Fe-Si-B amorphous alloys with high silicon concentration

Amorphous phase formation with good ductility has been found in Fe-Si-B ternary alloys with high silicon concentration using a melt-spinning technique. The formation range of these amorphous alloys is in the range 0 to 29 at % silicon and 5 to 26 at % boron, being much wider than the previously reported range (0 to 19 at % silicon and 10 to 26 at % boron). The crystallization temperature (T _x) and Vickers hardness (H _v) of the Fe-Si-B amorphous alloys containing more than 19 at % silicon increase significantly with increasing boron content, while the increase in silicon content causes a decrease inT _x andH _v. TheT _x andH _v of Fe₆₆Si₂₈B₆ alloy with the highest silicon concentration are 740 K and 500 DPN, respectively. The decreases inT _x andH _v with silicon content are interpreted owing to the increase in the contribution of the repulsive interaction between silicon and silicon against the attractive interactions between iron and silicon or boron. Furthermore, the silicon-rich amorphous phase has been found to crystallize by the almost simultaneous precipitation of the two equilibrium compounds of Fe₃Si and Fe₂B, Fe₂Si_0.4B_0.6 or Fe_4.9Si₂B.     

Metastable phases in vapour-deposited Al-Ru alloys

Vapour deposition of two Al-Ru alloys containing 10 and 15wt% Ru resulted in the formation of supersaturated solid solutions. Electron microscopy and electron diffraction techniques have been used to characterize this as-quenched structure and also those formed on subsequent decomposition. Elevated-temperature (650–800 K) annealing treatments resulted in the formation of a metastable intermediate phase. Electron diffraction evidence suggested that this phase can be assigned a simple cubic structure either of the Mo₃N₂-type witha=0.42 nm or of the Mg₂Zn₁₁-type witha=0.84 nm. Further annealing led to the formation of the equilibrium AI and AI₆ Ru phases. The morphology and structural features of transformation products and the crystal structure details of the metastable intermediate phase have been discussed.     

Crystallization of amorphous Ni60 Nb40−x Cr x alloys

Crystallization behaviour of amorphous Ni₆₀ Nb_40-x Cr _x (x = 0, 5, 10 and 13 at%) alloys was studied by differential Scanning calorimetry and X-ray diffraction measurements. It is shown that the addition of chromium reduces the crystallization temperature, stages of crystallization and activation energies associated with the crystallization sages of the Ni₆₀Nb₄₀ glass. Crystallization of the Ni₆₀Nb₄₀ glass occurred in three stages; in the initial stage a metastable M-phase formed in the amorphous matrix as reported earlier [1] . However, contrary to earlier observation [1], M -phase was not very stable and transformed together with some amorphous phase to the equilibrium Ni₃Nb phase in the second stage of crystallization. In the third stage, the remaining amorphous matrix transformed to the equilibrium NiNb phase. On addition of chromium the formation/stability of the M-phase was found to be suppressed and equilibrium NbCr₂ phase precipitated preferentially in the first stage. The second stage, corresponding to the formation of Ni₃Nb phase, remained almost unaltered. The third stage corresponding to the crystallization of NiNb phase disappeared completely at 13 at% Cr. In the fully crystallized samples the proportion of the NiNb phase decreased and that of NbCr₂ phase increased continuously with chromium concentration.     

Stability of amorphous Fe-W alloys in multilayer metallizations on silicon

Co-sputtered amorphous Fe_0.37 W_0.63 alloys were investigated for applications as diffusion barriers in multilayer metallizations on silicon. Interface reactions and recrystallization during thermal annealing at 400–800°C were studied by back-scattering spectrometry and X-ray diffraction. On SiO₂ substrates the recrystallization of these films occurs at approximately 700°C. On silicon the recrystallization is accompanied by the formation of a silicide layer containing FeSi₂ and WSi₂ phases. No detectable reaction is observed when the alloy film is amorphous. In contact with an overlay metal such as aluminum, copper, nickel or platinum the amorphous Fe_0.37 W_0.63 layer prevents direct interaction between the silicon substrate and an overlay metal film 1000 Å thick during thermal annealing for 30 min at 650°C. The lifetime of the barrier is limited by dissolution and compound formation at the interface and at grain boundaries of the overlay metal.     

Microstructures and crystallization of electroless Ni-P deposits

A study has been made of microstructures and crystallization of the electroless Ni-P deposits containing 11.3 to 23.0 at% P obtained from acidic nickel sulphate baths with sodium hypophosphite as a reducing agent by means of differential scanning calorimetry, X-ray diffractometry and hot stage transmission electron microscopy. The deposits containing low phosphorus content of 11.3 at% could be represented as an fcc Ni-P solid solution of 5 to 10 nm microcrystallites, whereas the deposits containing high phosphorus content were amorphous. The crystallization process of amorphous Ni-P solution involved more than one intermediate phases; precrystallized nickel or off-stoichiometric Ni₃(P, Ni) or Ni₅(P, Ni)₂ phase in which some phosphorus sites are replaced by nickel atoms. The final equilibrium phases were bct Ni₃P and fcc nickel crystals regardless of phosphorus content. The amorphous phase containing 20 to 22 at% phosphorus was the most stable among the amorphous Ni-P alloys.     

A TEM study on the microstructure of rapidly solidified Cu-Y alloys

The microstructure of rapidly solidified copper-yttrium alloys was studied by TEM. For the near-eutectic alloys, the following observations were made: melt-spun Cu-12.5 wt% Y whiskers showed a very fine eutectic of f.c.c. copper and hexagonal Cu₅Y plus isolated areas of a new metastable orthorhombic phase, Cu₉Y; melt-spun Cu-51.1 wt% Y whiskers contained an amorphous matrix plus precipitates of the equilibrium phase orthorhombic Cu₂Y. For far-from-eutectic alloys, glass formation was detected in the melt-spun whiskers of Cu-25.0 wt% Y alloy, while the melt-spun Cu-18.9 wt% Y whiskers showed a fine-grained single phase Cu₅Y, an easily formed metastable phase, and melt-spun Cu-41.6 wt% Y showed an ultrafine single phase Cu₂Y, an equilibrium orthorhombic phase.     

Preparation of nickel-based amorphous alloys with finely dispersed lead and lead-bismuth particles and their superconducting properties

The application of the melt-quenching technique to Ni-Si-B-Pb, Ni-P-B-Pb, Ni-Si-B-Pb-Bi and Ni-P-B-Pb-Bi alloys containing immiscible elements such as lead and bismuth has been tried and it has been found to result in the formation of a new type of material consisting of fine fcc Pb or hcp (Pb-Bi) + bct X(Pb-Bi) particles dispersed uniformly in the nickelbased amorphous matrix. The particle size and interparticle distance were 1 to 3 and 1 to 4 µm, respectively, for the lead phase, and less than 0.2 to 0.5 µm and 0.2 to 1.0 µm for the Pb-Bi phase. The uniform dispersion of such fine particles into the amorphous matrix was achieved in the composition range below about 6at% Pb and 7at% (Pb + Bi). Additionally, these amorphous alloys have been found to exhibit a superconductivity by the proximity effect of f c c Pb or (Pb-Bi) superconducting particles. The transition temperatureT _c was in the range 6.8 to 7.5 K for the Ni-Si(or P)-B-Pb alloys and 8.6 to 8.8 K for the Ni-Si (or P)-B-Pb-Bi alloys. The upper critical fieldH _c2 and the critical current densityJ _c for (Ni_0.8 P_0.1 B_0.1)₉₅ Pb₃ Bi₂ at 4.2 K were, respectively, about 1.6T and of the order of 7 X 10⁷ A m^–2 at zero applied field. Melt quenching of amorphous phase-forming alloys containing an immiscible element has thus been demonstrated, enabling us to produce amorphous composite materials exhibiting unique and useful characteristics which cannot be obtained in homogeneous amorphous alloys.     

Transformation behavior and thermal stability of Al-based systems prepared by ball milling

Transmission electron microscopy, X-ray diffraction, scanning electron microscopy, differential thermal analysis, and differential scanning calorimetry were used to investigate the transformation behavior of (Al_0.88Ni_0.08Co_0.04)_100−x,Zr_x, (wherex = 0 to 5 at. %) alloys during ball milling, and the thermal stability during the reverse process of return to equilibrium. The results have shown that the crystalline to amorphous transformation occurs only in compositions containing Zr. Mechanical grinding is shown to easily amorphize the Al₃Zr compound which enters in equilibrium with the fcc-Al and (Co, Ni)₂Al₉ phases for the compositions studied. The formation of an amorphous phase at the fcc-Al and Co₂Al₉ grain boundaries leads to a wetting transition, and with decreasing grain size the initially nanostructured Al₈₈Ni₈Co₄ alloy was found to progressively transform to an amorphous alloy. The crystallization temperature, the activation energy, and the crystallization enthalpy increase, while the melting temperature of the quaternary alloys decrease with increasing Zr substitution up to 5 at. %.     

On the performance of a class‐4 partial response digital communication system

Abstract

The dark conductivity of phosphorus‐doped amorphous‐silicon alloys (a‐Si:H:F) prepared by the RF plasma decomposition of a gaseous mixture of SiF₄, H₂ and diluted PH₃ is extremely high; it exceeds 10 (O‐cm)^‐1 with only a small amount of PH₃ (～500 ppm) added in the gas phase. These doping characteristics represent a significant improvement over the doping characteristics of a‐Si:H alloys prepared by a glow‐discharge of SiH₄. The improvement was found to be due to the fact that P‐doped a‐Si:H:F contains microcrystallites which are embedded in an amorphous network. The percolation process in these two‐phase systems gives rise to high conductivity. We have used transmission electron microscopy (TEM) and diffraction (TED) to determine the critical surface fraction, ρ_c, of crystallinity at the onset of extended conduction. The measured ρ_c is approximately 0.46. This percolation limit provides a basis for the analysis of the electrical properties of P‐doped a‐Si:H:F.     

Soft magnetic amorphous Fe–Zr–Si(Cu) boron-free alloys

Amorphous Fe₈₀Zr_xSi_20−x−yCu_y boron-free alloys, in which boron was completely replaced by silicon as a glass forming element, have been prepared in the form of ribbons by using the melt quenching technique. X-ray diffraction and Mössbauer spectroscopy measurements revealed that the as-quenched ribbons with the compositions with x = 6–10 at.% and y = 0, 1 at.% are fully or predominantly amorphous. Differential scanning calorimetry (DSC) measurements allowed the estimation of crystallization temperatures of the amorphous alloys. Soft magnetic properties have been studied by the specialized rf-Mössbauer technique. Since the rf-collapse effect observed is very sensitive to the local anisotropy fields it was possible to evaluate the soft magnetic properties of the amorphous alloys studied. The rf-Mössbauer studies were accompanied by conventional measurements of hysteresis loops from which the magnetization and coercive fields were estimated. It was found that amorphous Fe–Zr–Si(Cu) alloys are magnetically very soft, comparable with those of the conventional amorphous B-containing Fe-based alloys.     

The phase transformations and structure of Cu83.34Pt16.66 alloy

The structure and phase transformation of Cu_83.34Pt_16.66 alloy have been investigated by X-ray diffraction, TEM, differential thermal analysis, and measurements of electrical resistance and hardness. It was found that this alloy underwent a spinodal order decomposition. During heat treatment, superstructure with an f c c structure was formed rapidly and then progressively transformed to the stable ordered phase PtCu₅ with an fcr structure.     

Metastable phases in the Ni<Subscript>75</Subscript>Nb<Subscript>12</Subscript>B<Subscript>13</Subscript> alloy prepared under nonequilibrium conditions

The structure of Ni₇₅Nb₁₂B₁₃ alloys prepared by liquid quenching (LQ) and mechanical alloying (MA) has been studied by x-ray diffraction. The alloy prepared by LQ at a cooling rate of ～10⁶ K/s is shown to be fully amorphous, while MA yields an amorphous-crystalline material in which the predominant phase is an fcc Ni〈Nb,B〉 solid solution. The thermal stability of the alloys and their structural transformations on heating have been studied by differential scanning calorimetry. The amorphous phase obtained by LQ is shown to crystallize at 490°C. After heating to 720°C, the alloy consists of two equilibrium phases: Ni₂₁Nb₂B₆ (τ) and Ni₅Nb₃B₂ (z). Heating the MA alloy to 720°C leads to the formation of a stable τ-phase, while the Ni-based fcc solid solution remains supersaturated and, hence, metastable. Increasing the milling time leads to the formation of nanocrystalline τ and Ni₃B phases, in addition to the Ni-based fcc solid solution, which corresponds to the equilibrium phase composition of the Ni₇₅Nb₁₂B₁₃ alloy in the Ni-Nb-B phase diagram. The effect of high-energy milling on the phase composition of the alloy is similar to that of heat treatment.     

Compositional range,thermal stability,hardness and electrical resistivity of amorphous alloys in Al-Si (or Ge)-transition metal systems

An amorphous single phase was found to be formed in wide compositional ranges in rapidly solidified Al-Si-transition metal (M) and Al-Ge-M alloys. The compositional ranges are in the range from 12 to 53 at. % Si or Ge and 8 to 23% M and Al-Si-Co and Al-Ge-Fe alloys have the widest glass-formation ranges. Because the interaction between aluminium and silicon or germanium atoms is thought to be repulsive from the immiscible equilibrium phase diagrams, the glass formation is probably due to an attractive interaction of M-Si (or Ge) and Al-M pairs. Hardness, H _v, and crystallization temperature, T _x, increase with increasing M content and the highest values reach 1120 DPN and 715 K, while the change with silicon or germanium content is much smaller for H _v and is hardly seen for T _x. Additionally, the H _v and T _x have maximum values for Al-Si (or Ge)-M (M=Cr, Mn or Fe), decrease with the decrease and increase in the group number of M element and are the lowest for Al-Si (or Ge)-Ni alloys. The compositional dependence is interpreted under the assumption that T _x and H _v of the aluminium-based amorphous alloys are mainly dominated by the attractive interaction of M-(Si or Ge) and Al-M pairs. Room-temperature resistivity, _RT, increases in the range of 220 to 1940 cm with increasing silicon or germanium and M contents. The change in _RT with the group number of M elements shows a maximum phenomenon for manganese. It has thus been clarified that the characteristics of the Al-Si-M and Al-Ge-M amorphous alloys have the different compositional dependence as compared with those for conventional metalmetalloid amorphous alloys, probably because of the unusual interaction among the constituent elements.     

Alloy oxide equilibria in the Cr-Mn-O system

The phase boundaries of the Cr-Mn-O system have been investigated by alloy-oxide equilibria at 1173 and 1273 K and by isopiestic technique at 1323 K. The oxide phases which coexist in equilibrium with the Cr-Mn alloys are determined by x-ray diffraction studies. The results of the experiments indicate the presence of MnO in equilibrium with Mn-rich alloy whereas MnCr₂O₄ and Cr₂O₃ phases coexist with almost pure Cr. A three-phase equilibrium consisting of MnCr₂O₄ and MnO phases has been detected at the alloy composition X_Mn=0·252 at 1323 K. The composition of the alloy delineates the phase boundaries in the isothermal sections of the system. The results are interpreted by thermodynamic analysis of the Cr-Mn-O system using the data from the isopiestic measurements and those available in the literature.     

Characterisation of two Al–Fe based high temperature alloy powders

Abstract

Aluminium alloys containing additions of iron and cerium are among the alloys being developed as potential replacements for titanium based alloys for moderately high temperature applications. Development of these alloys is possible using rapid solidification technology, which results in a very fine distribution of dispersoids in the aluminium matrix. The microstructures of two rapidly solidified high temperature alloy powders of composition (wt-%) Al–6·7Fe–5·9Ce (alloy A) and Al–6·2Fe–5·9Ce–1·63Si (alloy B) have been characterised using transmission electron microscopy and the results are explained on the basis of some of the major solidification parameters, such as nucleation undercooling and recalescence. It was observed that most of the powder particles in the +10 to ?20 μm size range contained both microcellular and cellular regions, which could be explained in terms of an initial large undercooling followed by recalescence. The decomposition of the powder microstructure after exposing the powders to temperatures of 350, 420, and 500°C for 1 h was investigated using transmission electron microscopy. This work was complemented by phase identification studies using X-ray diffraction. The equilibrium precipitates Al₁₃Fe₄, Al₈Fe₂Si, and Al₃FeSi were detected in the powder microstructure of alloy B, whereas Al₁₃Fe₄ precipitates were detected in alloy A after high temperature exposure (500°C).

MST/1571     

Shape of grain size distributions during crystal grain nucleation in a-Si

The solid phase crystallization of chemical vapor deposited amorphous silicon films onto oxidized silicon wafers, induced by rapid thermal annealing has been extensively investigated. The morphological evolution of the amorphous towards the polycrystalline phase is investigated by transmission electron microscopy and it is interpreted in terms of a physical model containing few free parameters related to the thermodynamical properties of amorphous silicon and to the kinetic mechanisms of crystal grain growth at the early stages of transformation.     

Studies on structural transformation and magnetic properties in Sm2Co17 type alloys

Structural transformations and microstructural characterisation of Sm₂Co₁₇ alloys containing Fe, Cu and Zr at different stages of thermal processing have been investigated by X-ray diffraction, optical, scanning electron and transmission electron microscopes. Solution treated samples consist of a mixture of hexagonal TbCu₇ (1:7 H) and rhombohedral Th₂Zn₁₇ (2:17 R) structure types of 2:17 phase. After isothermal aging, TbCu₇ + Th₂Zn₁₇ structures transform into Th₂Zn₁₇ type structure with precipitation of Cu-rich hexagonal SmCo₅ (1:5 H) and Zr-rich platelet phases. In addition to the main phases, a soft magnetic phase of composition Zr₆(FeCo)₂₃ is formed in alloys containing higher Zr composition. Isothermal aging studies reveal that magnetic properties show a peak value when aged at 1108–1123 K for 10 h. TEM studies show cellular precipitate structure with cell interiors having 2:17 R structure, while the fully coherent cell boundaries have the 1:5 H structure. Zr-rich platelets which run across many cells and cell boundaries were found to have 1:7 H structure.     

TEM observatiion of a Cu-Mg age-hardenable alloy

The precipitation processes in Cu-Be, Cu-Co, Cu-Fe alloys have been thoroughly investigated; however, much less attention has been paid to studying the Cu-Mg system. In this work the decomposition of Cu-3.5 wt% Mg alloy during ageing was investigated by means of transmission electron microscopy (TEM). The microstructure of Cu-3.5 wt% Mg alloy aged at 340° C is characterized by the presence of fine dispersed coherent precipitates. On continued ageing the coherent precipitates disappear and a new transition phase with oblate octahedron morphology grows. At temperatures above 34O° C the equilibrium phase is formed by discontinuous precipitation. Ageing of Cu-3.5 wt% Mg alloy at temperatures above 450° C results in the formation of the equilibrium Cu₂Mg phase.     

Formation and microstructure of various thin film chromium silicide phases

The formation of the various chromium silicide phases, as predicted by the Cr-Si phase diagram for bulk materials, was studied when thin Cr/a-Si (where a-Si is amorphous silicon) bilayers were annealed in situ in a transmission electron microscope using a limited supply of a-Si. These results are compared with the silicide formed when unlimited a-Si or single-crystal silicon was used. The specimens were analysed using electron micrographs and diffraction patterns.The detailed studies of the bilayers were preceded by studies of single layers of chromium and a-Si. The chromium single layers proved to be continuous for films as thin as approximately 2 nm. The as-deposited films with thickness in excess of about 5 nm were crystalline with b.c.c. structure and comprised very small grains. A phase change occured at about 450°C from the b.c.c to a simple cubic lattice structure. The silicon single layers were completely amorphous in their as-deposited state and crystallized around 600°C. Very large grains formed.The self-supporting Cr/a-Si bilayers with a typical total thickness of about 50 nm, where the relative film thickness were adjusted to yield Cr:a-Si atomic ratios of 1:2, 1:1, 5:3 and 3:1, were prepared by sequential electron gun vacuum deposition of chromium and silicon onto photoresist-covered glass slides. In the early stages of phase formation, when both unreacted chromium and silicon were present, the CrSi₂ phase was formed at about 450°C for all of these specimens. This was the end phase for the 1:2 ratio specimen. For all the other specimens the metal- rich Cr₅Si₃ phase was next to grow at about 550°C. For the 5:3 ratio specimen this was the end phase.Upon further heating of specimens with a Cr:a-Si atomic ration of 3:1, a more chromium-rich phase of Cr₃Si was formed at about 650°C. In the 1:1 ratio specimen, however, the next and end phase observed was CrSi, also growing at about 650°C. The end phase was thus determined by the availability of chromium and silicon during the reactions and could be predicted from the phase diagram.When using an unlimited supply of a-Si (or of single-crystal silicon), instead of limiting it to the thickness necessary for the predetermined ratios mentioned above, the only phase that ever formed was CrSi₂.The grain sizes observed in the various final phase specimens were as follows: CrSi₂, 25 nm; Cr₃Si, 40 nm; Cr₅Si₃, 50 nm; CrSi, 100 nm.     

Microstructural evolution in silicon alloyed γ-Ti–Al alloys after thermomechanical processing

Abstract

The microstructures of silicon alloyed γ-Ti–Al alloys containing silicide particles have been studied after thermomechanical treatments to investigate microstructural evolution. Important parameters including temperature, forging strain, and sequence of thermomechanical treatments were systematically studied. Isothermal forging below the eutectoid temperature resulted in inhomogeneous dynamic recrystallisation with fine equiaxed grains in recrystallised areas and residual α₂ + γ lamellae elsewhere. Eutectic silicides play an important role in destruction of the as cast structure by promoting dynamic recrystallisation during deformation and static recrystallisation on subsequent annealing. There is evidence that silicon, in solution, also enhances recrystallisation. The presence of fine silicides produced by precipitation in the solid state restricts the size of grains produced by both dynamic and static recrystallisation. Silicon also alters significantly the phase equilibrium between the α and γ phases.