Isothermal aging–microstructure–property relationship of SmCo 2:17 magnets

Abstract

The isothermal aging–microstructure–magnetic property relationship of a SmCo 2:17 alloy was investigated in the present study. The alloy also contained Fe, Cu and Zr as major addition. Its magnetic properties, including intrinsic coecivity H _ci, remanence B _r and maximum energy product (BH)_max, were obtained from the measured second quadrant M–H hysteresis curves. The microstructure of the alloy was characterised using X-ray diffraction (XRD) and TEM. It was found that, as the one step aging temperature was increased from 850 to 900°C, the remanence of the SmCo 2:17 alloy magnet was increased while its coercivity was reduced. The maximum energy product (HB)_max of the magnet was significantly improved by applying a two step aging, consisting of aging first at 900°C followed by aging at 850°C. Based on XRD and TEM analyses, effects of the aging condition on the microstructure and magnetic properties were discussed. The microstructural change observed in the current work included cell size of the cellular structure and the degree of Cu segregation at the cell boundaries. Change in the degree of Cu segregation at the cell boundaries might be the major cause for the change in coercivity of the alloy aged in different aging conditions. The effect of aging condition on the remanence could be attributed to the effect of aging condition on cell size of the cellular structure, which affects the volume fraction of SmCo 2:17 phase in the alloy.     

Effects of Magnetic Annealing on Structure and Magnetic Properties of L10-FePt/Ag Films

The structural and magnetic properties of L1₀-FePt/Ag films were studied by X-ray diffraction and a vibrating sample magnetometer. The FeAg/Pt films were obtained by depositing FeAg thin films on thermally oxidized Si (001) substrates via magnetron sputtering and Pt layers on their surface after annealing FeAg thin films at 400 °C with and without an out-of-plane magnetic field of 10 kOe. These films were further annealed at various temperatures to obtain L1₀-FePt phase. The results indicated that the pre-annealing of FeAg thin films under 10 kOe magnetic field caused (001) orientation of Fe particles, and the deposition of Pt layer on such orientated underlayers reduced the ordering temperature of FePt in FeAg/Pt films, realizing the L1₀-FePt phase at 400 °C. The higher coercivity and ordering degree were also observed in the samples, compared with those pre-annealed without magnetic field at the same annealing condition.     

Influence of the structure on the high-temperature cyclic crack resistance of Ti-8Al-1.4Si-2.2Zr alloy

It is shown that, for various structural modifications of Ti-8Al-1.4Si-2.2Zr alloy, the elevation of temperature leads to the increase in the cyclic fracture toughness ΔK _fc and the decrease in the fatigue threshold ΔK _th. At 20°C, the cast material with coarse-grained lamellar structure exhibits better properties than the fine-grained modifications obtained by thermomechanical treatment under medium and high levels of loading. At 700°C, this is true only for medium amplitudes of the load. The procedure of quenching from the β-region of the alloy thermally deformed by 90% doubles the value of ΔK _th at a temperature of 20°C as compared with the cast alloy. At the same time, the value of ΔK _fc at 700°C becomes 1.6 times higher. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 3, pp. 50–56, May–June, 2008.     

Studies on thermally treated lanthanum-iron (atomic ratio 1∶9) oxyhydroxide

Mössbauer spectral (MS) and magnetic investigations on the products obtained by annealing coprecipitated La(III) and Fe(III) (atomic ratio 1∶9) oxyhydroxide gel to different temperatures in the range 100 to 1250°C are reported. The annealing temperatures were chosen on the basis of thermal analysis data. It is observed that the thermal behaviour of the bulk ferric gel is significantly influenced by La(III) incorporation. A quadrupole split doublet observed in the MS of the gel annealed from 100°C persists up to 570°C and for samples annealed from 850°C and above it is not discernable. However, a six-fingered hyperfine split pattern emerges even for the gel heated to 150°C indicating the onset of short-range ordering.H _n values ranging from 495 to 519 kOe for the samples heated to 570°C are in agreement with that for α-Fe₂O₃, however, their magnetic susceptibility per gram (χ_g) of the order of 10^?3 e.m.u. appears to be high for such systems. Annealing the gel from 850°C and above results in anH _n of about 526 kOe which is in better agreement with that of LaFeO₃, but the χ_g value falls to about 10^?5 e.m.u.     

Magnetic measurement of strain-induced martensite formation in Fe–30Ni steel

This investigation focuses on deformation-induced plasticity in Invar-type steel alloys. The effect is being studied in an austenitic model alloy, containing 30 wt-% of nickel. Its temperature dependent mechanical properties are being presented. Furthermore, the martensitic phase content has been determined by magnetic means in an alloy with two ferromagnetic phases for the first time. The results show that the α′-martensite formation within the austenitic phase with primarily wavy glide mechanism allows an increase in ductility of around 10% at the M_sσ temperature of ?5°C. This is the point of maximum uniform elongation. Near the M_s temperature, a microstructure of 70 vol.-% deformation-induced α′-martensite can be achieved.     

Phase transitions and hard magnetic properties for rapidly solidified MnAl alloys doped with C,B, and rare earth elements

MnAl alloys are attractive candidates to potentially replace rare earth hard magnets because of their superior mechanical strength, reasonable magnetic properties, and low cost. In this study, the phase transitions and magnetic properties of melt spun Mn₅₅Al₄₅ based alloys doped with C, B, and rare earth (RE) elements were investigated. As-spun Mn–Al, Mn–Al–C, and Mn–Al–C–RE ribbons possessed a hexagonal ε crystal structure. Phase transformations between the ε and the L1₀ (τ) phase are of interest. The ε → τ transformation occurred at ~500 °C and the reversed τ → ε transformation was observed at ~800 °C. Moderate carbon addition promoted the formation of the desired hard magnetic L1₀ τ-phase and improved the hard magnetic properties. The Curie temperature T _C of the τ phase is very sensitive to the C concentration. Dy or Pr doping in MnAlC alloy had no significant effect on T _C. Pr addition can slightly improve the magnetic properties of MnAlC alloy, especially J_S. Doping B could not enhance the magnetic properties of MnAl alloy since B is not able to stabilize either the ε phase or the L1₀ hard magnetic τ phase.     

Annealing Effect on Structural and Magnetic Properties of Cu2MnAl Heusler Alloy Films

Cu₂MnAl Heusler alloy films were grown on MgO (001) substrates by using the ion beam sputtering technique. The films were post-annealed at varying temperatures in order to investigate the influence of annealing on crystal structure and magnetic properties. The structural properties of Cu₂MnAl films have been investigated by using x-ray diffraction (XRD) and magnetic properties have been investigated by both vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR) techniques. The experimental data indicates that the crystal structure of the films strongly depends on the annealing temperature. When the films were annealed at 200?°C, the saturation magnetization (M _s =250?emu/cm³) achieved its maximum and the coercive field (H _c ??7?Oe) reached its minimum with B2 ordered structure. In addition, FMR results have revealed that the Cu₂MnAl film annealed 200?°C has the highest effective magnetization. The combination of structural and magnetic characterization indicates that the optimum growth temperature is 200?°C for the Cu₂MnAl Heusler alloy films on MgO substrates.     

Compositional effects on the properties of (1-x)BaTiO3-xBi0.5Na0.5TiO3 ceramics

The compositional effects on the crystal structure, phase transition, dielectric, ferroelectric and piezoelectric properties of the (1-x)BaTiO₃-x(Bi_0.5Na_0.5)TiO₃ solid solution ceramics were investigated. After sintering at 1200 °C for 2 h, the ceramics with different content of (Bi_0.5Na_0.5)TiO₃ (BNT) formed single-phase solid solutions with perovskite structure. The lattice constant c/a ratio of the solid solutions decreased as BNT content increased except that with 20 mol% BNT, which had the largest value of c/a ratio for all samples. The second phase transition corresponding to BaTiO₃ (BT) at about 5 °C shifted to low temperature and gradually disappeared as BNT increased. A new secondary phase transition appeared at 32 °C and 64 °C, respectively, when BNT content was 5 and 14 mol%. The Curie temperature, T _c, shifted in the temperature range between 112 °C and 166 °C, and the remanent polarization, P _r, decreased whereas the coercive field, E _c, increased as the BNT content increased. The relatively high value of piezoelectric coefficient d₃₃ together with a relatively high Curie temperature and low loss tangent and stable dielectric properties were obtained when the addition of BNT was 20 mol%.     

Study on squeeze casting of an in situ 5 vol.-% TiB2/2014 Al composite

Abstract

An in situ 5 vol.-% TiB₂/2014 composite was prepared by an exothermic reaction of K₂TiF₆, KBF₄ and Al melts. The effect of introduction of in situ formed TiB₂ particles on the squeeze-casting formability of the composite was discussed. The microstructural evolution and changes in the mechanical properties of the composite at different squeeze pressures were investigated. The results showed that a pouring temperature of 710°C, a die temperature of 200°C and a squeeze pressure of 90 MPa were found to be sufficient to get the qualified squeeze cast and maximum mechanical properties for an Al 2014 alloy. However, the pouring temperature, die temperature and squeeze pressure need to be increased to 780°C, 250°C and 120 MPa for the composite to get the qualified squeeze cast and maximum mechanical properties as a result of the effect of introduction of in situ formed TiB₂ particles on the solidification process, plasticity and fluidity of the composite. The microstructural refinement, elimination of casting defects such as shrinkage porosities and gas porosities and improved distribution of TiB₂ particles in the case of the composite result when pressure was applied during solidification. Compared with the gravity-cast composite, the tensile strength, yield strength and elongation of the squeeze-cast composite at 120 MPa increased by 21%, 16% and 200%.     

THE EFFECT OF GRAIN SIZE, STRAIN AND TEMPERATURE ON THE MONOTONIC STRESS-STRAIN BEHAVIOUR OF POLYCRYSTALLINE ALUMINIUM AND AL ALLOYS

Abstract The tensile yield and flow stresses of aluminium, A1-2.63Mg alloy and A1-2.07Li alloy at room temperature are shown to depend on the inverse square root of the polycrystal grain size and are described empirically by the Hall-Petch relation. The same relation describes the flow stress-grain size dependence for A1-2.07Li alloy at temperatures ranging from - 196°C to 400°C. The strain hardening in the friction stress of each material at 20°C is independent of the grain size, is approximately parabolic and is greatest for the precipitation strengthened A1-2.07Li alloy. The grain size contribution to the tensile flow stress is dependent on both the tensile strain and composition. The friction stress, σ₀, and slip band stress intensity parameter, k_ε, at yield, k_y, are both dependent on temperature. Low temperature suppresses dislocation annihilation and recovery processes, leading to planar pile-ups at grain boundaries and a hardening that is linear with strain. Weak hardening is observed at 250°C and 400°C due to extensive annihilation and recovery. The value of k_ε, at all temperatures falls following initial yielding with the generation of freshly unlocked sources.     

Effect of hot working variables on microstructure and brittle to ductile transition in Ti–46.5Al–3Nb–2Cr–0.2W gamma titanium aluminide

Abstract

The microstructure and mechanical properties of a γ-TiAl alloy (Ti–46.5Al–3Nb–2Cr–0.2W, in at.-%) were studied in two conditions: (a) after conventional forging in the +γ phase field and (b) after subsequent isothermal forging in the ₂+γ phase field. Tensile tests were conducted in the temperature range 800–1000°C and strain rate range of 10^-3–10^-1 s^-1. The microstructure of the alloy in condition 1 was non-homogeneous consisting of about 90 vol.-% of small γ grains (grain size of 3 to 20 µm) and 10 vol.-% of coarse grains or lamellar regions. The alloy in this condition showed a brittle to ductile transition at about 950°C and extensive cavitation during deformation above the transition temperature. The microstructure in condition 2 was much more uniform and finer, and the transition temperature was decreased to 850°C. The alloy in condition 2 showed better deformability and cavitation resistance than that in condition 1 and superplastic behaviour at temperatures 900–1000°C.     

Hydrothermal Synthesis and Characterization of CoFe2O4 Nanoparticles and Nanorods

This paper presents a low temperature (130 and 160 °C) synthesis route to prepare the spinel phase CoFe₂O₄ nanoparticles and nanorods. A one-dimensional (1-D) structure of Co-ferrite was successfully synthesized using Cetyl Trimethyl Ammonium Bromide (CTAB) as a surfactant at temperature 160 °C. Structural, electrical, and magnetic measurements have been performed using X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), and the Vibrating Sample Magnetometer (VSM). XRD patterns show a pure spinel (fcc) structure, showing a complete phase formation at a low temperature of 160 °C, without any subsequent sintering. Average crystallite sizes have been calculated by Sherrer’s and Williamson-Hall methods. As prepared CoFe₂O₄ nanorods exhibited a uniform shape of diameter 60–80 nm and 600–900 nm in length. The FTIR spectrum for Co-ferrite nanorods shows two intrinsic lattice absorption bands for tetrahedral and octahedral sublattices. DC electrical resistivity of CoFe₂O₄ nanorods is high up to ～10⁸ (Ω-cm), as compared to CoFe₂O₄ nanoparticles (～10⁷ Ω-cm) at 373 K. Dielectric parameters were measured using a LCR meter, in the frequency range of 1 kHz to 5 MHz. The real and imaginary part of the dielectric constant (ε′ and ε″) and dielectric loss tangent (tanδ) reduces for CoFe₂O₄ nanorods in comparison to nanoparticles, and has a value of 13.6 and 0.0416, respectively. Magnetic properties were characterized by VSM under a field of 10 kOe and showed that the 1-D structure reduces the magnetization of nanocrystalline CoFe₂O₄ from 65 emu/gm to 54 emu/gm.     

Design of powder metallurgy aluminium alloys for applications at elevated temperatures Part 2 Tensile properties of extruded and Conformed gas atomised powders

Abstract

The effects of aging treatments on the tensile properties and microstructure of Al–Cr–Zr–Mn powder metallurgy aluminium alloys prepared from high pressure gas atomised powders were investigated. The alloy compositions were designed to give powders with or without Al₁₃Cr₂ intermetallics in the <45 μm size fraction. The Al–5·2Cr–1·4Zr–1·3Mn alloy is typical of the former (concentrated alloy) and the Al–3·3Cr–0·7Zr–0·7Mn alloy of the latter (dilute alloy). The alloys were prepared using a canning/degassing/extrusion sequence or the Conform consolidation process. Measurements of micro hardness and electron microscopy were used to correlate the microstructure with the tensile properties. The extruded powders of both alloys exhibited better properties than those of the Conformed powders. A large contribution to the strength of the extruded materials is made by their stabilised fine grain size. The dilute alloys had consistently better ductility. Neither alloy retained its strength after prolonged aging at 400°C, but the results indicate that a service temperature of 300°C may be possible.

MST/1247b     

Grain refinement and superplastic behaviour of a modified 6061 aluminium alloy

Abstract

The superplastic properties and microstructure evolution of a 0.15%Zr and 0.7%Cu modified 6061 aluminium alloy were examined in tension at temperatures ranging from 475 to 600°C and strain rates ranging from 7 × 10^-6 to 2.8 × 10^-2 s^-1. The refined microstructure with an average grain size of about 11 μm was produced in thin sheets by a commercially viable thermomechanical process. It was shown that the modified 6061 alloy exhibits a moderate superplastic elongation of 580% in the entirely solid state at 570°C and ? = 2.8 × 10^-4 s^-1. Superior superplastic properties (elongation to failure of 1300% with a corresponding strain rate sensitivity coefficient m of about 0.65) were found at the same strain rate and a temperature of 590°C, which is higher than the incipient melting point of the 6061 alloy (~575°C). The microstructural evolution during superplastic deformation of the 6061 alloy has been studied quantitatively. The presence of a slight amount of liquid phase greatly promotes the superplastic properties of the 6061 alloy, reducing the cavitation level.     

TiH2-based Ti-1Al-8V-5Fe PM alloys with different addition methods of alloying elements

The powder metallurgy high-strength Ti-1Al-8V-5Fe alloy (Ti-185 alloy) was investigated in this paper with different addition methods of alloying elements, Al, V, Fe pure elements powder (EP) or 1Al-8V-5Fe ternary master alloy powder (MAP), based on TiH₂ powders at the sintering temperature from 1150 to 1350°C. The results indicate that the Ti-185 alloy with the 1Al-8V-5Fe master alloy (Ti-185 MAP alloy) possesses the higher relative sintered density, less oxygen content, and less α-phase volume fraction versus the Ti-185 alloy with Al, V, and Fe pure elements (Ti-185 EP alloy). No matter where the sintering temperature is 1150, 1250, or 1350°C, Ti-185 MAP alloy invariably has the higher yield strength and hardness which have a strong relationship to its higher density and less volume fraction of softer α-phase in comparison with Ti-185 EP alloy.     

Influence of temperature on structural and magnetic properties of Co0·5Mn0·5Fe2O4 ferrites

Co_0·5Mn_0·5Fe₂O₄ ferrites have been synthesized using a single-step sol-gel auto-combustion method in which the metal nitrate (MN)-to-citric acid (CA) ratio was adjusted to 0.5: 1 and pH to 7, respectively. The structural and magnetic properties of as-burnt and annealed samples were studied as a function of temperature. The inverse spinel structure was confirmed by X-ray diffraction (XRD) and crystallite size was estimated by the most intense peak (311) using Scherrer’s formula. Contrary to earlier studies reported in the literature, both as-burnt and annealed samples exhibit crystalline behaviour. Room temperature magnetic properties were studied using vibrating sample magnetometer (VSM) with field strengths up to ±10 kOe. Lattice constant and crystallite size increased as the annealing temperature was increased. However, the coercivity (H _c) initially increased and then decreased with the increase of crystallite size. The variation in coercivity is ascribed to the transition from a multi-domain to a single-domain configuration.     

Investigation on multiferroic properties of BiFeO3 ceramics

BiFeO₃ polycrystalline ceramics was prepared by solid-state reaction method and its structural, optical and magnetic properties were investigated. BiFeO₃ was synthesized in a wide range of temperature (825–880 °C) and a well crystalline phase was obtained at a sintering temperature of 870 °C. X-ray diffraction patterns of the samples were recorded and analyzed for the confirmation of crystal structure and the determination of the lattice parameters. The average grain size of the samples was found to be between 1–2 μm. The determined value of direct bandgap of BiFeO₃ ceramics was found to be 2.72 eV. The linear behavior of M-H curve at room temperature confirmed antiferromagetic properties of the BiFeO₃ (BFO). S shaped M-H curve was obtained at a temperature of 5 K. In the whole temperature measurement range (5–300 K) of M-T, no anomalies were observed due to high Curie temperature and Neel temperature of the BiFeO₃.     

Superplasticity in a 7055 aluminum alloy subjected to intense plastic deformation

Abstract

Superplasticity in a 7055 aluminum alloy subjected to intense plastic straining through equal channel angular extrusion (ECAE) was studied in tension over a range of strain rates from 1.4 × 10^-5 to 5.6 × 10^-2 s^-1 in the temperature interval 300 - 450 °C. The alloy had a grain size of ~ 1 μm. A maximum elongation to failure of ~750% occurred at a temperature of 425 °C and an initial strain rate of 5.6 × 10^-4 s^-1, with a strain rate sensitivity coefficient m of about 0.46. The highest m value was ~0.5 at a strain rate of 1.4 × 10^-3 s^-1 and T≥ 425 °C. Moderate superplastic properties with a total elongation of about 435% and m of ~0.4 were recorded in the temperature interval 350 - 400 °C; no cavitation was found. It was shown that the main feature of superplastic behaviour of the ECAE processed 7055 aluminum alloy is a low yield stress and strong strain hardening during the initial stages of superplastic deformation. Comparing the present results with the superplastic behaviour of the 7055 Al subjected to thermomechanical processing (TMP), the highest tensile elongation in the ECAE processed material occurred at lower temperatures because ECAE produces a finer grained structure.     

Structural and Magnetic Properties of Nanostructured Pr0.75 Y 0.25Co5 Powders Obtained by Mechanical Milling and Subsequent Annealing

Pr_0.75 Y _0.25Co₅-based as-cast alloys were processed by high-energy ball milling to obtain nanostructured powders with high coercivity. The powders obtained after 4 h of milling exhibited nearly amorphous behavior in X-ray diffraction patterns. DSC scans of the as-milled powders indicated a process of crystallization by broad, exothermic transition peak at 503 °C. Annealing of the milled powders at 850 °C for 2.5 min in high vacuum produced fine grains of size ranging 15–30 nm with optimal microstructure and hard magnetic properties. Magnetic measurements of the annealed powders evaluated a high intrinsic coercivity, _i H _c of 9.3 kOe, and a remanence ratio, M _r/ M _max of 0.72. The magnetic hardening was attributed to higher anisotropy field of the powders and microstructural uniformity achieved by the processing methodologies.