Formation of the nanostructure in amorphous alloys of the Al-Ni-Y system

The formation of the amorphous-nanocrystalline structure during the crystallization of metallic glasses of the Al-Ni-Y system has been investigated by X-ray diffractometry, differential scanning calorimetry, and transmission electron microscopy. It has been shown that the crystallization onset temperature depends on the chemical composition of the alloy, and the crystallization is accompanied by changes in the structure of the amorphous phase, such as the phase separation and the formation of amorphous regions of various compositions with different short-range orders. The nanocrystals nucleate in the amorphous phase by the heterogeneous mechanism. It has been found that the size of precipitated aluminum nanocrystals depends on the alloy composition. The amorphous-nanocrystalline structure formed during the thermal treatment is stable and remains unchanged during the isothermal annealing at 150°C for 500 h.     

Effect of the concentration of a rare-earth component on the parameters of the nanocrystalline structure in aluminum-based alloys

The effect of the concentration of a rare-earth component on the parameters of the nanocrystalline structure formed during crystallization of an amorphous phase in the Al₈₈Ni₆Y₆ and Al₈₈Ni₁₀Y₂ alloys is studied using X-ray diffraction and transmission electron microscopy. It is shown that, as the yttrium concentration increases, the nanocrystal size increases and the content of the nanocrystalline component of the structure decreases. The precipitation of nanocrystals is accompanied by separation of the amorphous matrix into regions with different radii of the first coordination spheres due to the enrichment or depletion with the rare-earth element. The parameters of the nanocrystalline structure support the assumption of the heterogeneous nucleation of the nanocrystals.     

The fine structure of FCC nanocrystals in Al-and Ni-based alloys

The structural perfection of nanocrystals in alloys of different chemical composition is studied by x-ray diffraction and high-resolution electron microscopy. In all the alloys studied, crystallization of the amorphous phase produces a nanocrystalline structure. The nanocrystal size depends on the chemical composition of the alloy and varies in aluminum-based alloys from 5 nm in Al₈₉Ni₅Y₆ to 12 nm in Al₈₂Ni₁₁Ce₃Si₄. Nanocrystals in nickel-based alloys vary in size from 15 to 25 nm. Al nanocrystals are predominantly defect-free, with microtwins observed only in some nanocrystals. The halfwidth of the diffraction lines is proportional to sec θ, which implies the small grain size provides the major contribution to the broadening. Nanocrystals in nickel alloys contain numerous twins, stacking faults, and dislocations.     

Phase separation and nanocrystallization in Al92Sm8 metallic glass

Crystallization of Al₉₂Sm₈ metallic glass was investigated in situ using combined small-angle and wide-angle X-ray scattering (SAXS/WAXS) techniques during isothermal annealing at temperatures close to crystallization point. A continuously growing interference maximum shifting progressively towards lower angles was found to develop in SAXS regime. Simultaneously taken WAXS spectra reveal formation of fcc-Al nanocrystalline phase. The analysis of the SAXS/WAXS data indicate that amorphous phase separation is responsible for the nanocrystalline microstructure formation. The primary fcc-Al crystals nucleate inside the Al-rich amorphous regions formed during alloy decomposition and their growth is constrained by the region size.     

Surface properties of Fe76Mo8Cu1B15 alloy after annealing

NANOPERM-type alloy Fe₇₆Mo₈Cu₁B₁₅ is investigated in amorphous and in partially crystallized state. Samples were prepared by 1 h isothermal annealing in vacuum at temperatures ranging from 330°C up to 700°C. Bulk and surface microstructural characteristics were studied using transmission and conversion electron Mössbauer effect techniques, respectively. Surface features were checked by the help of atomic force microscopy. Presence of nanocrystalline bcc-Fe phase was detected during the first crystallization stage. The crystallization process starts at 450°C and it is more pronounced in surface regions than in the bulk. With progressing crystallization, hyperfine parameters especially of the amorphous residual phase are altered. Distinctions in surface morphology are revealed between wheel and air side of the ribbon-shaped samples.     

Production, structure, and microhardness of nanocrystalline Ni-Mo-B alloys

Radiography, differential scanning calorimetry, luminescence and high-resolution electron microscopy are used to study the production, nanocrystalline structure, stability, and microhardness of alloys from the Ni-Mo-B system containing from 27 at. % to 31.5 at. % Mo and 10 at. % B. All studies of these alloys indicated that annealing at 600 °C leads to the creation of a granular phase consisting of FCC nanocrystallites with average grain sizes of 15–25 nm, depending on the chemical composition of the alloy. Annealing these nanocrystalline samples isothermally at a temperature of 600 °C has no appreciable effect on the grain size. Structurally, the nanocrystalline phase consists of grains of an FCC solid solution of Mo and B in Ni, dispersed in an amorphous matrix that isolates them from one another. The lattice parameters of the FCC nanocrystallites depend on the alloy composition and the duration of their isothermal anneal. Within this latter time, molybdenum and boron atoms diffuse from the FCC solid-solution lattice into the surrounding amorphous matrix. The stability of the nanocrystalline structure is determined by the thermal stability of the amorphous matrix, whose crystallization temperature increases with the isothermal annealing time due to enrichment by boron and molybdenum. As the structure forms, the alloy becomes harder as the nanocrystalline grains grow in size. This relation between hardness and grain size, which is opposite to the Hall-Petch law, is explained by hardening of the amorphous matrix due to changes in its chemical composition. Fiz. Tverd. Tela (St. Petersburg) 40, 10–16 (January 1998)     

Magnetic and Mössbauer Studies of Fe<Subscript>76</Subscript>Mo<Subscript>8</Subscript>Cu<Subscript>1</Subscript>B<Subscript>15</Subscript> Nanocrystalline Alloy

The influence of different degrees of crystallinity on the magnetic behaviour of heat-treated nanocrystalline Fe₇₆Mo₈Cu₁B₁₅ alloy has been investigated using a combination of Mössbauer spectrometry and magnetic measurements. The evolution of magnetically active regions and their growth with rising contents of nanocrystals are followed by distributions of hyperfine interactions. Combined electric quadrupole and magnetic dipole interactions corresponding to non-magnetic and magnetic regions inside the amorphous phase, respectively, were revealed. A deterioration of the soft-magnetic properties takes place for the samples exhibiting low fraction of crystallinity. The very good soft-magnetic behaviour is regained for the samples where the primary crystallization process is almost finished.     

Nanocrystallization of an amorphous Fe<Subscript>80</Subscript>B<Subscript>20</Subscript> alloy during severe plastic deformation

The structural evolution of an amorphous Fe₈₀B₂₀ alloy subjected to severe plastic deformation at room temperature or at 200°C was studied. Deformation leads to the formation of α-Fe nanocrystals in an amorphous phase. After room-temperature deformation, nanocrystals are localized in shear bands. After deformation at 200°C, the nanocrystal distribution over the alloy is more uniform. Possible causes of the crystallization of the amorphous phase during severe plastic deformation are discussed.     

非晶态Cu56Zr44合金的结构及其等温退火晶化过程的研究

利用X射线衍射技术、差示扫描量热分析技术和透射电子显微镜研究了非晶态Cu₅₆Zr₄₄合金的结构及其等温退火条件下的晶化过程.实验结果表明,非晶态Cu₅₆Zr₄₄合金在室温下的短程结构类似于硬球无规密堆积分布.在703K过冷液相区内等温退火时发现,当退火时间为3min时,晶化产物主要为Cu₈Zr₃相;当退火时间为6min时,Cu₈Zr₃关键词： 非晶态 56Zr₄₄合金')" href="#">Cu₅₆Zr₄₄合金 结构 等温退火     

Size effect on the structure of Al-and Ni-based nanocrystals

The structure of nanocrystals formed upon crystallization of amorphous alloys of the Ni-Mo-B and Al-Ni-RE systems (RE = Y, Yb, Ce) was studied using x-ray diffraction and high-resolution transmission electron microscopy. It is shown that the lattice parameters and the existence of structural defects depend on the alloy composition and heat treatment conditions. At the beginning of crystallization, all nanocrystals are defectless. After the first stage of crystallization is completed, the aluminum nanocrystals remain perfect, whereas the nanocrystals of molybdenum solid solution contain numerous defects. It is revealed that the nanocrystals of the same size in different systems are either defectless (Al₈₂Ni₁₁Ce₃Si₄, Al₈₈Ni₁₀Y₂, etc.) or contain numerous defects (Ni₇₀Mo₃₀)₉₀B₁₀.     

Grain growth process of two-phase nanocrystalline soft magnetic materials

In order to clarify the origin of the high thermal stability of the microstructure in bcc-Fe/amorphous two-phase nanocrystalline soft magnetic materials, we have investigated the changes in the magnetic and microstructural properties upon isothermal annealing at 898 K for an Fe₈₉Zr₇B₃Cu₁ alloy by means of transmission electron microscopy, Mössbauer spectroscopy and DC magnetometry. The mean grain size was found to remain almost unchanged at the early stage of annealing. However, rapid grain coarsening was evident at an annealing time of 7.2 ks where the intergranular amorphous phase begins to crystallize into Fe₂₃Zr₆. The grain growth process with a kinetic exponent of 1.6 is observed for the growth process beyond this annealing time, reflecting the disappearance of the intergranular amorphous phase. Our results confirm that the thermal stability of the bcc-Fe/amorphous two-phase nanocrystalline soft magnetic alloys is governed by the residual amorphous phase.     

Local atomic and magnetic structures of nanocrystalline Fe75Cr10B15 alloy

The crystal, local atomic and magnetic structures of Fe₇₅Cr₁₀B₁₅ alloys annealed at 440?C473°C for 5 min have been studied using X-ray diffraction and ⁵⁷Fe M?ssbauer spectroscopy. At the annealing temperature T _a = 440°C, nanocrystals of the ??-Fe phase (??1%) precipitate in the amorphous matrix of the alloy. The complete crystallization of the amorphous alloy occurs at T _a = 473°C with the formation of ??-Fe nanocrystals 26 ± 2 nm in size and nanocrystals of tetragonal boride t-Fe₃B 47 ± 2 nm in size. It has been found that chromium atoms are located in nanocrystals of the ??-Fe and y-Fe₃B types. The distribution functions of hyperfine fields in the nanocrystalline Fe₇₅Cr₁₀B₁₅ alloy reconstructed from the M?ssbauer spectra (at T _a = 473°C) show that there are three allowed states of iron atoms in the ??-Fe phase and three equally probable crystallographic nonequivalent states of iron in the t-(Fe,Cr)₃B phase. The chromium concentration x in the ??-Fe(Cr) phase is found to be ??10 at %. The substitution of chromium atoms for iron atoms in t-Fe₃B substantially decreases local magnetic moments of the iron atoms.     

Formation and structure of nanocrystals in bulk Zr<Subscript>50</Subscript>Ti<Subscript>16</Subscript>Cu<Subscript>15</Subscript>Ni<Subscript>19</Subscript> metallic glass

The possible formation of a nanocrystalline structure in controlled crystallization of a bulk Zr₅₀Ti₁₆Cu₁₅Ni₁₉ amorphous alloy has been studied using differential scanning calorimetry, transmission and high-resolution electron microscopy, and x-ray diffraction. It was established that crystallization of the alloy at temperatures above the glass formation point occurs in two stages and brings about the formation of a nanocrystalline structure consisting of three phases. Local spectral x-ray analysis identified the composition and structure of the phases formed.     

Crystallization behaviour in a new multicomponent Ti16.6Zr16.6Hf16.6Ni20Cu20Al10 metallic glass developed by the equiatomic substitution technique

A new amorphous Ti_16.6Zr_16.6Hf_16.6Ni₂₀Cu₂₀A1₁₀ alloy has been developed using the novel equiatomic substitution technique. Melt spinning Ti_16.6Zr_16.6Hf_16.6Ni₂₀Cu₂₀A1₁₀ forms an amorphous phase with a large supercooled liquid region, ΔT=70°C. After isothermal annealing within the supercooled liquid region for 3 h at 470°C, the amorphous alloy crystallizes to form a fine-scale distribution of 2–5 nm nanocrystals, and the supercooled liquid region increases to ΔT=108°C. Atomic-scale compositional analysis of this partially crystalline material using a three-dimensional atom probe (3DAP) is unable to detect any compositional difference between the nanocrystals and the remaining amorphous phase. After annealing for 1 hr at 620°C, the amorphous alloy crystallizes to form 20–50nm equiaxed grains of a hexagonal-type C14 Laves phase with lattice parameters a = 5.2Å and c = 9.0 Å. 3DAP analysis shows that this Laves phase has a composition very close to that of the initial amorphous phase, suggesting that the alloy crystallizes via a polymorphic rather than a primary crystallization mechanism, despite the complexity of the alloy composition.     

Structural transformations in the Al<Subscript>85</Subscript>Ni<Subscript>6.1</Subscript>Co<Subscript>2</Subscript>Gd<Subscript>6</Subscript>Si<Subscript>0.9</Subscript> amorphous alloy during multiple rolling

The effect of multiple rolling at room temperature on the structure and crystallization of the Al₈₅Ni_6.1Co₂Gd₆Si_0.9 amorphous alloy has been studied using transmission electron microscopy, differential scanning calorimetry, and X-ray diffraction. The total plastic strain is 33%. It has been shown that the deformation results in the formation of aluminum nanocrystals with the average size that does not exceed 10–15 nm. The nanocrystals are formed in regions of localization of plastic deformation. The deformation decreases the thermal effect of nanocrystallization (∼15%) as compared to the heat release at the first stage of crystallization of the unstrained sample. The morphology, structure, and distribution of precipitates have been investigated. Possible mechanisms of the formation of nanocrystals during the deformation have been discussed.     

Nanocrystal formation in gas-atomized amorphous Al85Ni10La5 alloy

An Al₈₅Ni₁₀La₅ amorphous alloy, produced via gas atomization, was selected to study the mechanisms of nanocrystallization induced by thermal exposure. High resolution transmission electron microscopy results indicated the presence of quenched-in Al nuclei in the amorphous matrix of the atomized powder. However, a eutectic-like reaction, which involved the formation of the Al, Al₁₁La₃, and Al₃Ni phases, was recorded in the first crystallization event (263°C) during differential scanning calorimetry continuous heating. Isothermal annealing experiments conducted below 263°C revealed that the formation of single fcc-Al phase occurred at 235°C. At higher temperatures, growth of the Al crystals occurred with formation of intermetallic phases, leading to a eutectic-like transformation behaviour at 263°C. During the first crystallization stage, nanocrystals were developed in the size range of 5 ～ 30 nm. During the second crystallization event (283°C), a bimodal size distribution of nanocrystals was formed with the smaller size in the range of around 10 ～ 30 nm and the larger size around 100 nm. The influence of pre-existing quenched-in Al nuclei on the microstructural evolution in the amorphous Al₈₅Ni₁₀La₅ alloy is discussed and the effect of the microstructural evolution on the hardening behaviour is described in detail.     

Effect of nanocrystallization on the mechanical and magnetic properties of finemet-type alloy (Fe<Subscript>78.5</Subscript>Si<Subscript>13.5</Subscript>B<Subscript>9</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript>)

The kinetics of primary crystallization and the effect of structural parameters of the precipitating nanocrystalline α-phase Fe-Si on changes in microhardness, coercive force, and saturation magnetization in an amorphous Finemet-type 5BDSR alloy (Fe_78.5Si_13.5B₉Nb₃Cu₁) obtained by melt quenching are studied. It is found that both an increase in bulk density and an increase in the average nanoparticle size contribute to the hardening of the amorphous/nanocrystalline alloy.     

Phase segregation and crystallization in the amorphous alloy Ni70Mo10P20

Structural evolution of the amorphous alloy Ni₇₀Mo₁₀P₂₀ has been studied by x-ray diffraction, and by following transmission and high-resolution electron microscopy annealing both above and below the glass-transition temperature. When annealed above this temperature, the amorphous phase undergoes segregation into regions about 100 nm in size having different chemical composition. Diffraction from such samples produces diffuse rings, and the scattering vector corresponding to the maximum intensity varies from point to point within the interval of 4.88 to 4.78 nm⁻¹. When occurring between the glass-transition and crystallization temperatures, crystallization produces groups of nanocrystals, 20–30 nm in size, which are in direct contact with one another and form a polymorphic mechanism. The crystallization mechanism changes when the annealing temperature is brought below the glass-transition point. At these temperatures the amorphous matrix crystallizes entectically with formation of eutectic colonies. Fiz. Tverd. Tela (St. Petersburg) 40, 1577–1580 (September 1998)     

Progress of laser spectroscopy at the IGISOL

Magnetic interactions and microstructure in the amorphous and nanocrystalline Fe₉₀Zr₇Cu₁B₂ alloy were investigated. These studies were carried out in the temperature range from 140 up to 380 K, using a Mössbauer spectrometer and completely automated set-up for measurements of magnetic properties. It has been found that the Curie temperature and quadrupole splitting decrease after annealing the sample at 573 K for 1 h (invar effect). However, this behaviour is not observed in nanocrystalline samples. In the early stages of crystallization (the volume fraction of the crystalline phase equal to about 0.06) α-Fe grains above the Curie temperature of amorphous matrix may be treated as non-interacting particles. The particle size estimated by Mössbauer spectra and magnetization curve analysis is equal to about 4 nm.