X.G. Li

Preparation and magnetic properties of ultrafine particles of FeNi alloys

Abstract Ultrafine particles (UFP) of six kinds of Feue5f8Ni alloys were synthesized by the method of hydrogen plasma reaction. The prepared UFP samples were examined by X-ray diffraction, electron transmission microscopy and magnetic measurement. The spherical Feue5f8Ni UFP alloys with a mean particle size less than 35 nm can be prepared with a production rate much higher than by conventional methods. The phase constitution of UFP alloys is different from the equilibrium phase diagram owing to rapid condensation of evaporated metal gases. Although the magnetization for the UFP alloys has almost the same temperature dependence as that of the bulk alloys, the saturation magnetization remarkably decreases as the bulk alloys change into the UFP alloys.

Thermal stability, oxidation behavior and magnetic properties of Fe–Co ultrafine particles prepared by hydrogen plasma–metal reaction

Abstract Six kinds of Fe–Co ultrafine particles (UFPs) with an average particle size of about 45 nm were prepared by a hydrogen plasma–metal reaction method (HPMR). Particle feature, thermal stability, oxidation and magnetic properties of these UFPs were investigated. Fe–Co UFPs are spherical in shape and have the same crystal structure as bulk samples, but the Co content in Fe–Co UFPs is slightly richer than that in the master samples. Characteristics such as particle shape, mean particle size and specific surface area do not vary remarkably with the Co content. Fe–Co UFPs are stable in Ar and air below 473xa0K, but oxidize quickly in air above 473xa0K and simultaneously vary from a spherical shape into acicular shape during oxidation. Although the saturation magnetizations of Fe–Co UFPs both before and after oxidation are smaller than those of bulk samples, they exhibit a composition dependence similar to that of the master alloys. High residual magnetization and coercive force are obtained for Fe–Co UFPs before and after oxidation.

Hydrogen absorption and corresponding changes in structure and thermal stability of Zr60Al10Ni30 amorphous alloy

Abstract Zr 60 Al 10 Ni 30 amorphous alloy exhibiting a wide supercooled liquid region was prepared by a melt–spinning technique. The hydrogen absorption properties and corresponding changes in structure and thermal stability were then investigated and compared with those in the crystalline state. Zr 60 Al 10 Ni 30 amorphous alloy absorbs hydrogen of 0.94∼1.13 H/M between 473 and 723 K, but releases little in the desorption process. The hydriding induces structural and thermal stability changes, which is dependent on the hydriding temperature and the absorbed hydrogen content. The hydrogen absorption behaviors in the amorphous state are seen to be quite different from those in the crystalline state.

Synthesis and magnetic properties of Fe–Co–Ni nanoparticles by hydrogen plasma–metal reaction

Abstract Nanoparticles of Fe–Co–Ni ternary alloys were prepared in six compositions by hydrogen plasma–metal reaction. Structures, particle features, oxidation behaviors and magnetic properties of the nanoparticles were investigated by XRD, TEM, DSC and SQUID. The nanoparticles having mean particle diameters of 34–43xa0nm can be prepared with desired composition in the whole range. The phase and magnetic diagrams for the nanoparticles have been drawn in the ternary alloys. The nanoparticles have nearly the same crystalline structures and lattice constants as those of the corresponding bulk alloys. The nanoparticles exhibit a ferromagnetism similar to the bulk alloys. The saturation magnetization is 10–30% lower, but the coercive force is much larger than those of the bulk alloys.

Spin distribution in plastically deformed Fe - Al intermetallic compounds II

The influence of plastic deformation on the magnetic properties has been studied in Fe - Al intermetallic compounds with composition between 30 and 40 at.% Al. The spontaneous magnetization, , increases considerably as a result of plastic deformation between 30 and 34 at.% Al concentration. The Fe - Al compounds containing between 35.0 and 50.0 at.% Al are paramagnetic before plastic deformation and as a result of plastic deformation the magnetic susceptibility increases remarkably. At the same time appears. The ferromagnetic clusters are induced by plastic deformation along the antiphase boundary (APB) ribbons between superpartial dislocations. The cluster along the APB ribbon shows a strong anisotropy whose easy direction of magnetization is within the glide plane. The easy direction of magnetization is consistent with that of the roll-induced anisotropy in alloys. The origin of the spin glass in these compounds is explained by the magnetic anisotropy depending on the atomic arrangement. The conditions of the transition to the ferromagnetic state are discussed.

Martensitic transformation due to plastic deformation and magnetic properties in SUS 304 stainless steel

Abstract The relation between magnetic properties and the dislocation structure is studied in SUS 304 stainless steels. The absolute value of the α-phase is obtained by the spontaneous magnetization whose result is consistent with the X-ray analysis. We ascertained the reason that the magnetization is difficult to be saturated; it causes the magnetic shape anisotropy. The needle shaped α-phase is created by plastic deformation, which causes the coercive force, which depends on the maximum value of the anisotropy. The magnetic anisotropy is also measured by torque measurements. The spontaneous magnetization gives the ratio of the α-phase in volume; it attains to about 45% at the 100% strain. The induced M s and the induced magnetic anisotropy recover by annealing above 700xa0K. The application to the nondestructive tests is discussed on the basis of the present investigation. The martensitic transformation depends on the internal stress, temperature and the applied stress. The first difficulty is that the magnetic parameters do not correspond simply to the dislocation density. The method due to magnetic measurements is difficult to apply the usage above 700xa0K, where almost dislocations remain in the substance but the α-phase recovers.

PARTICLE FEATURES, OXIDATION BEHAVIORS AND MAGNETIC PROPERTIES OF ULTRAFINE PARTICLES OF NI-CO ALLOY PREPARED BY HYDROGEN PLASMA METAL REACTION

Ni–Co ultrafine particles (UFPs) were prepared by hydrogen plasma metal reaction. Particle characteristics, oxidation, and magnetic properties were investigated and compared. fcc structured Ni–Co UFPs with a sphere shape were prepared over the whole composition range. The particles distribute from 5 to 100 nm in size with an average particle size of about 35 nm. They are thermally stable in air below 433 K, but oxidize quickly at the higher temperatures. The saturation magnetization of the UFPs is lower than that of the corresponding bulk alloy, especially in the Co rich side. Formation of oxide layer on the particle’s surface is an important reason for this reduction. Ni–Co UFPs possess large residual magnetization and coercive force above 60% Co. The composition dependence of the saturation magnetization and the coercive force changes remarkably around 60% Co, which implies a large variation in magnetic status around this composition.

Preparation, oxidation and magnetic properties of FeCr ultrafine powders by hydrogen plasma-metal reaction

Abstract Four kinds of Feue5f8Cr ultrafine particle (UFP) samples were synthesized by a hydrogen plasma—metal reaction method. The UFP samples were examined by X-ray diffraction, electron transmission microscopy and chemical analysis and specific surface area analysis. The oxidation and magnetic properties of as prepared UFP samples were investigated. The crystal structure of Feue5f8Cr UFP samples is the same as that of the master bulk samples, but the Cr content in Feue5f8Cr UFP samples is richer than that in bulk samples. The characteristics of Feue5f8Cr UFP samples such as particle shape, mean particle size and specific surface area show strong dependence on the Cr content. Oxidation for the Feue5f8Cr UFP occurs easily at a temperature even lower than 473 K and the oxided Feue5f8Cr UFP samples change from a acicular shape into angular shape with increasing Cr content. Although the saturation magnetization of the UFP samples is smaller than that of the corresponding bulk samples, the difference in saturation magnetization between the two kinds of samples becomes small as the Cr content increases.

High-temperature strength of Ni-doped IrAl with the B2 type ordered crystal structure

Abstract Constant-velocity and constant-load compression tests have been conducted to examine the mechanical behavior of polycrystalline Ir 1 − x Ni x Al intermetallic compounds with x = 0 to 1.0 at elevated temperatures. Ductility of Ir 1 − x Ni x Al is improved with increasing x , while the strength decreases with increasing x in the intermediate temperature range. The specific strength of Ir 1 − x Ni x Al is almost independent of x at temperatures higher than approximately 1473 K. Secondary creep of IrAl and Ir 0.2 Ni 0.8 Al (i.e. modified NiAl) exhibits class II and class I behavior, respectively. Creep strength of Ir 1 − x Ni x Al is about a magnitude of four higher than that of single-phase and multi-phase NiAl at a given applied stress. The Ir 1 − x Ni x Al intermetallic compound is superior to binary IrAl as a high-temperature structural material from the viewpoint of the specific strength, ductility and resistance to oxidation, especially at temperatures higher than 1473 K.

Preparation of fine Nb3Al powder by hydriding and dehydriding in an arc-melting chamber

Abstract The disintegration due to hydrogenation of Nbue5f8Al alloys with eight different compositions has been studied. The hydrogenation is performed directly in an arc-melting chamber without exposing Nbue5f8Al ingots to air after arc-melting. Because the active surface resulting from arc-melting is preserved, Nbue5f8Al alloys absorb a large amount of hydrogen and disintegrate rapidly into fine particles, even though the pressure of hydrogen is only at 0.1 MPa. This disintegration is closely related to the arc-melting atmosphere and the composition of the alloys. Structural changes of Nbue5f8Al alloys before and after hydrogenation and dehydrogenation have been examined by X-ray diffraction, electron microscopy and chemical analysis. A fine powder of single Nb 3 Al phase with an average particle size of less than 14 μm can easily be obtained by hydriding Nbue5f8Al alloy buttons with aluminum compositions from 22 to 28 at.% in an arc-melting chamber, followed by dehydrogenation at 1073 K for 3.6 ks.