Coercivity of 2:17 Based Permanent Magnets

High-quality permanent magnets either are based on large nucleation fields or strong pinning forces of domain walls (dws). In the case of 2:17 based magnets depending on the temperature range considered both hardening mechanisms have been discussed. The dominant hardening mechanisms at temperatures below around 700 K are repulsive or attractive pinning of domain walls at the cell walls (1:5 structure) between the nanostructured pyramidal cells (2:17 structure). Micromagnetic calculations of the pinning forces sensitively depend on the shape of the interaction potential between the dw and the microstructure. In the case of the 2:17 based magnets these calculations become rather complex if the domain wall width is of the order of the potential width. In this case the domain wall is modified due to the space dependent local anisotropy which changes from K₁^2:17 to K₁^1:5 of the cell boundary phase over a distance D of the intergranular phase boundary. Using the K₁(r)-profiles as determined from the chemical composition obtained by high-resolution EDX measurements the coercive field due to domain wall pinning is determined self-consistently as a function of the pinning-potential parameters taking into account the modification of the domain wall by the space dependent material parameters. It is shown that the coercive field of the high-temperature magnets as a function of the width of the cell boundary phase varies between 3 T for a sharp boundary (D = 0 nm) and 1 T for a wide boundary (D = 4 nm). On the basis of micromagnetism the condition for the dominance of pinning or nucleation is investigated. It is shown that in the case of 2:17 based high-temperature magnets with increasing temperature the hardening mechanisms change from repulsive to attractive pinning and nucleation starts at around 700 K. The temperature ranges where the transitions take place sensitively depend on the Cu content and on the annealing parameters.     

On the Selection of the Optimal Intercritical Annealing Temperature for Medium Mn TRIP Steel

A model is proposed to predict the room temperature austenite volume fraction as a function of the intercritical annealing temperature for medium Mn transformation-induced plasticity steel. The model takes into account the influence of the austenite composition on the martensite transformation kinetics and the influence of the intercritical annealing temperature dependence of the austenite grain size on the martensite start temperature. A maximum room temperature austenite volume fraction was obtained at a specific intercritical annealing temperature T _M. Ultrafine-grained ferrite and austenite were observed in samples intercritically annealed below the T _M temperature. The microstructure contained a large volume fraction of athermal martensite in samples annealed at an intercritical temperature higher than the T _M temperature.     

Chemical State, Site, Solid Solubility, and Magnetism of Fe in the Ferropericlase (Mg1–x Fe x )O Produced by Ball Milling of MgO and Fe

Ferropericlase (Mg_1–x Fe _x )O solid solution was prepared by ball milling of the mixture of MgO with a rock-salt structure and metal Fe powders in atmosphere and at room temperature. Differing from (Mg_1–x Fe _x )O prepared at high temperature by using MgO and FeO as starting materials, the solution of Fe in MgO is not continuous but limited in the ball milling process, and the solubility limit is less than 30 at. pct. About 92 pct of the Fe ions occupy the site of tetrahedral oxygen coordination in trivalent Fe (Fe³⁺) with high spin, whereas about 8 pct of the Fe ions occupy the site of octahedral oxygen coordination in bivalent Fe (Fe²⁺) with high spin. The Fe³⁺ and Fe²⁺ ions do not show a ferromagnetic but show a paramagnetic state. The as-milled (Mg_1–x Fe _x )O is metastable and decomposes to ferropericlase (Mg_1–y Fe _y )O (where y < x) and MgFe₂O₄ with spinel structure as annealed above 773 K (500  °C), and the content of Fe in the (Mg_1–y Fe _y )O increases with increasing annealing temperature. A bulk (Mg_1–x Fe _x )O was fabricated by annealing the as-milled (Mg_1–x Fe _x )O powders at 973 K (700  °C). It shows n-type conductivity, which is attributed to an electronic small polaron with an activation energy of 0.135 eV.     

The growth of antiphase domains in Cu3Au as studied by transmission electron microscopy

The changes in the antiphase domain structure of stoichiometric Cu₃Au during ordering after quenching from aboveT _c have been studied by means of transmission electron microscopy. The growth of the domains was found to satisfy aD ² ∞ t relation in agreement with previous X-ray diffraction measurements. The activation energy determined for the process (42 kcal per mole) is also in agreement with the previous work. An increase in quenching temperature gave no effect on the domain size achieved by a particular ordering treatment. Measurements of the domain size distribution at various stages during growth indicate that the domain structure is not completely irregular. Certain domain sizes which are multiples of a unit domain size are preferred. During the early stages of domain growth the behavior appears to be consistent with a diffusion model of the ordering process similar to that applied to spinodal decomposition.     

Effect of oxygen on microstructures of high-rate sputter-deposited Nb3Sn superconductors

The role of excess oxygen and deposition temperature on the microstructure and critical temperature (T _c ) has been studied in high-rate, sputter deposited Nb₃Sn. Excess oxygen does not significantly affect the grain size of the A15 phase whether it is deposited at low temperature followed by annealing or deposited directly at an elevated temperature. Oxygen does promote the formation of the amorphous phase during deposition at low temperature. During subsequent transformation of material sputter deposited at room temperature, gas bubbles form from the entrapped sputtering gas. Excess oxygen also promotes greater precipitate formation during deposition at elevated temperature. In no case was there a large change in the superconducting transition temperatureT _c ; however, the microstructural features may have significant effects on the critical current densityJ _c .     

Texture Evolution as Determined by In situ Neutron Diffraction During Annealing of Iron Deformed by Equal Channel Angular Pressing

In situ neutron diffraction experiments were performed to follow the annealing behavior of iron deformed by equal-channel angular pressing at room temperature using route B _c to a total von Mises strain of ε _vM = 9.2. The temperature was varied from room temperature to 1223 K (950 °C), while neutron diffraction data for quantitative texture analysis were collected at a given temperature when holding for 5 minutes. Pole figures and orientation distribution function maps from neutron diffraction and electron backscatter diffraction measurements were used to follow the changes in crystallographic texture and grain size during annealing. In situ neutron diffraction experiments allowed understanding and identifying texture-related changes that occur during recrystallization, grain growth, and phase transformation in iron.     

Kinetics of order-disorder transformations in the heusler AuAgZn2 alloy

The kinetics of order-disorder transformations in the Heusler AuAgZn₂ alloy was analyzed theoretically by taking into account the interchanges of atoms with neighboring vacancies. The variation of the degrees of the B2- and the L2₁-type orders, S₁ and S_H respectively, was calculated during isothermal annealing and during heating or cooling processes. During isothermal annealing, S₁, and S_Hshowed the relaxation phenomena. During heating and cooling processes, S_l, S_H−T curves showed hysteresis. Moreover, during heating, the S₁−T curve had a minimum at the critical temperature of the L2₁-type structure T_c2, which was dependent on the heating rate of specimen. The interchanges of atoms with vacancies between the 1st nearest neighbors as well as those between the 2nd nearest neighbors were found to play an important role on the kinetic behavior of ordering in the Heusler alloy.     

Tremendous enhancement of magnetic performance for Sm(CoFeCuZr)z magnet based on multiscale copper redistribution

Microstructure and magnetic properties were studied for the commercial Sm(CoFeCuZr)_z magnets before and after post annealing treatment. The results show that the phases composition and orientation of the magnet do not change after post annealing treatment, but the substantial redistribution of Cu element within multiscale (the microscale crystal grain and the nanoscale cellular structure) is observed simultaneously. In detail, along with the Cu redistribution, the thickness of the Cu-rich Sm(Co,Cu)₅ cell boundary becomes thinner, and the Cu concentration in the boundary increases sharply. The pinning field of domain walls and corresponding coercivity increase remarkably with slight remanence and maximum energy product loss, and the overall magnetic performance of (BH)_max (MGOe)+H_cj (kOe) increases by 54.3% as a result. Moreover, the thermal stability of the magnet improves as well. On the other hand, Cu-lean phenomenon was observed along the grain boundary region, triggering to magnetic domain reversal process and slightly undermining the squareness of the demagnetization curve of the magnet.     

Enhanced magnetic properties of Ce17Fe76.5Co1Zr0.5B6 alloys by magnetic field heat treatment

In the present work, Ce₁₇Fe_76.5Co₁Zr_0.5B₆ ribbons were prepared by a direct melt spinning method. The effects of chamber pressure and magnetic field annealing temperature on the magnetic properties and microstructures of the alloys were investigated. The grain size and content of Ce₂Fe₁₄B phase can be changed by adjusting the chamber pressure, and the optimal magnetic performance is obtained at 0.04 MPa. The magnetic properties can be influenced under magnetic field heat treatment. When the annealing temperature is lower than the Curie temperature, the refinement and a uniform distribution of the grains is obtained. The irreversible magnetic susceptibility curve reveals that magnetic field heat treatment enhances the exchange coupling interaction between grains of the Ce₂Fe₁₄B phase. When the magnetic field annealing temperature is 438 K, the alloy displays the optimal magnetic properties. Compared with the as-spun sample, the values of intrinsic coercivity (H_ci), remanence (B_r) and maximum energy product ((BH)_max) increase by 3.4%, 9.8% and 18.7%, respectively. This work provides an effective approach by which to enhance the magnetic properties of Ce–Fe–B alloys.     

Evolution of microstructures and optical properties of gadolinium oxide with oxygen flow rate and annealing temperature

In this study, the effects of oxygen flow rate and annealing temperature on Gd_2 O_3 structures and optical properties were systematically analyzed. Gd_2 O_3 films were deposited on both quartz and ZnS substrates by magnetron sputtering and then annealed under vacuum at 700, 800 and 900℃, Restructure and phase transformation from cubic to monoclinic occur at different temperatures depending on the oxygen flow rate. The optical band gap, which is more sensitive to the annealing temperature than oxygen flow rate changes from 5.32 to 5.65 eV. The refractive index is approximately 1.75 at 550 nm and is adjustable by the oxygen flow rate. The transmittance of the ZnS substrate with Gd_2 O_3 film exceeds 80% and reaches82% at the 7.5-9.5 μm range. When ZnS is coated on both sides, the transmittance is increased to approximately 90%. Our results indicate that Gd_2 O_3 films are promising new candidates for anti-reflective coatings in the infrared region.     

Crystal structure and hard magnetic properties of TbCu_7-type Sm_(0.98)Fe_(9.02–x)Ga_x nitrides

The compound Sm0.98Fe9.02–xGaxNδ(x=0, 0.25, 0.5, 0.75, 1) were prepared by melt-spun method and subsequent annealing and nitriding. The Rietveld analysis showed that the lattice expansion played an important role in improving the Curie temperature. An obvious development of the Curie temperature was obtained with the increased Ga content from x=0–1(ΔTc=90 oC). The optimum coercivity of nitrides was obtained at x=0.25 with the value Hcj=652 kA/m(8.15 kOe) after annealing, which corresponded to a reasonable distribution of grain sizes of both TbCu7-type SmFe9Nδ and α-Fe. However, an excess of Ga doping might lead to an abnormal growth of α-Fe, which in turn deteriorated the magnetic properties. It was concluded that a moderate Ga content was very effective in raising the coercivity and Curie temperament in the TbCu7-type Sm-Fe-N.     

Retardation of annealing embrittlement in iron-based glasses by microaddition of cerium

The effects of microadditions of cerium on the rate of annealing embrittlement, stress relief, and as-quenched magnetic domain structure in three iron-based metallic glasses have been investigated. Ribbons melt-spun from optimally doped castings of composition Fe₈₀B₁₆Si₂C₂ and Fe₈₀B₂₀ did not embrittle during annealing until the onset of crystallization, whereas Fe₇₈B₁₃Si₉ ribbons were unaffected by cerium additions. In the first two glasses optimal doping resulted in the disappearance of quenched-in stresses in the ribbons, as evidenced by an absence of maze domains, and, for the Fe₈₀B₂₀ alloy, a slight enhancement of stress relief rate for small times and temperatures. The effects of doping were qualitatively different in the Fe₇₈B₁₃Si₉ glass. We conclude that annealing embrittlement in the first two glasses is an impurity effect, and that the embrittling impurities are oxygen and/or sulfur, the dissolved concentrations of which are strongly reduced by cerium additions. That low concentrations (50–100 at. ppm) of these impurities are sufficient to cause annealing embrittlement suggests that impurity segregation may be involved in the embrittlement process.     

Change in composition and structure of chromium-carbonitride nanopowder on storage and heating in gases

The changes in composition and structure of chromium-carbonitride nanopowder on storage and heating in gases are investigated. On annealing in argon and nitrogen at 1273–1373 K, the carbonitride is converted to nitride Cr₃C₂; on annealing in hydrogen, the carbide Cr₇C₃ is produced. The conversion to Cr₃C₂ is accompanied by enlargement of the nanopowder as a result of solid-phase coalescence. The interaction of carbonitride with atmospheric gases (oxygen and water vapor), which involves adsorption and diffusion, is accompanied by considerable increase in oxidation within the first 24 h. The temperature at which oxidation begins in air depends on the size of the nanoparticles: it rises from 542 to 568 K with increase in size from 22 to 53 nm. The dependence of the degree of oxidation and the temperature at which oxidation begins on the particle size is determined.     

Comparison Between Different Processing Schedules for the Development of Ultrafine-Grained Dual-Phase Steel

A comparative study was carried out on the development of ultrafine-grained dual-phase (DP) (ferrite–martensite) structures in a low-carbon microalloyed steel processed using two thermomechanical processing routes, (i) intercritical deformation and (ii) warm-deformation and intercritical annealing. The samples were deformed using Gleeble3500^® simulator, maintaining a constant total strain (ε = 1) and strain rate ( $ \dot \varepsilon $  = 1/s). Evolution of microstructure and micro-texture was investigated by SEM, TEM, and EBSD. Ultrafine-grained DP structures could be formed by careful selection of deformation temperature, T _def (for intercritical deformation) or annealing temperature, T _anneal (for warm-deformation and annealing). Overall, the ferrite grain sizes ranged from 1.5 to 4.0 μm, and the sizes and fractions of the uniformly distributed fine-martensitic islands ranged from 1.5 to 3.0 μm and 15 to 45 pct, respectively. Dynamic strain-induced austenite-to-ferrite transformation followed by continuous (dynamic) recrystallization of the ferrite dictated the grain refinement during intercritical deformation, while, continuous (static) recrystallization by pronounced recovery dictated the grain refinement during the warm-deformation and the annealing. Regarding intercritical deformation, the samples cooled to T _def indicated finer grain size compared with the samples heated to T _def, which are explained in terms of the effects of strain partitioning on the ferrite and the heating during deformation. Alpha-fiber components dominated the texture in all the samples, and the fraction of high-angle boundaries (with >15 deg misorientation) increased with the increasing T _def or T _anneal, depending on the processing schedule. Fine carbide particles, microalloyed precipitates and austenitic islands played important roles in defining the mechanism of grain refinement that involved retarding conventional ferrite recrystallization and ferrite grain growth. With regard to the intercritical deformation, warm-deformation followed by annealing is a simpler process to control in the rolling mill; however, the need for high-power rolling mill and controlled annealing facility imposes industrial challenges.     

On the mechanism of grain formation during spray atomization and deposition

The mechanisms governing the morphological changes in the microstructure of spray atomized and deposited Ni₃Al were studied, with particular emphasis on the formation of a spheroidal grain morphology. Accordingly, the various microstructural features present in the spray deposited material were rationalized on the basis of thermal energy considerations. The formation of spheroidal grains was proposed to evolve from: (a) the homogenization of dendrites that did not deform extensively during deposition; and (b) the growth and coalescence of deformed or fractured dendrite fragments. Support for this suggestion was provided by experimental results and numerical analyses which show that the microstructure of Ni₃Al is exposed to a high temperature anneal during deposition. Moreover, the results show that during high temperature annealing, the deformed or fractured dendrite fragments that were initially present in the spray deposited materials grew and coalesced leading to the development of a spheroidal grain morphology. On the basis of a coarsening mechanism, the relative annealing time under a particular cooling rate may be calculated and converted into a spheroidal grain size, d_sph, from the following equation, d_sph = 15.2· Ṫ^−0.35. The experimental results were observed to concur with this relationship.     

Ordering transformations and mechanical properties of Ti3Ai and Ti3Al-Nb alloys

Phase transformations and the kinetics of domain growth were studied in near stoichiometric Ti₃Al and in a similar alloy containing about 5 at. pct Nb (Cb). The alloys were quenched from the β and from the α+ β fields and were subsequently annealed in the α₂ field to study the ordering transformation. The critical temperature (T _c) for ordering was found to be between 1125 and 1150° for both alloys. When quenched from aboveT _c the microstructure of the stoichiometric compound contained massive martensite with small antiphase domains of average size 8 × 10^— μm. On annealing the quenched structures in the range 700 to 1000°, domain coalescence occurred, the domains growing approximately as the square root of the annealing time. The activation energy for the domain growth process was found to be 64.6 ± 6 Kcal/mole (2.68 ± 0.25 × 10⁵ J/mole). On quenching the alloy containing Nb the β transforms to a fine acicular martensite. On annealing, antiphase domain coalescence within the martensite plates and the simultaneous recrystallization of the martensite resulted in a fine subgrain structure even after annealing at 900° for up to 3 h. The mechanical properties and the fracture modes of the two alloys tested at 700° were correlated with the observed microstructural changes. The effects of Nb in this alloy are to slow the domain growth kinetics, to reduce the planarity of slip, and to increase nonbasal slip activity. Formerly NRC Research Associate in the Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH     

Etude enthalpimetrique des differentes etapes du revenu des aciers a ressort 50CV4 et 61SC7: Stabilite de l'austenite residuelle,transformations η → χ et χ → θ

During annealing, the heat flux accompanying the matrix relaxation shows the following steps: formation of ϵ phase, then of η, both by carbon diffusion, transformation of η into χ then into θ by diffusion of the matrix atoms and finally coalescence of carbides. Residual austenite is transformed by immersion into liquid nitrogen, provided it has not been stabilized during a long wait at room temperature. It is shown that: the state (α + χ) corresponds to the enthalpy minimum and shows higher excess C_p than (α + θ); the transformation χ → θ spreads over a long time within a broad range of temperature; and the enthalpy-hardness correlation is roughly linear. The kinetic study allowed us to determine the activation energy for the transformation η → χ and to draw the curves of reaction progress versus time at different temperatures and so to simulate different types of annealings.     

Etude couplee calorimetrique et dilatometrique de l'enthalpie d'elimination des lacunes de trempe lors du revenu d'alliages ordonnes B2 Fe50+xAl50−x (−1 < x < 10)

The total enthalpy retained during quenching of B2 Fe_50+xAl_50−x alloys (− 1 < x < 10) has been measured during annealing by anisothermal thermal analysis from 373 to 873 K as a function of quenching temperature and aluminium concentration. Associated with dilatometrical measurements of vacancy concentration after quenching, these results enable us to determine directly the mean energy of elimination of a vacancy. The values of this energy which have been found are discussed as functions of vacancy concentration and order in the FeAl alloys.     

Coercivity kinetics upon step annealing of sintered Sm(Co_(0.88-x)Fe_xCu_(0.09)Zr_(0.03))_7 magnets

Behavior of the coercivity of the high-temperature Sm(Co_(0.88-x)Fe_xCu_(0.09)Zr_(0.03))_7 magnets depending on the temperature and time of annealing with the temperature decreasing stepwise from 700 to 400℃ was investigated.It is shown that the growth rate of coercivity abruptly increases at the initial stage of annealing in the vicinity of the Curie temperature of the SmCo_5 phase.The origin of the effect is the counter diffusion of Cu and Co atoms through dislocation tubes,which form because of enhanced stresses and a partial breakage of coherent coupling at the interface of the Sm_2 Co_(17) and SmCo_5 phases.Diffusive enrichment of the SmCo_5 phase in Cu close to the interface with Sm_2 Co_(17) leads to relaxation of stresses and increases in the gradient of the magnetic domain-wall energy and coercivity.     

Effect of the method of production of an Ni<Subscript>3</Subscript>Al/Mo layered composite material on its structure and properties

A stack of alternating 25 100-μ-thick Ni₃Al plates and 28 200-μm-thick Mo plates is subjected to hot isostatic pressing (HIP) at a temperature T = 1200°C and a pressure P = 150 MPa for τ = 2.5 h followed by hot rolling at 1050–950°C to a thickness of 2.3 mm. The stack is then subjected to cold rolling (CR) to a thickness of 0.5 mm without intermediate annealing, subsequent annealing during HIP at T = 1200°C, P = 150 MPa, and τ = 2.5 h, and CR to a thickness of 0.22 mm. Upon CR at a strain ε changing from 80.8 to 95.8%, the following specific structure forms in the longitudinal direction: molybdenum layers acquire a wavelike structure, can contact with each other, form “cells,” and retain almost the same thickness, and Ni₃Al alloy layers are rejected between the molybdenum layers to form a regular structure made of alternating thickenings and thinnings across the rolling direction. Annealing during HIP and subsequent CR to ε = 98.2% lead to the formation of zones with a broken alternation of layers in the longitudinal and transverse directions, which is related to different strain resistances of the (more refractory) molybdenum and Ni₃Al layers at 20°C. The adhesion between the layers is good, and no intermediate phases form at the interface. The ultimate bending strength of the 2.3-mm-thick workpiece at 20°C is 1000 ± 100 MPa, and the prepared material has a plasticity margin.