Superconducting properties and microstructure of crystallized Hf-Nb-Si and Hf-V-Si amorphous alloys

Amorphous Hf-Nb-Si and Hf-V-Si alloys have been produced by rapidly quenching the melts using a melt-spinning technique. The silicon content in the amorphous alloys was limited to 14 to 20 at% and the niobium or vanadium content was limited to 0 to 45 at% and 0 to 35 at%, respectively. These amorphous alloys did not show any superconducting transition down to liquid helium temperature (4.2 K). However, a transition was detected above 4.2 K after inducing crystallization in these alloys by annealing at appropriate temperatures. The highest superconducting transition temperatures, T _c, attained were 8.9 K for the Hf₄₅Nb₄₀Si₁₅ alloy annealed for 1 h at 1273 K and 6.7 K for the Hf₅₀V₃₅Si₁₅ alloy annealed for 1 h at 1173 K. The upper critical magnetic field, H _c2, at 4.2 K and the critical current density, J _c, at zero applied field and 4.2 K were about 5.1×10⁶A m^–1 (6.4 T) and more than 1×10⁴ A cm^–2 for the Hf₄₅Nb₄₀Si₁₅ alloy and more than 8.0×10⁶ A m^–1 (10 T) and 5×10³ A cm^–2 for the Hf₅₀V₃₅Si₁₅ alloy. Detailed transmission electron microscopic studies of the annealed structure of these amorphous alloys established that, after crystallization, these alloys contain a body-centred cubic -Hf(Nb) solid solution and body-centred tetragonal Nb₃Si phases in the Hf₄₅Nb₄₀Si₁₅ alloy and hexagonal Hf₅Si₃, face-centred cubic HfV₂ and cubic V₃Si phases in the Hf₅₀V₃₅Si₁₅ alloy. Since Nb₃Si and Hf₅Si₃ are not superconducting above 4.2 K, it has been concluded that superconductivity in these crystallized alloys is due to the precipitation of -Hf(Nb) solid solution in the Hf-Nb-Si alloys and to the precipitation of HfV₂ and V₃Si compounds in the Hf-V-Si alloys.     

Low-temperature fracture toughness study of Fe-7Al-27Mn-C alloys

This research studied the fracture toughness of the Fe-7Al-27Mn alloys with increasing carbon contents: 0.5% C, Fl alloy: 0.7% C, F2 alloy (with 4.0% Cr); and 1.0% C, F3 alloy. Fracture toughness experiments were conducted at temperatures of 25, – 50, – 100 and – 150 °C. It was found that plane-stress,K _C, values as measured by the R-curve method, decreased as the temperature dropped. F1 alloy possessed the highestK _C value at all temperatures among the three alloys. TheK _C values of the F2 and F3 alloys were similar at ambient temperatures, but F3 maintained the toughness property and ductility better at sub-zero temperatures. Quantitatively,K _IC values of the F2 alloy at – 150 °C were ca, 60% less than at 25 °C, but F1 and F3 alloys dropped by only ca. 30%. Using a compact-tension specimen, 20.0 mm thick, at –150°C only alloy F2 satisfied the requirement of plane-strain fracture toughness with aK _C value of 106 MPa m^1/2. The existence of Cr (4.0%) and the formation of a ferrite phase in an austenite matrix was responsible for the low toughness value observed.     

Microstructural evolution of FINEMET type alloys with chromium: An electron microscopy study

Microstructural changes in Fe_73.5–xCr_xCu₁Nb₃Si_13.5B₉ (0x5) alloys with thermal treatment were studied by electron microscopy. In a first stage, around 800 K, an Fe(Si) nanocrystalline phase is formed in the amorphous residual matrix. Crystallization onset is enhanced with the Cr content of the alloy. In a second stage, around 950 K, full crystallization of the samples leads to the formation of a body centred cubic (b.c.c.) boride-type unknown crystal phase with a lattice parameter of a=1.52 nm, and recrystallization of the previous Fe(Si) nanophase also occurs. No qualitative differences were found between dynamic and isothermal crystallization. The size effect for thin samples is limited to a lowering of crystallization temperatures. For isothermal nanocrystallization in the temperature range 775–900 K, the mean grain size of the nanocrystals increases for short annealing times to stabilize at a constant value of about 10–15 nm for long annealing times. The stabilized grain size increases with increasing annealing temperature and slightly decreases with the Cr content of the alloy.     

Investigations of Bi substitution in NdBa2Cu3Oy

Attempts to substitute Bi for Nd in orthorhombic NdBa₂Cu₃O _y , prepared in air or oxygen at about 950°C led instead to formation of Ba₂NdBiO₆, a new cubic compound witha=0.8703 nm. The possibility was then explored of preparing superconducting (Nd_1–x Bi _x )Ba₂Cu₃O _y , by first forming the tetragonal phase at 880–950°C in nitrogen or argon followed by reheating in oxygen or air at 250–500°C in order to insert the additional oxygen required to yield the orthorhombic form while avoiding oxidation of Bi³⁺ to Bi⁵⁺. X-ray diffraction studies, electrical conductivity measurements, and thermogravimetric analysis of products indicate that Bi does not enter the NdBa₂Cu₃O _y , lattice in either the tetragonal or the orthorhombic phase. Ba₂NdBiO₆ clearly forms on reheating in oxygen or air even at low temperatures, and evidence is presented that a poorly crystallized oxygen-deficient form of this compound is already present prior to the reheating.     

Thermal stability and crystallization behaviour of amorphous Zr-M-Si (M=IV–VIII group transition metals) alloys

By rapidly quenching with a melt-spinning apparatus, it has been possible to produce ductile amorphous single-phase ternary Zr_85–x M _x Si₁₅ (M=IV–VIII group transition metals) alloys in wide composition ranges. The crystallization temperature, activation energy for crystallization and hardness increase significantly only with the addition of group V and VI elements (V, Nb, Ta, Cr, Mo and W). Such a solute element effect could be interpreted on the basis that the chemical bonding between the solute elements and silicon is stronger for the group V and VI elements than for the other group elements. Crystallization studies of the amorphous Zr₈₅Si₁₅ and Zr₆₅Nb₂₀Si₁₅ alloys have been carried out through transmission electron microscopy and differential scanning calorimetry techniques. The binary alloy crystallizes by the uniform precipitation of b c c-Zr over the entire area of the amorphous matrix followed by the appearance of the metastable b c tetragonal Zr₃Si compound from the remaining amorphous phase. On the other hand, the ternary alloy transforms by the simultaneous precipitation of-Zr(Nb) and b c tetragonal Nb₃Si. The-Zr and Zr₃Si phases were found to be in metastable states, the equilibrium structure being a mixture of-Zr and Zr₄Si compound in the binary alloy.     

Preparation of KNbO3 by hydrothermal reaction

Perovskite-type KNbO₃ powder was prepared by hydrothermal reaction using Nb₂O₅ in KOH solution. A single phase of KNbO₃ was obtained when the molar ratio of KOH/Nb₂O₅ was above 20 and the reaction temperature was above 160 °C. Three types of KNbO₃ powder with the orthorhombic, tetragonal and cubic symmetries were obtained, depending on the reaction temperature and the ratio of KOH/Nb₂O₅. The molar ratios of K/Nb in the cubic and tetragonal phases were 0.91 and 0.94, respectively and that of the orthorhombic one was 0.98, and the mass loss was observed in the TG curves of tetragonal and cubic phases. The tetragonal and cubic phases were stabilized by OH⁻ and adsorbed water.     

Effect of heat treatment on precipitation behaviour in a Cu-Ni-Si-P alloy

The effects of applying different solution and ageing conditions on the electrical resistivity and precipitation behaviour of a Cu-1.3Ni-0.3Si-0.03P (wt%) alloy were studied. The electrical resistivity of solution-treated material is greatly reduced, by about 50%, by the ageing processes. The reduction in resistivity is due to depletion of solute atoms from the copper matrix by the formation of precipitates. Double ageing peaks appeared during isothermal ageing due to the formation of Ni₃P and Ni₂Si precipitates. The first maximum, due to the precipitation of Ni₃P, appeared at about 1 h of ageing time, while the second peak, due to Ni₂Si, appeared at around 10 h of ageing time when aged at 450°C. The precipitate Ni₃P forms early and the alloy starts to over-age before Ni₂Si precipitates and the alloy reaches maximum hardness. The maximum hardness produced by the precipitations of Ni₃P and Ni₂Si decreased with increasing ageing temperature from 450 to 550°C. The time to reach the maximum hardness due to Ni₃P precipitation became shorter, while that of Ni₂Si became longer, as the solution treatment temperature increased from 780 to 1020° C. The apparent activation energy for Ni₂Si precipitation was found to be about 80 kJ mol^–1 while that for Ni₃P precipitation was about 25 kJ mol^–1.     

Growth and Properties of K2TiNb2P2O13Crystals

K₂TiNb₂P₂O₁₃single crystals (monoclinic structure, a= 13.788 Å, b= 6.418 Å, c= 16.927 Å, = 97°) were grown from off-stoichiometric K₂O–TiO₂–Nb₂O₅–P₂O₅melts, and their habit was described. The 300°C electrical conductivity of the crystals was determined to be 5 × 10^–4S/cm.     

Crystallization-induced superconductivity in amorphous Ti-Ta-Si alloys

Amorphous alloys containing 0 to 40 at% Ta and 15 to 20 at% Si have been produced in the ternary Ti-Ta-Si system by rapidly quenching the melts using a melt-spinning technique. The amorphous alloys did not show any superconducting transition down to liquid helium temperature (4.2 K). However, a transition was detected above 4.2 K after inducing crystallization in these alloys by annealing at appropriate temperatures. The superconducting transition temperature, T _c, increased with increasing tantalum content and showed the highest value of 7.6 K for the Ti₄₅Ta₄₀Si₁₅ alloy annealed for 1 h at 1073 K. An upper critical magnetic field, H _c2 of 4.7×10⁶ Am^–1 at 4.2 K and a critical current density, J _c, of 1.5×10⁴ A cm^–2 at zero applied field and 4.2 K were recorded for this alloy. Detailed electron microscopic studies of the crystallization behaviour of the amorphous alloys established that a supersaturated solid solution of tantalum in -Ti with a bcc structure forms first, followed by the precipitation of the bc tetragonal Ta₃Si compound. Since Ta₃Si is not superconducting above 4.2 K, it has been concluded that superconductivity in the crystallized alloys is due to the precipitation of -Ti(Ta) solid solution.     

The high-temperature thermal expansion of BiPbSrCaCuO superconductor and the oxide components (Bi2O3, PbO,CaO, CuO)

The thermal expansion of superconducting Bi_1.6Pb_0.4Sr₂Ca₂Cu₃Ox (BiPbSrCaCuO) and its oxide components Bi₂O₃, PbO, CaO and CuO have been studied by high-temperature dilatometric measurements (30–800°C). The thermal expansion coefficient for the BiPbSrCaCuO superconductor in the range 150–830°C is =6.4×10^–6K^–1. The temperature dependences of L/L of pressed Bi₂O₃ reveals sharp changes of length on heating (T ₁=712°C), and on cooling (T ₂=637°C and T ₃=577°C), caused by the phase transition monoclinic-cubic (T ₁) and by reverse transitions via a metastable phase (T ₂ and T ₃). By thermal expansion measurements of melted Bi₂O₃ it is shown that hysteresis in the forward and the reverse phase transitions may be partly caused by grain boundary effect in pressed Bi₂O₃. The thermal expansion of red PbO reveals a sharp decrease in L/L, on heating (T ₁=490°C), related with the phase transition of tetragonal (red, a=0.3962 nm, c=0.5025 nm)-orthorhombic (yellow, a=0.5489 nm, b=0.4756 nm, c=0.5895 nm). The possible causes of irreversibility of the phase transition in PbO are discussed. In the range 50–740°C the coefficient of thermal expansion of pressed Bi₂O₃ (_m=3.6 × 10^–6 and _c=16.6×10^–6K^–1 for monoclinic and cubic Bi₂O₃ respectively), the melted Bi₂O₃ (_m=7.6×10^–6 and _c=11.5×10^–6K^–1), PbO (_t=9.4×10⁶ and _or=3.3×10^–6K^–1 for tetragonal and orthorhombic PbO respectively), CaO (=6.1×10^–6K^–1) and CuO (=4.3×10^–6K^–1) are presented.     

Oxidation behavior of FeAl alloys with and without titanium

The influence of Ti addition on the high temperature oxidation behavior of FeAl intermetallic alloys in air at 1000°C and 1100°C has been investigated. The oxidation kinetics of FeAl alloys was examined by the weight gain method and oxide products were examined by XRD, SEM, EDS and EPMA. The results showed that the oxidation kinetic curves of both Ti-doped and binary Fe-36.5Al alloys could be described as different parabolas that followed the formula: (W/S)² = K _p t + C. The parabolic rate constant, K _p values are approximately 2.4 and 3.3 mg² cm^–4 h^–1 for Fe-36.5Al alloy and about 1.3 and 2.0 mg² cm^–4 h^–1 for Fe-36.5Al-2Ti alloy when oxidizing at 1000°C and 1100°C respectively. The difference between Fe-36.5Al and Fe-36.5Al-2Ti alloy is not only in the surface morphology but also in the phase components. In the surface there is only -Al₂O₃ oxide for Fe-36.5Al alloy while there are -Al₂O₃ and TiO oxides for Fe-36.5Al-2Ti alloy. The effects of Ti addition on the oxidation resistance of FeAl alloy were addressed based on the microstructural evidence.     

Mechanical and elastic properties of transparent nanocrystalline TeO2-based glass-ceramics

Mechanical and elastic properties of transparent TeO₂-based glass-ceramics (15K₂O · 15Nb₂O₅ · 70TeO₂) consisting of nanocrystalline particles (each particle size: 40–50 nm) and showing optical second harmonic generation were evaluated by means of usual Vickers indentation and nanoindentation tests. The precursor glass has Vickers hardness H _v of 2.9 GPa, Young's modulus E of 54.7 GPa, the fracture toughness K _c of 0.25 MPam^1/2 and Poisson's ratio of 0.24. The transparent nanocrystalline glass-ceramic heat-treated at 420°C for 1 h has H _v = 3.8 GPa, E = 75.9 GPa and K _c = 0.34 MPam^1/2, and the opaque glass-ceramic heat-treated at 475°C for 1 h has H _v = 4.5 GPa, E = 82.9 GPa and K _c = 0.68 MPam^1/2, demonstrating that poor mechanical and elastic properties of the precursor TeO₂-based glass are improved through sufficient crystallization. The fracture surface energy, brittleness and elastic recoveries (about 44%) after unloading (the maximum load: 30 mN) of transparent nanocrystalline glass-ceramics are almost the same as those of the precursor glass, implying that the interaction among nanocrystalline particles is not so strong.     

Top-cooling-solution-growth and characterization of piezoelectric 0.955Pb(Zn1/3Nb2/3)O3-0.045PbTiO3 [PZNT] single crystals

A technique of top-cooling-solution-growth (TCSG) has been developed to grow the piezo-/ferroelectric perovskite single crystals of 0.955Pb(Zn_1/3Nb_2/3)O₃-0.045PbTiO₃ [PZNT95.5/4.5]. The flux composition and concentration, and the thermal parameters have been optimized, leading to the growth of high quality PZNT crystals with a size up to 20 × 15 × 10 mm³. The perovskite crystals are found to form upon slow cooling down to 1020°C, while the undesirable pyrochlore crystals of Pb_1.5Nb₂O_6.5-type start growing upon further cooling from 1020°C to 950°C. By controlling the growth pathway, the formation of the pyrochlore phase can be avoided. The dielectric properties of the grown PZNT95.5/4.5 crystals have been measured as a function of temperature at various frequencies. Upon heating, the phase transition for the rhombohedral R3m to the tetragonal P4mm phase takes place at 132°C, while the tetragonal to cubic phase transition occurs at 160°C. The TCSG developed in this work provides an alternative technique to grow PZNT piezocrystals of medium size at low cost for transducer applications.     

The mechanism and kinetics of growth of the superconducting compound Nb3Sn

A study has been made of the mechanism and kinetics of formation of Nb₃Sn from the elemental components. The Nb₃Sn forms partly by diffusion and partly by a solution/ deposition mechanism which depends on thermal gradient mass transfer. The effect of this is to modify the growth equation to x = kt ^0.36 over the temperature range 950 to 1150° C. The temperature dependence of these two processes, given by the difference between the activation energies for diffusion and solution, is –9.7 kcal/g atom (–0.42 eV/atom) so that the thickness of the Nb₃Sn layer produced in any given time decreases with increasing temperature.Various experimental factors are discussed in terms of their influence on the rate of growth of the layer.     

Microstructure and internal friction of spray deposited Al-3.3Fe-10.7Si alloy

Al-3.3Fe-10.7Si alloy has been experimentally made with spray deposition technology. The internal friction of the alloy which was directly associated with the microstructures under spray deposited, extruted and heat treated conditions has been investigated using a low frequency inverted torsion pendulum over the temperature region of 10–300 °C. An internal friction peak was observed in the temperature range 50–250 °C in the present alloy. The Q^-1 peak decreased after extruted and in subsequent to the earliness of isothermal annealing, which was found to be directly attributed to the precipitation of FeAl₂ and Al– Fe– Si intermetallics from the supersaturated aluminium alloy matrix. We suggest that the internal friction peak in the alloy originates from grain boundary relaxation, but the grain boundary relaxation can also be affected by FeAl₂ and Al– Fe– Si intermetallics at the grain boundaries, which will impede grain boundary sliding.     

The effects of quenching just after deposition for metastable compounds

It has been demonstrated that high rate d.c. magnetron sputtering can easily prepare materials such as Nb₃Ge and Nb₃Si in a metastable state. This sputtering-system was made in an effort to stabilize the metstable phases. Specimens were quenched just after sputtering from 1100° C to liquid N₂ temperature at the rate of 3500° C min^–1. The relation between quenching rate and superconducting onset temperture, T _c0, the transition width, T _c, and the lattice constant, a ₀ were examined. The results show that quenching just after sputtering is very useful in preparing metastable materials.     

Characterization of crystallization in Metglas 2826 MB alloy

The crystallization behaviour of the Metglas 2826 MB alloy (Fe₄₀Ni₃₈Mo₄B₁₈) has been studied using resistance measurements and X-ray diffraction techniques. Three annealing sequences were used to follow the process. Samples were annealed isothermally (a) at 780° C in a vacuum of 2×10^–5 torr for times in the range 1 sec to 4 h, (b) for 2 h in an argon atmosphere at temperatures where the resistance curve indicated phase changes to occur, and (c) for 300 h in 100 torr of helium at 400, 600, 700 and 850° C. From these annealing sequences it was found that the alloy did not crystallize below 410° C and followed a crystallization process of: amorphous Fe₄₀Ni₃₈Mo₄B₁₈ Fe _x Ni_23–x B₆ (cubic)+glassy matrix Fe _x Ni_23–x B₆+(Fe, Ni) (FCC) (Fe, Ni)₃B(bct). This series of transformations was followed for Sequences (a) and (c) above, but was slightly different for Sequence (b). An orthorhombic (Fe, Ni)₃ B phase was found in the samples annealed in a vacuum of 2×10^–5 torr.Trademark of Allied Chemical Co.     

Temperature effects in the fracture of PMMA

Experiments are described in which the fracture toughness,K _c, of PMMA has been determined in the temperature range –190 to + 80° C and over the crack speed range of 10^–2 to 10² mm sec^–1. Single edge notch tension was used for instability measurements but the other data were obtained using the double torsion method. In the range –80 to + 80°C the variations inK _c may be described in terms of modulus changes and a constant crack opening displacement criterion. Crack instabilities are correlated with isothermal-adiabatic transitions at the crack tip. Below –80° C there is an inverted rate dependence associated with thermal effects during post-instability crack propagation.     

Ordering of Fe-Si phases

The microstructure of Fe-Si alloys containing 8 and 15.5 at % Si and heat-treated between 550 and 1200°C is studied by transmission electron microscopy, and the phase composition of alloys containing 19 and 23 at % Si is determined by x-ray diffraction. The Fe-15.5 at % Si alloy heat-treated above 700°C is found to consist of a disordered solid solution and B2 phase. The B2 particles can be thought of as portions of {100} layers consisting entirely of Si atoms and sandwiched between {100} layers of Fe atoms, that is, as a two dimensional phase. At t 675°C, a compositionally modulated microstructure develops in which the Si-enriched zones have the Fe₃ Si stoichiometry and DO₃ structure. At high temperatures, the Fe-19 at % Si alloy consists of the and B2 phases, and the Fe-23 at % Si alloy consists of the and DO₃ phases. These findings are at variance with the generally accepted Fe-Si phase diagram.Translated from Neorganicheskie Materialy, Vol. 41, No. 1, 2005, pp. 28–35.Original Russian Text Copyright © 2005 by Ustinovshchikov, Sapegina.     

Study of the crystallization kinetics in amorphous Fe73.5Cu1Nb3Si13.5B9 alloy

The kinetics of the crystallization of Fe_73.5Cu₁Nb₃Si_13.5B₉ amorphous alloy was studied using Mössbauer spectroscopy and X-ray diffractometry. During isothermal annealing of the samples, two phases were observed: a crystalline D0₃-FeSi alloy with fine grains and an amorphous phase enriched with niobium, boron and copper. It was found that the growth rate of the particles of the crystalline alloy was controlled by diffusion. For longer annealing time or at higher annealing temperature the growth process was found to be suppressed, probably by the niobium atoms. The activation energy obtained for the crystallization was about 143 kJ mol^–1.