Structural, microstructural and magnetic properties of amorphous/nanocrystalline Ni63Fe13Mo4Nb20 powders prepared by mechanical alloying

This paper focuses on the magnetic, structural and microstructural studies of amorphous/nanocrystalline Ni₆₃Fe₁₃Mo₄Nb₂₀ powders prepared by mechanical alloying. The ball-milling of Ni, Fe, Mo and Nb powders leads to alloying the element powders, the nanocrystalline and an amorphization matrix with Mo element up to 120 h followed by the strain and thermal-induced nucleation of a single nanocrystalline Ni-based phase from the amorphous matrix at 190 h. The results showed that the saturation magnetization decreases as a result of the electronic interactions between magnetic and non-magnetic elements and finally increases by the partial crystallization of the amorphous matrix. The coercive force increases as the milling time increases and finally decreases due to sub-grains formation.     

Microstructure and martensitic transformation of TiNiNbB shape memory alloys

In present work, microstructure, martensitic transformation and mechanical properties of Ti₄₄Ni_47−xNb₉B_x (x = 0, 0.5, 1, 5 at.%) alloys were investigated as a function of B content. The results show that the addition of B significantly influences the microstructure of the alloys. The microstructure of Ti₄₄Ni₄₇Nb₉ alloy consists of B2 parent phase matrix and β-Nb phase. When the B content is 0.5 at.%, Nb₃B₂ phase presents. With further increasing B content to above 1 at.%, TiB and NbB phases present instead of Nb₃B₂ phase. With increasing B content, the transformation temperatures increase due to the reduced Ni/Ti ratio and Nb content in the matrix. The mechanical properties can be optimized by the addition of 1 at.% B.     

Kinetics and formation mechanism of amorphous Fe52Nb48 alloy powder fabricated by mechanical alloying

A single phase amorphous Fe₅₂Nb₄₈ alloy has been synthesized through a solid state interdiffusion of pure polycrystalline Fe and Nb powders at room temperature, using a high-energy ball-milling technique. The mechanisms of metallic glass formation and competing crystallization processes in the mechanically deformed composite powders have been investigated by means of X-ray diffraction, Mössbauer spectroscopy, differential thermal analysis, scanning electron microscopy and transmission electron microscopy. The numerous intimate layered composite particles of the diffusion couples that formed during the first and intermediate stages of milling time (0–56 ks), are intermixed to form amorphous phase(s) upon heating to about 625 K by so-called thermally assisted solid state amorphization, TASSA. The amorphization heat of formation for binary system via the TASSA, ΔH_a, was measured directly as a function of the milling time. Comparable with the TASSA, homogeneous amorphous alloys were fabricated directly without heating the composite multilayered particles upon milling these particles for longer milling time (86 ks–144 ks). The amorphization reaction here is attributed to the mechanical driven solid state amorphization. This single amorphous phase transforms into an order phase (μ phase) upon heating at 1088 K (crystallization temperature, T_x) with enthalpy change of crystallization, ΔH_x, of −8.3 kJ mol⁻¹.     

Solid State Interaction in Ni-Mo-B System During Mechanical Alloying and Annealing

This work presents the results of a study of Ni_87?x Mo _x B₁₃ alloys (x?=?7, 10 and 14?at.%), which were obtained by mechanical alloying (MA) of elemental powder mixtures in a MAPF-2M high-energy planetary ball mill. The x-ray diffraction analysis and differential scanning calorimetry measurements were used. The single-phase fcc solid solutions of Mo and B in Ni were formed by MA of Ni-Mo-B mixtures of various compositions for 6-8?h. The coherent domain sizes of solid solutions calculated from the x-ray peak widths were 12-14?nm. The exothermic effects on the DSC curves, which corresponded to the phase transformations of supersaturated Ni(Mo,B) solid solutions, were observed during heating of the synthesized alloys. After heating to 700?°C, the alloys contained a fcc Ni(Mo) phase and a metastable hexagonal MoB₄ phase. Thermodynamically stable phase composition of Ni₈₀Mo₇B₁₃ and Ni₇₇Mo₁₀B₁₃ alloys, containing three phases: fcc Ni (Mo), Ni₂₁Mo₂B₆ with cubic lattice and Ni₃B with orthorhombic lattice, was reached after the isothermal annealing at 1000?°C. The ratio between the amounts of these phases in the alloys corresponds to their location in a three-phase area of the Ni-Mo-B equilibrium phase diagram.     

High hydrogen permeability in the Nb-rich Nb–Ti–Ni alloy

The microstructure and the hydrogen permeability of the Nb-rich Nb–Ti–Ni alloy, i.e., the Nb₅₆Ti₂₃Ni₂₁ alloy were investigated and compared with those of the Nb₄₀Ti₃₀Ni₃₀ alloy. The Nb₅₆Ti₂₃Ni₂₁ alloy consisted of a combination of the primary phase bcc- (Nb, Ti) solid solution with the eutectic phase {bcc- (Nb, Ti) + B2-TiNi}. The volume fraction of the former and the latter phases were 62 and 38 vol.%, respectively. The Nb₅₆Ti₂₃Ni₂₁ alloy showed the higher Φ value of 3.47 × 10⁻⁸ (mol H₂ m⁻¹ s⁻¹ Pa^−0.5) at 673 K, which is 1.8 times higher than that of the Nb₄₀Ti₃₀Ni₃₀ alloy, which has been reported to be highest in the Nb–Ti–Ni system. The present work demonstrated that the Nb-rich Nb–Ti–Ni alloys consisting of only the primary phase bcc- (Nb, Ti) and the eutectic phase {bcc- (Nb, Ti) + B2-TiNi} are promising for the hydrogen permeation membrane.     

Electrode characteristics of two-phase glass-forming Ni–Nb–Y alloys

Two-phase amorphous and two-phase crystalline Ni_58.2Nb_20.25Y_21.25 alloy samples with coexisting Y-rich and Nb-rich phase fractions were produced by melt-spinning and copper mould casting. Their electrochemical reactivity in the as-quenched or mechanically polished state and after HNO₃/HF solution treatment was investigated in nitrogen-purged 0.1 M NaOH solution (pH = 13) by means of potentiodynamic polarisation experiments. In the as-quenched state the two-phase amorphous samples are blocked by a strong passive layer. The main effect of a HNO₃/HF treatment is a selective dissolution of Y-rich phases leading to an excavation of Nb-rich phase regions, i.e. amorphous Ni–Nb–(Y) spheres or Nb₆Ni₇ grains. The enhanced reactivity of etched Ni_58.2Nb_20.25Y_21.25 ribbons is mainly related to the high reactivity of the residual Y-rich matrix phase, while the Nb-rich spheres with smooth and non-defective surface appear quite inactive. In contrast, in the case of the etched crystalline counterpart the Nb₆Ni₇ grains provide a highly defective and thus, active surface, which can contribute to an enhanced electrochemical reactivity. The high cathodic performance of a commercial Ni foam for the hydrogen evolution reaction is reached for etched two-phase amorphous and crystalline Ni_58.2Nb_20.25Y_21.25 alloys.     

Microstructural characterization and amorphous phase formation in Co40Fe22Ta8B30 powders produced by mechanical alloying

In this work, microstructural evolution and amorphous phase formation in Co₄₀Fe₂₂Ta₈B₃₀ alloy produced by mechanical alloying (MA) of the elemental powder mixture under argon gas atmosphere was investigated. Milling time had a profound effect on the phase transformation, microstructure, morphological evolution and thermal behavior of the powders. These effects were studied by the X-ray powder diffraction (XRD) in reflection mode using Cu Kα and in transmission configuration using synchrotron radiation, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The results showed that at the early stage of the milling, microstructure consisted of nanocrystalline bcc-(Fe, Co) phases and unreacted tantalum.Further milling, produced an amorphous phase, which became a dominant phase with a fraction of 96 wt% after 200 h milling. The DSC profile of 200 h milled powders demonstrated a huge and broad exothermic hump due to the structural relaxation, followed by a single exothermic peak, indicating the crystallization of the amorphous phase. Further XRD studies in transmission mode by synchrotron radiation revealed that the crystalline products were (Co, Fe)_20.82Ta_2.18B₆, (Co, Fe)₂₁ Ta₂ B₆, and (Co, Fe)₃B₂. The amorphization mechanisms were discussed in terms of severe grain refinement, atomic size effect, the concept of local topological instability and the heat of mixing of the reactants.     

Magnetic and structural studies of mechanically alloyed (Fe50Co50)62Nb8B30 powder mixtures

Amorphous (Fe₅₀Co₅₀)₆₂Nb₈B₃₀ powder mixture was prepared by mechanical alloying from elemental Fe, Co, B and Nb powders in a planetary ball mill under argon atmosphere. Structural, thermal and magnetic properties were performed on the milled powders by means of X-ray diffraction, differential scanning calorimetry and magnetic measurements. The amorphous state is reached after 125 h of milling. The excess enthalpy due to the high density of defects is released at temperature below 300 °C. Crystallisation and growth of crystal domains are the dominating processes at high temperatures. The saturation magnetisation decreases rapidly during the first 25 h of milling to about 15.24 A m²/kg and remains nearly constant on further milling. Coercivity, H_c, value of about 160 Oe is obtained after 125 h of milling.     

The correlation of microstructural development and thermal stability of mechanically alloyed multicomponent Fe-Co-Ni-Zr-B alloys

The microstructural evolution of multicomponent Fe_70-x-yCo_xNi_yZr₁₀B₂₀ (x = 0, 7, 21; y = 7, 14, 21, 28) alloys during mechanical alloying (MA) has been studied using XRD, SEM and TEM. Mixtures of elemental and pre-alloyed powders have been transformed initially into the single supersaturated bcc α-Fe solid solution phase for the alloys investigated. Subsequently, an amorphous phase has been obtained in Co-free alloys and Co-containing alloys with high Ni/Co ratios of 1 and 3. However, no amorphous phase was detected in another Co-containing alloy with a lower Ni/Co ratio (e.g. 0.33). The thermal stability of the as-milled powders has been investigated by a combination of DSC and the Pendulum magnetometer experiments. The DSC studies provide information on the thermodynamics and kinetics of crystallization of amorphous structure as a function of alloying contents. The Pendulum magnetometer studies reveal the phase transformation from nanocrystalline bcc α-Fe solid solution to amorphous structure during MA and the thermomagnetization behavior of the as-milled powder.     

Nanocrystalline Fe–Nb–(B,Ge) alloys from ball milling: Microstructure,thermal stability and magnetic properties

Fe₇₅B₂₀Nb₅, Fe₇₅Ge₁₀B₁₀Nb₅ and Fe₇₅Ge₂₀Nb₅ alloys were prepared by ball milling from pure powders and their microstructure and magnetic properties were studied. A nanocrystalline solid solution of α-Fe type is the main phase formed, although traces of some intermetallics were found in the Fe–B–Nb alloy. The local arrangements of Fe atoms in Ge containing alloys continuously evolve with milling time. The obtained powders are thermally stable even heating up to 773 K. After heating up to 1073 K, intermetallic compounds are detected. The best soft magnetic properties are achieved after heating up to 773 K, due to stress relaxation of the nanocrystalline microstructure (for Fe–Ge–Nb alloy, coercivity ∼ 600 A/m).     

New data on phase equilibria in the Nb-rich region of the Nb-B system

The currently accepted Nb-B phase diagram shows Nb_ss (solid solution), Nb₃B₂, NbB, Nb₅B₆, Nb₃B₄, NbB₂, B, and liquid L as the stable phases in this system. There is a general agreement in the literature about the stability of the NbB, Nb₃B₄, and NbB₂ phases. However, the stability of Nb₃B₂, Nb₅B₆, and Nb₂B₃ phases is arguable. The aim of this work was to reevaluate the phase equilibria in the Nb-rich region (0-50at.% B) of the Nb-B system. The alloys were arc melted from high purity materials and heat-treated at 1700 °C under high vacuum. The samples were characterized by scanning electron microscopy/back-scattered electron image (SEM/BSE) and x-ray diffraction (XRD). The most important findings were: (1) no liquid formation was observed during heat-treatments of the alloys at 1700 °C; (2) the eutectic reaction in the Nb-rich region is L ↔ Nb_ss+ NbB with liquid eutectic composition close to 16 at.%B; and (3) the Nb₃B₂-phase is formed through the peritectoid reaction Nb_ss+ NbB ↔ Nb₃B₂. These results support the phase diagram proposed by Rudy [1969 Rud] for the Nb-rich region, which is not in agreement with the currently accepted Nb-B phase diagram.     

New data on phase equilibria in the Nb-rich region of the Nb-B system

The currently accepted Nb-B phase diagram shows Nb_ss (solid solution), Nb₃B₂, NbB, Nb₅B₆, Nb₃B₄, NbB₂, B, and liquid L as the stable phases in this system. There is a general agreement in the literature about the stability of the NbB, Nb₃B₄, and NbB₂ phases. However, the stability of Nb₃B₂, Nb₅B₆, and Nb₂B₃ phases is arguable. The aim of this work was to reevaluate the phase equilibria in the Nb-rich region (0-50at.% B) of the Nb-B system. The alloys were arc melted from high purity materials and heat-treated at 1700 °C under high vacuum. The samples were characterized by scanning electron microscopy/back-scattered electron image (SEM/BSE) and x-ray diffraction (XRD). The most important findings were: (1) no liquid formation was observed during heat-treatments of the alloys at 1700 °C; (2) the eutectic reaction in the Nb-rich region is L ↔ Nb_ss+ NbB with liquid eutectic composition close to 16 at.%B; and (3) the Nb₃B₂-phase is formed through the peritectoid reaction Nb_ss+ NbB ↔ Nb₃B₂. These results support the phase diagram proposed by Rudy [1969 Rud] for the Nb-rich region, which is not in agreement with the currently accepted Nb-B phase diagram.     

In-situ XAFS study on structures and devitrifications of Ni–B nano-amorphous alloys

The local structures and the devitrifications of Ni₇₀B₃₀ nano-amorphous alloys prepared by chemical reduction method were studied by in-situ X-ray absorption fine structure (XAFS). The results indicate that the preparation temperature strongly affects the local structures of Ni₇₀B₃₀ amorphous alloys. The coordination geometry surrounding Ni atoms is a structure of amorphous Ni-like for the Ni₇₀B₃₀ (273 K) while the Ni₇₀B₃₀ (313 K) has amorphous Ni₃B-like structure. In-situ XAFS analysis revealed that the Ni₇₀B₃₀ amorphous alloy prepared at the low temperature of 273 K is directly crystallized into the metallic Ni in a wide temperature range from 498 to 598 K. However, the crystallization of the sample prepared at the high temperature of 313 K is accomplished in two steps: the generation of intermediate product of crystalline Ni₃B starting from 548 K and finishing at about 573 K; the unique formation of crystalline Ni at about 598 K.     

Microstructure of hard magnetic bccFe/NdFeB nanocomposite alloys

The microstructure of amorphous-phase remaining bccFe/NdFeB nanocomposite Nd_vFe_balCo₈Nb_xCu_yB_z (V = 6–8, x = 0–2.5, y = 0–0.5 and z = 6–7 at%) magnet alloys, which were prepared by melt spinning, was investigated by means of high resolution transmission electron microscopy (HR-TEM), three-dimensional atom probing (3DAP), and Mössbauer spectroscopy. It was found by HR-TEM that a small amount of amorphous phase remains in the intergranular region between crystal grains of bccFe and Nd₂Fe₁₄B₁ even after heat treatment at 740°C. The results of 3DP showed that Nb and B atoms are significantly enriched in the remaining amorphous phase. Mössbauer spectroscoopy revealed that the intergranular region is composed of not only so called amorphous phase but also several thin layered phases which have a rather strong hyperfine field distinctivity, and that the relative existing ratios of both the total intergranular phases and the Nd₂Fe₁₄B₁ phase increase with increasing Nb content. The coercivity-enhancing effect of Nb addition is discussed in this paper.     

Thermodynamic modeling of the Nb–B system

In the present work, the Nb–B binary system was thermodynamically optimized. The stable phases in this system are BCC (niobium), Nb₃B₂, NbB, Nb₃B₄, Nb₅B₆, NbB₂, B (boron) and liquid L. The borides Nb₃B₂, NbB, Nb₃B₄ and Nb₅B₆ and the B (boron) were modeled as stoichiometric phases and the liquid L, BCC (niobium) and NbB₂ as solutions, using the sublattices model, with their excess terms described by the Redlich–Kister polynomials. The Gibbs energy coefficients were optimized based on the experimental values of enthalpy of formation, low temperature specific heat, liquidus temperatures and temperatures of invariant transformations. The calculated Nb–B diagram reproduces well the experimental values from the literature.     

Thermally induced crystallization of Fe73.5Cu1Nb3Si15.5B7 amorphous alloy

Thermally induced crystallization of Fe_73.5Cu₁Nb₃Si_15.5B₇ amorphous alloy occurs in two well-separated stages: the first, around 475 °C, corresponds to formation of α-Fe(Si)/Fe₃Si and Fe₂B phases from the amorphous matrix, while the second, around 625 °C, corresponds to formation of Fe₁₆Nb₆Si₇ and Fe₂Si phases out of the already formed α-Fe(Si)/Fe₃Si phase. Mössbauer spectroscopy suggests that the initial crystallization occurs through formation of several intermediate phases leading to the formation of stable α-Fe(Si)/Fe₃Si and Fe₂B phases, as well as formation of smaller amounts of Fe₁₆Nb₆Si₇ phase. X-ray diffraction (XRD) and electron microscopy suggest that the presence of Cu and Nb, as well as relatively high Si content in the as-prepared alloy causes inhibition of crystal growth at annealing temperatures below 625 °C, meaning that coalescence of smaller crystalline grains is the principal mechanism of crystal growth at higher annealing temperatures. The second stage of crystallization, at higher temperatures, is characterized by appearance of Fe₂Si phase and a significant increase in phase content of Fe₁₆Nb₆Si₇ phase. Kinetic and thermodynamic parameters for individual steps of crystallization suggest that the steps which occur in the same temperature region share some similarities in mechanism. This is further supported by investigation of dimensionality of crystal growth of individual phases, using both Matusita–Sakka method of analysis of DSC data and texture analysis using XRD data.     

热处理对机械合金化Ni/Ti冷喷涂层金属间化合物形成的影响

选择了三种球磨时间制备的Ni/Ti机械合金化粉末,通过冷喷涂制备了不同结构的Ni/Ti涂层.涂层组织结构采用扫描电镜(SEM)和X射线衍射(XRD)进行了表征分析.试验发现,随着粉末球磨时间的增加,热处理后的冷喷涂合金转变为金属间化合物的温度下降,涂层的组成相由Ni3Ti,B2-NiTi和Ti2Ni逐渐变成Ni3Ti和Ti2Ni;随着热处理温度的增加,涂层组织中不同成分的金属间化合物的相对量会发生一定改变.结果表明,热处理过程中形成的B2-NiTi金属间化合物在冷却时表现出较高的稳定性.     

Effects of Hydrogen on Ni47Ti44Nb9 Shape Memory Alloy

The effects of hydrogen on the transformation characteristics of Ni₄₇Ti₄₄Nb₉ shape memory alloy were investigated. Cathodic hydrogen charging was performed at a current density of 20 mA/cm² in 0.5 mol/L H₂SO₄ solution at room temperature. The transformations of the hydrogenated specimens were characterized by differential scanning calorimetry, x-ray diffraction, and electrical resistivity measurement in details. For the hydrogen-charged NiTiNb alloy, the original reversible transformation between B2 and B19′ phase disappeared. Meanwhile, new transformation around 120 °C was present. This high temperature reversible transformation was confirmed to be the transformation between β-NbH phase and α-Nb(H) solid solution phase.     

Neutron diffraction study in amorphous Ni30Ta70 alloy powders by mechanical alloying

A mixture of elemental Ni and Ta powders with an atomic ratio of 3∶7 was subjected to mechanical alloying (MA). An amorphous Ni₃₀Ta₇₀ alloy was formed after 80 hrs of milling, the amorphization by rapid quenching technique of which has not been reported. The atomic structural changes were observed by neutron diffraction in the amorphization process during MA. The radial distribution function RDF(r) shows that peaks of fcc-Ni and bcc-Ta crystal broaden first and gradually approach those characteristic of an amorphous phase with increasing MA time. A local atomic environment around Ni and Ta atoms was studied by analyzing the first peak in the total pair distribution function g(r) after the completion of amorphization. We reach our conclusion from this analysis that the amorphization in the Ni₃₀Ta₇₀ alloy takes place by the penetration of smaller Ni atoms into the bcc-Ta lattice.     

Structure and thermal behavior of multicomponent Fe68−xNixZr15Nb5B12 (x = 5, 10, 15, 20) alloys

Multicomponent Fe_68−xNi_xZr₁₅Nb₅B₁₂ (x = 5, 10, 15, 20) alloy powders milled for 60 h were prepared by mechanical alloying (MA). The structure and crystallization behavior were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential thermal analysis (DTA). Ni enhances the amorphisation of alloy powders. Particle size increases with increasing Ni content. Both onset crystallization temperature T_x and the first crystallization peak temperature T_p of the four alloys shift to a higher temperature with increasing heating rate while melting temperature (T_m) is just the opposite. Fe_68−xNi_xZr₁₅Nb₅B₁₂ (x = 5, 10, 15, 20) alloys all have a large supercooled liquid region ΔT_x. The supercooled liquid region ΔT_x increases and the crystallization activation energy E decreases with increasing Ni content.