FeSiBPNbCu alloys with high glass-forming ability and good soft magnetic properties

Effects of Cu addition on the glass-forming ability (GFA), thermal stability, magnetic properties and crystallization process of (Fe_0.76Si_0.09B_0.1P_0.05)_99−xNb₁Cu_x (x = 0, 0.25, 0.5, 0.75, 1) alloys were investigated. The introduction of Cu effectively stimulates the precipitation of the α-Fe(Si) without obvious deterioration of the GFA, and successfully modifies the simultaneous precipitation of α-Fe(Si), Fe₂B and Fe₃(B,P) phases in (Fe_0.76Si_0.09B_0.1P_0.05)₉₉Nb₁ alloy into separable precipitation of each phase at different temperatures during annealing, leading to the enhancement of soft magnetic properties. The saturation magnetic flux density of the representative (Fe_0.76Si_0.09B_0.1P_0.05)_98.25Nb₁Cu_0.75 alloy could be enhanced from 1.43 to 1.51 T after annealing at 530 °C for 10 min due to the precipitation of α-Fe(Si) nanoparticles with a diameter of about 22 nm dispersing randomly in the amorphous matrix. The integration of high GFA and excellent soft magnetic properties makes the FeSiBPNbCu alloys promising soft magnetic materials for industrial applications.     

The corrosion behavior of amorphous and nanocrystalline Fe73.5Cu1Nb3Si15.5B7 alloy

The potentiodynamic polarization curves in 0.5 M NaCl solution before and after crystallization of Fe_73.5Cu₁Nb₃Si_15.5B₇ alloy have been studied in relation to the microstructure and alloy composition. It was shown that the corrosion resistance of the alloy strongly depending on these two factors. The observed decrease in corrosion resistance of the alloy after the heat treatment up to 480 °C in comparison to the corrosion resistance of the alloy in the as prepared state is attributed to the increased inhomogeneity of the alloy that coincides with the first appearance of Fe₃Si phase. Further heating (up to 600 °C) resulted in an increase in the number of Fe₃Si nanocrystallites and the appearance of a FeCu₄ phase. After annealing at 600 °C the lowest corrosion rate, 0.004 mm a⁻¹, was observed. Annealing of the samples at higher temperatures (>600 °C) induced formation of six crystalline phases which proved detrimental to the corrosion resistance of the Fe_73.5Cu₁Nb₃Si_15.5B₇ alloy. Solid corrosion products were identified on the surface of the samples after anodic polarization.     

Characterization of silicide phases formed during pack siliconizing coating on the Nb-1Zr-0.1C alloy

The present paper describes the morphology, chemistry and crystallography of the phases observed in the silicide coatings produced by pack cementation technique on Nb based alloys. Cross-sectional microstructures examined by transmission electron microscopy and scanning electron microscopy techniques have shown that the coating has two silicide layers: NbSi₂ and Nb₅Si₃. NbSi₂ formed at the surface of the sample and Nb₅Si₃ formed in between the substrate (Nb alloy) and NbSi₂ coating layer. Electron diffraction analyses revealed that NbSi₂ has hexagonal crystal structure with lattice parameters as a = 0.48 nm and c = 0.66 nm and Nb₅Si₃ has tetragonal crystal structure with lattice parameters as a = 0.65 nm and c = 1.19 nm. Nb₅Si₃ showed fine equiaxed grains, whereas, NbSi₂ exhibited duplex morphology having columnar grain morphology near to the Nb₅Si₃ layer and large equiaxed grains at the surface of the coating sample. The presence of duplex morphology was explained by estimating diffusion of various species and it was shown that columnar morphology of grains could be attributed to outward diffusion of Nb and equiaxed grains to inward diffusion of Si. In the case of Nb₅Si₃, growth takes place due to single element Si diffusion, leading to development of single equiaxed grain morphology of the Nb₅Si₃ phase.     

Influence of thermal treatment on structure development and mechanical properties of amorphous Fe73.5Cu1Nb3Si15.5B7 ribbon

Ribbon-shaped amorphous samples with the stoichiometric composition Fe_73.5Cu₁Nb₃Si_15.5B₇ prepared by the melt spinning process were annealed at temperatures ranging from 693 K to 1123 K for 1 h under vacuum. In the early annealing stage, the alloy undergoes a specific nucleation process where Cu clusters precipitate from an amorphous matrix. Further heating initiates the partial crystallization of alloy forming the α-Fe–Si nanocrystallites. Subsequent Vickers hardness tests showed high values depending on the annealing temperature. It was found that the hardening process includes two stages. This behavior correlates well with results of density dislocation calculations. A crystallite size of 10 nm for the α-Fe–Si particles correlated very well with a maximum hardness of the material.     

Effect of ion irradiation on the nanocrystallization and magnetic properties of soft magnetic Fe7<Subscript>2.5</Subscript>Cu<Subscript>1</Subscript>Nb<Subscript>2</Subscript>Mo<Subscript>1.5</Subscript>Si<Subscript>14</Subscript>B<Subscript>9</Subscript> alloy

The effect of accelerated Ar⁺ ions on the crystallization process and magnetic properties of nanocrystalline Fe_72.5Cu₁Nb₂Mo_1.5Si₁₄B₉ alloy has been studied using X-ray diffraction analysis, transmission electron microscopy, thermomagnetic analysis, and other magnetic methods. Irradiation by Ar⁺ ions with an energy of 30 keV and a fluence of 3.75 × 10¹⁵ cm^–2 at short-term heating to a temperature of 620 K (which is 150 K below the thermal threshold of crystallization) leads to the complete crystallization of amorphous alloy, which is accompanied by the precipitation of the α-Fe(Si) solid solution crystals (close in composition to Fe₈₀Si₂₀), Fe₃Si stable phase, and metastable hexagonal phases. The crystallization caused by irradiation leads to an increase in the grain size and changes the morphology of grain boundaries and volume fraction of crystalline phases, which is accompanied by changes in the magnetic properties.     

Influence of thermal treatment on structure and microhardness of Fe75Ni2Si8B13C2 amorphous alloy

Correlation between hardness of amorphous Fe₇₅Ni₂Si₈B₁₃C₂ alloy and thermally induced structural transformations has been investigated by measuring microhardness in a series of samples heated at different temperatures from 25 to 1000 °C. The alloy has a relatively high hardness in the amorphous state, due to its chemical composition involving silicon, boron and carbon. As the alloy begins to crystallize, microhardness increased and reached a plateau in 500–650 °C temperature region, due to formation composite structure involving the small nanocrystals of α-Fe(Si) and Fe₂B phases dispersed in the amorphous matrix. After treatment at higher temperatures, the nanocomposite structure is replaced by a more granulated structure, leading to decline in microhardness.     

The oxidation behavior of a novel Ni-free Zr–Cu-based bulk metallic glass composite in the supercooled liquid and crystallization states

The oxidation behavior of a novel Ni-free (Zr₄₈Cu₃₂Al₈Ag₈,Ta₄)Si_0.75 bulk metallic glass composite (BMG-C) in dry air in the supercooled liquid state (SLS at 430 °C and 480 °C) and the crystallization state (CS at 520 °C and 560 °C) for 100 h were studied herein.Test results showed that the oxidation kinetics of the BMG-C in the SLS and CS followed a multi-stage oxidation rate law. The scales forming in both the SLS and CS consist of t-ZrO₂, and m-ZrO₂, CuO and Ag. In the CS, additional Al₂O₃ was observed. In the substrate area, Cu₁₀Zr₇ crystalline likely formed in the amorphous substrate in the SLS. In the CS, more crystallization phases were found in the substrate, including additional CuZr₂ observed at temperatures ≥520 °C; AlCu₂Zr was observed at 560 °C.The Ta-reinforced phase in the BMG-C was more likely to react with Si in the scales forming Ta₂Si at temperatures ≥480 °C, which resulted in cracks in the Ta. Furthermore, channels between Ta precipitates and the matrix might exist, facilitating oxygen diffusivity. As the oxidation temperature (e.g., CS) and test duration were increased, the effects of the cracks and the channels became more significant and were responsible for the fast-growth oxidation in the final stage of the test.     

Study of the role of Ta and Cr additions in the microstructure of Nb–Ti–Si–Al in situ composites

Niobium silicide-based in situ composites are Nb-base alloys with high Si content that have the potential for higher temperature capability than the Ni-base superalloys. Microstructure-property studies of these alloys have been the subject of many research programmes, where the differentiation between the αNb₅Si₃ and βNb₅Si₃ is usually not clear, even though it is essential to understanding the solidification of the alloys and the stability of their microstructures at high temperatures. In this work, the effects of Cr (5 or 8 at.%) and Ta (6 at.%) in the microstructures of as-cast and heat-treated Nb–24Ti–18Si–5Al in situ composites have been studied. The main phases observed in the as-cast and heat-treated (100 h at 1400 or 1500 °C) alloys were the niobium solid solution, (Nb,Ti)_ss, the niobium 5–3 silicides, αNb₅Si₃ and βNb₅Si₃, and a Cr-rich C14 silicide Laves phase. During solidification, Al additions promoted the formation of βNb₅Si₃, while the Cr additions caused the appearance of the C14 silicide Laves phase that was probably formed congruently from the remaining liquid. During heat treatment, the βNb₅Si₃ phase transformed to αNb₅Si₃ according to the reaction βNb₅Si₃→αNb₅Si₃+(Nb,Ti)_ss. The Cr addition lowered the melting temperature of the alloys as liquation was observed after 100 h at 1500 °C in the two Cr-rich alloys. Ta and Cr retard the βNb₅Si₃→αNb₅Si₃+(Nb,Ti)_ss transformation. Solid state diffusion was sluggish in the presence of Ta, but the Ta addition did not destabilize the three-phase equilibrium among (Nb,Ti)_ss, αNb₅Si₃ and the C14 silicide Laves phase, in the Nb–24Ti–18Si–6Ta–8Cr–4Al alloy.     

A study of the microstructures and oxidation of Nb–Si–Cr–Al–Mo in situ composites alloyed with Ti,Hf and Sn

The effects of alloying on the microstructures, solidification path, phase stability and oxidation kinetics of Nb_ss/Nb₅Si₃ base in situ composites of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system have been investigated in this study. All the studied alloys are classified as hyper-eutectic Nb silicide base in situ composites and have lower densities compared to nickel-based superalloys. The Nb₃Si silicide formed in the Hf-free alloys and transformed to Nb_ss and αNb₅Si₃ during heat treatment at 1500 °C. This transformation was enhanced by the addition of Ti. The Nb_ss and Nb₅Si₃ were the equilibrium phases in the microstructures of the Hf-free alloys. In the presence of Ti, the βNb₅Si₃ only partially transformed to αNb₅Si₃, suggesting that Ti stabilises the βNb₅Si₃ to lower temperatures (at least to 1300 °C). Furthermore, alloying with Hf stabilised the hexagonal γNb₅Si₃ (Mn₅Si₃-type) silicide in the Hf-containing alloys. The addition of Sn promoted the formation of the Si-rich C14 Laves phase and stabilised it at 1300 °C. This is attributed to the Sn addition decreasing the solubility of Cr in the Nb_ss of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system whilst increasing the Si solubility. The Si solubility in the C14 Laves phase was in the range ∼6.6 to 10.5 at%. The lattice parameter of the Nb_ss in each alloy increased after heat treatment signifying the redistribution of solutes between the Nb_ss and the intermetallic phases. The oxidation resistance of the alloys at 800 °C and 1200 °C increased significantly by alloying with Ti and Sn. Pest oxidation behaviour was exhibited by the Nb–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–2Mo (heat treated) and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (heat treated) alloys at 800 °C. Pesting was eliminated in the alloy Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn at 800 °C, indicating that the addition of Sn plays an important role in controlling the pest oxidation behaviour at intermediate temperatures. The oxidation behaviour of all the alloys at 800 °C and 1200 °C was controlled by the oxidation of the Nb_ss and was sensitive to the area fraction of Nb_ss in the alloy.     

Cu clustering and Si partitioning in the early crystallization stage of an Fe73.5Si13.5B9Nb3Cu1 amorphous alloy

Solute clustering and partitioning behavior in the early crystallization stage of an Fe_73.5Si_13.5B₉Nb₃Cu₁ amorphous alloy have been studied by employing a three-dimensional atom probe (3DAP) and a high resolution electron microscope (HREM). Results from the 3DAP have clearly shown that Cu atom clusters are present in the amorphous state after annealing below the crystallization temperature. The density of these clusters is in the order of 10²⁴/m³, which is comparable to that of the α-Fe grains in the optimum nanocrystalline microstructure. In the early stage of primary crystallization, Cu clusters are in direct contact with the α-Fe nanocrystals, suggesting that the α-Fe primary particles are heterogeneously nucleated at the site of Cu clusters. In the early stage of crystallization, the concentration of Si is lower in the primary crystal than in the amorphous matrix phase, unlike in the late stage of the primary crystallization, where Si partitions into the α-Fe phase with a composition of approximately 20 at.%.     

Nanocrystallization of Pd74Si18Au8 metallic glass

The crystallization process of Pd₇₄Si₁₈Au₈ amorphous alloy has been investigated by transmission electron microscopy, small angle X-ray scattering and three-dimensional atom probe techniques. Although literature suggests that the alloy decomposes into two glassy phases prior to the crystallization, we found that the crystallization occurs directly from a single amorphous phase by the primary crystallization of fcc Pd–Au solid solution, followed by the polymorphous crystallization of the remaining amorphous phase to a Pd₃Si phase.     

Improvement of oxidation resistance of arc-melted Mo76Si14B10 by microstructure control upon minor Fe addition

Addition of Fe refines the microstructure of Mo_76-xSi₁₄B₁₀Fe_x (x = 0, 0.5, 1 at.%) composites containing α-Mo, Mo₃Si and Mo₅SiB₂ phases, increases the hardness from 950 Hv (x = 0) to 1031 Hv (x = 1), and improves the oxidation resistance at temperature in the range of 800–1300 °C. The hardness of the base alloy substrate decreases only by <7% than that of as-solidified ingots, indicating good microstructural stability of the composite for high temperature application.     

Experimental isothermal section at 1200 °C of V–Cr–Si system

In the present work, the isothermal section at 1200 °C of the V–Cr–Si phase diagram was experimentally studied. The samples were prepared by arc melting and characterized using scanning electron microscopy/energy-dispersive spectroscopy and electron probe microanalyzer. The continuous solution phases (Cr,V)₃Si, (Cr,V)₅Si₃ and (Cr,V)Si₂ were confirmed. The solubilities of Cr in V₆Si₅ and of V in CrSi have been measured. A ternary phase (Cr,V)₁₁Si₈ was observed and its homogeneity range, which is at constant Si ratio, was determined at 1200 °C.     

The influence of structural peculiarities of rapidly quenched bands of the Fe76.1Cu1.0Nb3.0Si13.0B6.1 alloy on their electrochemical behavior

X-ray diffraction analysis, auger spectroscopy and electrochemical measurement showed that, by varying the temperature of starting melt, it is possible to change the surface crystallization degree of quenched metallic glass of the Fe_76.1Cu_1.0Nb_3.0Si_13.8B_6.1 composition, while the presence of crystallites of an ordered α-Fe(Si) solid solution and the air-oxide film formed during the quenching of bands allows improving their corrosion-electrochemical characteristics.     

Correlation between microstructure and properties of fine grained Mo–Mo3Si–Mo5SiB2 alloys

Three phase α-Mo–Mo₃Si–Mo₅SiB₂ alloys of various compositions, namely Mo–6Si–5B, Mo–9Si–8B, Mo–10Si–10B and Mo–13Si–12B (at.%) were processed by a powder metallurgical (PM) route. Increasing the Si and B concentration in these Mo–Si–B alloys resulted in increasing volume fractions of the intermetallic phases Mo₃Si (A15) and Mo₅SiB₂ (T2) and the distribution of the three phases present in these alloys was dependent on the volume fractions of the individual phases. Above volume fractions of about fifty percent, bcc Mo solid solution (α-Mo) formed the matrix. Consequently, Mo–6Si–5B and Mo–9Si–8B alloys, which possessed a continuous α-Mo matrix provided increased fracture toughness at ambient temperatures. Additionally, a decreased BDTT of about 950 °C is caused by the homogeneous α-Mo matrix. In contrast, Mo–13Si–12B with 65 vol.% of the intermetallic phases that formed the matrix phase in this material had a BDTT value higher than 1100 °C, while the strength at elevated temperatures up to 1300 °C was significantly increased compared to alloys that have the α-Mo matrix. Alloy compositions with ≥50 vol.% of intermetallic phases (corresponding to alloys containing a minimum of 9 at.% Si and 8 at.% B) were oxidation resistant with minimal mass loss under cyclic conditions for 150 h at 1100 °C due to the formation of a dense borosilicate glass layer that protects the material surface.     

Effect of tin addition on Nb–Si-based in situ composites. Part II: Oxidation behaviour

This paper reports the effects of adding from 2 to 8 at.% tin on the oxidation behaviour of Nb/Nb₅Si₃ composites at 815 °C and at higher temperatures (1100 and 1200 °C). The role of tin in the elimination of pesting and in the oxidation process at high temperatures was established. The consumption of elements with a higher affinity for oxygen than Sn induces the accumulation of tin at the oxide/internal oxidation zone boundary. Low melting point phases (NbSn₂ and/or pure Sn) form at 815 °C, whereas a layer of M₅Si₃ and Nb₅SiSn₂ forms at 1100 and 1200 °C. Once these products are formed, they generate an oxygen diffusion barrier and allow the elimination of pesting. However, for long oxidation processes at 1100 °C, the oxidation rate of Nb/Nb₅Si₃ composites containing tin should be higher than that for tin-free composites. Moreover, some oxidation results have suggested that the presence of A15-(Nb,Ti)₃(Sn,Ti) in the microstructure of composites with at.%Sn > 2 can severely impact the low temperature fracture toughness of these composites.     

Microstrucural characterization of gas atomized Fe73.5Si13.5B9Nb3Cu1 and Fe97Si3 alloys

Powder particles of Fe_73.5Si_13.5B₉Nb₃Cu₁ and Fe₉₇Si₃ soft magnetic alloys have been prepared by gas atomization. The gas atomized powder was microstructurally characterized and the dependence of coercivity with the composition and powder particle size investigated. As-atomized powder particles of both compositions were constituted by a bcc α-Fe (Si) solid solution. The Fe_73.5Si_13.5B₉Nb₃Cu₁ powder particles presented a grain microstructure with dendrite structure, which dendrite arms were enriched in Nb. The coercivity increased as the particle size decreased, with a minimum coercivity, of 5 Oe, measured in the Fe₉₇Si₃ alloy in the range of 50–100 μm powder particle size. The coercive fields were quite higher in the Fe_73.5Si_13.5B₉Nb₃Cu₁ than in the Fe₉₇Si₃ powder, due to the Nb addition, which produced a phase segregation that leads to a noticeable magnetic hardening.     

Structural transformations of amorphous iron-based alloys upon abrasive and thermal treatments

Wear resistance and structural changes have been investigated in amorphous alloys Fe₆₄Co₃₀Si₃B₃ and Fe_73.5Nb₃Cu₁Si_13.5B₉ upon wear using a fixed abrasive. The structural studies have been performed by the methods of metallography, electron microscopy, and Mössbauer spectroscopy. It has been shown that the abrasive resistance of amorphous alloys is 1.6–3.1 times lower than that of high-carbon tool steels, which have a close level of hardness. The low abrasive wear resistance of amorphous alloys is caused by the deformation softening of the alloy surface in the process of wear. The major volume of the deformed surface layer of the alloys preserves the amorphous state. Its structural changes upon wear are characterized by the formation of inhomogeneities (fragments with a size of 10–50 nm) and by a decrease in the width of the strongest “halo” in the selected-area electron-diffractions patterns. In the amorphous matrix of the Fe₆₄Co₃₀Si₃B₃ alloy, a strong magnetic texture is formed and a redistribution of atoms occurs, which leads to an increase in the local shortrange order corresponding to FeB, Fe₂B, Fe₃B and α-Fe phases. In microvolumes of a thin (several μm) surface layer, the formation of a nanocrystalline structure (on the order of several volume %) was revealed. A tempering of the Fe_73.5Cu₁Nb₃S_13.5B₉ alloy at temperatures below 500°C does not affect the hardness and wear resistance of the alloy. At 500°C, there occurs an increase in microhardness and wear resistance of the Fe_73.5Cu₁Nb₃S_13.5B₉ alloy as a result of the formation in it of a nanocrystalline structure with the retention of a certain amount of the amorphous phase. The complete crystallization of the alloy at 540°C increases the brittleness of the alloy, which leads to a sharp reduction in its wear resistance.     

Interdiffusion and reaction between Zr and Al alloys from 425° to 625 °C

Zirconium has recently garnered attention for use as a diffusion barrier between U–Mo nuclear fuels and Al cladding alloys. Interdiffusion and reactions between Zr and Al, Al-2 wt.% Si, Al-5 wt.% Si or AA6061 were investigated using solid-to-solid diffusion couples annealed in the temperature range of 425° to 625 °C. In the binary Al and Zr system, the Al₃Zr and Al₂Zr phases were identified, and the activation energy for the growth of the Al₃Zr phase was determined to be 347 kJ/mol. Negligible diffusional interactions were observed for diffusion couples between Zr vs. Al-2 wt.% Si, Al-5 wt.% Si and AA6061 annealed at or below 475 °C. In diffusion couples with the binary Al–Si alloys at 560 °C, a significant variation in the development of the phase constituents was observed including the thick τ₁ (Al₅SiZr₂) with Si content up to 12 at.%, and thin layers of (Si,Al)₂Zr, (Al,Si)₃Zr, Al₃SiZr₂ and Al₂Zr phases. The use of AA6061 as a terminal alloy resulted in the development of both τ₁ (Al₅SiZr₂) and (Al,Si)₃Zr phases with a very thin layer of (Al,Si)₂Zr. At 560 °C, with increasing Si content in the Al–Si alloy, an increase in the overall rate of diffusional interaction was observed; however, the diffusional interaction of Zr in contact with multicomponent AA6061 with 0.4–0.8 wt.% Si was most rapid.     

Influence of vibrational entropy on structural stability of Nb–Si and Mo–Si systems at elevated temperatures

The heats of formation of stable and metastable phases of the Nb–Si and Mo–Si systems were studied using density functional theory (DFT). The high-temperature behavior of the competing phases was studied by performing additional phonon calculations. Our theoretical results rationalize the major differences observed in the behavior of the Nb–Si and Mo–Si systems: Nb₃Si is only stable at temperatures above 2043 K, whereas Mo₃Si is always stable; Nb₅Si₃ and MoSi₂ undergo phase changes at elevated temperatures, in contrast to Mo₅Si₃ and NbSi₂. These differences are qualitatively explained by including the vibrational entropy to the free energies within the harmonic approximation. In particular, the softer shear moduli of the Nb₅Si₃ and MoSi₂ βphases cause their stabilities over the α phases at elevated temperature.