Optimization of the microstructure and corrosion resistance of Finemet type alloys in KOH solutions

The effect of Cr replacement for Fe on the kinetics of Cu clustering and on the electrochemical behavior of Fe_73.5−XCu₁Nb₃Si_13.5B₉Cr_X amorphous alloys has been studied for improving the corrosion resistance of Fe–Si–B–Nb–Cu Finemet type alloys. The corrosion testing of these materials was performed in 0.5 M KOH solutions using AC impedance measurements. These experimental results are discussed from the thermodynamical point of view, and the optimized microstructure to achieve a high corrosion resistance has been found.     

Change in corrosion behavior of amorphous NiP alloys by alloying with chromium,molybdenum or tungsten

An attempt was made to improve the corrosion resistance of amorphous nickel-based alloys by the addition of chromium, molybdenum or tungsten. The addition of these elements was more or less effective in decreasing the corrosion rate in 1 M HCl and 10% FeCl₃·6H₂O solutions at 30°C. In particular the addition of a certain amount of chromium is most effective and leads to spontaneous passivation in the HCl solution with immunity to pitting corrosion. The addition of molybdenum or tungsten lowers the corrosion rate. The amorphous NiP alloys with and without addition of molybdenum or tungsten are not passivated by anodic polarization even in sulfuric acid or phosphoric acid, forming thick porous surface films and suffer pitting corrosion in acidic chloride solutions.     

Potentiodynamic polarization studies on amorphous Zr46.75Ti8.25Cu7.5Ni10Be27.5, Zr65Cu17.5Ni10Al7.5, Zr67Ni33 and Ti60Ni40 in aqueous HNO3 solutions

Potentiodynamic polarization studies were carried out on virgin specimens of Zr-based bulk amorphous alloys Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 and Zr₆₅Cu_17.5Ni₁₀Al_7.5, and conventional-type binary amorphous alloys Zr₆₇Ni₃₃ and Ti₆₀Ni₄₀ in solutions of 0.2 M, 0.5 M and 1.0 M HNO₃ at room temperature. The values of the corrosion current density (I_corr) for the bulk amorphous alloy Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 were found to be comparable with those of Zr₆₅Cu_17.5Ni₁₀Al_7.5 in 0.2 M and 0.5 M HNO₃, but the value of I_corr for the former was almost three times more than that of the latter in 1.0 M HNO₃. In the case of conventional binary amorphous alloys, Ti₆₀Ni₄₀ showed lower value of I_corr as compared to Zr₆₇Ni₃₃ in 0.5 M and 1.0 M HNO₃ and a comparable value of I_corr in 0.2 M HNO₃. In general, the binary Ti₆₀Ni₄₀ displayed the best corrosion resistance among all the alloys in all the cases and the corrosion current density (I_corr) for all the alloys was found to increase with the increasing concentration of nitric acid. It is noticed that the bulk amorphous alloys do not possess superior corrosion resistance as compared to conventional binary amorphous alloys in aqueous HNO₃ solutions. The observed differences in their corrosion behavior are attributed to different alloy constituents and composition of the alloys investigated.     

X-ray diffraction study of the effects of boron on occurrence of deformation faults in Ni3Al alloy

Ni₃Al and Ni₃Al alloys containing varying amounts of boron in L1₂ phase were prepared. All the samples were suitably annealed and then cold-worked by hand filing. X-ray diffraction data are collected on annealed and cold-worked samples. The deformation fault probabilities have been evaluated by two methods, viz. peak shift analysis of powder pattern X-ray reflections of annealed and cold-worked samples, and lattice parameter change between annealed and corresponding cold-worked samples. The values obtained by both the methods are in reasonable agreement. It has been found that the deformation fault probability of Ni₃Al and Ni₃Al containing boron alloys are very high compared to Ag and Cu base alloys. Further the deformation fault probability of Ni₃Al is more sensitive to boron addition. It has been found that at least 1.22 at. % of boron goes into the Ni₃Al lattice.     

Change in corrosion behavior of amorphous FePC alloys by alloying with various metallic elements

Comprehensive work to examine the effect of metallic alloying elements M on the corrosion behavior of amorphous FeM13 P7 C alloys was performed. All alloying elements except manganese are effective in increasing the corrosion resistance of amorphous alloys in various acidic and neutral solutions, regardless of anionic species, pH and oxidizability. The addition of elements more active than iron to amorphous FePC alloy decrease the corrosion rate in an active region during immersion tests, depending upon the passivation capabilities of the alloying elements themselves. When elements nobler than iron are added, the nobler is the alloying element, the higher is the corrosion resistance of amorphous FeM13 P7 C alloys.     

Corrosion-resistant amorphous FeC alloys containing chromium and/or molybdenum

An attempt was made to improve corrosion resistance of amorphous iron-base alloys, which contain only carbon as a metalloid element, by the addition of chromium and/or molybdenum. The addition of both metallic elements is effective in decreasing the corrosion rate. Especially, the addition of certain amounts of chromium leads to spontaneous passivation even in 1 N HCl. The corrosion resistance of amorphous FeCrC alloys is intermediate between those of amorphous FeCrPC and FeCrB alloys. The all-amorphous alloys examined are characterized by immunity to pitting corrosion, even by anodic polarization in 1 N HCl.     

Effect of boron on the crystallization of amorphous Si–(B–)C–N polymer-derived ceramics

Amorphous Si–(B–)C–N polymer-derived ceramics (PDCs) with a boron content ranging from 0 to 8.3 at.%, were synthesized by thermolysis of boron-modified poly(methylvinylsilazane). Correlation of the boron content and the thermal stability of these materials in the course of annealing were investigated using high temperature thermal gravimetric analysis (HT-TGA). Furthermore, the initial crystallization of the as-thermolyzed amorphous ceramics was studied by X-ray diffraction (XRD) measurements. The increase of boron content promotes the crystallization of SiC, and inhibits the crystallization of Si₃N₄. Moreover, the ratio of α-Si₃N₄/β-Si₃N₄ in crystalline ceramic decreases with increasing boron content. Thermodynamic modeling proves the influence of boron content on driving force for crystallization. The available thermodynamic model of amorphous Si–C–N domains, nano-crystalline silicon nitride, and nano-crystalline silicon carbide treated as the separated phases has been used to interpret these results.     

Ductility improvement of Zr-Cu-Ni-Al glassy alloy

We studied the relationship between the phase diagrams and formation of crystalline inclusions in cast samples in order to improve the ductility of Zr-Cu-Ni-Al bulk amorphous alloy. A Zr₅₀Cu₄₀Al₁₀ bulk amorphous alloy with a composition close to the ternary eutectic point has no crystalline inclusions and possesses superior mechanical properties of tensile strength σ_B=2000 MPa, Young’s modulus E=107 GPa and Vickers hardness HV=580. Oxygen embrittlement of Zr-Cu-Ni-Al bulk amorphous alloys can be avoided in the Zr₅₀Cu₃₀Ni₁₀Al₁₀ bulk amorphous alloy, which exhibits superior ductility and resistance to oxidation. Furthermore, we tried to improve ductility by cold rolling by making use of the apparent work-softening phenomenon of Zr-Cu-Ni-Al bulk amorphous alloys. Preliminary studies indicated that cold rolling to produce a fine slip-band structure, which enabled uniform deformation and superior deformability, is an important procedure for improving the ductility of Zr-Cu-Ni-Al bulk amorphous alloys.     

Multinuclear NMR study of phosphosilicate gels derived from POCl3 and Si(OC2H5)4

Solid state ¹H, ²⁹Si and ³¹P MAS NMR have been used to investigate the microstructure of phosphosilicate gels prepared by a modified sol-gel method involving hydrolysis of silicon precursors in a solely aqueous environment at 50 °C. Gels with molar compositions 5, 10, 20 and 30 mol% P₂O₅ in P₂O₅-SiO₂ were studied. After drying to 400 °C the gels have very similar structures formed by a siloxane framework containing silanol groups and trapped molecules of orthophosphoric acid together with a very small amount, of pyrophosphoric acid. Unlike the gel samples previously synthesized by the hydrolysis of the silicon precursor in alcoholic solution at room temperature, the co-polymerization of phosphorus and silicon is much reduced. Although co-polymerization increases with phosphorus content, it still represents less than 50% of the phosphorus in the 30 mol% P₂O₅ gel. Furthermore there is no evidence for six-coordinated silicon in the glassy matrix.     

First-principle molecular dynamics study of the structural and electronic properties of liquid and amorphous Ni-Al alloys

Atomic and electronic structures of liquid and rapidly quenched Ni-Al (NiAl₃, NiAl, and Ni₃Al) alloys were studied by ab initio molecular dynamics simulation. Pair correlation function, structure factor, bond pair, electron population, and density of states were calculated. It is found that amorphous Ni-Al alloys can be prepared by rapid quenching of liquid alloys, with the former bearing similar structures to the latter, although amorphous alloys have a more ordered structure. Bond pair analysis indicates that both the liquid and amorphous Ni-Al alloys consist mainly of 1441, 1431, 1421, and 1422 pairs of tetrahedral local order. The positions of the first peaks of the Al-Ni pair correlation functions are lower than the sum of the metallic radii of Ni and Al, suggesting the occurrence of chemical bonding between Ni and Al in Ni-Al alloys. Electronic structure analysis further revealed that the interaction between the d-electrons of Ni and the p-electrons of Al is responsible for the bonding. The main peak positions of the total DOS of amorphous Ni-Al alloys become more and more positive when the content of Ni in the alloys increases.     

Structure,Crystallization and Electrochemical Corrosion Behaviour of Amorphous Cu66Ti34 Alloy

The effect of thermally induced structural changes on electrochemical corrosion behaviour of amorphous Cu₆₆Ti₃₄ alloy is studied using electrochemical potentiodynamic polarization technique in conjunction with XRD, TEM and DTA. The heat treatment has been carried out at two temperatures: 300 °C — where the process of structural relaxation is only possible and 500 °C — where crystallization of the alloy occurs. As model corrosive media 1N H₂SO₄ and 1N HNO₃ solutions are used. The short range order of the amorphous samples is studied with the total pair correlation function. Some changes of the interatomic distances and a tendency to increasing of the density of the amorphous structure after low temperature heating are established. The crystallization leads to formation of Cu₃Ti₂, β-Cu₃Ti and Cu₂Ti crystalline phases. It has been found that structural relaxation may have a beneficial effect on the susceptibility of the amorphous alloy to passivation, while crystallization lowers considerably its corrosion resistance.     

Boron removal from metallurgical-grade silicon using lithium containing slag

The possibility of extracting boron impurity from metallurgical-grade silicon by lithium containing slag refining for solar cell application was investigated. The distribution coefficients of boron between CaO–SiO₂–Li₂O and CaO–SiO₂–LiF slags were examined. The boron content in the refined silicon was studied under different conditions of mass ratio of slag to silicon and refining time. The results showed that a small amount of Li₂O or LiF had significant effect on the distribution coefficients of boron. The removal of boron impurity using CaO–SiO₂–LiF system was more effective than using CaO–SiO₂–Li₂O system. Values of boron content in the refined silicon did not decrease significantly when the mass ratio exceeded 4, while it had been obviously falling with the increase in refining time. When the mass ratio of CaO–SiO₂–Li₂O slag to metallurgical-grade silicon was 4, the boron content in silicon was successfully reduced from 2.2 × 10^? 5 to 1.3 × 10^? 6 after slag refining for 2 h at 1823 K.     

Thermal embrittlement of Fe-based amorphous ribbons

Thermal embrittlement of amorphous Fe_78?xNi_xSi₁₀B₁₂ and Co₇₈Si₁₀B₁₂ alloys has been studied. The majority of Fe-based amorphous alloys became brittle after annealing to temperatures significantly lower than their crystallization temperatures, whereas the non-Fe-based amorphous alloys remained ductile after annealing up to their crystallization temperatures. The present results emphasize on the key difference between the embrittlement behavior of Fe-based and non-Fe-based amorphous alloys. We point out a possible correlation between different phenomena like Invar effect, compositional inhomogenity, and residual stress playing a critical role in the thermal embrittlement of Fe-based amorphous alloys.     

Influence of outphase Cu50Ti50 amorphous alloy addition on microstructural evolution of mechanically alloyed Zr66.7Ni33.3 amorphous alloy

The influence of outphase Cu₅₀Ti₅₀ amorphous alloy addition on microstructural evolution of Zr_66.7Ni_33.3 amorphous alloy has been investigated using a mechanical alloying method. It has been found that the milling induced microstructural evolution is related to the change of peak positions of the first maximum on X-ray diffraction patterns of the as-obtained amorphous alloys. With increasing milling time, the 3 wt.% Cu₅₀Ti₅₀ addition can give rise to the cyclic amorphization transformation of the as-milled alloy. The mechanical stability of the mixing amorphous phase can be greatly enhanced with increasing Cu₅₀Ti₅₀ addition up to 10 wt.%. Moreover, the addition of outphase Cu₅₀Ti₅₀ amorphous alloy not only increases the onset crystallization temperature of Zr_66.7Ni_33.3 amorphous alloy but also alters its crystallization mode. The effect of outphase amorphous addition on the mechanical stability of the Zr_66.7Ni_33.3 amorphous phase has been discussed based upon the bond order theory.     

On the deposition mechanism of boron in silicon CVD epitaxy

The deposition mechanism of boron doping in CVD silicon epitaxy has been investigated by exposing silicon substrates to B₂H₆ H₂ doping gas mixtures at epitaxy temperatures and examining the effect by dopant profile measuring in an afterwards intrinsically in-situ deposited epitaxial silicon layer. It has been shown that boron is deposited increasing its concentration on the surface linearly with prolonged exposition time and desorbed by purging the surface in pure hydrogen. In the latter case its content decreases linearly proportional to the predeposited concentration. The desorbed boron builds up a secondary doping source which maintains a parasitic boron flow for reincorporation during following layer growth.     

Effect of microalloying of Nb on corrosion resistance and thermal stability of ZrCu-based bulk metallic glasses

Bulk metallic glasses (BMGs) of Zr_46−xNb_xCu_37.6Ag_8.4Al₈ with x = 0, 0.5, 1, 2 and 4 at.% were prepared by copper mould casting. The corrosion resistance of the ZrCu-based BMGs with different Nb contents was carefully examined by weight loss measurements and potentiodynamic polarization tests in 3 mass% NaCl, 1 N HCl and 1 N H₂SO₄ solutions, respectively. Nb addition improves the newly developed BMGs’ corrosion resistance in chloride-containing solutions and the alloys all exhibit excellent corrosion resistance in 1 N H₂SO₄. The corrosion behavior of the alloy containing 0 and 4 at.% Nb in phosphate-buffered solution was examined by electrochemical polarization tests. The influence of Nb addition on glass forming ability (GFA), thermal stability and mechanical property was investigated by X-ray diffraction, differential scanning calorimetry and compression tests, respectively. It is found that the addition of Nb can deteriorate the GFA and thermal stability of the base system, but little effect is observed on the mechanical properties, e.g., yielding strength and plasticity, of the ZrCu-based BMG alloys.     

Estimation of diffusivity governing primary nanocrystallisation and its relation to thermal stability of amorphous phases

Thermal stability and changes of both the average size of Al nanocrystals and their crystallised volume fraction formed in a series of the amorphous Al–(Ni,Co,Fe)–(Gd,Y,Tb) alloys as well as of α-Fe(Si) in the Fe_73.5Si_13.5B₉Cu₁Nb₃ alloy under isothermal conditions have been experimentally studied by a combination of X-ray diffraction (XRD) analysis, differential scanning calorimetry (DSC) and electrical resistance measurements. The experimental data have been fitted with the analytical models describing the diffusion-limited growth of nanocrystals and nanocrystallisation kinetics accounting for impingement of diffusion fields and the values of the effective diffusivity governing this process have been estimated. The obtained values of the diffusivity in Al-based amorphous alloys follow the Arrhenius-type dependencies with correlation between the pre-exponential factors and the activation energies somewhat different from that found for impurity diffusion coefficients in amorphous alloys. It has been established that at the onset crystallisation temperatures varying from 453 to 778 K, the values of the effective diffusivity in the investigated amorphous alloys are in the narrow range of 1.7–4.7 × 10^? 20 m² s^? 1, which indicates a crucial role of the effective diffusivity for the thermal stability of nanocrystals forming amorphous alloys and the possible reason for this is discussed.     

The effects of microalloying on thermal stability,mechanical property and corrosion resistance of Mg-based bulk metallic glasses

(Mg₆₅Cu₂₀Y₁₀Zn₅)₉₈ M₂ (M = Ti, Cr) bulk metallic glasses with diameter of 3 mm were prepared by copper mold casting. The effects of Ti or Cr on thermal stability, mechanical property and corrosion resistance were systematically investigated. It's shown that the Glass forming ability of the Mg₆₅Cu₂₀Y₁₀Zn₅ bulk metallic glasses slightly decreases with the addition of the elements. Whereas, the strength and corrosion resistance significantly increase. The superior corrosion resistance of the amorphous sample containing Ti or Cr is presented for the forming of homogeneous passive layer with highly protective oxides.     

Influence of nickel content on the electrochemical behavior of Finemet type amorphous and nanocrystalline alloys

The aim of this work has been the systematic study of the influence of partial substitution of Fe by Ni in the Ni_xFe_73.5−xSi_13.5B₉Nb₃Cu₁ alloy within the range 0 ⩽ x ⩽ 10% atom (x = 0, 2, 4, 6, 8, 10) on electrochemical behavior, corrosion rate and the structural changes in amorphous and nanocrystalline alloys. The amorphous nature of the alloys was confirmed by X-ray diffraction and the chemical compositions were determined by ICP. The glass transition and kinetic crystallization of amorphous alloys were studied by DSC. The technique of XPS was used for evaluating the chemical states of elements present in native oxide films. The electrochemical behavior of amorphous and nanocrystalline alloys have been investigated in 0.5 M KOH using cyclic voltammetry. The experimental results show that the formation of different nanocrystalline phases is not excessively transformed by the addition of small amount of nickel and the electrochemical behavior is improved as nickel content increased.     

Trap-limited diffusion of hydrogen in precursor derived amorphous Si–B–C–N-ceramics

The tracer diffusion of hydrogen is studied in precursor derived amorphous Si–C–N and Si–B–C–N ceramics using deuterium as a tracer and secondary ion mass spectrometry (SIMS). Since the amorphous ceramics are separated in carbon rich phases (amorphous carbon and amorphous C(BN)_x, respectively) and silicon rich phases (amorphous Si₃N₄ and amorphous Si_3+(1/4)xC_xN_4−x, respectively) we additionally measured the diffusivities of hydrogen in amorphous carbon, in amorphous SiC and in amorphous C–B–N films. The silicon rich phases are identified as diffusion paths for hydrogen in the precursor derived ceramics. Diffusion of hydrogen in these materials is explained with a trap limited diffusion mechanism with a single trap level. We found activation enthalpies of about 2 eV for the precursor derived ceramics, where the activation enthalpy is the sum of a migration enthalpy and a binding enthalpy. The low values for the pre-exponential factors of less than 10⁻⁷ m²/s can be explained with an appropriate expression for the entropy factor.