Ni-Nb Alloys Research Articles

The oxidation of Ni-Nb alloys at low-oxygen pressures at 600?800�C

The oxidation of Ni-Nb alloys at low-oxygen pressures at 600?800�C

Concentration dependence of the local order in amorphous NiNb alloys

Abstract The atomic structure of amorphous NiNb alloys has been studied via molecular-dynamic simulations on the basis of effective interatomic potentials calculated using the hybridized nearly-free-electron-tight-binding-bond technique. The partial static structure factors and pair correlation functions at four different compositions are compared with results from neutron diffraction with isotopic substitution. Theory and experiment agree in finding a certain chemical order (similar to that in the crystalline NiNb compounds) and strong NiNb bonds that are contracted compared with the average NiNi and NbNb bond lengths. The chemical short-range order depends only weakly on composition. In addition to the laboratory experiment, the computer simulation also yields information on the partial triplet correlation functions. The bond-angle distributions indicate that the local atomic arrangement is essentially of a polytetrahedral type, with broad peaks in the total bond-angle distribution close to the icosahedral bond-angles. In the partial bond-angle distributions, the peaks are slightly shifted as a consequence of size differences and chemical order. This shift is again similar to the bond angles in the coordination polyhedra of the crystalline Frank-Kasper phase. Compared with NiZr and NiY and Ni-metalloid glasses, NiNb glasses are found to be more random and show no sign of the trigonal prismatic order characteristic of these systems. The physical origin of these structural trends is discussed.

Mechanical alloying of NiNb in different atmospheres

The formation of NiNb alloys prepared by mechanical alloying (MA) in different atmospheric conditions was examined by X-ray diffraction. Ball milling of elemental Ni and Nb powders under an argon atmosphere for alloys with Nb contents up to 15 at.% forms a supersaturated solid solution on the base of Ni. Amorphous alloys are formed if the Nb content is more than 20 at.%. The interaction of metallic powders Ni and Nb was absent and a b.c.c. phase was formed for all compositions up to pure Nb, when MA of NiNb powders was performed in the presence of water, alcohol or acetone in Ar and air atmospheres. This phase, identified as the interstitial phase of oxygen in niobium, has the lattice parameter a = 3.390 A ̊ and an O content of about 34 at.%.

Model Calculations of the Structure of Ni-Nb Metallic Glasses

Abstract The partial pair correlation functions of amorphous NixNb100-x alloys (x = 40, 50, 56, and 63) were simulated by computer modelling. A Cluster Relaxation procedure yielded reasonable agree­ ment with the experimental functions when non-additive pair potentials with preference for Ni-Nb atomic pairs were applied. With the use of the Reverse Monte Carlo method perfect agreement could be achieved. The impact of the chemical effect on the short range order is discussed by comparison with a statistical reference cluster. The analysis of the model clusters showed that the chemical short range order effect in amorphous Ni-Nb alloys, increasing with the Ni concentration, does not in­ volve specific stereochemical atomic arrangements. There is no correspondence to the short range order in crystalline Ni3Nb. The structure of Ni-Nb metallic glasses can be described as a polytetra-hedral packing with preferred hetero-coordination.

The oxidation-sulfidation of Ni-Nb alloys at 600–800 °C in H 2-H 2S-CO 2 mixtures

The corrosion of two Ni-Nb alloys containing 15 and 30 wt% Nb has been studied at 600–800 °C in H 2-H 2S-CO 2 mixtures providing a sulfur pressure of 10 −8 atm and oxygen pressures ranging between 10 −24 atm and 10 −20 atm. The corrosion kinetics were approximately linear under all conditions, except for Ni-30Nb at 600 °C, when nearly parabolic kinetics were observed. At 600 °C the scales presented an outer layer of Ni sulfide, an innermost layer of mixed Ni-Nb double sulfide and Ni metal as well as a complex intermediate layer containing a mixture of sulfides of both Nb and Ni with niobium oxide. At 700 and 800 °C the scales formed on both alloys contained a complex mixture of Ni sulfide, double sulfide and pure Ni metal but no oxide, very likely due to the formation of a liquid Ni-S solution which allowed a very fast transport of reactants through the scales. The Nb addition was able to reduce the corrosion rate at 600 °C but was practically inefficient at the higher temperatures.

The corrosion of NiNb alloys in H 2H 2S mixtures at 600–800°C

The sulfidation of two NiNb alloys containing 15 and 30 wt% Nb in H 2H 2S mixtures providing a sulfur pressure of 10 −8 atm was studied at 600–800°C. The corrosion kinetics were nearly parabolic initially and then linear at 600°C but practically linear from the start at 700 and 800°C. At 600°C the scales presented an outer layer of Ni sulfide and an inner complex region containing both Ni and Nb. At 700 and 800°C only a single complex region was observed, due to the formation of a liquid NiS solution which allowed a very fast transport of reactants through the scales. The Nb addition was able to reduce the corrosion rate at 600°C due to a gradual replacement of nickel sulfide by the more protective double NiNb sulfide but was practically inefficient at the higher temperatures due to the appearance of the liquid phase.

Mechanism of the <I>A<SUB>s</SUB></I> Temperature Increase by Pre-deformation in Thermoelastic Alloys

The Ti–Ni–Nb shape memory alloys are attracting recent attention for the wide transformation temperature hysteresis, which is caused by the As increase due to pre-deformation, since it is suitable for pipe couplings. To find out the origin for the As increase by pre-deformation, similar investigation was made by tensile tests, electrical resistance measurements, optical microscopy and transmission electron microscopy (TEM), for a Cu-13.8%Al-4.0%Ni (mass%) single crystal, and Ti-50%Ni and Ti-50.5%Ni (at%) polycrystals, which exhibit the thermoelastic martensitic transformation. The increase of As due to pre-deformation was confirmed in these alloys as well. When the single crystal was elongated to the end of the first stage of the stress-strain curve in the martensitic state, which corresponds to the rearrangement of martensite variants without permanent strain, As increased. On the other hand, the As increase in polycrystals was found to be closely related with the introduction of the permanent strain. Furthermore, the effect of pre-deformation was found to be erased by the subsequent reverse transformation, as is reported for Ti–Ni–Nb alloys. For that effect, we propose a mechanism, by focusing attention on the elastic strain energy stored during the thermoelastic martensitic transformation, which can explain the observed effect well.

The corrosion behavior of Ni-Nb alloys in a mixed gas of H2-H2O-H2S

The corrosion behavior of Ni-Nb alloys containing up to 40 wt.% Nb was studied over the temperature range of 550–800°C in a mixed H2/H2O/H2S gas. The scales formed on all alloys were multilayered. The outer scale was single-phase Ni3S2, while the structure and constitution of the inner scale depended on alloy composition and reaction conditions. Internal oxidation has been found in Ni-20Nb and Ni-30Nb, external oxidation has been observed on Ni-34Nb. Platinum markers were located at the interface between the outer scale and inner scale. The decrease in corrosion rate with increasing Nb content may be attributed to the presence of increasing amounts of Ni-Nb double sulfides as well as to the presence of Nb2O5 in the inner region of the scale.

The limits of glass formation by pulsed laser quenching in NbNi alloys

This paper reports on the growth velocity of a crystal which from a liquid depends on the kinetics of atomic processes at the interface and the rate at which the heat of crystallization is removed from the interface. In the case of rapid quenching using pulsed lasers, the rate of heat removal following the laser pulse is very high due to the very small size of the liquid compared to the surrounding solid. The isotherm velocities are typically {gt}100 ms{sup {minus}1} and hence the crystal growth velocity is determined by the kinetics of atomic processes at the liquid- solid interface. The kinetics at the interface can be limited either by short range or long range atomic rearrangements depending on the alloy composition. The melt duration in a nanosecond laser quenching technique is {gt}5 ns. Atomic rearrangements requiring long range diffusive jumps are not possible in these time scales as the maximum diffusive speed in a liquid is only 10 ms{sup {minus}1}. Hence the crystal growth in dilute alloys has to be partitionless, occurring at velocities {gt}10 ms{sup {minus}1}. However, as the solute content in the alloy increases continuously, the long range diffusive constraints are expected to severly limit the crystalmore » growth velocity. Since the isotherm velocities in the case of pulsed laser quenching are orders of magnitude higher than the diffusion limited crystal growth velocity, it should result in the formation of an amorphous or glass phase. Spaepen and Lin have observed a sharp transition from partitionless crystal growth to glass formation in the Fe-B system with as low as 5 at.% B due to the short range order required to form B clusters. In the case of metal-metal systems such a strong short range order is not always known to exist and hence the transition may not be as sharp as in the metal-metalloid systems.« less

Stress Relaxation and the Formation of Silicides in the Process of the Crystallization of NI-NB Films on SI

Thin films of amorphous Ni-Nb alloys are of interest since they can be used as diffusion barriers for integrated circuits on Si. A native SiO2 layer is an effective barrier for Ni diffusion but it deformation during the crystallization of the alloy film lead to the appearence of diffusion fluxes through it and the following formation of silicides. This study concerns the direct evidence of the action of stresses in the process of the crystallization of Ni-Nb films on Si and the structure of forming NiSi2 islands.

Interaction of thin films of Ni and Ni—Nb alloys with Si the action of the native SiO2 layer

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Devitrification products of rapidly solidified Ni-Nb amorphous alloys.

Rapid solidification of Ni-Nb alloys promotes the formation of amorphous structure. Preliminary results indicate promising elastic properties and high fracture strength for the metallic glass. Knowledge of the thermal stability of the amorphus alloy and the changes in properties with temperature is therefore of prime importance. In this work rapidly solidified Ni-Nb alloys were analyzed with transmission electron microscopy (TEM) during in-situ heating experiments and after isothermal annealing of bulk samples. Differential thermal analysis (DTA), scanning electron microscopy (SEM) and x-ray diffraction (XRD) techniques were also used to characterize both the solidification and devitrification sequences.Samples of Ni-44 at.% Nb were electromagnetically levitated, melted, and rapidly solidified by splatquenching between two copper chill plates. The resulting samples were 100 to 200 μm thick discs of 2 to 3 cm diameter. TEM specimens were either ion-milled or alternatively electropolished in a methanol-10% sulphuric acid solution at 20 V and −40°C.

The reaction of amorphous Ni-Nb films with Si in the presence of a native SiO2 layer

Amorphous Ni-Nb alloys are of potential interest as diffusion barriers for high temperature metallization for VLSI. In the present work amorphous Ni-Nb films were sputter deposited on Si(100) and their interaction with a substrate was studied in the temperature range (200-700)°C. The crystallization of films was observed on the plan-view specimens heated in-situ in Philips-400ST microscope. Cross-sectional objects were prepared to study the structure of interfaces.The crystallization temperature of Ni5 0 Ni5 0 and Ni8 0 Nb2 0 films was found to be equal to 675°C and 525°C correspondingly. The crystallization of Ni5 0 Ni5 0 films is followed by the formation of Ni6Nb7 and Ni3Nb nucleus. Ni8 0Nb2 0 films crystallise with the formation of Ni and Ni3Nb crystals. No interaction of both films with Si substrate was observed on plan-view specimens up to 700°C, that is due to the barrier action of the native SiO2 layer.

EXAFS study on solid state amorphization process of NiNb alloy by mechanical alloying

The main purpose of this work is to investigate the micromechanism of the solid state amorphization of metals by mechanical alloying. The changes of radial distribution functions around the atomic species of a NiNb mixture powder through ball milling were studied by means of EXAFS. The atomic structural change during crystalline to amorphous phase was discussed.

Annealing of Amorphous Ni-Nb/Cu Overlayer Films - Thermal Groove Kinetics

ABSTRACTRecent studies have shown that sputter deposited amorphous NixNb1−x (60≤×≤75) alloy films are effective diffusion barriers for operation at temperatures up to 6(X) °C[1]. The thermal stability of the amorphous films has been established on Si and GaAs substrates and may be modified by interactions wilji polycrystalline metal overlayer films. In the current work a detailed microstructural examination by cross-sectional TEM was performed on the annealing behavior of 50 nm polycrystalline Cu films on amorphous Ni60Nb40. Specifically, the development of grooves at the intersection of grain boundaries in the Cu film with both the free surface and the amorphous Ni60Nb40 base was monitored to evaluate the low temperature interfacial diffusion processes. Based upon measurements of the groove dimensions and dihedral angle during vacuum annealing at 450 °C, the surface diffusion coefficient of Cu was determined as 7.4 × 10-20 m2/sec and the surface energy for Cu derived from groove kinetics was in good agreement with other reported determinations. In addition, the grain boundary grooves that were present at the Cu/amorphous Ni60Nb40 interface in the as-deposited condition were observed to be eliminated during annealing. Again, by following the kinetics of groove healing the interfacial diffusivity was determined to be of the order of 10-28m2/sec at 500 °C and the interfacial energy for the amorphous film/Cu surface was estimated to be ≥ 340 mJ/cm2. The benefits of utilizing amorphous Ni-Nb alloys as underlays to retard thermal grooving and electromigration failure in copper arc discussed.

The surface reactivity of NbNi 3 and Nb 2Ni 3 alloys toward oxygen and its role in modeling catalysts

The surface reactivities of NbNi 3 and Nb 2Ni 3 toward oxygen have been investigated using X-ray photoelectron spectroscopy and in situ reaction. At low temperature and/or low oxygen activities, Nb oxidizes preferentially. At high temperature (greater than 250° C) and high oxygen activity Ni preferentially oxidizes with the product, NiO, overgrowing the niobium oxide. NiO reduces easily to Ni° with loss of surface coverage. It has been found that Ni does not wet Nb 2O 5 as well as it wets TiO 2 and ZrO 2. The role of alloy oxidation, in particular the Ni-Nb alloy, in modeling catalysts is discussed.

High-temperature sulfidation behavior of Ni-Nb alloys

The sulfidation properties of Ni-Nb alloys containing additions of niobium up to 40 wt.% have been studied at $$P_{S_2 } = 0.01$$ atm over the temperature range 550-700 °C. The sulfidation reactions followed the parabolic rate law; the sulfidation rates decreased with increasing amounts of niobium. An Arrhenius plot of the rate constants gave activation energies of 25.0+3.5 kcal/ mole. The scales formed on Ni-Nb alloys were multilayered, generally consisting of an outer layer of nickel sulfide ( NiS1+x and Ni3S2) and an inner complex layer of NiNb3S6 plus NbS2. The position of the original metal surface was notedy platinum-wire marker experiments to be the interface between the inner andouter layers. The location of the marker indicates that the outer layer, generally greater in thickness than the inner layer, grew by outward diffusion of the nickel cations, and the inner layer formed probably by the inward diffusion of sulfur. Neither preferential sulfidation nor internal sulfidation was observed. The development of the scale structures from the transient stage to steady state was also studied.

SEM Observation on Solid State Amorphization of Ni–Nb Alloy by Ball Milling

The main purpose of this work is to investigate the mechanism of solid state amorphization by mechanical alloying of Ni–Nb alloy. The solid state amorphization process of a Ni–Nb mixture powder by mechanical alloying was studied by means of the SEM observation and DTA measurement. The solid state amorphization reaction proceeds through the alternating two steps. In the first step, a lamellar structure is formed by kneading introducing many defects. In the second step, diffusion occurs through the defects, using the collision energy of balls. Then the elements are distributed uniformly, forming an amorphous structure simultaneously in the lamellar region. When the whole crystalline structure changes to an amorphous structure, the solid state amorphization reaction is complete.

Structural modification and ductilization of a metallic glass by mechanical pulverization

Observations on mechanically pulverized Fe 78Si 9B 13 metallic glass ribbon are presented. Pulverized ribbon that has been initially embrittled by relaxation annealing at least partly recovered the malleability that is characteristic of the glass in its as-spun state, as the pulverization was extended to longer times. Scanning electron microscopy observations on the powder particles revealed clear evidence of plastic deformation. Following the increase in the Curie temperature T c associated with the enhancement of short-range order during relaxation, T c was found to decrease towards its initial value for the as-cast ribbon on extended pulverization, consistent with disordering of the glass by plastic deformation. The mechanism by which the initially brittle powder is able to absorb energy and disorder is briefly discussed and comparisons are drawn with the amorphizing of initially stable crystalline NiTi and NiNb alloys by ball milling.

Oxidation of amorphous Ni–Nb films: Surface spectroscopy studies

Sputter deposited amorphous Ni–Nb alloy films have been proposed as thin-film metallization layers for high-temperature (>300 °C) compound semiconductor applications. We have used x-ray photoelectron spectroscopy (XPS) to investigate the oxidation behavior of 1-μm-thick, sputter deposited, amorphous Ni60 Nb40 films exposed to oxygen or room air at temperatures in the range 30–500 °C for times of 5 min to 45 h. The initial stages of oxidation were examined in situ at oxygen partial pressures below 1×10−6 Torr. At all temperatures, Nb2 O5 formed on the surface; this remained the dominant corrosion product for specimens oxidized in air for 45 h at temperatures up to 200 °C. Angle-resolved XPS measurements indicated that a thin (5–10 Å) NiO layer formed on the surface of the Nb2 O5 after long exposures at these temperatures. Specimens oxidized at temperatures above 240 °C, however, quickly developed a thick (>100 Å), crystalline NiO layer on top of the Nb2O5.