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.     

Microstructure and superconductivity in annealed Cu-Nb-(Ti,Zr, Hf) ternary amorphous alloys obtained by liquid quenching

Melt-quenched Cu-Nb-(Ti, Zr, Hf) ternary alloys have been found to be amorphous possessing high strength and good bend ductility. The niobium content in the amorphous alloys was limited to less than 35 at % and the titanium, zirconium or hafnium contents from 25 to 50 at %. The Cu₄₀Nb₃₀(Ti, Hf)₃₀ alloys showed a superconducting transition above the liquid helium temperature (4.2 K) after annealing at appropriate temperatures. The highest transition temperatures attained were 5.6 K for the Cu₄₀Nb₃₀Ti₃₀ alloy annealed for 1 h at 873 K and 8.4 K for the Cu₄₀Nb₃₀Hf₃₀ alloy annealed for 1 h at 1073 K. In addition, these alloys exhibited upper critical magnetic fields of 1.8 to 2.3×10⁶ Am^–1 at 4.2 K and critical current densities of 2×10³ to 1×10⁴ A cm^–2 at zero applied field and 4.2 K. Since the structure of the superconducting samples consisted of ordered phases based on a b c c lattice with a lattice parameter of 0.31 nm, it was concluded that the superconductivity in the Cu₄₀Nb₃₀Ti₃₀ and Cu₄₀Nb₃₀Hf₃₀ alloys was due to the precipitation of the metastable ordered b c c phases.     

Effect of annealing on the superconducting properties of two amorphous alloys: Nb70Zr15Si15 and Zr85Si15

The changes in the superconducting and electronic properties of amorphous Nb₇₀Zr₁₅Si₁₅ and Zr₈₅Si₁₅ alloys with annealing were examined with an aim to evaluate the effect of structural relaxation on the superconductivity of metal-metalloid type amorphous alloys.T _c rises once from 3.99 to 4.42 K on annealing at temperatures below about 473 K for the Nb-Zr-Si alloy and from 2.71 to 2.75 K at temperatures below about 373 K for the Zr-Si alloy, and with further rising annealing temperature,t _d, lowers monotonically to a final relaxed value (3.15 K for Nb₇₀Zr₁₅Si₁₅ and 2.49 K for Zr₈₅Si₁₅), which is independent of the previous thermal cycling. These results indicate that the thermal relaxation of an amorphous phase occurs through at least two stages. The lowering ofT _c occurs exponentially witht _d, and an activation energy for the relaxation process and the frequency of jump over the barrier were estimated to be about 2.03 eV and 2.4×10¹⁴ sec^–1 for Nb₇₀Zr₁₅Si₁₅ and about 1.28 eV and 1.2×10¹¹ sec^–1 for Zr₈₅Si₁₅, respectively. The high frequencies indicate that the relaxations occur more or less independently of each other in a non-co-operative manner. The dressed density of electronic states at the Fermi level,N(E _f) (1+), which was calculated from the measured values of _n and (dH _c2dT)T_c, exhibited a similar annealing temperature dependence to that ofT _c. From this the change inT _c on thermal relaxation was interpreted as due to the changes in and/orN(E _f). From the depressions ofJ _c(H) and fluxoid pinning force on annealing in a temperature range of 473 to 873 K, it was concluded that the structural relaxation from a less homogeneous quenched-in state to a homogeneous stable state occurred on the scale of coherence length (7.5 nm) during the annealing.     

Effect of composition on the crystallization and magnetic properties of Fe–Co–Si–B–Nb amorphous alloys

The crystallization behavior and magnetic properties of Fe₆₂Co₁₀Si_15−x B_18−y Nb_{(x + y)−5} amorphous alloys with x = 0–5 and y = 0–5 were examined. Primary crystallization temperature of Fe₆₂Co₁₀Si_15−x B₁₃Nb _x and Fe₆₂Co₁₀Si₁₀B_18−y Nb _y alloys increased with the addition of Nb. The primary and secondary crystallization temperatures were well separated when the Nb content is above 4 at%. The alloys with 15–18 at% B showed a distinct supercooled region. The Nb addition decreased the Curie temperature as well as room temperature saturation magnetization. The glassy-type Fe₆₂Co₁₀Si₁₀B₁₈ alloy exhibited good soft magnetic properties as well as a supercooled liquid region of 39 K. The finding of the glassy-type Fe-based alloy without Nb element exhibiting high Bs above 1.4 T is promising for future use as a soft magnetic glass material.     

Microstructure and superconducting properties of melt-quenched insoluble Al-Pb and Al-Pb-Bi alloys

In order to obtain aluminium-based superconducting alloys including finely dispersed lead or Pb-Bi particles, the application of the melt-quenching technique has been tried for Al-Pb, Al-Si-Pb and Al-Si-Pb-Bi alloys containing immiscible elements such as lead and bismuth. It has been found to result in the preparation of superconducting materials consisting of fcc Pb or h c p (Pb-Bi) particles dispersed finely and densely in the aluminium-based matrix in each composition range below about 2 at % Pb for Al-Pb alloys and 5 at % Pb or (Pb + Bi) for (Al_0.9 Si_0.1)_100–x Pb _x and (Al_0.9Si_0.1)_100–x (Pb_0.6Bi_0.4) _x alloys. The particle size and interparticle distance were 40 nm and 40 to 100 nm, respectively, within the grains, and 100 nm and below 30 nm, respectively, at the grain boundaries for the lead phase in Al₉₈ Pb₂ alloy. Particle size was 15 to 60 nm and interpartide distance 30 to 60 nm for the Pb- Bi phase in (Al_0.9 Si_0.1)₉₅(Pb_0.6 Bi_0.4)₅ Transition temperature,T _c was 4.16 K for Al₉₈Pb₂, 3.94K for (Al_0.9Si_0.1)₉₅ Pb₅ and 7.75 K for (Al_0.9Si_0.1)₉₅(Pb_0.6Bi_0.4)₅. The upper critical magnetic field,H _c2, and critical current density,J _c, for (Al_0.9Si_0.1)₉₅(Pb_0.6Bi_0.4)₅ were 0.22 T at 4.2 K and 1.67 × 10⁷ Am^–2atzeroappliedheld and 4.2 K. The appearance of the superconductivity for the aluminium -based alloys was interpreted as due to the formation of superconducting percolation path along the tangled dislocations, sub-boundaries and/or grain boundaries where Pb and Pb-Bi phases precipitated preferentially.     

Nonequilibrium phases in rapidly quenched Co-C-Si and Ni-C-Si alloys

It has been found that the rapid quenching of Co-C-Si and Ni-C-Si alloys results in the formation of an amorphous phase in the range above 10 at% C and 12 to 23 at% Si in the Co-C-Si system, and a nonequilibrium ordered b c c phase with a lattice parameter of 0.2744 nm in the range above 4 at% C and 15 to 21 at% Si in the Ni-C-Si system. Since the interaction between cobalt and carbon is repulsive, the glass formation in the high metalloid concentration range in the Co-C-Si system is thought to be attributed to a strongly attractive interaction between metalloid atoms (carbon and silicon). Crystallization temperature and Vickers hardness of the amorphous alloys are in the range of 671 to 708 K and 833 to 942 diamond pyramid number (DPN) respectively. Furthermore, the amorphous alloys exhibit a soft ferromagnetism and the Curie temperature, saturated magnetization under an applied field of 100 Oe, coercive force and permeability at 1 kHz are 395 to 432 K, 4.33 to 5.50 kG, 0.031 to 0.210 Oe and 31 000, respectively, in the as-quenched state. The effective permeability of (Co_0.94Fe_0.06)_67.5C_12.5Si₂₀ amorphous alloy is higher than that of Co₆₇Fe₄Si₁₉B₁₀ amorphous alloy with zero magnetostrain at frequencies above 200 kHz. Accordingly, the Co-C-Si amorphous alloys newly found in the present work are very attractive as a soft ferromagnetic material with good characteristics in the high frequency range.     

Effect of Nb addition on the glass forming ability and mechanical properties in the Ni-Zr-Al-Hf-Nb alloys

The effect of substituting Nb for Zr on the glass forming ability (GFA) and mechanical properties of Ni₆₀ Al₆Hf₇ (x = 0 -- 14) alloys was investigated. The substitution of Zr with Nb of 0 -- 14 at% improved the GFA. When increasing the Nb content x from 0 -- 14, the glass transition temperature, T_g, and the crystallization temperature, , of melt-spun Ni₆₀ Al₆Hf₇ (x = 0 -- 14) alloys increased from 833 and 863 K to 877 and 914 K, respectively. The Ni₆₀ Al₆Hf₇ (x = 8, 10, 12 and 14) alloys exhibited high (>0.615), (>0.4) values and a wide supercooled liquid region, (= – T_g) (>36 K) enabling the fabrication of bulk metallic glass (BMG) with a diameter above 1 mm. As such, Ni₆₀Zr₁₇Al₆Hf₇Nb₁₀ and Ni₆₀Zr₁₅Al₆Hf₇Nb₁₂ alloys with a maximum diameter of 2 mm could be fabricated by injection casting. These bulk amorphous alloys also exhibited good mechanical properties, where the true ultimate compressive strength, strain and Vickers hardness (H_v) were approximately 3.0 GPa, 2.08% and 713, respectively, for the amorphous Ni₆₀Zr₁₇Al₆Hf₇Nb₁₀ alloy, and 3.1 GPa, 2.22% and 687, respectively, for the amorphous Ni₆₀Zr₁₅Al₆Hf₇Nb₁₂ alloy.     

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.     

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.     

The influence of composition on the formation and stability of Ni-Si-B metallic glasses

The influence of composition on the formation and stability of Ni-based glassy alloys containing Si and B has been investigated systematically. Depending on the SiB ratio certain compositions, with total metalloid contents in the range 17 to 49 at% (a wider range than has hitherto been reported for metal-metalloid glasses), have been vitrified by melt-spinning to an average thickness of 17 m. For metalloid concentrations greater than 36 to 40 at% the amorphous phase is brittle in the as-quenched state. The highest crystallization temperatures, T _x, occur at 32 to 38 at% metalloid, the actual value of T _x again depending on the SiB ratio. It is shown that, over a wide composition range, the glass-forming boundary corresponds closely with a value for the reduced crystallization temperature isometric of 0.52. This value corresponds to a critical cooling rate for glass formation of about 10⁶ K sec^–1, predicted from kinetic theories and assuming that T _x is a good estimate of the glass transition temperature, T _g. This agrees quite closely with the cooling rate for approximately 17 m thick tape predicted from thermal studies of the melt-spinning process. Hence, T _x/T _liq usefully describes the glass-forming ability (GFA) for much of the composition range studied, although preliminary results suggest that near the centre of the glass-forming range the GFA may be over-estimated. Substitution of Si by other metalloid elements from Groups IIIb–Vb in Ni₇₈Si₁₀B₁₂ glassy alloy generally decreases the thermal-stability.     

Superconductivity in dilute Cu-Nb-Sn alloys containing Nb3Sn precipitates

The dilute Cu-Nb-Sn alloys containing small amounts of Nb and Sn less than 1 at % exhibited superconductivity after quenching from the liquid state and ageing. The best superconducting properties ( andJ _c=130 A cm^–2) in a Cu-0.30 at % Nb 0.15 at % Sn alloy were obtained when the sample was aged at 550° C for 384 h. This sample exhibited a structure of fine Nb₃Sn precipitates of 200 to 500Å diameter distributed homogeneously in the Cu matrix, and therefore it was concluded that superconductivity in these alloys resulted from the proximity effect of Nb₃Sn particles. In spite of the similar structure obtained by ageing at 800° C, the Cu-Nb-Sn alloys showed inferior superconducting properties compared to the Cu-0.4 at % Nb alloy and this would be explained qualitatively by the difference in the mean free path in the two alloys.     

Microstructure and superconducting properties of rapidly-quenched copper-based alloys containing immiscible lead and bismuth elements

Copper-based superconducting alloys including finely dispersed f c c lead or h c p (Pb- Bi) particles in f c c copper matrix have been obtained by rapid quenching (Cu-M)_100-x Pb_x and (Cu-M)_100-x (Pb_0.6Bi_{0.4 x} (M = aluminium, silicon or tin;x < 10 at%) alloys containing immiscible elements such as lead and bismuth. The particle size and interparticle distance were about 30 to 130 nm and 20 to 200 nm for had particles and about 30 to 60 nm and 30 to 150 nm for (Pb- Bi) particles. The transition temperature,T _c, was in the range of 3.2 to 5.5 K for the Cu-M-Pb alloys and 6.2 to 6.3 K for the Cu-M-Pb-Bi alloys. Critical magnetic field,H _c2, and critical current density,J _c, for the later alloys were 0.47 to 0.93T at 4.2 K and 1.1 × 10⁵ to 2.7 × 10⁵ Am^–2 at zero applied field and 4.21 K. The mechanism of the appearance of such a soft-type superconductivity for the rapidly quenched copper-based alloys was discussed, and inferred to be due to the formation of a percolation path of a superconducting lead or Pb-Bi phase along the grain boundaries, sub-boundaries and/or tangled dislocations where the lead or Pb-Bi phase precipitated preferentially, rather than the proximity effect based on lead or Pb-Bi particles.     

Preparation of nickel-based amorphous alloys with finely dispersed lead and lead-bismuth particles and their superconducting properties

The application of the melt-quenching technique to Ni-Si-B-Pb, Ni-P-B-Pb, Ni-Si-B-Pb-Bi and Ni-P-B-Pb-Bi alloys containing immiscible elements such as lead and bismuth has been tried and it has been found to result in the formation of a new type of material consisting of fine fcc Pb or hcp (Pb-Bi) + bct X(Pb-Bi) particles dispersed uniformly in the nickelbased amorphous matrix. The particle size and interparticle distance were 1 to 3 and 1 to 4 µm, respectively, for the lead phase, and less than 0.2 to 0.5 µm and 0.2 to 1.0 µm for the Pb-Bi phase. The uniform dispersion of such fine particles into the amorphous matrix was achieved in the composition range below about 6at% Pb and 7at% (Pb + Bi). Additionally, these amorphous alloys have been found to exhibit a superconductivity by the proximity effect of f c c Pb or (Pb-Bi) superconducting particles. The transition temperatureT _c was in the range 6.8 to 7.5 K for the Ni-Si(or P)-B-Pb alloys and 8.6 to 8.8 K for the Ni-Si (or P)-B-Pb-Bi alloys. The upper critical fieldH _c2 and the critical current densityJ _c for (Ni_0.8 P_0.1 B_0.1)₉₅ Pb₃ Bi₂ at 4.2 K were, respectively, about 1.6T and of the order of 7 X 10⁷ A m^–2 at zero applied field. Melt quenching of amorphous phase-forming alloys containing an immiscible element has thus been demonstrated, enabling us to produce amorphous composite materials exhibiting unique and useful characteristics which cannot be obtained in homogeneous amorphous alloys.     

Non-equilibrium crystalline superconductors in Zr-Si binary alloys rapidly quenched from melts

The new non-equilibrium superconductor with a b c c structure has been found in rapidly quenched Zr-Si alloys. The silicon content in the b c c alloys was limited to the narrow range between 8 and 11 at%. The b c c alloys showed a superconducting transition whose temperature increased from 3.20 to 3.84 K with decreasing silicon content. The upper critical magnetic field and the critical current density for Zr₉₂Si₈ alloy were of the order of 3.58 × 10⁶ Am^–1 at 2.0 K and 3.3 × 10^–2 Am^–2 at 2.42 K in the absence of an applied field. The upper critical field gradient at the transition temperature and the electrical resistivity at 4.2 K were about — 1.82 × 10^–1 Am^–1 K^–1 and about 150 cm. The Ginzburg-Landau parameter and coherence length _GL (0) were estimated to be about 65 and 6.3 nm, respectively, from these experimental values by using the Ginzburg-Landau-Abrikosov-Gorkov theory and hence it is concluded that the present b c c alloys are extremely soft type-II superconductors with a high degree of dirtiness.     

Crystallization characteristics of an amorphous Nb81Si19 alloy under high pressure and formation of the A15 phase

Amorphous Nb-19 at% Si alloy, prepared by rapid quenching from the molten state, was annealed while being subjected to a pressure of 10 GPA. X-ray diffraction investigations on the alloy specimens quenched to ambient conditions have shown that pressure greatly alters the crystallization characteristics and the cubic A15 (Nb₃Si)-phase forms in preference to the tetragonal Nb₃Si-phase at temperatures in the range from 710° C to 800° C. Up to 680° C, the component atoms do not show any tendency towards ordering upon crystallization and the body-centred tetragonal solid solution forms; while, at 830° C, niobium atoms diffuse to form the body-centred cubic Nb precipitates. Superconducting properties have been measured for the single-phase A15 structure with the lattice parametera=0.5155 nm with the results that the transition temperature,T _C, is 3.4 K and the temperature coefficient of the upper critical field,H _C2, is 1.2 MA m^–1 K^–1 (15 kOe K^–1).     

Superconductivity of Zr-Nb-Si amorphous alloys

Superconducting amorphous alloys with high strength and good ductility have been found in rapidly quenched alloys of the Zr-Nb-Si system. These alloys were produced in a continuous ribbon form of 1 to 2 mm width and 0.02 to 0.03 mm thickness using a modified single roller quenching apparatus. The amorphous alloys were formed over the whole composition range between zirconium and niobium, but the silicon content was limited to the relatively narrow range between about 12 and 24 at%. All the amorphous alloys showed a superconducting transition whose temperature, T _c, increased from 2.31 to 4.20 K with increasing niobium content or with decreasing silicon content. The upper critical magnetic field, H _c2, and the critical current density, J _c, for Zr₁₅Nb₇₀Si₁₅ alloy were of the order of 4.5 Tesla(T) and 5.5×10⁶ A m^–2 at 1.5 K in the absence of applied field. The upper critical field gradient at T _c, , and the electrical resistivity at 4.2 K, _n, decreased from 2.89 to 2.10 T K^–1 and from 2.70 to 1.80m, respectively, with the amount of niobium. The Debye temperature, _D, the electron-phonon coupling constant, , and the bare density of electronic states at the Fermi level, N(E _f ) were calculated from the experimentally measured values of _n, , Young's modulus and density by using the strong-coupling theories. From the comparison of T _c with their calculated parameters, it was found that is the most dominant parameter for T _c. The GL parameter, , and the GL coherence length, _GL(0), were estimated to be 70 to 100 and about 7.6 nm, respectively, from the experimental values of and _n by using the GLAG theory and hence it is concluded that the present amorphous alloys are an extremely dirty type-II superconductor having a very weak flux pinning force.     

The effects of carbon on the magnetic properties of nanocrystalline Fe-based alloys

We investigate the magnetic properties of nanocrystalline Fe_73.5Cu₁M₃Si_13.5B₉ (M=Nb or Mo) alloys when C is substituted for B up to 2 at%. It is found that the permeability and coercivity deteriorate with the content of C in the case of both M=Nb and Mo. The saturation magnetization also deteriorates as C is substituted for B in the case of M=Mo but it improves linearly with the C content in the case of M=Nb. This increase in the saturation magnetization of the Fe-Cu-Nb-Si-B alloy with C addition can provide an opportunity to overcome one of the main disadvantages, low magnetic flux density, of the alloy. In the latter part of the work we also investigate the magnetic properties of Fe_76.5–y Cu₁Nb_y (Si_0.5B_0.4C_0.1)_22.5 (0y3) alloys, particular emphasis being given to the role of Nb in the presence of C. It is found that C may help Nb to suppress the growth of -Fe grains in the alloy.     

A review of mechanical behaviour and stress effects in hard superconductors

The mechanical properties of type II superconducting materials are reviewed as well as the effect of stress on the superconducting properties of these materials. The bcc alloys Nb-Ti and Nb-Zr exhibit good strength and extensive ductility at room temperature. Mechanical tests on these alloys at 4.2 K revealed serrated stress-strain curves, non-linear elastic effects, and reduced ductility. The non-linear behaviour is probably due to twinning and de-twinning or a reversible stress-induced martensitic transformation. The brittle A-15 compound superconductors, such as Nb₃Sn and V₃ Ga, exhibit unusual elastic properties and structural instabilities at cryogenic temperatures. p]Multifilamentary composites consisting of superconducting filaments in a normal metal matrix are normally used for superconducting devices. The mechanical properties of alloy and compound composites, tapes, as well as composites of niobium carbonitride chemically vapour deposited on high strength carbon fibres are presented. Hysteretic stress-strain behaviour in the metal matrix composites produces significant heat generation, an effect which may lead to degradation in performance of high field magnets. Measurements of the critical current density, J_c, under stress in a magnetic field are reported. Modest stress-reversible degradation in J_c is observed in Nb-Ti composites while more serious degradation is found in Nb₃Sn sample.The importance of mechanical behaviour on device performance is discussed.     

Studies of superconducting A-15 vanadium-based alloys

Superconductivity of binary alloys spanning the A-15 compounds V₃Si, V₃Ge, V₃Ga, and V₃Al and the pseudobinary derivatives of these stoichiometric compounds was surveyed by studying samples prepared by rf-sputtering from alloy cathodes. The possible formation of the hypothetical A-15 binaries V₃P, V₃B, and V₃C and their pseudobinary formation with V₃Si was also explored. Efforts to form these hypothetical alloys were not successful. The T _c's were measured resistively and the structure and lattice constants were determined by x-ray analysis. A maximum T _cof 11.7 K was obtained for A-15 V₃Al, and the importance of a suitable deposition temperature and high sputtering pressures was examined. It is proposed that large variations of T _camong pseudobinary alloys imply a rapid variation of the density of states at the Fermi level.     

Irradiation damage of A15-Nb3Si and Nb7Si

The behavior of the superconducting temperatureT _c and the lattice parametera ₀ of polycrystalline Nb₃Si and Nb₇Si thin films as a function of 300-kV -particle fluence was investigated. It was found that theT _c depression of A15 Nb₃Si is of the same size and nature as for other A15 materials. In contrast, the initialT _c degradation of Nb₇Si is a factor of three larger. The observed minimum in theT _c vs fluence dependence of Nb₃Si is discussed and a simple qualitative model is presented.On leave of absence from the Institute of Physics, Czechoslovak Academy of Sciences, Prague, Czechoslovakia.