Ni3 Nb Research Articles

Hot tensile deformation behaviors and fracture characteristics of a typical Ni-based superalloy

The hot tensile deformation behaviors and fracture characteristics of a typical Ni-based superalloy are studied by uniaxial tensile tests under the deformation temperature range of 920–1040°C and strain rate range of 0.01–0.001s−1. Effects of deformation parameters on the flow behavior, microstructural evolution and fracture characteristics are discussed in detail. The results show that the flow behaviors are significantly affected by the deformation temperature, strain and strain rate. Under relatively low deformation temperatures (920, 950 and 980°C), the flow curves are composed of three distinct stages, i.e., work hardening, steady stress and flow softening stages. The flow curves show the typical DRX characteristics under relatively high deformation temperatures (1010 and 1040°C). With the increase of deformation temperature or the decrease of strain rate, the fraction of recrystallized grains increases. The synthetical effects of localized necking and microvoid coalescence cause the fracture of the studied superalloy under all the deformation conditions. δ phase (Ni3 Nb) and carbides are the nucleus for the formation of microvoids. Also, δ phase plays an important role in the coalescence of microvoids.

Alloying behavior of Ni 3M-type GCP compounds

The site preference of ternary additions in Ni 3M-type GCP compounds was determined from the direction of solubility lobe of the GCP phase on the ternary phase diagram that have been experimentally reported. In Ni 3Nb (D0 a), Co and Cu preferred the substitution for Ni-site, Ti, V and W the substitution for Nb-site, and Fe the substitution for both sites. In Ni 3V (D0 22), Co preferred the substitution for Ni-site, Cr the substitution for both sites, and Ti the substitution for V-site. In Ni 3Ti (D0 24), Fe, Co, Cu, and Si preferred the substitution for Ni-site, Nb, Mo and V the substitution for Ti-site. The thermodynamic model, which was based on the change in total bonding energy of the host compound by a small addition of ternary solute, was applied to predict the site preference of ternary additions. The bond energy of each nearest neighbor pair used in the thermodynamic calculation was derived from the heat of compound formation by Miedema's formula. The agreement between the thermodynamic model and the result of the literature search was excellent. From both experimental and theoretical results, it was shown in three Ni 3M-type GCP compounds that both transition and B-subgroup elements have two possibilities, i.e., the case of substitution for Ni-site or the case for M-site, depending on the relative value of two interaction energies.

Plastic deformation behavior of Ni 3(Ti 0.7Nb 0.3) single crystals with D0 19 structure

We have investigated the plastic deformation behavior of Ni 3(Ti 0.7Nb 0.3) crystals with the D0 19 structure in the temperature range of −196 to 1200 °C. The ( 0001 ) < 2 1 ¯ 1 ¯ 0 > basal slip, { 1 1 ¯ 00 } < 11 2 ¯ 0 > prism slip, and { 2 ¯ 111 } < 2 ¯ 11 6 ¯ > pyramidal slip are operative depending on the temperature and loading orientation. A strong yield stress anomaly (YSA) is caused by slip on the basal plane similar to that observed in other hcp-derivative geometrically close-packed (GCP) compounds such as D0 a-Ni 3Nb and D0 24-Ni 3Ti, due to the Kear–Wilsdorf (K–W) locking of screw dislocations. The features of YSA by basal slip, such as the profile of the temperature vs. critical resolved shear stress (CRSS) curve are quite similar in those Ni 3(Ti 0.7Nb 0.3), Ni 3Nb, and Ni 3Ti crystals; nevertheless, the profiles of the temperature vs. CRSS curves observed in the case of prism slip are significantly different for each crystal. The results indicate that in the case of these hcp-derivative GCP compounds, although the K–W locking, which is the origin of YSA, accompanies the thermally activated cross-slip of the screw partial dislocations from the (0001) primary slip plane onto the { 01 1 ¯ 0 } prism plane, the mobility of dislocations on the prism plane does not directly influence the YSA behavior of basal slip.

Microstructure and mechanical properties of the superalloy ATI Allvac ® 718Plus™

ATI Allvac ® 718Plus™ is a novel nickel-based superalloy, which was designed for heavy-duty applications in aerospace turbines. In the present study the high-resolution investigation techniques, atom probe tomography, electron microscopy and in situ high-temperature small-angle neutron scattering were used for a comprehensive microstructural characterization. The alloy contains nanometer-sized spherical γ′ phase precipitates (Ni 3(Al,Ti)) and plate-shaped δ phase precipitates (Ni 3Nb) of micrometer size. The precipitation kinetics of the γ′ phase can be described by a classical model for coarsening. The precipitation strongly influences the mechanical properties and is of high scientific and technological interest.

Oxygen enhanced crack growth in nickel-based superalloys and materials damage prognosis

This paper summarizes the results from a comprehensive multidisciplinary study to better understand the role of niobium and other strengthening elements in enhancing crack growth by oxygen in nickel-based superalloys at high temperatures, and considers its importance for materials damage prognosis and life cycle engineering in high temperature service. Three γ′ strengthened powder metallurgy (P/M) alloys, with 0, 2.5 and 5 wt pct Nb and comparable volume fractions (about 53 vol pct) of γ′′ precipitates, were specially designed for this study. Coordinated crack growth, microstructural and surface chemistry studies were conducted on the alloys. They were complemented by oxidation studies of Nb, Ni 3Nb, NbC, Ni 3Al and Ni 3Ti, and analyses of fracture surfaces of interrupted crack growth specimens by X-ray photoelectron spectroscopy (XPS). The findings taken in toto show that oxygen enhancement of crack growth is the result of the formation of a brittle film of surface oxides along grain boundaries and interfaces ahead of the crack tip by the preferential oxidation of Nb, Ti and Al in the Nb-rich carbides and Ni 3Al, Ni 3Ti and Ni 3Nb (in Inconel 718) precipitates. The results also showed that the oxidation of Nb-rich carbides alone can significantly enhance crack growth in oxygen. The findings are discussed in relation to the previously proposed crack growth mechanisms, and their applications.

Determination of isothermal sections of the Co-Nb-Ni and Ni-Mo-Re ternary systems

The isothermal sections of the Co-Nb-Ni ternary system at 1373 K and the Ni-Mo-Re ternary system at 1473 K were determined by means of diffusion triple and electron probe microanalysis (EPMA). The results indicate that there are three three-phase regions found in the Co-Nb-Ni ternary system at 1373 K: (Ni, Co) + NbCo 3 + Ni 3Nb, NbCo 3 + NbCo 2 + Ni 3Nb, and NbCo 2 + μ + Ni 3Nb; and four three-phase regions found in the Ni-Mo-Re ternary system at 1473 K: Re + Ni + χ, Ni + NiMo + σ, σ + χ + Ni, and σ + Mo + NiMo. No ternary compound is observed in the two isothermal sections. The isothermal sections are contrasted with the previous study.

The effect of second-phase dispersions on mechanical property of Ni 3Si based multi-phase intermetallic alloys

Using alloys whose initial microstructures are composed of Ni 3Si(L1 2), Ni 3Si(L1 2) + Ni 3Ti(D0 24) and Ni 3Si(L1 2) + Ni 3Nb(D0 a), aging phenomenon and the associated high-temperature tensile property were investigated. The plate-like Ni 3Ti (D0 24) phase was precipitated from the Ni 3Si (L1 2) matrix at high temperature. It was shown by micro-hardness measurement that age hardening behavior due to the precipitation of the Ni 3Ti (D0 24) phase occurs in all alloys in the temperature range from 923 K to 1323 K. It was however shown by tensile test that the precipitated Ni 3Ti (D0 24) phase is not so much effective in improving the mechanical properties of alloys whose initial microstructures are composed of Ni 3Si(L1 2) + Ni 3Nb(D0 a) or Ni 3Si(L1 2) + Ni 3Ti(D0 24). In alloys whose initial microstructures are composed of Ni 3Si(L1 2) + Ni 3Nb(D0 a), a good combination of tensile strength and tensile elongation was found over a wide of test temperature regardless of presence or absence of the precipitated Ni 3Ti (D0 24) phase. It is suggested that the primary coarse Ni 3Nb (D0 a) phase precipitates are beneficial to improving the mechanical properties.

The isothermal section of the Co–Ni–Nb ternary system at 1123 K determined by diffusion triple technique

An isothermal section of the Co–Ni–Nb system at 1123 K was experimentally determined using diffusion triple technique together with electron probe microanalysis (EPMA). Two binary compounds, Ni 6Nb 7 and Co 7Nb 6 form a continuous solid solution (Ni,Co) 6Nb 7 in the ternary. Ni can partially substitute Co in Co 2Nb and Nb in Co 3Nb. Co can partially substitute Ni in Ni 3Nb. One ternary phase named as “X” has been observed with homogeneity range covering 20–68 at.% Co and 23–26 at.% Nb. Furthermore, five three-phase equilibria, Co 3Nb + Co 2Nb + X, X + (Ni,Co) 6Nb 7 + Co 2Nb, X + Co 3Nb + γ(Co,Ni), Ni 3Nb + X + (Ni,Co) 6Nb 7 and Ni 3Nb + X + γ(Co,Ni) have been determined in this isothermal section.

Finite-element analysis of microstructure evolution in the cogging of an Alloy 718 ingot

Microstructural changes during the cogging of an Alloy 718 ignot processed by (vacuum induction melting)/(vacuum arc re-melting) were predicted using the combined approach of three-dimensional finite element simulation and modeling for the recrystallization and grain growth. In order to establish recrystallization and grain growth models for Alloy 718 ingot, hot compression and isothermal heat treatment tests were carried out at different temperatures and strain rates. The constitutive equations for metadynamic and static-recrystallization behavior were generated using the experimental data collected from various references. From the experimental results, it was found that the dissolution temperature of δ-phase (Ni 3Nb) played an important role in the microstructural refinement of Alloy 718 ingot. The predicted grain size and its distribution were compared with the microstructures of the cogged billet, and the reliability of the prediction was discussed.

Effect of boron and carbon on thermomechanical fatigue of IN 718 superalloy: Part II: Deformation microstructures

The influence of loading mode, stress range and addition of B and C on the deformation microstructures of IN 718 superalloy produced by thermomechanical fatigue (TMF) was investigated. The results showed that microstructural characteristics and cyclic deformation mechanisms were significantly influenced by the loading mode. In the in-phase (IP) TMF, twinning was the predominant deformation mode in IN 718 with different concentrations of B and C, and only occasional slip activity was observed in some localized regions. The typical IP-TMF deformation substructure consisted of a group or multiple groups of parallel twin bands on {1 1 1} twinning planes. In contrast, out-of-phase (OP) TMF was accompanied by a pronounced planar slip, and the characteristic deformation substructure was planar slip bands with occasionally observed twins. Twinning was more detrimental than slip during TMF deformation and caused a reduced TMF life in IP than in OP deformation mode. M 23C 6 type carbide particles were observed to retard the migration of grain boundaries by pinning them during TMF cycling. Coarsening of γ″ (Ni 3Nb) precipitates was observed to occur in the alloy with a lower concentration of boron. The dependence of TMF deformation behavior on the loading mode was rationalized on the basis of the deformation microstructures.

Influence of modified processing on structure and properties of hot isostatically pressed superalloy Inconel 718

Inert gas atomized (IGA) superalloy Inconel 718 powder was consolidated by hot isostatic pressing (HIPing) at 1200 °C under 120 MPa pressure for 3 h. The HIPed alloy heat treated as per the aerospace materials specification (AMS) 5662J standard schedule, viz. solution treatment (ST) at 980 °C for 1 h/water quenching (WQ) to room temperature (RT) and a two-step ageing treatment (AT) at 720 °C for 8 h/furnace cooling (FC) at 55 °C h −1 to 620 °C and holding at 620 °C for 8 h and air cooling (AC) to room temperature has exhibited the yield strength (YS) and ultimate tensile strength (UTS) comparable to that of the conventionally processed (forged and heat treated) IN 718. However, its ductility and stress rupture properties at 650 °C were found to be poor due to the presence of prior particle boundary (PPB) networks decorated with highly stable oxides (Al 2O 3 and TiO 2) and brittle MC (Nb, Ti)C carbides. To mitigate this problem, the HIPed alloy was subjected to solution treatment at 1270 °C for 1 h followed by re-HIPing at 1100 °C/130 MPa/3 h before heat-treating it as per AMS 5662J standard schedule. Optical and scanning electron microscopy (SEM) of the alloy processed under modified HIPing and heat treatment conditions have shown the dissolution of MC-carbides, breaking up of PPB networks and formation of equiaxed grains with an average diameter of about 50 μm. Transmission electron microscopy (TEM) of this alloy has revealed uniform distribution of γ″ and γ′ strengthening precipitates in the γ-matrix and the presence of δ(Ni 3Nb) phase as well as very fine oxide particles near the grain boundaries. The tensile properties of the alloy processed under modified conditions have shown quite satisfactory levels of YS and UTS combined with a significantly improved elongation (EL) values at room temperature (19.5%) and at 650 °C (8.0%). The improvement in alloy ductility was found to correlate well with the fractography of the tensile tested specimens, which showed the predominance of transgranular fracture with fine dimples at room temperature and fine dimples together with the particle boundary facets at 650 °C. The stress rupture properties of modified processed alloy at 650 °C and at a stress level of 690 MPa have shown a vastly improved rupture life of 80 h combined with 5% ductility. The improvement in tensile and stress rupture properties accomplished by the modified processing makes it possible to explore the near net shape capability of HIP technology to its full potential in the development of alloy 718 components.

Effect of a heterogeneous distribution of particles on the formation of banded grain structure in wrought Alloy 718

Alloy 718 is known to be sensitive to interdendritic segregation formed during ingot solidification. The occurrence of banded grain structures under heat treating conditions close to 1000 ° C related to interdendritic segregation is often reported. In order to have a better understanding of this microstructural evolution, an extensive experimental program has been carried out. Consequently, a model taking into account the selective dissolution of δ -phase (Ni 3Nb) is proposed. A grain growth simulation by Monte-Carlo method is then used to illustrate the grain structure evolution in a banded particle distribution. By comparing experimental data and computer simulation, the relationship between the Monte-Carlo step and the real time is assessed and the range of parameters when heterogeneous microstructures appear is specified.

Correlation of radiation-induced changes in mechanical properties and microstructural development of Alloy 718 irradiated with mixed spectra of high-energy protons and spallation neutrons

Alloy 718 is a γ ′(Ni 3(Al,Ti))–γ″(Ni 3Nb) hardenable superalloy with attractive strength, and corrosion resistance. This alloy is a candidate material for use in accelerator production of tritium (APT) target and blanket applications, where it would have to withstand low-temperature irradiation by high-energy protons and spallation neutrons. The existing data base, relevant to such irradiation conditions, is very limited. Alloy 718 has therefore been exposed to a particle flux and spectrum at the Los Alamos Neutron Science Center (LANSCE), closely matching those expected in the APT target and blanket applications. The yield stress of Alloy 718 increases with increasing dose up to ∼0.5 dpa , and then decreases with further increase in dose. The uniform elongation, however, drastically decreases with increasing dose at very low doses (<0.5 dpa), and does not recover when the alloy later softens somewhat. Transmission electron microscopy (TEM) investigation of Alloy 718 shows that superlattice spots corresponding to the age-hardening precipitate phases γ ′ and γ″ are lost from the diffraction patterns for Alloy 718 by only 0.6 dpa, the lowest proton-induced dose level achieved in this experiment. Examination of samples that were neutron irradiated to doses of only ∼0.1 dpa showed that precipitates are faintly visible in diffraction patterns but are rapidly becoming invisible. It is proposed that the γ ′ and γ″ first become disordered (by <0.6 dpa), but remain as solute-rich aggregates that still contribute to the hardness at relatively low dpa levels, and then are gradually dispersed at higher doses.

High temperature oxidation of Nb, NbC and Ni 3Nb and oxygen enhanced crack growth

Previous studies have shown that oxygen increased the rates of crack growth in Ni base superalloys at high temperatures (> 800 K) by up to four orders of magnitude over those observed in an inert environment for alloys such as inconel 718, Rene-95 and x 750. In this study, high temperature oxidation studies of pure Nb, NbC and Ni{sub 3}Nb using XPS were undertaken (a) to provide direct confirmation of the oxidation of NbC, (b) to explore the possible role of Ni{sub 3}Nb and (c) to assess the relative reactivity of the latter two phases, along with Nb, with oxygen. Nb, NbC and Ni{sub 3}Nb were oxidized as a function of temperature and constant oxygen exposure, and the results are presented in this communication.

Radiation-Induced Phase Instabilities and Their Effects on Hardening and Solute Segregation in Precipitation-Strengthened Alloy 718

ABSTRACTA classic example of radiation-induced phase instability and degraded mechanical properties occurs in γ′-γ″-strengthened alloy 718. During irradiation with neutrons or protons at ∼ 30 to 288°C, the Ni 3Nb γ″ particles disappear after low doses. The γ′ (present only in the matrix) also disappears after &lt;0.6 dpa at 30 to 55°C, but at 288°C it persists to higher doses and eventually reprecipitates as new γ′ with changed composition. Hardness of the alloy is unaffected by disappearance of the γ″, but decreases appreciably at 288°C as the original γ′ particles dissolve. Fine-probe compositional measurements in a TEM showed that the softening coincides with solute redistribution and reprecipitation rather than with the phase disappearance. Compositional changes at grain boundaries included leveling of the thermally segregated Mo as well as strong Ni enrichment and loss of Nb after high doses. The complex phase stability and solute redistribution behavior reflects mainly ballistic mixing at 30-70°C irradiation temperatures and the influence of significant thermal diffusivities at the higher temperatures.

The corrosion of two NiNb alloys under 1 atm O 2 at 600–800 °C

The oxidation of two NiNb alloys containing 15 and 30 wt% Nb has been studied at 600–800 °C in pure oxygen under 1 atm O 2 at 600–800 °C. The scales formed on both alloys under all conditions show an external scale, generally duplex, containing an outermost layer of nearly pure NiO and an innermost region of NiO mixed with the double NiNb oxide NiNb 2O 6. Moreover, the samples corroded at all temperatures also show a region of internal oxidation composed of a mixture of alpha nickel and niobium oxides (Nb 2O 5 or/and NbO 2), which formed from both alloy phases Ni 8Nb and Ni 3Nb. No important depletion of niobium was observed in the alloy close to the interface with the zone of internal oxidation, while the depth of this region is generally much higher than measured for the corrosion of the same alloys under low oxygen pressures at the same temperatures. The corrosion mechanism of these alloys is examined with special reference to the effects of the low solubility of niobium in nickel.

Influence of heat treatments on microstructure, mechanical properties, and corrosion resistance of weld alloy 625

The effects of heat treatments of the industrial type (eight-hour hold times at temperatures between 600 °C and 1000 °C) on the structural, mechanical, and corrosion resistance characteristics of weld alloy 625 have been studied. During the heat treatment, the mean concentration ratios of Nb, Mo, Si, Cr, Ni, and Fe elements between the interdendritic spaces and dendrite cores show little evolution up to 850 °C. Beyond that temperature, this ratio approximates 1, and the composition heterogeneity has practically disappeared at 1000 °C. An eight-hour heat treatment at temperatures between 650 °C and 750 °C results in increased mechanical strength values and reduced ductility and impact strength linked to the precipitation of body-centered tetragonal metastable intermetallic γ″ Ni3Nb phase in the interdendritic spaces. An eight-hour treatment in the temperature range between 750 °C and 950 °C has catastrophic effects on all mechanical characteristics in relation with the precipitation, in the interdendritic spaces, of the stable orthorhombic intermetallic δ Ni3(Nb, Mo, Cr, Fe, Ti) phase. At 1000 °C, the ductility and impact strength are restored. However, the higher the heat treatment temperature, the weaker the mechanical strength. Heat treatments have no effect on the pitting resistance of weld alloy 625 in sea water. The comparison of the results of this study on weld alloy 625 with those previously obtained on forged metal 625 shows that heat treatments below 650 °C and above 1000 °C are the sole treatments to avoid embrittlement and impairment of the corrosion resistance characteristics of alloy 625.

Multiphase diffusion in the CoNb and NiNb systems: Part I. Solid-solid phase equilibria and growth of intermetallic phases

Diffusion couples of cobalt or nickel and niobium were employed to study solid-state diffusion reaction (multiphase diffusion). The formation and growth of intermetallic phases was investigated by means of optical metallography and electron probe microanalysis. Parabolic layer growth of Co 7Nb 2 and Co 2N b— the dominant phases in the diffusion zone of CoNb pairs—was observed. The growth constants were determined at various temperatures. Slow growth of the CoNb compound was also observed, but the layer thickness never exceeded a few μm. In the diffusion zones of NiNb pairs the two expected intermetallic phases NiNb and Ni 3Nb were present. The phase diagrams of CoNb and NiNb have been carefully redetermined for the temperature regions of solid-solid equilibrium.

Multiphase diffusion in the CoNb and NiNb systems: Part II. Interdiffusion

Interdiffusion coefficients for the intermetallic compounds Co 2Nb, CoNb, NiNb, Ni 3Nb and for the Co-rich primary solution of the CoNb system were determined using the Boltzmann-Matano method. The interdiffusion coefficient of the cubic Laves phase Co 2Nb shows a maximum near 73 at.% Co. The activation enthalpies vary from 173 to 157 kJ mol −1 for 70–73 at.% Co and the pre-exponential factor is 4 × 10 −8 m 2s −1. Diffusion very likely occurs via thermal vacancies. Diffusion in the Co-rich primary solution of CoNb is described by a pre-exponential factor of 8· × 10 −6 m 2 s −1 and an activation enthalpy of 252 kJ mol −1. Interdiffusion in NiNb is independent of composition and is described by an activation enthalpy of 271 kJ mol −1 and a pre-exponential factor of 1·9 × 10 −4 m 2 s −1. Coefficients of interdiffusion for the line compound Co 7Nb 2 have been deduced using Wagner's method.

Comportement à 700°C de métaux et alliages soumis à NbCl5 gazeux

The electrochemical reduction of NbCl5 performed at 700°C in a molten salt bath for obtaining niobium has shown that all metallic elements present in the cell are corroded. In such conditions the electrolysis produces only a Ni3 Nb alloy and never pure metallic niobium. To determine the effect of NbCl5 , we have studied the oxidation of metals (M = Ni, Cr, Nb, Fe, Ti) and alloys (NSMC-NS21C-NS22S-NS30-Inconel 600) put in vapors of niobium pentachloride. For the pure metal M this corrosion is limited by the formation of pure or mixed dichloride (M, Nb)Cl2 . Nickel and chromium are the most resistant in this atmosphere. The alloys with melai M as the main component show a much more important corrosion than pure metal. More ever the amount of corrosion is always too weak to explain the big production of NiCl2 that appears when a cell is continously feed with NbCl5 .We study now the same metals and alloys put in a molten salt bath having differents concentrations in NbCl5 .