Thermal stability and magnetic properties of Gd–Fe–Al bulk amorphous alloys

Gd₆₅Fe₂₀Al₁₅, Gd₆₅Fe₁₅Al₂₀ and Gd₇₀Fe₁₅Al₁₅ bulk amorphous alloys were produced by copper mold casting method with the maximum diameters of 2, 1 and 1 mm, respectively. The crystallization temperature (T_x) and melting temperature (T_m) of the Gd₆₅Fe₂₀Al₁₅ bulk amorphous alloy are 808 and 943 K, respectively. Accordingly, the temperature interval of T_m and T_x, ΔT_m (=T_m − T_x), is as small as 135 K and the reduced crystallization temperature (T_x/T_m) is as high as 0.86. The small ΔT_m and high T_x/T_m values are presumed to be the origin for the achievement of the high amorphous-forming ability of the Gd–Fe–Al bulk amorphous alloy. The Gd₆₅Fe₂₀Al₁₅, Gd₆₅Fe₁₅Al₂₀ and Gd₇₀Fe₁₅Al₁₅ bulk amorphous cylinders with a diameter of 1 mm exhibit superparamagnetism at room temperature, while the amorphous ribbon shows the paramagnetism at room temperature. Finally, the mechanical properties of Gd₆₅Fe₂₀Al₁₅ bulk amorphous alloys are investigated.     

Magnetic properties and microstructural characteristics of bulk Nd–Al–Fe–Co glassy alloys

Bulk Nd–Al–Fe–Co glassy alloys with diameter up to 5 mm were investigated by magnetic measurements, magnetic force microscopy (MFM) and high resolution electron microscopy (HREM) at room temperature. The results from the measurement of vibrating sample magnetometer show that these samples with compositions Nd₆₅Al₁₀Fe_25-xCo_x (x=0–10 at.%) and Nd₆₀Al₁₀Fe₂₀Co₁₀ display hard magnetic properties with H_C of 300 kAm⁻¹, M_S of 10 Am² kg⁻¹, and M_r of 7 Am² kg⁻¹. The MFM measurements of the Nd₆₀Al₁₀Fe₂₀Co₁₀ bulk metallic glass (BMG) reveal the existence of magnetic domains with a period of about 0.36 μm, and the ordered clusters with the averaged size of about 5 nm was observed by the HREM on the sample. The domain structure or cluster is believed to be associated with the appearance of hard-magnetic properties in this alloy system. The existence of the large-size domains demonstrates that magnetic moment of a great deal of ordered atomic clusters in the BMG has been aligned by exchange-coupling.     

Perpendicular magnetic anisotropy in nanocomposite Nd–Fe–B/FeCo bilayer films

Nd–Fe–B/FeCo bilayer films with Mo underlayers and overlayers have been investigated. All the samples have perpendicular anisotropy and the magnetization is found to increase with increasing FeCo layer thickness (d_FeCo) and the coercivity decreases with increasing d_FeCo. The maximum energy-product is 20 MGOe for d_FeCo = 5 nm. The enhancement of the remanence and energy products in the bilayer films is attributed to the exchange coupling between the magnetically soft and hard phases.     

Crystallization processes and phase evolution in amorphous Fe–Pt–Nb–B alloys

Fe–Pt system is nowadays widely studied due to its potential applications as magnetic recording media. The hard magnetic FePt L1₀ phase has extremely promising potential as permanent magnet with high magnetocrystalline anisotropy. Of recent interest is also the developing of the hard magnetic phase from an amorphous precursor by appropriate crystallization processes. The melt-spun amorphous Fe₆₈Pt₁₃Nb₂B₁₇ alloy has been submitted to dynamical annealing and its phase transformation during the process has been monitored by differential scanning calorimetry and in situ energy-dispersive X-ray diffraction of the synchrotron radiation. In the first stage of crystallization, -Fe and cubic FePt phases are formed from the amorphous precursor. At around 600 °C superlattice Bragg reflections corresponding to tetragonal FePt are indexed in the XRD spectra and -Fe phase diminishes drastically. Finally, between 900 °C and 975 °C the tetragonal superlattice peaks disappear and cubic FePt phase is formed again. This reversible order–disorder transformation is accompanied by a strong uniaxial lattice expansion of the cubic FePt unit cell. The system show promising features for the co-existence of hard and soft exchange coupled magnetic phases crystallized from FePt-based amorphous precursors.     

Influence of oxygen content on grain growth in Pr–Fe–B/Nd–Fe–B sintered magnets

An investigation into the effects of intermediate/variable final oxygen contents, in relation to the resultant microstructures and final magnetic properties of Pr–Fe–B- and Nd–Fe–B-type magnets in the as-sintered state has been undertaken. For low oxygen contents the Pr–Fe–B and Nd–Fe–B sintered magnets exhibited excessive grain growth on sintering. A high oxygen content resulted in the elimination of abnormal grain growth for both materials, on sintering, where the level of oxygen contamination appeared to affect the grain growth process, although not the grain growth mechanism. Differences, however, in grain growth behaviour for the Pr–Fe–B and Nd–Fe–B sintered magnets have been observed. A changing oxygen content resulted in a variable grain size and grain size distribution for both the Pr–Fe–B and Nd–Fe–B sintered magnets, resulting in a progressive change in intrinsic coercivity, with oxygen content.     

Mechanical properties and fracture mechanism study of sintered Nd–Fe–B alloy

The mechanical properties including bending strength (σ_bb) and fracture toughness (K_IC) of sintered Nd–Fe–B magnets have been examined in this paper. Alloyed magnet with Dy, Co, Nb additions shows the higher magnetic properties and bending strength but the lower fracture toughness than the ternary magnet. The zero plastic energy in bending procedure presents the purely brittle nature of Nd–Fe–B magnet. Fracture mechanism has been discussed on the base of the in situ SEM observation of micro-crack nucleation and FE-SEM (field emission scanning electron microscope) observation of indentation cracks. The fracture mechanism of sintered Nd–Fe–B alloy is concluded as two stages: the damage-accumulating stage and the crack-propagating stage. Some suggestions for improving toughness of sintered Nd–Fe–B magnets are mentioned.     

Corrosion resistance of Nd‐Fe‐B permanent magnetic alloys. Part 1: Role of alloying elements

The electrochemical behaviour of Nd‐Fe‐B magnetic alloys was investigated in acid and neutral solutions. The differences in the chemical composition of these materials have distinct influence on the corrosion rate and polarization behaviour. Small additions of cobalt, aluminium and gallium increase the corrosion resistance of the magnetic alloys at high cathodic potential. It was also observed that the increase of hydrogen evolution rate on the surface of the magnetic materials raises the rate of dissolution of these magnets. The electrostatic surface potential was examined by scanning probe microscopy. A relation between the electrostatic surface potential and electrochemical behaviour of these alloys was found. The high values of electrostatic surface potential of the intergranular phases reflect higher corrosion attack. Auger electron spectroscopy was used to analyse the surface layer which formed during anodic polarization of the magnet containing alloying additives. The result indicates the formation of (Nd,Fe)‐oxide with small amounts of cobalt and aluminium.     

Interstitial distribution in Fe–Al and Fe–Cr quenched and aged alloys:: Computer simulation and internal friction study

Two types of physical approaches for simulation of the Snoek-type relaxation in low and high alloyed iron are examined to explain the experimental results obtained for Fe–Al–C and Fe–C–Cr alloys. The first approach developed by Smirnov–Tomilin is to calculate all octahedral positions available for interstitial atoms with different amount of substitute atoms in the first coordination shell and to simulate the loss maximum as a sum of all partial peaks according to the above mentioned interstice positions. The second approach takes into account the all pairwise interatomic interaction between solute atoms in a few coordination shells due to their interatomic elastic and ‘chemical’ interaction according to Khachaturyan–Blanter theory. The change of activation energy of ‘diffusion under the stress’ for interstitial atoms in that case is not a linear function of substitutional concentration in solution. Both physical models (short- and long-range interatomic interaction) for the Snoek-type relaxation in quenched ternary alloys (Fe–C–Me) are examined from the viewpoint of a distance of interatomic interaction taken into account and checked using experiments. It is shown that contrary to the second approach, the first type of calculations is reasonable for relatively low alloyed solid solution only. Decomposition (Fe–Cr) and ordering (Fe–Al) change the parameters of atomic distribution in bcc solid solution and lead to the corresponding change in the Snoek relaxation parameters. The use of an adequate physical model and structure parameters allows to explain corresponding effects and, vice versa, the internal friction spectrum allows to estimate quantitatively atom redistribution in alloyed ferrite.     

Magnetomechanical damping in Fe–Cr alloys and effect of Al and Mo addition

The damping capacity of ferromagnetic Fe–Cr based alloys containing 0–8% Al and 0–4% Mo has been investigated over a wide range of frequencies and amplitudes using a cantilever device. The resonance curves of the flat beam samples under various driving forces were recorded from which several parameters including the damping capacity (Q⁻¹), ΔE effect, internal stress (σ_i) and magnetostriction constant (λ_s), were determined. The variation of these parameters with chemical composition and thermal treatment are analyzed. The results showed that the magnetostriction constant is the key parameter to describe variation of the damping capacity in these alloys. Lower concentration of Al and Mo enhanced the damping capacity several times but higher concentration above 4% decreased it drastically. Addition of Mo resulted in higher damping capacity than did Al.     

Solidification of levitated Nd–Fe–B alloy droplets at significant bulk undercoolings

Nd–Fe–B alloys with composition of Nd₁₆Fe₇₆B₈, Nd₁₈Fe₇₃B₉ and Nd₂₂Fe₆₇B₁₁ were melted and solidified using an electromagnetic levitation technique. Two types of solidification behavior were observed depending on the bulk undercooling achieved prior to solidification. The samples with small undercoolings were solidified by the predominant primary formation of the Nd₂Fe₁₄B compound, whereas those with large undercoolings were solidified by the primary formation of the metastable Nd₂Fe₁₇B_x compound (x1) plus the subsequent formation of Nd₂Fe₁₄B. The critical undercooling for the primary Nd₂Fe₁₇B_x formation was determined to be 40, 70 and 130 K in the three alloy compositions, respectively. The liquidus temperature of the metastable Nd₂Fe₁₇B_x compound was estimated to be 1423, 1393 and 1283 K, respectively. The Nd₂Fe₁₇B_x compound was found to decompose into a mixture of Fe plus Nd₂Fe₁₄B due to the slow cooling rates of the samples. It was suggested that the metastable Nd₂Fe₁₇B_x compound may have a lower interfacial energy than that of the stable Nd₂Fe₁₄B compound, hence being favored in nucleation-controlled phase selection at large undercoolings.     

The microstructure of liquid immiscible Fe–Cu-based in situ formed amorphous/crystalline composite

In the microstructures of slowly and rapidly cooled liquid of the immiscible alloy Fe₃₀Cu₃₂Ni₁₀Si₁₃Sn₄B₉Y₂ two distinct regions were observed following arc melting and slow cooling, confirming that liquid/liquid phase separation had occurred. Rapid cooling from a temperature within the liquid immiscibility gap, melt spinning, resulted in an amorphous/crystalline composite, formed from the previously melted Fe- and Cu-rich regions, respectively. Transmission electron microscopic studies of this melt-spun ribbon revealed the glassy nature of the Fe-rich matrix, as well as of the Fe-rich spheres formed within the previously existing Cu-rich liquid.     

Structure and phase transformations in Fe–Ni–Mn alloys nanostructured by mechanical alloying

Ternary Fe₈₆Ni_xMn_14−x alloys, where x = 0, 2, 4, 6, 8, 10, 12, 14, 16 at.%, were prepared by the mechanical alloying (MA) of elemental powders in a high-energy planetary ball mill. X-ray diffraction analysis and Mössbauer spectroscopy were used to investigate the structure and phase composition of samples. Thermo-magnetic measurements were used to study the phase transformation temperatures. The MA results in the formation of bcc α-Fe and fcc γ-Fe based solid solutions, the hcp phase was not observed after MA. As-milled alloys were annealed with further cooling to ambient or liquid nitrogen temperatures. A significant decrease in martensitic points for the MA alloys was observed that was attributed to the nanocrystalline structure formation.     

Dependence of coercivity on the intergranular phase for nanocrystalline Nd–Fe–B magnet

Assume that the intergranular phase (IP) existing between adjacent grains is a weak magnetic phase, and could weaken or interrupt the intergrain exchange-coupling interaction (IECI). Using our proposed cubic-grain anisotropy model, we investigate the effects of IP's thickness d, and its anisotropy constant K₁(0) on the coercivity of nanocrystalline Nd–Fe–B magnet. Calculation results indicate that the coercivity increases with increasing d, but decreases with increasing K₁(0). When d = 1.2 nm and K₁(0) = 0.5K₁ (K₁ is the common anisotropy constant of the bulk Nd–Fe–B material), our calculated results are consistent with available experimental data.     

Influence of aluminium content on retarding hydrogen transport in Fe–Al binary alloys

The electrochemical hydrogen permeation measurement was utilized to determine the effective diffusivity, permeation rate and apparent solubility of hydrogen in a series of Fe–Al binary alloys at 25°C. The experimental results reveal that both the hydrogen effective diffusivity and permeation rate in these alloys are significantly dropped when Al content is more than 15–20 at.%. The hydrogen apparent solubility in these alloys is reduced with the increment of Al content, even though their lattice parameters and strain fields in these alloys are increased. For these Fe–Al binary alloys, the retardation factor of hydrogen is mainly dependent upon Al content. Both oxide film and the degree of structural order are the major retardation factors on hydrogen transport in Fe–Al binary alloys.     

Effects of Nb and C additions on the crystallization behavior, microstructure and magnetic properties of B-rich nanocrystalline Nd–Fe–B ribbons

The effects of Nb and C additions on the crystallization behavior, microstructure and magnetic properties of B-rich Nd_9.4Fe_79.6−xNb_xB_11−yC_y (x = 0, 2, and 4; y = 0, 0.5, and 1.5) alloy ribbons have been investigated. The results show that Nb and C additions change the crystallization behavior of Nd_9.4Fe_79.6B₁₁, avoid the formation of metastable Nd₂Fe₂₃B₃ phase, leading to the simultaneously precipitation of α-Fe and Nd₂Fe₁₄B phases. The results also show that Nb and C additions suppress the formation and growth of the soft α-Fe phases, leading to the presence of a large amount of Nd₂Fe₁₄B phases. Nb and C additions also refine the structure, and thus increase the exchange coupling interaction between the soft and hard phases. Excellent magnetic properties of B_r = 0.85 T, _iH_c = 1106 kA/m, and (BH)_max = 117 kJ/m³ have been achieved in Nd_9.4Fe_75.6Nb₄B_10.5C_0.5 alloy ribbons.     

Dependence of the brittle-to-ductile transition temperature (BDTT) on the Al content of Fe–Al alloys

The brittle-to-ductile transition temperature (BDTT) of binary Fe–Al alloys with between 9.6 and 45 at.% Al was investigated in the as-cast state by four-point bending tests. An increase of the BDTT was observed with increasing aluminium content between 9.6 and 19.8 at.%. Up to 41.3 at.%, the BDTT did not change significantly. A sharp increase of the BDTT occurred between 41.3 and 45 at.% Al. Transgranular cleavage was observed at a composition of 25 at.% Al, mixed-mode fracture between 39.6 and 41.3 at.% Al and intergranular fracture at 45 at.% Al. The results indicate that the increase in BDTT is correlated with the transition from mixed-mode to intergranular fracture.     

Phase transformations in iron-rich Tb–Fe–Si alloys

RRhO₃ (R=rare earth except Ce and Pm) was prepared by a solid-state reaction, and its crystallographic, magnetic, and electric properties were investigated. RRhO₃ has an orthorhombic perovskite-type structure of the space group Pbnm. RRhO₃ shows Curie–Weiss paramagnetism above 5 K. On the other hand, EuRhO₃ shows antiferromagnetic behavior. The Rh³⁺ ion, which seems to be in the low-spin state, has a very small effective magnetic moment (P_eff=0.295 μ_B/ion). The P_eff value of the RRhO₃ compounds shows the same dependence on the number of 4f electrons as the gJ value of the rare-earth ions. The rare-earth ions make a major contribution to the magnetic moment of RRhO₃. The resistivity of all RRhO₃ shows an activation-type (or semiconductor-like) temperature dependence.     

Structural transformations in quenched Fe–Ga alloys

It has been speculated that the large increase in magnetostriction in Fe–Ga alloys results from local short-range ordering of the Ga atoms along specific crystallographic directions in the disordered Fe structure. The structural transitions associated with different cooling rates from the high temperature disordered state were investigated with X-ray diffraction of oriented single crystals of Fe–19 at% Ga. Results are presented for long-range ordering during slow cooling and indirect evidence of local short-range ordering of Ga atoms in the disordered state when the alloys are quenched is also presented. In the latter case, the short-range ordering of Ga atoms leads to a tetragonal distortion of the lattice. The dependence of the magnetostrictive response of Fe–Ga alloys on thermal history has been found to be directly related to these structural transformations in Fe–19 at% Ga alloys and experimental support for the proposed magnetostriction model based on Ga–Ga pairing along [100] crystallographic directions is presented.     

Microstructures and magnetic properties of hydrogenation disproportionation desorption recombination-processed Nd–Fe–B materials with different Nd content of 11.0 and 12.6 at.%

The hydrogenation disproportionation desorption recombination (HDDR) process was performed on the generally used alloy composition of Nd_12.6Fe_63.1Co_17.4Zr_0.1Ga_0.3B_6.5 and a low rare earth content alloy composition of Nd_11.0Fe_65.0Co_17.8Zr_0.1Ga_0.3B_5.8. A detailed evaluation was made of the relationship between the microstructure and magnetic properties of these HDDR-processed magnetic powders with respect to their different rare earth element concentrations. The HDDR-processed powders of both alloy compositions were transformed to the Nd₂Fe₁₄B phase consisting of fine recombined crystal grains of around 400–500 nm in size and maintained the anisotropic magnetic characteristic that was present before HDDR processing. However, reduction of the rare earth content drastically reduced coercivity, and the alloy composition of Nd_11.0Fe_65.0Co_17.8Zr_0.1Ga_0.3B_5.8 did not manifest magnetic properties. From the results of an examination of their microstructures, it was inferred that the coercivity decreased due to a decline in the concentration of the rare earth element at the grain boundaries of the fine Nd₂Fe₁₄B grains with the reduction of the rare earth content of the alloys. Accordingly, in magnetic powders obtained by the HDDR process, the nucleation type of coercivity mechanism predominates, in which rare earth-rich regions present at the grain boundaries of fine Nd₂Fe₁₄B grains play a large role in the manifestation of coercivity.