In situ HVEM studies on the effects of electron-irradiation on the thermal stability of Ni-based amorphous alloys

In situ high voltage electron microscope (HVEM) studies on the thermal stability of splat-cooled Ni-based amorphous alloys were made and three kinds of accelerated crystallization modes were observed during bombardment by focused 1000 keV electrons. In one case the crystalline grains induced by irradiation were coarser near the edge of the irradiated region (IR) than in other parts. This type of crystallization was observed in Ni₇₅B₁₇Si₈ and Ni₇₅B₁₅Si₈C₂ amorphous alloys subjected to a continuous increase in temperature of irradiation. The second case was one where there was no appreciable difference in size distribution of the crystalline grains throughout the IR. This type was observed in a Ni₈₀P₁₀B₁₀ amorphous alloy which again was subjected to a continuous increase in irradiation temperature and also observed in Ni₇₅B₁₇Si₈ amorphous alloy irradiated during isothermal annealing. In these two cases, the crystalline grains induced during irradiation did not cover the whole of the IR before crystallization started in the unirradiated region. In the third case, however, the amorphous phase completely disappeared from the IR before crystallization in the unirradiated region occurred. This type of crystallization was observed in Ni₈₀P₂₀ amorphous alloy whilst the temperature was being increased continuously during irradiation.     

Microstructural evolution of FINEMET type alloys with chromium: An electron microscopy study

Microstructural changes in Fe_73.5–xCr_xCu₁Nb₃Si_13.5B₉ (0x5) alloys with thermal treatment were studied by electron microscopy. In a first stage, around 800 K, an Fe(Si) nanocrystalline phase is formed in the amorphous residual matrix. Crystallization onset is enhanced with the Cr content of the alloy. In a second stage, around 950 K, full crystallization of the samples leads to the formation of a body centred cubic (b.c.c.) boride-type unknown crystal phase with a lattice parameter of a=1.52 nm, and recrystallization of the previous Fe(Si) nanophase also occurs. No qualitative differences were found between dynamic and isothermal crystallization. The size effect for thin samples is limited to a lowering of crystallization temperatures. For isothermal nanocrystallization in the temperature range 775–900 K, the mean grain size of the nanocrystals increases for short annealing times to stabilize at a constant value of about 10–15 nm for long annealing times. The stabilized grain size increases with increasing annealing temperature and slightly decreases with the Cr content of the alloy.     

Effect of Cr-doping on crystallization sequence and magnetic properties of Fe3B/Nd2Fe14B nanocomposite permanent magnets

Crystallization sequence starting from Nd_xFe_{82 − x}B₁₈ and Nd_xFe_{79 − x}Cr₃B₁₈(x = 3.5 – 5.5 at%) amorphous alloys has been investigated using X-ray diffractometer. In the alloys with Nd concentration around 5 at%, the intermediate compound Nd₂Fe₂₃B₃ first crystallizes along with Fe₃B from the amorphous state. Then, the Nd₂Fe₂₃B₃ phase decomposes to form a mixture of Nd₂Fe₁₄B, Fe₃B and αFe for the Cr-bearing systems, whereas it breaks up into another mixture of αFe and NdFe₄B₄ for the Cr-free systems. Atom probe field ion microscopy analysis has revealed that Cr is preferentially partitioned into the Fe₃B phase during the crystallization process, which may stabilize the Fe₃B-containing mixture exhibited in the Cr-doped systems. We can conclude that a large fraction of Nd₂Fe₁₄B produced from the distinct decomposition scheme of the intermediate Nd₂Fe₂₃B₃ phase exhibited in the Cr-doped systems brings about the high coercivity materials.     

Influence of silicon on the oxidation of iron-based amorphous alloys

The room-temperature oxidation behaviour of three different iron-based amorphous alloys, Fe₇₈B₁₃Si₉, Fe₇₉B₁₆Si₅ and Fe₈₁ B_13.5Si_3.5C₂, was studied by Auger electron spectroscopy and X-ray photoelectron spectroscopy. The oxides which formed were characterized and the influence of alloy composition on the oxidation rate was determined. The presence of chromium in the 5 at% Si specimen increased the oxidation rate of the boron in the alloy. Oxidation-induced segregation of boron led to boron enrichment in the oxide layer. No positive influence of silicon content on the oxidation behaviour could be detected.     

The influence of laser annealing on the crystallization processes in amorphous Co-rich alloys

Crystallization of Co-rich amorphous alloys via thermal and laser annealing has been studied using X-ray diffraction and high resolution electron microscopy techniques. It has been found that microstructural properties of the resulting alloys can differ significantly for the two thermal treatments depending on their Co content. Co₇₈Fe₂Mn₂Si₉B₉, Co₇₂Fe₈B₂₀ and Co₅₀Fe₂₈Si₁₀B₁₂ amorphous alloys have been studied and only Co₇₂Fe₈B₂₀ has shown very different crystallization processes after thermal and laser annealing. The results of comparative analysis of the influence of laser and thermal annealing on optical and structural properties of the samples are discussed.     

Role of aluminium addition on structure of Fe substituted Fe73·5 − x Si13·5B9Nb3Cu1Al x alloy ribbons

The investigation has dealt with the structure and magnetic properties of rapidly solidified and annealed Fe_73·5???x Si_13·5B₉Nb₃Cu₁Al _x (x?=?0, 2, 4, 6 at%) ribbons prepared by melt spinning. Complete amorphous structure was obtained in as-spun ribbons of x?=?0 and 2 at% compositions, whereas structure of ribbons containing higher Al was found to be partially crystalline. Detailed thermal analyses of the alloys and the melt spun ribbons revealed that the glass forming ability in the form of ${{ \textit{T}}}_{\mathbf{x}}{/}{{ \textit{T}}}_{\mathbf{l}}$ (ratio between crystallization and liquidus temperature) is the highest for 2 at% Al alloys and decreases with further addition of Al. Annealing of all as spun ribbons resulted in the precipitation of nanocrystalline phase embedded in amorphous matrix in the form of either ${ \textit{DO}}_{{ 3}}$ phase or bcc ${\upalpha}$ -Fe(Si/Al) solid solution depending on the initial composition of the alloy. Only bcc ${\upalpha}$ -Fe(Si/Al) solid solution was formed in 2 at% Al ribbons whereas ordered DO₃ structure was found to be stabilized in other ribbons including 0 at% Al. A detailed study on determination of precision lattice parameter of nanocrystalline phase revealed that the lattice parameter increases with the addition of Al indicating the partitioning behaviour of Al in nanocrystalline phase.     

Nonequilibrium phases in rapidly quenched Co-C-Si and Ni-C-Si alloys

It has been found that the rapid quenching of Co-C-Si and Ni-C-Si alloys results in the formation of an amorphous phase in the range above 10 at% C and 12 to 23 at% Si in the Co-C-Si system, and a nonequilibrium ordered b c c phase with a lattice parameter of 0.2744 nm in the range above 4 at% C and 15 to 21 at% Si in the Ni-C-Si system. Since the interaction between cobalt and carbon is repulsive, the glass formation in the high metalloid concentration range in the Co-C-Si system is thought to be attributed to a strongly attractive interaction between metalloid atoms (carbon and silicon). Crystallization temperature and Vickers hardness of the amorphous alloys are in the range of 671 to 708 K and 833 to 942 diamond pyramid number (DPN) respectively. Furthermore, the amorphous alloys exhibit a soft ferromagnetism and the Curie temperature, saturated magnetization under an applied field of 100 Oe, coercive force and permeability at 1 kHz are 395 to 432 K, 4.33 to 5.50 kG, 0.031 to 0.210 Oe and 31 000, respectively, in the as-quenched state. The effective permeability of (Co_0.94Fe_0.06)_67.5C_12.5Si₂₀ amorphous alloy is higher than that of Co₆₇Fe₄Si₁₉B₁₀ amorphous alloy with zero magnetostrain at frequencies above 200 kHz. Accordingly, the Co-C-Si amorphous alloys newly found in the present work are very attractive as a soft ferromagnetic material with good characteristics in the high frequency range.     

Combustion synthesis of (Mo1 − xCrx)Si2 (x = 0.00–0.30) alloys in SHS mode

Combustion synthesis was adopted to successfully synthesize molybdenum–silicon–chromium (Mo?Si?Cr) alloys by the mode of self-propagating high-temperature synthesis (SHS). The experimental study of combustion synthesis of Mo?Si?Cr alloys was conducted on elemental powder compacts. Powder compacts with nominal compositions including MoSi₂, (Mo_0.95Cr_0.05)Si₂, (Mo_0.90Cr_0.10)Si₂, (Mo_0.85Cr_0.15)Si₂, (Mo_0.80Cr_0.20)Si₂, (Mo_0.75Cr_0.25)Si₂ and (Mo_0.70Cr_0.30)Si₂ were employed in combustion synthesis experiments. The combustion mode, combustion temperature, flame-front propagation velocity and product structure were investigated. The results showed that Mo?Si?Cr alloys were synthesized by an unsteady state combustion mode with a spiral-trajectory reaction front. The peak combustion temperature reduced with the addition of Cr to Mo–Si system. The flame-front propagation velocity decreased with an increase in Cr content of the powder compact. The X-ray diffraction (XRD) results showed that the crystal structure of the combustion product changed from Cll_b-type structure (Mo_0.90Cr_0.10)Si₂ to C40-type structure (Mo_0.85Cr_0.15)Si₂ with increase in Cr content of Mo–Cr–Si alloys. The intensities of diffraction peaks of the C40-type phase gradually increased with increase in Cr content.     

Influence of Ag admixtures on the crystallization of amorphous Fe75Si9B16

This work studies the influence of the Ag admixtures on the crystallization of the amorphous Fe₇₅Si₉B₁₆ alloy, with the aid of electric and magnetic measurements. It is concluded that the solid solubility of the Ag in these alloys is very small, reaching 2 at% at most. Because of microsegregation, the presence of even these minimal admixtures accelerates the crystallization procedure.     

Nanocrystallization of amorphous CoSiBFeNb alloys

The effect of Fe and Nb contents on the crystallization behavior of CoSiBFeNb alloys was studied using Mössbauer spectroscopy, transmission electron microscopy and X-ray diffraction. It was found that the structure formed by primary crystallization in the (Co₇₇Si_13.5B_9.5)₈₈Fe₇Nb₅ alloy is similar to that of the famous Fe_73.5Si_1.35B₉Cu₁Nb₃ nanocrystalline alloy. The data obtained are discussed regarding a random network of associates as a base structural element of amorphous metallic alloys.     

Reversible structural relaxation of amorphous Ni70TM5Si10B15 (TM = V,Cr, Mn,Fe, Co,Ni) alloys

The structural relaxation of amorphous Ni₇TM₅Si₁₀B₁₅ (TM = V, Cr, Mn, Fe, Co, Ni) alloy was investigated by the electrical resistance measurement under isochronal annealing. The reference Ni₇₅Si₁₀B₁₅ alloy showed no reversible changes in electrical resistance. All alloys other than TM = Ni exhibited a reversible change which is peculiar to the chemical short-range ordering. Only in TM = Cr alloy was the reversible part predominantly observed after pre-annealing, and in other alloys the irreversible change was also seen. The degree of reversible change below 623 K was in the order Fe > V > Cr > Co > Mn. This origin is discussed on the basis of Ni-TM correlation, which is considered to play an important role in ordering, and TM-metalloid correlation, which is considered to hinder the formation of an ordered phase.     

Superconducting properties and microstructure of crystallized Hf-Nb-Si and Hf-V-Si amorphous alloys

Amorphous Hf-Nb-Si and Hf-V-Si alloys have been produced by rapidly quenching the melts using a melt-spinning technique. The silicon content in the amorphous alloys was limited to 14 to 20 at% and the niobium or vanadium content was limited to 0 to 45 at% and 0 to 35 at%, respectively. These amorphous alloys did not show any superconducting transition down to liquid helium temperature (4.2 K). However, a transition was detected above 4.2 K after inducing crystallization in these alloys by annealing at appropriate temperatures. The highest superconducting transition temperatures, T _c, attained were 8.9 K for the Hf₄₅Nb₄₀Si₁₅ alloy annealed for 1 h at 1273 K and 6.7 K for the Hf₅₀V₃₅Si₁₅ alloy annealed for 1 h at 1173 K. The upper critical magnetic field, H _c2, at 4.2 K and the critical current density, J _c, at zero applied field and 4.2 K were about 5.1×10⁶A m^–1 (6.4 T) and more than 1×10⁴ A cm^–2 for the Hf₄₅Nb₄₀Si₁₅ alloy and more than 8.0×10⁶ A m^–1 (10 T) and 5×10³ A cm^–2 for the Hf₅₀V₃₅Si₁₅ alloy. Detailed transmission electron microscopic studies of the annealed structure of these amorphous alloys established that, after crystallization, these alloys contain a body-centred cubic -Hf(Nb) solid solution and body-centred tetragonal Nb₃Si phases in the Hf₄₅Nb₄₀Si₁₅ alloy and hexagonal Hf₅Si₃, face-centred cubic HfV₂ and cubic V₃Si phases in the Hf₅₀V₃₅Si₁₅ alloy. Since Nb₃Si and Hf₅Si₃ are not superconducting above 4.2 K, it has been concluded that superconductivity in these crystallized alloys is due to the precipitation of -Hf(Nb) solid solution in the Hf-Nb-Si alloys and to the precipitation of HfV₂ and V₃Si compounds in the Hf-V-Si alloys.     

Microstructure and thermal stability of Al–Cr–X (X=Ni,Mo, Si) powders obtained by centrifugal atomisation

Abstract

The microstructure and the thermal stability of rapid solidified centrifugally atomised AI–3Cr–X (X=1Ni, 3Ni, 0·3Mo, 1Si, or 3Si, at.-%) powders were studied. Three main types of microstructure were observed in the powders: cellular, globular, and rosettelike. Some powders exhibited a mixture of these. In the atomised state the alloys usually had two phases, intermetallic Al₁₃Cr₂ and α-Al solid solution. Thermal stability was studied for a range of temperatures from 20 to 500°C. The phase Al₃Ni appeared in the nickel containing alloys and grew upon heat treatment. The molybdenum containing alloy did not show any noticeable change upon heat treatment. With respect to the silicon containing alloys, the intermetallic Al₁₃Cr₂ transformed into Al₁₃Cr₄Si₄ at high temperatures. On the basis of bibliographic information a nucleation map was calculated relating the prevalence of the intermetallic Al₁₃Cr₂ phase and the α-Al phase to the particle diameter and the chromium concentration of powder obtained by centrifugal atomisation.

MST/3273     

Effect of transition metal (TM) on the crystallization and the magnetic properties of Fe<Subscript>62</Subscript>Co<Subscript>10</Subscript>Si<Subscript>10</Subscript>B<Subscript>13</Subscript>Nb<Subscript>4</Subscript>TM<Subscript>1 </Subscript>melt-spun ribbons

Crystallization and magnetic behavior of melt-spun Fe₆₂Co₁₀Si₁₀B₁₃Nb₄TM₁ amorphous ribbon where TM = Ni, Cr, V, Pd, Pt, Ti, Ta and Zr were examined. The alloy with Pt as transition metal showed the lowest crystallization temperature of 823 K among the studied alloys. Significant increase in crystallization temperature was observed when the atomic radius of the substituted transition metal was varied from that of Pt. High Curie temperature and high saturation magnetization were recognized for the alloys containing Pd, Pt or Ti. The amorphous alloys except the alloys containing Ti or V showed good soft magnetic properties.     

Effect of composition on the crystallization and magnetic properties of Fe–Co–Si–B–Nb amorphous alloys

The crystallization behavior and magnetic properties of Fe₆₂Co₁₀Si_15−x B_18−y Nb_{(x + y)−5} amorphous alloys with x = 0–5 and y = 0–5 were examined. Primary crystallization temperature of Fe₆₂Co₁₀Si_15−x B₁₃Nb _x and Fe₆₂Co₁₀Si₁₀B_18−y Nb _y alloys increased with the addition of Nb. The primary and secondary crystallization temperatures were well separated when the Nb content is above 4 at%. The alloys with 15–18 at% B showed a distinct supercooled region. The Nb addition decreased the Curie temperature as well as room temperature saturation magnetization. The glassy-type Fe₆₂Co₁₀Si₁₀B₁₈ alloy exhibited good soft magnetic properties as well as a supercooled liquid region of 39 K. The finding of the glassy-type Fe-based alloy without Nb element exhibiting high Bs above 1.4 T is promising for future use as a soft magnetic glass material.     

The Influence of Vanadium on Magnetism and Magnetocaloric Properties of Fe 8 0 − x V x B12Si8 (x = 8, 10, and 13.7) Amorphous Alloys

Amorphous soft magnetic Fe_80?x V _x B₁₂Si₈ ribbons (0 ≤ x ≤ 14) have been fabricated by melt spinning technique, and their magnetic and magnetocaloric properties have been studied. The value of magnetocaloric effect has been determined from the measurements of magnetization as a function of temperature and an external magnetic field. The addition of vanadium to the ternary Fe₈₀B₁₂Si₈ alloy results in a decrease of the Curie temperature of amorphous alloys, T _C, from 473.5 to 335 K. With an increasing V content, the average magnetic moment of Fe atom and the magnetic entropy change also decrease. Fe_66.3V_13.7B₁₂Si₈ alloy exhibits the highest refrigeration capacity of 93.7 J kg^?1 and moderate peak magnetic entropy of 1.034 J kg^?1 K^?1 (T _C = 335 K) under the maximum applied field of 2 T. The results from this work showed that V containing amorphous alloy 13.7 at. % is an interesting material and potential candidate for magnetic refrigerants working near room temperature. The observed ?ΔS_M ^max values compare favorably with other amorphous Fe-based alloys.     

Micro-hardness, microstructures and thermal stability of (Ti,Cr,Al,Si)N films deposited by cathodic arc method

(Ti,Cr,Al,Si)N films were deposited by cathodic arc method using TiCrAlSi alloy cathodes. It was found that the microstructures of (Ti,Cr,Al,Si)N were closely related to (Al+Si) content. The crystal structure of (Ti,Cr,Al,Si)N was NaCl-type structure up to the (Al+Si) content of 0.60, where it changed to a hexagonal structure. The maximum hardness of 33 GPa was obtained at the lowest (Al+Si) content of 0.56, still in the cubic structure. The micro-hardness decreased down to 28 GPa as the crystal structure changed from NaCl-type to wurtzite-type.To investigate the thermal stabilities of (Ti,Cr,Al,Si)N, the films were annealed in a vacuum furnace. In Ti_0.20Cr_0.20Al_0.55Si_0.05N with cubic structure, the phase segregation occurred by annealing at over 900 °C, while Ti_0.22Cr_0.22Al_0.44Si_0.12N remained in cubic phase up to 1000 °C. The micro-hardness of Ti_0.20Cr_0.20Al_0.55Si_0.05N increased and that of Ti_0.22Cr_0.22Al_0.44Si_0.12N decreased at 1000 °C. Ti_0.20Cr_0.11Al_0.58Si_0.11N with a cubic and hexagonal mixture phase held its (c,h)-mixture phase up to 1000 °C, while there was an indication of an increase both in micro-hardness and in cubic ratio after annealing.In this paper, the micro-hardness and microstructure of (Ti,Cr,Al,Si)N are discussed as a function of annealing temperature and investigated by X-ray diffraction and electron microscopy.     

Crystallization behaviour of Ni-Co-base metallic glasses by isochronal and isothermal annealing

The crystallization behaviour of three amorphous alloys, Co₅₀Ni₂₅Si₁₅B₁₀, Ni₅₀Co₂₅Si₁₅B₁₀ and Ni₅₀Co₂₅P₁₅B₁₀, was studied by means of differential thermal analysis in conjunction with scanning transmission electron microscopy. Isochronal annealing showed a strong dependence of crystallization on scan rate over the range of 1.99 to 20.70 K min^–1. At high Co/Ni ratios, a sequential two-stage crystallization process involving primary MS-I phase followed by MS-II phase precipitation was observed. At low Co/Ni ratios MS-I and MS-II crystallization were concurrent and inseparable. Replacement of the metalloid Si with P as the glass-former dramatically reduced the activation energy for crystallization as well as the crystallization temperature. A mechanistic understanding of these findings was pursued in light of TEM/STEM microanalysis     

Development of FeSiB/CoSiB Bilayered Melt-spun Ribbon by?Melt-spinning Technique

The investigation addresses the bilayer melt-spun ribbons in which the positively magnetostrictive layer Fe_77.5Si_7.5B₁₅ is toward the free surface, while the negatively magnetostrictive layer of Co_72.5Si_12.5B₁₅ forms the one in contact with the quench wheel. Scanning electron microscopy (SEM) elemental scan showed drastic changes in concentration profiles of Co, Fe and Si across the bilayer, indicating the efficacy of rapid quenching by melt spinning. Crystallization onsets, saturation magnetization and dilatation studies of the single-layered alloys distinctly reflected the properties of the bilayered ribbons. The thermal variation of coercivity below crystallization onset in the case of a bilayer revealed a softening mechanism due to reduction of stresses. Ribbon deflection studies showed interesting results.     

Effects of Cr contents in Fe-based bulk metallic glasses on the glass forming ability and the corrosion resistance

Fe-based bulk metallic glasses of Fe_69.9?xC_7.1Si_3.3B_5.5P_8.7Cr_xMo_2.5Al_2.0Co_1.0 (x = 0.0, 2.3–12.3) with high glass forming ability and good corrosion resistance were fabricated using industrial raw materials. Glass forming ability of the Fe-based bulk metallic glasses tends to decrease with the Cr content, while the corrosion resistance increases with the Cr content. A homogeneous passive layer on the amorphous sample with 12.3 at% Cr can be formed leading to superior corrosion resistance of the amorphous sample to an austenitic stainless steel (SUS304) in the 0.5 M H₂SO₄ and 1N HCl solutions at 298 K. Fe-based bulk metallic glasses with an optimum combination of glass forming ability and corrosion resistance can be produced in large quantities through a systematic control of the Cr content for extensive practical applications.