosion performance of Mg–Li–Al–Zn anodes with high Al composition

This study investigated the electrochemical and corrosion performance of Mg–Li–Al–Zn anodes with Al compositions of 3 wt.% and 9 wt.%. Mg–Li–Al–Zn alloy with 9 wt.% Al had a relatively negative open-circuit potential and a high discharge voltage in MgCl₂ electrolyte, owing to the distribution of numerous AlLi particles in the matrix of the alloy. AlLi particles were believed to transform to Al particles during the corrosion of the Mg–Li–Al–Zn anode. The high-Al anode material exhibited good corrosion performance since a dense and continuous Mg(OH)₂/Al composite layer covered the surface of the high-Al anode. Experimentally, increasing the Li⁺ concentration in the electrolyte improved the corrosion performance of the Mg anode.     

Improving the strength and the toughness of Mg–Cu–(Y,Gd) bulk metallic glass by minor addition of Nb

Mg₆₅Cu₂₅Re₁₀ (Re = Y, Gd) and Mg₆₄Cu₂₅Nb₁Re₁₀ (Re = Y, Gd) bulk metallic glasses (BMGs) have been fabricated by copper mould casting. It is shown that the minor addition of Nb can not only increase the thermal stability but also improve the fracture strength and the toughness of the Mg-based BMG alloys greatly. The fracture strength of Mg₆₄Cu₂₅Nb₁Y₁₀ and Mg₆₄Cu₂₅Nb₁Gd₁₀ reaches as high as 1023 MPa and 973 MPa, respectively. The Young's modulus has also been increased by the addition of Nb. From the fracture morphologies of the Nb bearing Mg-based BMG alloys, it is known that the size of plastic deformation zone can be increased to micrometer scale. This work proves that it is possible to improve the strength and toughness of Mg-based BMG by adding an element having positive heat of mixing with the constituent elements.     

Phase equilibria on the ternary Mg–Mn–Ce system at the Mg-rich corner

Three isopleths at the Mg-rich corner of Mg–Mn–Ce ternary system were investigated via thermal analysis, SEM/EPMA and XRD. A ternary eutectic reaction was observed at 1 wt.% Mn and 23 wt.% Ce and 592 °C. A solid-solution type ternary intermetallic compound, (Mg,Mn)₁₂Ce, was observed with 0.5 at% solid solubility of Mn in the tetragonal Mg₁₂Ce. With the aid of thermodynamic modeling and experiments, a revised phase diagram for the binary Mg–Ce system and the isopleths of 0.6, 1.8 and 2.5 wt.% Mn were proposed up to 25 wt.% Ce.     

Magnetic properties of (Fe)1−x–(Al2O3)x and (Fe50Ni50)1−x–(Al2O3)x nanocomposite magnetic media synthesized using gel like Al2O3 matrix

Composites with ferromagnetic nanoparticles, Fe and Fe₅₀Ni₅₀, dispersed in Al₂O₃ have been synthesized by a solution phase technique. The structure and magnetic properties of these composites with varying fractions of Al₂O₃ have been investigated. Both Fe and Fe₅₀Ni₅₀ nanoparticles are amorphous in the as-prepared state and become crystalline on heat treating with near equilibrium lattice parameters of 0.287 nm and 0.358 nm respectively. The interparticle distance increases with increasing Al₂O₃ from 0 wt.% to 20 wt.%. The size of Fe nanoparticles is 40 nm while the Fe₅₀Ni₅₀ nanoparticles are 20 nm in size. The Fe and Fe₅₀Ni₅₀ nanoparticles dispersed composites are found to be ferromagnetic at room temperature both in the as-prepared and heat treated conditions with clear coercive fields of 5.5–35 × 10³ A m⁻¹. The saturation magnetization increases by orders of magnitude on heat treatment, for e.g. from <1.0 emu g⁻¹ to 143.4 emu g⁻¹ for Fe–15 wt.% Al₂O₃ and 95.6 emu g⁻¹ for Fe₅₀Ni₅₀–15 wt.% Al₂O₃. The Fe-composites exhibit a Curie transition at 1000 K while the Fe₅₀Ni₅₀ composites exhibit a transition at 880 K, both temperatures close to bulk values.     

Properties of Mn-doped FINEMET

Structural, thermodynamical and magnetic properties of Fe_73.5−xSi_13.5B₉Cu₁Nb₃Mn_x amorphous alloys, with Mn content x=1,3–15, were studied by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC), Mössbauer spectroscopy (MS) and energy-dispersive X-rays (EDX), as-quenched and after annealing. The alloys with x≤7 suffer primary (above 550 °C) and secondary (below 680 °C) crystallisation, whereas the alloys with x≥9 only at 580 °C. Mn doping results in increase (for x≤7) and then (for x≥9) in decrease of grain size. The hyperfine field of the amorphous precursor and remainder substantially decrease with increase of Mn content x, whereas the fields in crystallites remain nearly independent of x. A paramagnetic component appears at x9 and grows with x.     

Creep feed grinding of gamma titanium aluminide and burn resistant titanium alloys using SiC abrasive

Following a brief introduction to titanium alloys and their machinability, the cutting performance of a gamma titanium aluminide intermetallic (γ-TiAl) alloy: Ti–45Al–8Nb–0.2C wt% and a burn resistant titanium (BuRTi) alloy: Ti–25V–15Cr–2Al–0.2C wt%, is compared with creep feed grinding using SiC abrasive. The work utilised 2 separate L9 Taguchi fractional factorial arrays. Typically G-ratios were a factor of 10× greater for γ-TiAl than BuRTi, with on average 10% lower maximum power and 25% lower maximum specific energy for the γ-TiAl alloy. A combination of a moderately high wheel speed: 35 m/s, low depth of cut: 1.25 mm and low feed rate: 150 mm/min, produced the lowest average workpiece surface roughness (Ra1.4 μm). Workpiece surface integrity evaluation indicated that with lower operating parameter levels, in particular a wheel speed of 15 m/s, surfaces free of burn and cracks could be produced, while at higher wheel speeds: 35 m/s, extensive workpiece surface burn was evident, with the γ-TiAl alloy suffering extensive cracking. Microhardness measurements showed in some instances slightly increased workpiece surface hardness of around 50–60HK_0.025 for the BuRTi alloy and 200HK_0.025 for the γ-TiAl material over respective bulk hardness values of 375HK_0.025 and 400HK_0.025.     

Milling and subsequent thermal annealing effects on the microstructural and magnetic properties of nanostructured Fe90Co10 and Fe65Co35 powders

The influence of milling and subsequent annealing on the microstructural and magnetic properties of Fe₉₀Co₁₀ and Fe₆₅Co₃₅ alloys is investigated. After milling for 8 h a body-centred cubic nanostructured Fe–Co alloy forms with an average crystallite size of about 12 nm. The magnetization saturation (M_S) increases 16% for Fe₆₅Co₃₅ and 5% for Fe₉₀Co₁₀ alloys by milling for 8 h. Subsequent annealing of Fe₉₀Co₁₀ and Fe₆₅Co₃₅ powders for 105 min at 550 °C improves the M_S about 6 and 11%, respectively. Before annealing, the coercivity increases (up to 60 Oe) by milling for 3 h, followed by a reduction on milling for longer periods (45 h). At the initial stage of the heating, a sharp decrease in H_C to 8–10 Oe occurs due to the relief of internal strain. Further heating leads to an increase in the coercivity (intermediate times) followed by a slight diminution on heating for final stage.     

Early oxidation behaviors of Mg–Y alloys at high temperatures

The early oxidation behaviors of Mg–Y alloys (Y = 0.82, 1.09, 4.31 and 25.00 wt.%) oxidized in pure O₂ have been investigated at high temperatures. The results showed that the oxidation behaviors of the Mg–Y alloys (Y = 4.31 and 25.00 wt.%) obeyed a parabolic law, while that of the Mg–Y (Y = 0.82 and 1.09 wt.%) exhibited both parabolic and linear kinetics depending on the oxidation temperature. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses indicated that an oxide film with a single structure composed of MgO and Y₂O₃ had formed. Moreover, the higher the oxidation temperature was, the thicker the oxide film was. Finally, the corresponding oxidation mechanism has been discussed, and the improved oxidation resistance of the Mg–Y alloys can be due to the formation of a continuous Mg-dissolving Y₂O₃ protective film.     

dc-Magnetic susceptibility and EPR studies of vapour phase grown Cd1−xCoxTe crystals

Cd_1−xCo_xTe crystals (x = 0.001, 0.003, 0.005, 0.007 and 0.009) were grown by vapour phase technique. The grown diluted magnetic semiconducting (DMS) crystals were subjected to magnetization and dc-magnetic susceptibilities at room temperature. EPR spectra were recorded at 20 K for samples of all compositions. EPR spectra exhibited a broad resonance band around g  5.43. All the studies indicated the paramagnetic nature of the samples.     

Influence of thickness on the material characteristics of reactively sputtered W2N layer and electrical properties of W/W2N/SiO2/Si capacitors

The material characteristics of W₂N layer and electrical properties of W/W₂N/SiO₂/Si metal–oxide–semiconductor (MOS) capacitors with different W₂N thickness upon annealing in N₂ + H₂ ambient at 500 °C for 20 min are investigated. The nitrogen concentration of W₂N for the W/W₂N stack with thin W₂N layer (≤10 nm) is lower than that for the W/W₂N stack with thick W₂N layer (≥15 nm). In addition, the crystallinity of W₂N in the W/W₂N (15 nm) stack is better than that in the W/W₂N (10 nm) stack. For all capacitors, the oxide charges decrease significantly after annealing and the amount of oxide charges is independent of the W₂N thickness. However, the work function (Φ_m) of the W/W₂N (≤10 nm) stack (4.6 eV) is smaller than that of W/W₂N (15 nm) stack (5.0 eV). The Φ_m of W/W₂N (15 nm) stack is close to that of W₂N single layer. After annealing, the Φ_m of W/W₂N (15 nm) stack and W₂N single layer decrease, especially for the W₂N single layer. But for the W/W₂N (≤10 nm) stack, the Φ_m increases after annealing.     

Si-induced superconductivity and structural transformations in DyRh4B4

DyRh₄B₄ has been known to crystallize in the primitive tetragonal (pt)-structure and to exhibit a ferromagnetic transition at 12 K, the highest magnetic transition temperature in the entire series of the RRh₄B₄ materials [For an extensive review on the magnetic and superconducting properties on the ternary superconductors, including RRh₄B₄, see, for example, Ø. Fischer, in: K.H.J. Buschow, E.P. Wohlfarth (Eds.), Ferromagnetic Materials, vol. 5, Elsevier Science Publishers B.V., 1990 (Chapter 6)]. We show here that our silicon-added samples of the nominal composition DyRh₄B₄Si_0.2 exhibit superconductivity below T_c  4.5 K and an antiferromagnetic transition below T_N  2.7 K. The 12 K transition observed in the pt-DyRh₄B₄ is completely suppressed. Our annealed samples mainly consist of domains of the chemical composition DyRh_3.9B_4.2Si_0.08. These domains contain two crystallographic phases belonging to the body-centred tetragonal (bct)-structure and the orthorhombic (o)-structure. We have reasons to suggest that superconductivity and antiferromagnetic ordering arise from bct-DyRh₄B₄ phase and, therefore, coexist below T_N  2.7 K.     

The role of Zr-rich cores in strength differential effect in an extruded Mg–Zn–Zr alloy

This work examines the effects of extrusion parameters namely ratio and temperature on recrystallization behavior of a Mg–Zn–Zr alloy, and their consequent effects on anisotropy in the mechanical properties. Upon extrusion, the characteristic Zr-rich cores that do not recrystallize form the so-called “soft stringers” which are deformed bands elongated in the extrusion direction and curled around the extrusion axis. At higher extrusion ratio, there is more twinning contribution and the DRX response is improved, making the recryatallized grains finer and increasing the proportion of recrystallized Zr-rich cores. A basal texture after extrusion and the directional activation of tensile twinning cause anisotropy in the mechanical properties. In addition, the microstructural features such as large unrecrystallized regions and coarse crystallized grains also contribute in the strength differential effect. Further slip in the strain-hardened unrecrystallized grains is inhibited while twin activation under favorable orientation becomes easier in the coarse recrystallized grains. A higher proportion of large unrecrystallized and coarse crystallized gains in the case of lower extrusion ratio result in a much higher strength differential effect (100 MPa) in comparison to the one caused by the crystallographic texture only (25 MPa).     

Ru-based bimetallic alloys for hydrogen generation by hydrolysis of sodium tetrahydroborate

The present study contemplates the application of Ru-based bimetallic alloys for hydrogen generation by hydrolysis of sodium tetrahydroborate (NaBH₄). Ru and Pt, RuCu, RuPd, RuAg and RuPt (atomic ratio 1:1), PtAg, and Ru_xPt_y (atomic ratios x:y of 2:1 or 1:2), all supported over titanium oxide, were prepared. Their activity decreased in the order RuRu₂Pt₁ > RuPtRu₁Pt₂ > RuPd > RuAgPt > RuCu > PtAg. Alloying Ru with an inactive metal like Cu, Pd or Ag did not improve the performances of Ru. The catalytic ability of Ru₂Pt₁-TiO₂ is in the range of the highest values reported so far in the literature with a hydrogen generation rate of 15.2 L(H₂) min⁻¹ g⁻¹(RuPt). After separation from the reaction medium and rinsing with deionised water, the used Ru₂Pt₁-TiO₂ catalyst was re-evaluated and almost the same catalytic activities as fresh catalyst were obtained during several cycles.     

Effect of Pt on the superconducting and magnetic properties of ErNi2B2C

We show that the variation of T_c in Er(Ni_1−xPt_x)₂B₂C (T_c  10.6 K and T_N  5.7 K for x = 0) as a function of x proceeds in two steps: strong decrease of T_c for initial values of x (0 ≤ x < 0.10, T_c = 7.3 K at x = 0.1) and, thereafter, a relatively much weaker drop (almost a plateau) of T_c with further increase of x. T_N exhibits a slight, almost linear, decrease over the entire range of x studied here; T_N = 4.7 K for x = 0.2. Our results for x = 0.10 are in sharp disagreement with the results, namely, T_c < T_N, as reported by Felner et al. [I. Felner, D. Schmitt, B. Barbara, C. Godart, E. Alleno, J. Solid State Chem. 133 (1997), 5].     

A theoretical interpretation of the pressure–composition isotherms of RNi5 (, Pr, Nd and Sm) systems based on statistical mechanics

A simple model to understand the phase behavior of RNi₅–H (R=La, Pr, Nd and Sm) systems is proposed based on the statistical mechanics. The essential parameters in the present model, hydrogen site energies and lattice relaxation term, were optimized so as to reproduce the experimentally determined pressure–composition isotherms (PCIs) of these four alloys. The qualitative difference in the PCI of LaNi₅ from other alloys – at room temperature the former tends to show a single plateau while the latter have two plateaux – is explicable in terms of hydrogen site energies as follows. In case of LaNi₅, four hydrogen sites, which are considered to belong to 3f, 12n, 6m and 12o sites, are stable in α phase, leading to a wide single plateau upto N6 where N is in RNi₅H_N below room temperature. At higher temperature, small differences in site energies for these sites brings the appearance of the second plateau. In case of other three alloys, on the other hand, the 6m site (or 12o) is less stable than LaNi₅ in the α phase. After obtaining enough lattice relaxation energy by around N=4 through hydrogenation, the 6m site becomes favorable for hydrogen, bringing the sencond plateau spanning from N4 to N6. Predicted T–C phase diagrams are also shown.