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STABILIZING STRATEGY FOR HIGH-ENTROPY ALLOYS Scientists at City University of Hong Kong discovered that tailoring the concentration of cobalt in high entropy alloys can prevent nanopar-ticles from rapid coarsening at [...]

This paper aims to compare the tribo-mechanical properties and structure-property relationships of a wear resistant cobalt-based alloy produced via two different manufacturing routes, namely sand casting and powder consolidation by hot isostatic pressing (HIPing). The alloy had a nominal wt % composition of Co-33Cr-17.5W-2.5C, which is similar to the composition of commercially available Stellite 20 alloy. The high tungsten and carbon contents provide resistance to severe abrasive and sliding wear. However, the coarse carbide structure of the cast alloy also gives rise to brittleness. Hence this research was conducted to comprehend if the carbide refinement and corresponding changes in the microstructure, caused by changing the processing route to HIPing, could provide additional merits in the tribo-mechanical performance of this alloy. The HIPed alloy possessed a much finer microstructure than the cast alloy. Both alloys had similar hardness, but the impact resistance of the HIPed alloy was an order of magnitude higher than the cast counterpart. Despite similar abrasive and sliding wear resistance of both alloys, their main wear mechanisms were different due to their different carbide morphologies. Brittle fracture of the carbides and ploughing of the matrix were the main wear mechanisms for the cast alloy, whereas ploughing and carbide pullout were the dominant wear mechanisms for the HIPed alloy. The HIPed alloy showed significant improvement in contact fatigue performance, indicating its superior impact and fatigue resistance without compromising the hardness and sliding/abrasive wear resistance, which makes it suitable for relatively higher stress applications. [DOI: 10.1115/1.2736450] Keywords: Stellite 20, HIPing, abrasive wear, sliding wear, fatigue

Research describing reactions of synthesis of Co-Pt alloys in the form of core-shell and solid solution is presented. Particular attention is given to reactions of transmetallation and ways in which they lead to the construction of these alloys.

A new study investigates cobalt silicide formation in the presence of a Ti/Co/SiO2/Si system.

A study on the use of aluminum alloy interconnections in active matrix liquid crystal device metallization processes conducted hillock formation investigations on the use of Ni or Co layers in bilayer aluminum alloy metallization stacks. Results confirm the advantages of using bilayer Ni/Al-1%Cu and Co/Al-1%Cu metallization lines in preventing hillock formation. Experimental methodology, findings and conclusions are presented.

A study was conducted to analyze different crystallization states of melt-spun Co80B20 alloys. Transmission electron microscopy, high-resolution transmission electron microscopy and magneto-optical Kerr effect were utilized to carry out the analysis. Experimental results indicated that the volume fraction and median grain size of the Co3B decreases during increases in the annealing temperature.

The correlation between the magnetic viscosity in CoPtCr thin film media and the concentration of local fcc-like regions was described. Results showed that the concentration of the defects was varied by growth on different underlayers and was measured with grazing incidence x-ray scattering using synchrotron radiation. It was shown that a substantial percentage of local fcc regions in a cobalt alloy film leads to a significant magnetic viscosity effects at modest magnetic fields.

The favored orientation of hexagonal close packed hcp grains in magnetically annealed Co and Co-20%Ni alloy was investigated by both Schulz's x-ray transmission and reflection procedures for verifying the source of the magnetic anisotropy caused by the magnetic annealing effect. The favored orientation was first observed at a low polar angle alpha < 50 deg area for both Co and Co-20%Ni alloy. The mechanism of the magnetic anisotropy caused by the field cooling effect was effectively explained qualitatively and quantitatively by the favored orientation of the grains.

Intergrain magnetic coupling and microstructure in three classes of cobalt-based alloys were investigated for longitudinal magnetic recording. CoPtCr, CoPtCrTa and CoPtCrB alloys were used to measure how intergranular exchange interactions are changed by doping the alloys with additives such as boron and tantalum. Experimental results showed that the ternary alloy displays the strongest intergrain coupling. On the other hand, the degree of exchange interactions in quaternary alloys is virtually the same.

Scanning tunneling microscopy in air is used to investigate the grain structure and interatomic distances in annealed Hf2Fe and in annealed and nonannealed Hf2Co polycrystalline isostructural phases with Ti2Ni type. The findings indicate a phase transition in morphological transformations during the annealing of Hf2Co. They also suggest that the more resolved charge distribution of Hf2Fe is more 'metallic' and that similar behavior of valence electrons of the ions in Fe(Co) explains the similar intensities of the bright spots indicating the characteristic positions of Hf and Fe(Co) under the same scanning conditions.

A study was conducted to examine the reactivities of several phosphate diesters coordinated to a dinuclear Co(III) complex. Results indicated that the hydrolysis of the phosphate diesters are supported by an intramolecular oxide attack on the bridging phosphate. Findings also showed no significant proton transfer from the solvent to the leaving group in the rate-determining state.

Halogenated synthetic organic compounds are widespread contaminants of the environment; however, little is known concerning their potential dehalogenation by extracellular corrinoids. Our primary objective was to determine if corrinoids sorbed to sod colloids are redox active and thus viable electron transfer mediators of contaminant dehalogenation in reduced soils and sediments. Dicyanocobinamide, cyanocobalamin (vitamin [B.sub.12]), and aquocobalamin were sorbed onto [Ca.sup.2+]-, [K.sup.+]-, and [Na.sup.+]-hectorite and sorption isotherms determined. Additional assessment of sorption reactions was performed using x-ray diffraction (XRD) and infrared (IR) spectroscopy. Redox states of the sorbed corrinoids in hectorite suspensions were monitored using ultraviolet and visible (UV-VIS) spectrophotometry. Corrinoid molecular size and charge, as well as the saturating cation, controlled corrinoid sorption. The XRD results indicated that Co corrinoids intercalated hectorite, expanding the basal spacing from 1.45 to 2.21 nm for [Ca.sup.2+]-, 1.17 to 2.70 nm for [K.sup.+]-, and 1.31 to 2.81 nm for [Na.sup.+]-hectorite. The IR spectra of bound corrinoids yielded little information on binding mechanisms, but indicated that axial ligands remained intact. The central metal of bound corrinoids was reduced to Co(I) by Ti(III) and reoxidized to Co(III) by introduction of [O.sub.2], thus demonstrating that redox activity of sorbed corrinoids was maintained and reversible. Microbially produced extracellular corrinoids may act as potential electron transfer mediators when in association with clays, possibly participating in the reductive dehalogenation of organic contaminants.

Research was conducted to study the relationship between roughness and magnetic features of cobalt films deposited on plasma-etched Si(100) substrates. Atomic force microscopy was utilized to image the surface morphologies of the roughened Si(100) substrates while the longitudinal hysteresis loops were determined under an external magnetic field. Results showed a twofold symmetry between the ferromagnetic resonance field and linewidth. The magnetic force images of the films also indicated fragmentation of domains.

A study was conducted on cobalt silicide layers formed by ion beam irradiation at a temperature between room temperature and 100 degrees C. The results indicate that the growth is diffusion-limited and attributed to radiation-enhanced diffusion with an activation energy of 0.16 eV. Moreover, the number of intermixed silicon atoms is roughly proportional to the nuclear energy deposition rate at the initial Co/Si interface.

The magnetic properties of CoCrPt is discussed relative to sputtering power dependence. Several x-ray experimental methods were used for structural analysis.Correltation between strucural poper

A phase sensitive detector-based technique enables an assessment of the variations in magnetic susceptibility of equi-atomic CoPt alloy subjected annealing up o 850 K with atomic ordering. The Curie-Weiss law is obeyed by magnetic susceptibility in the paramagnetic region. The Co and Pt magnetizations mutually reorient, leading to maxima at 630 and 710 K, while the change in axis from (111) to (001) causes the maximum at 470 K.

Co/Al2O3 multilayers prepared by radio-frequency sputtering techniques on sapphire (1120) substrates exhibit CO percolation thresholds between 15 and 2.5 nanometers. When the multilayers are more than 3 nanometers thick they exhibit continuous metallic layers. The growth of the Co/Al2O3 occurs in a polycrystalline manner.

In the past, in sealed-off Lyman-[Alpha] radiation sources (121.57 nm), uranium hydride was used as the hydrogen reservoir. We found that the zirconium-cobalt alloy ZrCo, which has similar thermodynamic properties, can also be used for hydrogen storage in such lamps. Like uranium, ZrCo acts as a getter for atmospheric contaminants. The advantage of the use of ZrCo lies in much easier and safer handling during production and disposal of the lamps. Using ZrCo, we succeeded in producing radiation sources with a large Lyman-[Alpha] radiation output and high spectral purity, which were successfully applied in a Lyman-[Alpha] fluorescence hygrometer for stratospheric observations.

A standard dc arc evaporation method facilitates the synthesis of nanometer size crystalline particles of Co3C, beta-Co, alpha-Fe, Ni and gamma-Fe in multiwalled carbon nanocages. X-ray energy dispersive spectroscopic analysis of the crystals yields a spectrum that displays carbon- and metal-associated peaks only. The crystalline size distribution of the encapsulated crystals with gamma-Fe and alpha-Fe smaller than 15nm does not agree with that of crystals with alpha-and gamma-Fe larger than 15nm. The influence of encapsulation on particle growth process accounts for the difference.

Texture development as a function of film thickness (5-980 nm) was investigated for two series of [Co.sub.81][Cr.sub.19] films. In general, the films were strongly textured. The orientation ratio, [OR.sub.c], was used to describe the strength of the texture. Experimental data showed that for the Series A films (5-200 nm), the [OR.sub.c] value increased with increasing film thickness, while for Series B films (46-980 nm), the [OR.sub.c] as a function of the film thickness described a single peak curve, with its maximum near 130 nm. The calculated local orientation ratios [OR.sub.cx] for both the Series A and B films had maxima near 110 nm. The strain in Co-Cr films also changes with the film thickness. In the case of the Series B films, the strain along the film normal gradually changed from a tensile to a compressive strain with increasing film thickness, and near 130 nm the film was in a stress-free condition. It was also discovered that for Co-Cr films thinner than 46 nm, the aspect ratio of the grains approaches 1 and the typical columnar structure of grains is not observed, although (0002) fibre texture still exists.

Fe-Co alloy thin films produced by direct current magnetron sputtering is studied to determine its structural and magnetic properties. Uniaxial magnetic anisotropy is exhibited by these films, and the easy axis is at right angles to the direction of incidence. Increase in sputtering gas pressure and increasing cobalt content at higher temperatures enhanced the sputtering field.

Amorphous CoSi2 is studied to determine the effect of ion beam irradiation on crystalline kinetics of amorphous CoSi2. Three dimensional growth of existing nuclei influences the crystallization kinetics of as-deposited CoSi2. The existing nuclei is amorphized by ion beam irradiation of as-deposited CoSi2. Direct amorphization is caused by heavy-ion irradiation. Variation in the growth mode from three-dimensional to two-dimensional decreases the density of preexisting nuclei.

Cobalt-Copper (Co-Cu) superlattices are investigated to determine their magnetoresistance, antiferromagnetic coupling, and crystallographic orientation as a function of thickness. Increase in the thickness of mixed Co-Cu interfaces causes a weakening in the antiferromagnetic coupling and increases the spin-independent scattering of free electrons. Superlattice period and morphology are not affected by the increase in thickness. The formation of a 0.15 nanometer mixed region of Co-Cu at the interface produces a reduction in saturation magnetoresistance from 27% to 4%.

Auger electron spectroscopy, four-point probe resistivity calculations, cross sectional transmission electron microscopy and X-ray diffraction spectroscopy aid in the analysis of phase transitions during solid-state creation of cobalt germanide phases at various temperatures. Quick thermal chemical vapor deposition deposits Ge on silicon substrates, and reacting Co within thin layers of Ge produces germanides. At a temperature of 300 degrees Celsius, Co5Ge7 phase is formed, and at 425 degrees Celsius, CoGe2 phase is formed.

The effect of Cr incorporation on the magnetic characteristics of a Cu-Co alloy was investigated using a vibrating sample magnetometer. The results showed that the addition of Cr into the alloy results in a temperature-dependent magnetic moment. Magnetic anisotropy was small after Cr was added. The magnetic clustrs in the Cu-Co-Cr alloy contained Cr atoms.

X-ray diffraction has been used to measure the stress and the in-plane crystallographic texture of Co alloy films on grooved substrates. The preference for the c-axis to lie parallel to the groove direction has been quantified. A small in-plane stress was measured which displayed an anisotropy between the two directions. The magnitude of this stress appears to be large enough (according to a simple calculation) to contribute to observed coercive anisotropies. The techniques used for extracting the in-plane crystallographic texture and the value of the in-plane stress from the x-ray measurements are discussed.

A study of the relationships between the microstructure and magnetic properties of cobalt alloy magnetic thin films and chromium underlayers deposited onto circumferentially textured NiP/Al substrates is presented. The variation of the chromium microstructure as a function of argon pressure and substrate temperature during sputtering is analyzed using high resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (TEM). This combination shows that the layers grow in columns which have increasing physical separation with increasing pressure and decreasing temperature. Crystallographic grain separation, however, follows a different trend because intercolumnar crystallographic alignment decreases with increasing temperature. TEM is also used to show how the growth of the chromium underlayer affects the cobalt alloy layer and to analyze the resulting microstructures in different magnetic alloy layers. The noise properties of the magnetic alloy are correlated with the observed size of the physically and crystallographically distinct units.

Bloch Line (BL) pair propagation using a field-access scheme has been investigated in the straight part of stripe domains stabilized around completely etched grooves. Potential wells to define the bit position for the BL-pairs were produced by the stray field from Co-Pt thin-film patterns with high coercivity. By applying a series of asymmetric triangular bias pulsed field, BL-pair was propagated at 500 kHz with a 14% bias field margin. At 1 MHz, however, the bias field margin was only 7%. This reduction was due to the long relaxation time of the domain walls. The drive field margins depended on the orientation of the propagation direction relative to the magnetized direction of the Co-Pt bit patterns. This dependence became small when the in-plane bias field was decreased.

The mixing of iron and cobalt during mechanical alloying was investigated using Mossbauer spectroscopy, x-ray diffraction and saturation magnetization measurements. The results showed that decreases in the layer thickness resulted in interdiffusion of the elements and the formation of a disordered crystalline Fe-Co solid solution. The magnitude of the heat mixing was not influential in the reaction kinetics.

Phase formation and magnetic properties of FeCo powders were analyzed using x-ray diffraction, differential scanning calorimetry and measurements of the saturation magnetization and the coercivity. The results show that there was true formation of fcc and bcc solid solutions in mechanical alloying. However, therewas no improvement observed in the soft magnetic properties by the nanocrystalline state.

The CoZrCr alloy for high speed write heads is discussed. It is shown that the soft magnetic properties of CoZrCr amorphous alloy was achieved on sloped surfaces with 75 V substrate bias. These films possess high saturation magnetization at 13.6 kG and high resistivity at 90 micro-ohm-cm. These films also exhibited improved magnetic properties on sloped surfaces and improved corrosion properties via deposition at higher power.

A study was conducted to analyze the dependence of the time decay of magnetization on orientation for longitudinal cobalt-alloy thin film media. The coercivity orientation ration was controlled using by the level of mechanical texture. Experimental results indicated that anisotropic interactions and magnetic anisotropy distributions influence the time decay of magnetization along the circumferential and radial directions.

A study was conducted to analyze a novel epitaxy process to fabricate highly oriented cobalt-alloy films using single crystal silicon substrates. The uniaxial anisotropy constants of the unicrystal Co and CoCrTa films were determined from torque and hard axis hysteresis. The growth of the unicrystal cobalt-alloy films were carried out on Cr/Ag templates sputter deposited on hydrofluoric acid.

A study was conducted to analyze highly oriented perpendicular cobalt-alloy media on Si(111) substrates. The orientation relationship was obtained by x-ray pole figure measurements. Experimental results indicated rocking curve measurement data supporting improved c-axis orientation of the cobalt alloy. Findings also showed that the magnetic characteristics of the films were enhanced by introducing single crystal templates.

The basic tungsten carbide-cobalt (WC-Co) material has been modified over the years to produce a variety of cemented carbides for applications such as metal cutting, mining, rock drilling, dies, wear […]

Thermomagnetic and x-ray diffraction analyses of the magnetic and structural properties of Sm2Fe14-xCoxSi2-based quasiternary compounds reveal that Co addition enhances Curie temperature and saturation magnetization while N addition enhances anisotropy and Curie temperature. Saturation magnetization reaches a maximum of 124 emu per grams when x equals about 4. Increase in N addition promotes Sm sublattice contribution to uniaxial anisotropy.

The influence of thermal treatments on the magnetostriction constant of [Vitrovac.sup.R] 6025 amorphous alloys is reported. Annealing conditions (673 and 523 K at several annealing times) were chosen to result in nanocrystallization. The presence of nanocrystals led to a significant magnetic softening. No changes were observed on the saturation magnetization value at room temperature on the treated samples, as compared with the as-quenched sample. The magnetostriction constant, [[Lambda].sub.s], shifts from -0.14 X [10.sup.-6] for the as-quenched sample to positive values for the treated alloys. Samples annealed at 673K exhibited two components of [[Lambda].sub.s] which depended on the tensile stress, [Sigma]; one for low stress, and the other for high stress values, with the change at [Sigma] = 400 MPa. This behavior is interpreted in terms of a two-phase model, where one phase is the Fe-enriched residual amorphous fraction, and the other one is formed by Co-rich nanocrystals. For annealing at 723K, a well-known relationship, [[Lambda].sub.s]([Sigma]) = [[Lambda].sub.s](0) - A[Sigma], was observed, with no resolution of components. This difference is interpreted on the basis of the nanocrystals size: for 673K, the nanocrystals are smaller than the exchange length, while they are larger for 723K.

Neutron-scattering and magnetization measurements in dilute PdCo alloys reveal a 7.92 microB ferromagnetic cloud with 2.02 microB Co moment for different concentrations of Co. A magnetic model is developed with Heisenberg-type of interactions between Monte Carlo simulations and the clouds to represent the cloud measurements. Neutron scattering has a critical component in unpolarized neutron and calculated and experimental critical temperatures are also consistent.

Melt spinning of rare-earth cobalt alloys yields ribbons which have the hexagonal PrCo5 phase as the major phase. Consolidation of these ribbons at very high pressures and temperatures yields fully-dense hot-pressed magnets with isotropic magnetic properties. Nearly 75% saturation magnetization is exhibited by these magnets.

The Neel model of magnetic relaxation is investigated to determine the magnetic properties of dilute Co-Ag heterogeneous alloy films. Narrow particle size distribution is revealed by determination of the static properties. Frequency dependence for susceptibility peak is determined by dynamic susceptibility measurements.

The magnetoresistance (MR) in Co/Ru sandwiches depends on temperature. The dependence is weak for thin Ru layer, but is stronger for thicker Ru layers. The saturation MR exhibits three oscillations as functions of Ru layers. The saturation MR decreases when the temperature is between 4.2 and 300 K and increases beyond 300 K.

The suppression of oxidation in Co/Cu multilayer films increases the magnetoresistance ratio. The antiferromagnetic coupling is increased when there is no oxidation of the interfaces between Co and Cu in the multilayers. Oxidation is prevented by the deposition of the film in a low background pressure with an Fe buffer layer.

Spin-valves for the amorphous ferrimagnetic TbCo alloy with thicknesses of 200 to 400 angstroms for use at the exchange coupling layer are manufactured. The variation in the TbCo thickness or substrate bias causes the change in exchange fields. The coupling across the copper layer degrades the coercivity of the pinned Permalloy.

Interlayer exchange coupling of sputtered Co-Re superlattices exhibit a short period oscillation superimposed on a long period oscillation. The hcp-textured samples exhibit this effect when their C axis is perpendicular to the film plane. The Bruno and Chappert theory provides a description of the variation of the exchange coupling strength with spacer thickness. Multiperiodic exchange influences magnetoresistance and magnetization measurements.

Tensile components for films deposited by rf sputtering are increased by incremental deposition of Co layers due to a difference between the lattice constants of Co and Pd. Studies using atomic force microscope show that the increase in tensile components increases the total film thickness and the surface roughness.

Co/Cr(001) superlattices have hcp-Co(11.0) layers oriented to Cr(001) planes in parallel and the C-axis aligned parallel to the Cr(110). A variation in the easy axis of magnetization from in plane to a perpendicular direction is exhibited by the precursive structural changes of the Co/Cr(001) to the body centered cubic-Co phase.

Polar Kerr hysteresis loop measurements of a molecular beam epitaxy (MBE) grown Au/Co/Au wedge/Co/Au samples deposited on Cu(111) single crystals yield the oscillatory interlayer coupling in the samples. The period of coupling is directly proportional to the Au thickness implying the presence of preasymptotic behavior. The period is about 14 angstroms larger than the theoretically predicted value of 11 angstroms.