The eutectic grain boundary diffusion process was applied to a 2-mm-thick hot-deformed Nd-Fe-B magnets using Nd62Dy20Al18 alloy as a diffusion source, realizing the coercivity enhancement from 0.91 T to 2.75 T with relatively small remanence deterioration from 1.50 T to 1.30 T. In contrast, the conventional grain boundary diffusion process using Dy-vapor resulted in the degradation of coercivity as the grains are catastrophically coarsened at the processing temperature of 900 °C. Scanning transmission electron microscopy showed the formation of Dy-rich shell at the sides of the Nd2Fe14B grains and the diffusion of Al into the Nd2Fe14B grains, explaining the significant improvement in coercivity.Download high-res image (284KB)Download full-size image

We have grown Sm(Fe1 − xCox)12 (x = 0, 0.1 and 0.2) films epitaxially on V(001) buffered MgO(001) single crystalline substrates in order to investigate the intrinsic hard magnetic properties of the Co substituted SmFe12 phase with the ThMn12 structure. We found Sm(Fe0.8Co0.2)12 has excellent intrinsic hard magnetic properties with spontaneous magnetization of 1.78 T, anisotropy field of 12 T and Curie temperature of 859 K, all of which are superior to those for Nd2Fe14B. Hence, the Sm(Fe0.8Co0.2)12 compound is promising as permanent magnet material, if it can be stabilized in a bulk form.Download high-res image (378KB)Download full-size image

We observed the magnetization reversals of exchange-coupled and exchange-decoupled Nd-Fe-B sintered magnets with coercivities of 1.16 and 1.80 T, respectively, by magneto-optical Kerr effect (MOKE) microscopy. The cascade propagation of reversed magnetic domain throughout multiple grains was observed in the standard sintered magnet, in which Nd2Fe14B grains are believed to be exchange-coupled through ferromagnetic grain boundary phase. In the exchange-decoupled Nd-rich Ga-doped sintered magnet, the magnetization reversal is suppressed by thick nonmagnetic grain boundary phase. The EBSD revealed 11° misalignment of 2:14:1 grains in the standard sintered magnet, while higher misalignment of 18° was found in the exchange-decoupled Nd-rich Ga-doped magnet. Micromagnetic simulations incorporating such microstructural features reproduced experimentally observed demagnetization curves fairly well including the enhancement of the coercivity and the deterioration of the squareness in the exchange decoupled sintered magnet.Download high-res image (211KB)Download full-size image

•Coercivity of hot-deformed Nd-Fe-B magnets is enhanced by the infiltration of various R-TM eutectic alloys.•The sample infiltrated with Nd90Al10 shows the highest coercivity of 2.5 T at room temperature.•At 200 °C, Nd70Cu30 diffusion-processed sample possesses the highest coercivity of 0.7 T.Nd-M (M = Al, Cu, Ga, Zn, Mn) alloys with compositions close to eutectic points were investigated as diffusion sources for the grain boundary diffusion process to hot-deformed Nd-Fe-B magnets. Coercivity enhancement was observed for most of the alloys. Among them, the sample processed with Nd90Al10 exhibited the highest coercivity of 2.5 T at room temperature. However, the sample processed with Nd70Cu30 exhibited the highest coercivity of 0.7 T at 200 °C. Microstructural observations using scanning transmission electron microscope (STEM) showed that nonferromagnetic Nd-rich intergranular phase envelops the Nd2Fe14B grains after the diffusion process. Abnormal grain growth and the dissolution of Al into the Nd2Fe14B grains were observed in the sample processed with Nd90Al10, which explains its inferior thermal stability of coercivity compared to the sample processed with Nd70Cu30. The coercivity enhancement and poor thermal stability of the coercivity of the Nd90Al10 diffusion-processed sample are discussed based on microstructure studies by transmission electron microscopy.

The NdFe12Nx compound with a ThMn12 structure (space group I4/mmm) was successfully synthesized by nitriding an NdFe12 layer grown on a W underlayer on a single-crystalline MgO(001) substrate. The c-axis expanded from 0.480 to 0.492 nm while the a-axis showed a slight contraction from 0.852 to 0.849 nm after the nitriding. Excellent intrinsic hard magnetic properties of μ0Ms ≈ 1.66 ± 0.08 T, μ0Ha ≈ 8 T, and Tc ≈ 550 °C, which are superior to those of Nd2Fe14B, were obtained.

Among various ThMn12-type rare-earth iron compounds, NdFe11TiN has been known for its large magnetization and high anisotropy field, although their values are smaller than those of Nd2Fe14B and Sm2Fe17N3. Recent first-principles calculations predicted that NdFe12N has a substantially larger magnetization than NdFe11TiN with a comparable anisotropy field. To validate this prediction, the NdFe12Nx phase was successfully synthesized by nitriding the NdFe12 film that was grown on a W underlayer on a MgO(001) substrate. The NdFe12Nx phase was found to have a saturation magnetization, anisotropy field, and Curie temperature superior to those of Nd2Fe14B.

The application of expansion constraint during the grain-boundary diffusion process using Nd70Cu30 eutectic alloy minimizes the remanence loss for the coercivity enhancement by optimizing the volume fraction of the Nd(Cu)-rich non-ferromagnetic intergranular layer. The diffusion-processed sample exhibits μ0Hc ∼ 2 T, μ0Mr = 1.36 T and (BH)max = 358 kJ m−3 at room temperature. Because of the low-temperature coefficient of coercivity, the magnet showed (BH)max ∼ 191 kJ m−3 at 200 °C, which is superior to that of Dy-containing high-coercivity sintered magnets.

Two single-phase Mg–Gd alloys (0.01 and 0.06 at.% Gd) were identically hot-deformed through an indirect extrusion process. High-angle annular dark-field scanning transmission microscopy observations revealed that the Gd-richer alloy, which possessed a weaker texture, had substantial Gd-solute clustering and grain boundary segregation, while the more Gd-dilute alloy, which showed a markedly stronger texture, did not. It is suggested that clustering and grain boundary segregation play a role in the modification and consequential weakening of the overall texture.

Twin roll cast and hot rolled Mg–6.2 wt%Zn alloys microalloyed with Zr, Ca, and Ag show tensile yield strength exceeding 300 MPa in the T6 (peak-aged) condition with reasonable formability in the T4 condition. The addition of Zr and Ca plays a critical role in the development of weak textured recrystallized microstructure in Mg–6.2 wt%Zn alloys so Mg–6.2Zn–0.5Zr–0.2Ca (wt%) alloy shows equivalent mechanical properties with Mg–6.2Zn–0.5Zr–0.2Ca–0.4Ag (wt%) alloy even without expensive Ag.

The effect of sole and combined additions of Ag and Ca in enhancing the age hardening response in a Mg–2.4Zn (at%) alloy have been studied by systematic microstructure investigations using transmission electron microscopy (TEM) and three dimensional atom probe (3DAP). In the early aging stage of a Mg–2.4Zn–0.1Ag–0.1Ca (at%) alloy at 160 °C, Zn-rich Guinier Preston (G.P.) zones form with Ag and Ca enrichment. Further aging lead to the formation of fine β′1 precipitates with Ag and Ca enrichment. We confirmed that the G.P. zones do not form in the Mg–2.4Zn (at%) binary alloy at 160 °C, but form after a prolonged aging at 70 °C. This suggests that the combined addition of Ag and Ca shifts the metastable solvus for the G.P. zones to a higher temperature, thereby making it possible to form G.P. zones even at the artificial aging temperature of 160 °C. Since G.P. zones act as nucleation sites for the β′1 precipitates, the peak-aged microstructure is refined substantially by the addition of Ag and Ca.

Newly developed TAZ1031 and TAZ1031-0.1Na alloys were extruded at different temperatures and extrusion speeds, and the resulting microstructure, texture, and mechanical properties were evaluated comparing to those of AZ31 alloy extruded with the same conditions. Tensile yield strength of TAZ1031 ranged from 214 to 319 MPa with elongations of 6–15% depending on extrusion conditions. The TAZ1031-0.1Na alloy exhibited even higher yield strength of ~336 MPa at the expense of ductility. The alloys were found to show good yield anisotropy of ~1 when extruded at 250 °C. The high strength is attributed to the fine grain size as a result of the recrystallization accompanied by dynamic precipitation.

The effect of Al additions and microalloying on the age hardening response of Mg–Sn alloys has been investigated to optimize the composition for the highest age hardening response. The addition of 3 at%Al lead to the best enhancement in peak hardness being 72 HV at 200 °C. The age hardening response was further enhanced from 73 to 81 HV by microalloying with 0.5 Zn, leading to the optimized alloy composition of Mg–2.2Sn–3Al–0.5Zn (at%) or Mg–9.8Sn–3.0Al–1.2Zn (wt%), i.e., TAZ1031. The enhancement in the peak strength and aging behavior is attributed to the solid solution hardening due to Al, the refinement of the Mg2Sn precipitates, the occurrence of Mg17Al12 precipitates, and the increase in the number density of non-basal Mg2Sn precipitates.

Si-rich intermetallic particles stabilize the fine grained microstructure in extruded Mg–6Zn–1Si–0.5Mn alloy, resulting in a yield strength (YS) of ∼200 MPa, ultimate tensile strength (UTS) of ∼310 MPa with an elongation of ∼20%. Ca additions further refine the grain size leading to an increase in UTS by 30 MPa with no decrease in YS and elongation. The extruded Mg–6Zn–1Si–0.5Mn alloy can be age-hardened with the precipitation of fine [0 0 0 1]Mg rod-like β1′ (Mg4Zn7) particles and YS ∼ 290 MPa and UTS ∼ 320 MPa were achieved in the peak aged condition. The mechanical properties were further enhanced by pre-aging the alloy at 70 °C due to the refinement of the precipitate microstructure with YS ∼ 350 MPa and UTS ∼ 360 MPa.Highlights► The extruded Mg–6.Zn–1Si–0.5Mn(–Ca) had a yield strength of ∼200 MPa. ► The duplex aging increased the yield strength to 340 MPa. ► The addition of Ca to this alloy enhanced the work hardening and refined grain size. ► Ca additions did not affect the precipitation hardening. ► The strengthening due to [0 0 0 1] rods of MgZn2.

Dual-phase 9Cr-ODS (oxide dispersion-strengthened) steel consisting of residual-α ferrite and α′ martensite has excellent high-temperature strength. This study describes the microstructure of dual-phase 9Cr-ODS steels characterized by atom-probe tomography in order to compare oxide-particle dispersion states in each phase. This revealed that nano-size oxide particles were of the same chemical composition and that their mean size was about 3 nm in each phase. On the other hand, the number density in the residual-α phase was about four times higher than that of the α′ phase. These results indicate that the dense distribution of the oxide particles in the residual-α phase contribute to the excellent high-temperature strength of 9Cr-ODS steel.

Laser assisted field evaporation using ultraviolet (UV) wavelength gives rise to better mass resolution and signal-to-noise ratio in atom probe mass spectra of metals, semiconductors and insulators compared to infrared and green lasers. Combined with the site specific specimen preparation techniques using the lift-out and annular Ga ion milling in a focused ion beam machine, a wide variety of materials including insulating oxides can be quantitatively analyzed by the three-dimensional atom probe using UV laser assisted field evaporation. After discussing laser irradiation conditions for optimized atom probe analyses, recent atom probe tomography results on oxides, semiconductor devices and grain boundaries of sintered magnets are presented.Research highlights► Application of ultraviolet (UV) femtosecond pulsed laser in a three dimensional atom probe (3DAP). ► Improved mass resolution and signal-to-noise ratio in atom probe mass spectra using UV laser. ► UV laser facilitates 3DAP analysis of insulating oxides. ► Quantitative analysis of wide variety of materials including insulating oxides using UV femotosecond laser.

Nanocrystalline Al–5 at.% Fe alloy powders produced by mechanical alloying were consolidated by spark plasma sintering. The sintered sample showed high strength >1000 MPa with a large plastic strain of 15% at room temperature and 500 MPa at 350 °C. Microstructure characterizations by transmission electron microscopy and atom probe tomography revealed that the sintered samples are composed of α-Al and Al6Fe nanocrystalline regions with 90 nm in diameter and a minor fraction of Al13Fe4 phase and coarsened 0.5–1 μm α-Al grains. This bimodally grained feature is attributed to the relatively large plastic strain for the strength level of 1000 MPa at room temperature.

Three different processing routes were explored to develop Al–Zr nanocomposite alloys using mechanical alloying and spark plasma sintering methods. Depending on the route of milling adopted, the powder in the as-milled condition consisted of either a solid solution of Zr in Al or a mixture of Al-solid solution and Al3Zr (L12) phases. The alloys after sintering consisted of Al and Al3Zr (L12) with grain sizes of less than 100 nm. These nanocomposite alloys exhibited a high compressive strength of 1 GPa with 10% plasticity. The high strength observed in these alloys was explained on the basis of the retention of nanometer sized grains and also the fine dispersion of the L12 phase. On the other hand, the good amount of plasticity was explained to be due to excellent bonding between the powder particles and the presence of coarse Al grains in the matrix.

Nanocrystalline iron synthesized by mechanical alloying and spark plasma sintering exhibited high yield and ultimate compressive strengths of 2.59 and 3.08 GPa, respectively, with a total true strain of 0.05. Partial recrystallization led to a true strain of 0.4 with a lower strength of 2.25 GPa. A bimodal grain size distribution consisting of average fine grains of 2.5 μm and nanograins of 85 nm is attributed to the combined high strength and large plastic strain.

The cell boundary phase of the Sm(Co0.725Fe0.1Cu0.12Zr0.04)7.4 commercial sintered magnet in the early stage of the isothermal heat treatment at 800 °C was identified to be the SmCo5 phase by the nanobeam diffraction technique. This finding contradicts the recently reported conclusion that it consists of the Sm5Co19 phase.

The precipitation process of an Mg–0.3Ca–0.3Zn (in at.%) alloy has been investigated by transmission electron microscopy and the chemical compositions of the precipitates have been characterized by a three-dimensional atom probe. The reason for the pronounced precipitation hardening in the alloy compared to the Mg–0.3Ca alloy is discussed.

Fully dense bulk nanocrystalline Fe–0.8wt.%C alloy was synthesized by spark plasma sintering of mechanically milled Fe–C nanocrystalline powder. The sample sintered at 600 °C was composed of 150 nm ferrite grains with nanocrystalline cementite dispersoids, whose compression yield strength, fracture strength, and plastic strain were 1900 MPa, 3500 MPa, and 40%, respectively.

Three-dimensional atom probe analysis of the nanocrystalline ferrite produced by mechanically milling pearlitic steel indicated that carbon atoms are strongly segregated at the grain boundaries, and approximately 1.0 at.% carbon are dissolved in the nanocrystalline ferrite. Annealing at 400 °C led to the rejection of carbon from the grains and the precipitation of cementite at the grain boundaries, while the nanocrystalline microstructure was retained.

The precipitation sequence of a Mg–2.1Gd–0.6Y–0.2Zr (at.%) alloy at 200 °C was investigated by transmission electron microscopy (TEM), and the chemical compositions of the precipitates observed in different stages of the sequence were analyzed by the three-dimensional atom probe (3DAP) technique. The precipitation sequence is described as supersaturated solid solution (SSS) → β″ (D019 Mg3X) → β′ (bco Mg15X3) → β phase (Mg5X). There are structural similarities in each phase and the phase evolution occurs continuously by decreasing the rare earth content when β″ transforms to β′.

•Improvement of (0 0 1)-texture of prototype FePt-C granular films for heat heat assisted magnetic recording media.•Insertion of Cr buffer layer improves the crystallographic textures of the MgO underlayers, thereby reduces in-plane component in the FePt-C recording layer.•The growth in the grain size of the MgO underlayer as well as the (0 0 1)-texture of the MgO underlayer are the key factor in reducing the in-plane component in the FePt-C recording layer.FePt-C granular films deposited on MgO underlayers are the prototype media for heat-assisted magnetic recording. To reduce the in-plane magnetic component in the FePt-C media, we investigated the effect of Cr buffer layers on the crystallographic textures of the MgO underlayers and the resultant magnetic properties of the FePt-C layers. By growing a MgO underlayer on a Cr buffer layer, the (0 0 1) texture of the MgO underlayer is improved, on which the in-plane component of a FePt-C film is substantially reduced. We conclude that the growth in the grain size of the MgO underlayer is the key factor in reducing the in-plane component in the FePt-C recording layer.