Ni Specimens Research Articles

Characterization of inhomogeneous elastic deformation with x- ray diffraction

In standard engineering practice, the stress/strain responses of polycrystalline specimens are analyzed using models based on the equations of continuum mechanics. In general, these equa-tions implicitly assume that the material is quasi-isotropic. This assumption requires a specimen whose volume exceeds a critical. “representative volume.” Domains that are smaller than the representative volume do not exhibit quasi-isotropic behavior. In such volumes, the compati-bility conditions dictated by the local material distribution can cause significant perturbations in the applied stress/strain field. Consequently, the constants linking stress to strain in small volumes may contain additional configurational terms. Due to the complex local material dis-tribution in most real samples, these configurational terms are extremely hard to calculate an-alytically. Experimental measurement of the local deformation is also nontrivial, and thus far there have been no measurements of the configurational constants in the elastic regime. An X-ray method that links the local strain perturbations in diffracting volumes to the overall ap-plied strain is described. This method is then applied to α-brass and Ni specimens. It is seen that the average reaction strains within various diffracting subsets can be significantly different from each other and that such strains can be substantial fractions of the applied strain.

Influence of work hardening on the reactive stress in a TiNi shape memory alloy

The reactive stresses in tensile-prestrained specimens of a hot-rolled TiNi shape memory alloy containing 49.5 at.% Ni (specimens A) were studied. The investigated alloy consisted of the martensitic structure at room temperature. Some specimens were further cold rolled with a reduction of 15% (specimens B). The reactive stresses attained with the work-hardened specimens are markedly higher than those with the hot-rolled specimens.

Effects of galvanostatic treatments on hot corrosion of Ni

Several electrochemical polarization tests have been developed to assess the hot corrosion resistance of gas turbine materials [1-3]. Shores [4] has developed a screening test based on the corrosion current density estimated from the anodic part of the polarization curve after galvanostatic polarization. The corrosion current density of some super-alloys determined by this method correlated well with the penetration rates measured by the burner rig test. Erodos et al. [5] proposed an electrochemical screening test based on the measurement of activation potential after a galvanostatic pretreatment similar to that by Shores. The activation potential after galvanostatic polarization was reported to have a linear correlation with the depth of corrosion determined metallo-graphically after the test. However, these works [4, 5] were conducted in air containing no oxidents such as SO2 or SO3, different from the gas turbine operating environments, and the work by Shores was criticized for low repeatibility [5]. This letter describes the results of a study on the effect of galvanostatic treatment on the corrosion current density of Ni and its scatter measured by the conventional Tafel method, in molten Na2SO 4 in contact with gas atmospheres containing Ar, 02 and SO3. Commercial Ni wire of 99.995% purity was used in this investigation. Some specimens were preoxidized for times of 10 to 180 min in still air at 900 °C. The reference electrode was made of pure Ag wire dipped into a 10 m/o Ag2SO4-Na2SO 4 solution contained in a mullite tube, and the counter electrode was made of pure Au wire. Specimens were exposed to a reagent grade molten Na2SO 4 at 910 °C in Ar, air and Pt-catalysed 0.2% SO2-O 2 gas atmospheres. The flow rate of gases except the air used in static condition was 400 ml/min. For the determination of corrosion current density I~or~, potentials were scanned at a rate of 0.3 mV/s from E~o~r 250 mV to Ecor~ + 250 mV, using a Princeton Applied Research model 273 potentiostat. For the galvanostatic treatment, currents were applied to the specimens for 2 h either at a density of 5 mA cm -2 for the cathodic treatment or at +5 mAcm -2 for the anodic treatment. The results of hot corrosion tests of clean Ni specimens in various conditions are summarized in Table I. Values of Icor~ are remarkably dependent upon the gas atomospheres in contact with the melt. The highest value of Ico~r was observed in specimens tested in Pt-catalysed 0.2% SO2-O2 gas containing both SO3 and O2, and the lowest in Ar gas

Coke Formation on Preoxidized Iron and Nickel in a CH4-H2Gas Atmosphere at 1,000°C

Coke formation on preoxidized iron (Fe) and nickel (Ni) specimen was studied in a 15% methane-hydrogen gas (CH4-H2) atmosphere at 1,00°C by thermogravimetry. Ground specimen of different surface finish were investigated also. An accelerated heterogeneous reaction resulted when the Fe specimen were preoxidized, resulting in a heavy coke deposition. This resulted from formation of an active “metallic” surface generated by the reduction of preoxidized scales. For Ni specimen, acceleration resulting from preoxidation was small. The different surface activities between the preoxidized Fe and Ni specimen were not attributed to the different surface areas. Based on the test results, a practical remedy to alleviate deposition of coke in high-temperature cracking environments was proposed.

Magnetic properties of nanocrystalline nickel

The ferromagnetic properties of compaction-prepared nanocrystalline Ni specimens, (crystallite size 10 nm) were investigated in order to study the correlation between the disordered interfacial structure and the macroscopic properties. The magnetic moment of the atoms in the interfaces is decreased to 0.34 μB/atom (0.6 μB/atom in bulk Ni) and the Curie temperature T ci =545 K of the interfacial components is lower than the value T cb =630 K of the Ni bulk crystal. Both results are due to open structure of the interfaces and will be discussed within a band model of itinerant ferromagnetism. The temperature variation of the coercivity H c of the nanocrystalline specimens suggest a model of weak magnetic coupling of single-domain particles with a non-spherical shape.

溶融炭酸塩の関与するニッケルの高温腐食における不純物の効果

The high temperature corrosion of Ni with molten carbonates coating was studied under 33%O2-67%CO2 atmosphere at the temperature range 873K-1073K.The corrosion behavior depended on the impurities of Ni specimen. For 99.997% Ni the hot corrosion occurred below 1023K. For 99.7% Ni that contained 43 ppm Fe the hot corrosion took place below 973K. However, for 99.7% Ni that contained more than 335 ppm Fe the severe corrosion could not be obser ved at any temperature.The hot corrosion was closely related to the solubility of NiO in the carbonate melt. The iron impurity in Ni might suppress the solubility of NiO.

Effect of surface oxygen on reemission of deuterium implanted in nickel

We have investigated the effect of oxygen on deuterium reemission from Ni specimens with surface oxide and/or under simultaneous oxygen exposure at 10 −6 to 10 −3 Pa. Above 500 K the D 2 reemission rate seems to be reduced by the surface oxide, and the normalized reemission rate remains just below 100% for a time duration that depends on the amount of surface oxygen. Finally it reaches 100% with no observed enhancement in the amount of retained deuterium. This delay is probably due to the simultaneous D 2O production of reemitted deuterium reacting with surface oxygen. We infer, therefore, that the deuterium release rate from Ni is not greatly influenced by surface oxygen, although some deuterium is released in the form of D 2O depending on the irradiation temperature and the amount of oxygen on the surface.

Release processes of He implanted in Cu and Ni studied by a monoenergetic positron beam

Variable-energy positron-beam studies have been carried out on 70-keV He-implanted Cu and Ni specimens. Doppler broadening profiles of the positron annihilation were measured at a number of different incident positron energies. Observed drastic change of a lineshape parameter induced by He implantation was attributed to the positron annihilation in He-vacancy complexes. From isochronal annealing experiments, it was found that He atoms implanted into the Ni specimen deformed prior to the He implantation were trapped by vacancy-type defects and could not grow up to stable He bubbles. Just after a recovery of such complexes, He atoms were released and diffused back toward the surface of the specimen. As a result, the re-emission of He for the deformed Ni occurred at lower temperature than that for the annealed Ni.

Ferromagnetic domain patterns of Ni and Fe polycrystals in the course of the magnetic hysteresis loop

Origin and change of ferromagnetic domain patterns on the surface of polycrystalline Ni and Fe specimens in the course of the magnetic hysteresis loop have been investigated by means of the Bitter colloid technique. Comparing Ni and Fe, similarities and dissimilarities of typical domain patterns and their changes are reported.

Active sites in oxide-promoted metal catalysts: CO chemisorption on AlO x-modified Ni(111)

CO chemisorption on both clean Ni(111) and AlO x ( x is between 0.5 and 1.1)-modified Ni(111) surfaces at 200 K has been studied by Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and high-resolution electron energy loss spectroscopy (HREELS). The AIO x /Ni(111) specimen was prepared by evaporating AI onto the Ni surface and then oxidizing and reducing the specimen at 700 K. TDS results showed that the presence of surface AIO x species suppresses CO chemisorption and that the suppression is almost a linear function of the AlO x coverage. The most prominent CO desorption peak on Ni occurs at 415 K. This peak intensity decreases with increasing Al0 x coverage, indicating a site-blocking effect. An additional CO desorption peak at 300 K was found on the AIO x -modified Ni surface. This 300 K peak intensity attains a maximum at some intermediate AIO x coverage. HREELS results showed a new CO stretching frequency of 1613 cm −1 for CO adsorbed on the A1O x /Ni surface. The disappearance of this 1613-cm −1 energy loss peak upon heating the surface to 325 K suggests that both the 300 K TDS peak and the 1613-cm − 1 energy loss peak represent the same adsorbed CO state, which is attributed to CO absorbed on A1O x /Ni perimeter sites. Surface morphology of the oxide islands and significance of the perimeter sites are discussed.

Radiation damage and copper distribution in 14 MeV copper-ion-implanted nickel — TEM and AEM analyses in cross-section

14 MeV Cu ions have been implanted into a pure Ni specimen at 500 ° C to a dose of 6 × 10 20 ions/m 2. TEM and AEM analyses were performed in cross-section to investigate the effect of implanted Cu on the formation of defect clusters. The TEM result has been compared with that obtained in another Ni specimen which was irradiated with 14 MeV Ni ions to the same damage level at the same temperature. While voids formed throughout the entire damage range in the Ni ion-irradiated sample, they mainly appeared at the near-surface region and at the peak damage depth in the Cu-ion-implanted specimen. A high density of dislocation loops formed in the region where implanted Cu ions were detected by AEM. The AEM result of the implanted Cu concentration profile has been compared with a Monte Carlo calculation.

Morphological changes associated with the R-phase and martensitic transformations in Ti-Ni single crystals.

The martensitic transformation and the R-phase transition were observed successively upon cooling and heating both in solution-treated and age-treated Ti–Ni specimens, and they wre correlated with the electrical resistance vs. temperature curves for both cases. Specific self-accommodating morphologies were observed both for the R-phase and martensite variants in an age-treated specimen, while no specific self-accommodating morphology was observed for martensite variants in a solution-treated specimen, the reason for the difference being discussed. The interaction between the R-phase and martensite variants was also observed to affect the morphology of the R-phase variants upon heating.

Use of pure nickel and LiOH for thermal energy storage

The solid to liquid phase transformation of LiOH has been proposed as an ideal candidate thermal energy storage media for a Rankine Cycle powered electrical generation unit envisioned in Space Station based solar dynamic systems. Due to the corrosive nature of molten hydroxides, long term containment of LiOH is of concern. Pure nickel is thought to be a suitably resistant material, and a program has been instituted to measure the effects of prolonged exposure of liquid and gaseous LiOH on the mechanical properties of pure nickel alloys. Results to date indicate that negligible weight and thickness changes occurred in Ni alloys exposed to LiOH for as long as 2500 hr at 775 K, and essentially no difference in 77–900 K tensile properties could be detected between LiOH exposed and vacuum annealed Ni specimens. Although there was little sign of outward damage, microstructural examination revealed that all hydroxide contaminated tensile test specimens had surface connected intergranular cracks along the gage lengths. Two other potential problems, which have strong implications with respect to a LiOH/Ni energy storage system, were also noted during the corrosion experiments. In particular stress corrosion cracking of weld joints in pressurized vessels and permeation of hydrogen through nickel, which leads to the loss of LiOH, were observed.

Nature and stability of tem observed defects in 14-MeV neutron-irradiated Au and Cu

Cryo-transfer transmission electron microscopy and isochronal annealing experiments have been made on low temperature 14-MeV neutron-irradiated Au, Cu, Cu-0.1 at% Ge and Cu-0.2 at% Ni specimens. A two and a half dimensional stereo analysis has revealed the effects of specimen thickness and neutron dose on the formation of cascade defects. Interstitial-type defects disappear and some of the submicroscopic vacancy-type defects grow and become visible at the stage-III temperature because of migrating vacancies. Both disappearance and appearance of interstitial-type defects are observed below stage-III, which may be due to beam effects in the electron microscope. Above stage-III, residual interstitial-type defects disappear by absorbing vacancies at the temperature at which vacancy-type defects thermally emit these vacancies and disappear. Stable vacancy-type stacking fault tetrahedra seem to be transformed from vacancy-type clusters during isochronal annealing.

Effects of purity on damage evolution in Ni irradiated by 14 MeV neutrons

Abstract Ni specimens with two different purities were irradiated at 290°C by D-T neutrons using RTNS-II to a fluence of 6.4 × 1022 n/m2 and their damage structure was examined by means of electron microscopy. Voids, stacking fault tetrahedra (SFT), aggregates of vacancies and interstitial loops were observed but the density and size depended not only on fluence but also on purity. At relatively low fluence, dense SFT, which may be nucleated with the assistance of impurity atoms, are formed in the lower purity Ni, while voids are dominant vacancy clusters in the higher purity Ni. The present observation showed that the interstitial loops grow abruptly when vacancy clusters formed inside. In such cases, formation of voids is enhanced due to the strong bias effects of the growing interstitial loops.

CO chemisorption over MnO x-modified Ni(111): Importance of perimeter sites in strong metal-support interaction

CO chemisorption on both clean Ni(111) and MnO x ( x is between 0.2 and 0.5)-modified Ni(111) surfaces at 200 °K have been studied by Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and high-resolution electron energy loss spectroscopy (HREELS). The MnO x Ni specimen was prepared by evaporating MnO 2 powder to the Ni surface and then reducing the specimen at 700 °K. TDS results showed that the presence of surface MnO x species suppresses CO chemisorption and that the suppression is almost a linear function of the MnO x coverage. The most prominent CO desorption peak on Ni occurs at 415 °K. This peak intensity decreased with increasing MnO x coverage, indicating a site-blocking effect. An additional CO desorption peak at 305 °K was found on the MnO x -modified Ni surface. This 305 °K peak intensity attains a maximum at some intermediate MnO x coverage. HREELS showed a new C-O stretching frequency of 1620 cm −1 for CO adsorbed on the MnO x Ni surface. The disappearance of this 1620-cm −1 energy loss peak upon heating the surface to 325 °K suggests that both the 305 °K TDS peak and the 1620-cm −1 energy loss peak represent the same adsorbed CO state, which is attributed to CO adsorbed on MnO x Ni perimeter sites. Surface morphology of the oxide islands and significance of the perimeter sites are discussed along with experimental results.

Ni-FeおよびNi-Co系fcc合金の水素脆化

To clarify the correlation between the hydrogen embrittlement and the hydride formation, cathodically hydrogen charged Ni-Fe and Ni-Co binary alloys with fcc structure have been investigated by means of X-ray diffraction and tensile test. The results are summarized as follows: (1) The hydrogen embrittlement of these alloys occurred always only when the hydrides were formed. (2) The degree of the embrittlement was dependent on the magnitude of the miscibility gap of the alloy-hydrogen systems and the amount of the hydride formed. (3) The embrittlement was enhanced by the dislocation transport of hydrogen. The enhancement was remarkable, when the amount of the hydride formed was small. (4) Hydrogen charging produced dislocations of high density and grain boundary microcracks in Ni specimens, only when the hydride was formed.From these results the embrittlement was ascribed to the lattice expansion which accompanied the precipitation of the hydride phase as in the case of Ni-Cu and Pd-Ag alloys.

Investigation of the TSEE from Cu, Ni and Their Alloys

Thermally stimulated exoelectron (TSEE) from oxidized surfaces of Cu, Ni and their alloys and of the same specimens surface-treated with 0.1N KOH was studied in a vacuum (1.5×10-3 Pa) using a secondary electron multiplier. The temperature at which the glow curve began to appear (starting temperature) was higher in the case of a Ni content between 40 and 20%. The strain in the surface layer was observed by an X-ray diffraction technique. It was found that the starting temperature has some relation to the surface strain; the strain reached a maximum at a Ni content between 40 and 20%. The TSEE from specimens with 0.1N KOH was about 100 times more intensive than those with no KOH. The surface composition of the samples before and after the examinations were investigated by AES. It was found that the oxide surface film which was thick for Ni specimens decreased monotonously with an increase in the Cu content.

POSITRON ANNIHILATION STUDY OF CRYSTAL DEFECTS DEVELOPED DURING FATIGUE PROCESS OF PURE POLYCRYSTALLINE Ni

Positron annihilation lifetime spectra and Doppler broadening linesliapes of positron annihilation radiation energy were measured for the tension-compression fatigued Ni specimens under constant strain amplitude and for the cold rolled ones which were used for comparison. A few of the formers were selected for TEM observation. A lifetime component that can be associated with e+ lifetime in vacancy clusters developed during fatigue process was resolved by using a multi-exponential fitting method. It is 209, 255 and >300 ps on average in diferent ranges of fatigue respectively. The change of its relative intensity seems associated with increase of the concentration of the small vacancy clusters at early stage and growth of its dimension afterwards.Hence, this work provides a experimental evidence for the formation and clustering of vacancies as well as the development of a great number of jogged dislocations in fatigued Ni. The concentration of these defects were estimated by using a simple three state e+ trapping model. This work also provides a illustration of the utility of multi-exponential fitting method to complex lifetime spectra and to the above mentioned trapping model.

Compaction and Sintering Phenomena in Titanium—Nickel Shape Memory Alloys

AbstractTi–Ni specimens of the equiatomic composition have been processed by conventional powder metallurgy techniques. The effects of compaction pressure, sintering temperature, and powder particle size on dimensional changes of sintered compacts are reported. During sintering, anisotropy of dimensional change occurs, with expansion in the radial and contraction in the axial direction. Densities decrease during sintering. From metallographic evidence it is suggested that these observations are connected with the difference in interdiffusion rates of Ti and Ni and the segregation of powder particles in the green compacts. The sintered compacts demonstrate a well defined shape memory behaviour, with martensite nucleation being enhanced by the presence of pores. PM/0358