Katsuhisa Nagayama

Stoichiometry of Quasicrystalline Al–Mn

Electron microscopy has shown that in rapidly solidified Al–(14–20) at.%Mn alloys a new metastable phase of a quasicrystalline phase with the icosahedral symmetry, which was first discovered by Shechtman et al . (Phys. Rev. Lett. 53 (1984) 1951), is formed but with a considerable amount of fcc Al second phase in alloys with lower Mn content. From the change with Mn content of the intensity of fcc Al lines in the powder X-ray diffraction spectrum, the stoichiometry of the Al–Mu quasicrystalline phase is concluded to be nearly Al 4 Mn (∼20.5 at.%Mn), being at variance with the reported composition of Al 6 Mn.

Structure and Stability of Quasicrystalline Al–Mn Alloys

The quasicrystalline (qc.) phase with the icosahedral symmetry in melt-spun Al 100- x Mn x ( x =14, 17, 20) has been investigated by X-ray diffraction, TEM and DSC. Nearly single phase quasicrystal is obtainable for Al 80 Mn 20 when the quenching rate is sufficiently fast, while slower quenching rate produces an alternative metastable phase called T phase. On heating, the qc. phase in Al 80 Mn 20 changes to T phase at around 300°C and then at around 600°C to the stable hexagonal Al 4 Mn, whereas the qc. phase in Al 86 Mn 14 crystallizes directly to the stable orthorhombic Al 6 Mn. Heat evolution of the latter reaction is 0.40 kcal/mol, while those of the qc. phase to T phase and T phase to the hex.-Al 4 Mn in Al 80 Mn 20 are 0.12 and 0.17 kcal/mol, respectively. Atomic model of the qc. phase with a stoichiometric composition of Al 78.35 Mn 21.65 is proposed.

Magnetic Properties of Amorphous Fe-Nd Alloys*

Melt-spun amorphous Fe 1- x Nd x alloys (0.11≤ x ≤0.60) were investigated by the magnetization measurement up to 143 kOe and the Mossbauer spectroscopy. The coercive field at 4.2 K increases abruptly at around x =0.30 with increasing x , amounting to 53 kOe at x =0.40. The hyperfine field of 57 Fe at 77 K is independent of x and about 300 kOe on an average, suggesting that the magnetic moment of Fe is about 2.0 µ B /atom throughout this system. On this assumption and the measured magnetization at 4.2 K, it is estimated that the average Nd moment is about 3.2 µ B /atom at the low x limit. This value corresponds to the free ion value of Nd 3+ . The Nd moment decreases with increasing x to one half, 1.6 µ B /atom. A ferromagnetic cluster model with large random anisotropy is proposed for the high coercivity alloys of this system. Magnetization curves at 4.2 K are well explained by the model, by an adjustment of the parameters for the anisotropy energy of the cluster and the inter-cluster coupling. The concen...

Microstructure of Cu-added Pr-Fe-B magnets: Crystallization of antiferromagnetic Pr6Fe13Cu in the boundary region

Abstract The relations between microstructures and magnetic properties have been investigated in Pr17Fe76B5.5Cu1.5 magnets produced by a process of casting, hot-pressing and heat treatment. The crystallization of Pr6Fe13Cu during heat treatment at 753 K for 2 h increases the coercivity. This explains the enhancement of coercivity with the addition of small amounts of Cu. Pr6Fe13Cu has a tetragonal structure, space group 14/mcm, and lattice parameters a = b = 0.81 nm, c = 2.31 nm. Mossbauer spectroscopy and VSM measurements indicate that Pr6Fe13Cu is an antiferromagnetic compound with a Neel temperature of 391 K. Pr6Fe13Cu crystallizes in a peritectic reaction at 918 K, and is stable below that temperature. In Pr-Fe-B-Cu magnets, Pr6Fe13Cu is formed only within a low boron composition range (below 5.5 at%B). In the system Nd-Fe-Cu a compound Nd6Fe13Cu with the same crystal structure is observed.

Magnetization of Fe-Nd amorphous alloys with random anisotropy

Abstract Magnetization curves of the high coercivity amorphous Fe 0.5 Nd 0.5 alloy at low temperature were analysed on the molecular field approximation of a random anisotropy ferromagnet. Existence is estimated of clusters of the order of 100 atoms.

Appearance of high-coercivity in Fe-Nd amorphous alloys

Magnetic properties of melt-spun Fe 1-x Nd x amorphous alloys are examined. Magnetically homogeneous alloys were obtained in the range of 0.15 \leq x \leq 0.60 . The Curie point is lowered by decrease of x down to room temperature. The internal field at 57Fe nuclei is almost constant, ≃ 300 kOe, at 77 K, suggesting that the Fe moment is nearly constant in this range. The coercive field at 4.2 K is as small as ≃ 1 kOe for x = 0.2 but increases rather abruptly for larger x, up to more than 50 kOe for x \gsim 0.4 . Large random uniaxial anisotropy of Nd is supposed to be the cause of such a high coercivity.

Morphological transition in crystallization of Si from undercooled melt

Using CO2 laser equipped electro-magnetic levitator, we carried out the crystallization of Si at undercoolings from 0 K to 200 K. From the point of the interface morphologies, the relationship between growth velocities and undercoolings was classified into two regions, I and II, respectively. In region I where the undercooling is approximately less than 100 K, needle-like thin plate crystals whose interface consists of faceted plane were observed. In region II, the morphology of growing crystals changed to massive dendrites. Although the interface morphologies look quite different between region I and II, the growth velocities are expressed by two dimensional (2D) nucleation-controlled growth model, and at undercoolings larger than 150 K, the growth velocities asymptotically close to the analysis of the mono-parametric linear kinetics growth model. In this stage, the kinetic coefficient of 0.1 m/sK is equivalent to that derived by the diffusion-controlled growth model. This result means that with increase of undercooling, the rate-determining factor changes from 2D nucleation on the faceted interface to random incorporation of atoms on the rough interface.