Development of nanocrystalline Fe–Co alloys using mechanical alloying

Abstract

Nanostructured alloys have considerable potential as soft magnetic materials. In these materials a small magnetic anisotropy is desired, which necessitates the choice of cubic crystalline phases of Fe, Co, Ni, etc. In the present work, Fe–50 at.-%Co alloys were prepared using mechanical alloying (MA) in a planetary ball mill under a controlled environment. The influence of milling parameters on the crystallinity and crystal size in the alloys was studied. The particle size and morphology were also investigated using SEM. In addition, a thermal treatment was employed for partial sintering of some of the MA powders. The crystal size in both MA powders and compacted samples was measured using X-ray diffraction. It was shown that the crystal size could be reduced to less than 15 nm in these alloys. The nanocrystalline material obtained was also evaluated for magnetic behaviour.     

Thermal stability and mechanical properties of nanocrystalline Fe–Ni–Zr alloys prepared by mechanical alloying

The thermal stability of nanostructured Fe_100?x?y Ni _x Zr _y alloys with Zr additions up to 4 at.% was investigated. This expands upon our previous results for Fe–Ni base alloys that were limited to 1 at.% Zr addition. Emphasis was placed on understanding the effects of composition and microstructural evolution on grain growth and mechanical properties after annealing at temperatures near and above the bcc-to-fcc transformation. Results reveal that microstructural stability can be lost due to the bcc-to-fcc transformation (occurring at 700 °C) by the sudden appearance of abnormally grown fcc grains. However, it was determined that grain growth can be suppressed kinetically at higher temperatures for high Zr content alloys due to the precipitation of intermetallic compounds. Eventually, at higher temperatures and regardless of composition, the retention of nanocrystallinity was lost, leaving behind fine micron grains filled with nanoscale intermetallic precipitates. Despite the increase in grain size, the in situ formed precipitates were found to induce an Orowan hardening effect rivaling that predicted by Hall–Petch hardening for the smallest grain sizes. The transition from grain size strengthening to precipitation strengthening is reported for these alloys. The large grain size and high precipitation hardening result in a material that exhibits high strength and significant plastic straining capacity.     

Preparation of NiAl–TiC nanocomposite by mechanical alloying

NiAl–TiC nanocomposite was successfully synthesized via a ball-milled mixture of Ni, Al, Ti, and graphite powders. The structural and morphological evolutions of the powders were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Results show that NiAl–TiC composite was obtained after 6 h of milling. The mean grain sizes of 6 and 10 nm were attained for NiAl and TiC at the end of milling, respectively. An annealing of 3 h milled sample at 600 °C led to the formation of Ni (Al, Ti, C) solid solution. NiAl–TiC nanocomposite that was formed in the 12 h milled sample is stable during an annealing at 600 °C. The mean grain size of NiAl at the 12 h milled powder increased during annealing at 600 °C. Maximum micro hardness value of 870 kg/mm² (8.7 GPa) was acquired from the 12 h milled powder. SEM images and particle size measurement showed that very fine spheroid particles (1 μm) were procured at the end of milling.     

The mechanical alloying mechanism of various Fe2O3–Al–Fe systems

The mixed powders of in situ Al₂O₃ particles and Fe (Al) solid solution were prepared via self-propagating combustion reaction initiated by mechanical alloying (MA), and the MA mechanism of several Fe₂O₃–Al–Fe systems with different Al₂O₃ mass fractions were studied. The adiabatic temperature (T_ad) of each system was calculated to estimate whether the self-propagating combustion reaction could be initiated in theory. The microstructure of the mixed powders was investigated by SEM, EDS and TEM. The phase analysis was evaluated by XRD, and the Fe lattice parameter was calculated from the XRD patterns. The results showed that with the addition of Fe during the MA process, the activation period was prolonged and the sharp increase of temperature occurred, and when the Al₂O₃ mass fraction was decreased to 10.94%, the self-propagating combustion reaction could not occur in theory and practice. When there was no added Fe, the final product was homogeneous Fe (Al) solid solution.     

Nanocrystalline FeNi solid solutions prepared by mechanical alloying

Mechanical alloying of FeNi36 was performed by the ball milling process. XRD studies revealed that the alloy was crystallized in a stable FCC phase. Structural analysis of the samples pointed out a crystallite size to 10 nm. Thermogravimetric measurements allowed the detection of both CO and CO₂. During thermomagnetic analyses, we observed unusual response of our samples.     

Oxidation behaviour of nanocrystalline Fe–Ni–Cr–Al alloy coatings

Abstract

Nanocrystalline Fe–Ni–Cr–Al alloy coatings with ～4 wt-%Al were produced using the unbalanced magnetron sputter deposition technique with a composite 310S stainless steel target embedded with aluminium plugs. The oxidation behaviour of the coatings was studied, during which complete external α-Al₂O₃ scales were formed. During isothermal oxidation tests at 950, 1000, and 1050°C, the oxidation kinetics followed an essentially parabolic rate law, and the oxidation constants were measured to be 2·06 × 10^-3, 4·23 × 10^-3, and 1·14 × 10^-2 mg² cm^-4 h^-1 respectively. During a cyclic oxidation test at 1000°C the α-Al₂O₃ scale showed good scale spallation resistance. The surface hardness of the coatings was measured with a Knoop indentor before and after oxidation. After oxidation, the coating surface hardness was still significantly higher than that of the uncoated specimen, demonstrating the potential this coating has in the improvement of high temperature erosion resistance.     

Study on martensitic transformation of mechanically alloyed nanocrystalline Fe–Ni

Phase transformation of Fe–Ni powders with different nickel content during mechanical alloying was studied, as well as reverse transformation of mechanically alloyed nanocrystalline Fe–Ni upon heating. Results show that nickel content plays an important role in the phase transformation tendency during mechanical alloying. When heated at 300 °C, neither grain size nor phase changes in Fe–30 wt.% Ni milled for 80 h, indicating the nanometer-sized martensite is very stable below 300 °C. When the temperature increases to 350 °C, concurrently with grain growth reverse transformation takes place. The reverse transformation temperature of mechanically alloyed nanocrystalline Fe–Ni is higher than that of bulk alloys.     

Electro-discharge consolidation of nanocrystalline Nb—Al powders produced by mechanical alloying

Nanocrystalline powders of a 77 at% Nb—Al system prepared by mechanical alloying were subjected to external pressures of up to 450 MPa in a ceramic die, producing a green density between 75% and 85% of theoretical. These powders were consolidated into a solid bulk with a relative density of up to 99% by discharging a high-voltage, high-density current pulse (discharge time <500 s and input energy of 0.5–1.0 kJ g-1). The consolidated bulk was still a mixture of two nanocrystalline intermetallic phases of Nb2Al and Nb3Al. The resultant grain sizes ranged from 13–33 nm. A negative Hall–Petch relation between Vickers microhardness and grain size was clearly observed, indicating that grain-size softening occurs.     

Amorphous Ti–Cu–Ni–Al alloys prepared by mechanical alloying

Ti _x (CuNi)_90–x Al₁₀ (x = 50, 55, 60) amorphous powder alloys were synthesized by mechanical alloying technique. The evolution of amorphization during milling and subsequent heat treatment was investigated by scanning electron microscopy, X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. The fully amorphous powders were obtained in the Ti₅₀Cu₂₀Ni₂₀Al₁₀, Ti₅₅Cu_17.5Ni_17.5Al₁₀ and Ti₆₀Cu₁₅Ni₁₅Al₁₀ alloys after milling for 30, 20 and 15 h, respectively. Differential scanning calorimetry revealed that thermal stability increased with the increasing (CuNi) content: Ti₆₀Cu₁₅Ni₁₅Al₁₀, Ti₅₅Cu_17.5Ni_17.5Al₁₀ and Ti₅₀Cu₂₀Ni₂₀Al₁₀. Heating of the three amorphous alloys at 800 K for 10 min results in the formation of the NiTi, NiTi₂ and CuTi₂ intermetallic phases.     

Synthesis of nanocrystalline LaFeO3 powders via glucose sol–gel route

LaFeO₃ were prepared by sol–gel process using glucose as a complexing agent. Thermogravimetric (TG), differential thermal analysis (DTA), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Field emission scanning electron microscope (FSEM) and Transmission electron microscope (TEM) techniques were used to characterize precursor and derived oxide powders. The effect of the mol ratios of glucose to metal ions (glucose/M) on the formation of LaFeO₃ was investigated. XRD analysis showed that single-phase and well-crystallized LaFeO₃ was obtained from precursor with glucose/M = 3:5 and 3:10 at 500 °C. LaFeO₃ nanoparticles with a diameter of about 30 nm were obtained. However, for precursor with glucose/M = 3:20, pure LaFeO₃ was not obtained at temperature below 600 °C.     

The effect of Ti addition on alloying and formation of nanocrystalline structure in Fe–Al system

In this work, the effect of Ti addition on alloying and formation of nanocrystalline structure in Fe–Al system was studied by utilizing mechanical alloying (MA) process. Structural and morphological evolutions of powder particles were studied by X-ray diffractometry, microhardness measurements, and scanning electron microscopy. In both Fe₇₅Al₂₅ and Fe₅₀Al₂₅Ti₂₅ systems MA led to the formation of Fe-based solid solution which transformed to the corresponding intermetallic compounds after longer milling times. The results indicated that the Ti addition in Fe–Al system affects the phase transition during mechanical alloying, the final crystallite size, the mean powder particle size, the hardness value and ordering of DO₃ structure after annealing. The crystallite size of Fe₃Al and (Fe,Ti)₃Al phases after 100 h of milling time were 35 and 12 nm, respectively. The Fe₃Al intermetallic compound exhibited the hardness value of 700 Hv which is significantly smaller than 1050 Hv obtained for (Fe,Ti)₃Al intermetallic compound.     

Production of Fe3Al–TiC nanocomposite by mechanical alloying of FeTi2–Al–C powders

Abstract

The aim of the present work was to produce Fe₃Al/TiC nanocomposite by mechanical alloying of the FeTi₂30Al10C60 (in at-%) powder mixture. The morphology and the phase transformations in the powder during milling were examined as a function of milling time. The phase constituents of the product were evaluated by X-ray diffraction (XRD). The morphological evolution during mechanical alloying was analysed using scanning electron microscopy (SEM). The results obtained show that high energy ball milling, as performed in the present work, leads to the formation of a bcc phase identified as Fe(Al) solid solution and an fcc phase identified as TiC and that both phases are nanocrystalline. Subsequently, the milled powders were sintered at 873 K. The XRD investigations of the powders revealed that after sintering, the material remained nanocrystalline and that there were no phase changes, except for the ordering of Fe(Al), i.e. formation of Fe₃Al intermetallic compound, during the sintering process.     

The microstructure and mechanical properties of Fe–Cu materials fabricated by pressure-less-shaping of nanocrystalline powders

The microstructure of Fe-40%_wtCu nanocrystalline powders, prepared by mechanical alloying, was studied before and after the consolidation process. Pressure-less-shaping (PS) was used to consolidate the powders. The PS technique, similar to metal injection moulding (MIM), does not require external pressure in order to fill up the mould. The key factor of the process of consolidation is the use as binder a hybrid inorganic–organic monomer, formed by the reaction of zirconium propoxide and 2-hydroxy ethyl methacrylate. This type of monomer, mixed with the metallic powders, formed slurry having low viscosity, which was easily poured into mould. The binder stiffened upon polymerization. Some pieces were produced through debinding and sintering, both performed under inert atmosphere in order to avoid metal oxidation. Different microstructure and density were observed depending on the maximum sintering temperatures, ranging from 904 to 1,120 °C. In the sample sintered at 1,120 °C, the crystalline domains of the copper phase were of about 40 nm.     

Microstructure and mechanical properties of Al-base composites by addition of Al–Ni–Co decagonal quasicrystalline particles through a mechanical stirring route

In the present study, the A356Al-base composite materials were fabricated by introducing 2.5, 5, 7.5, 10 mass% of Al–Ni–Co decagonal quasicrystalline particles using the mechanical stirring method. The microstructures, mechanical properties, and Brinell hardness of these composites were investigated in detail by means of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It is found that serious compositional diffusion occurs between the Al₇₂Ni₁₂Co₁₆ quasicrystalline particles and the Al melt. Microstructural analysis of all as-cast composites shows that the structure of the quasicrystal disappears and is replaced by the formation of two crystalline phases, Co-rich θ-phase and Ni-rich γ-phase which all contain Al, Si, Ni, and Co. The particle sizes of the two crystalline phases are much smaller than that of the original decagonal quasicrystalline phase. The composites exhibit improvement of 10.5–24% and 20–25% in yield strength and Brinell hardness, respectively, while the percent elongation decreases obviously. Examination of the fracture surface of the as-cast A356Al-base composites shows that they exhibit typical brittle fracture mode.     

Influence of solution treatment temperature on mechanical properties of a Fe–Ni–Cr alloy

Influence of solution treatment temperature on mechanical properties of a Fe–Ni–Cr alloy was studied in this work. The results indicate that the strength and the ductile properties are optimum after solution treatment at 1000 °C followed by conventional two-step aging, but decrease markedly with the increase of solution temperature. Microstructure analyses show that TiC phase particles in the microstructure partly dissolves into the matrix when the solution treatment temperature is higher than 1100 °C, resulting in the coarsening of austenitic grain. Flake-like M₃B₂ phase precipitates at the grain boundary in the specimens solution-treated at temperatures higher than 1050 °C and its formation induces the mechanical properties to be worse.     

Preparation and electromagnetic characteristics of silica coated Fe–Ni–Mo alloy flakes

The Fe–Ni–Mo alloy flakes were firstly prepared from water atomized powders. Subsequently, by a facile sol–gel technique using tetraethylorthosilicate (Si(OC₂H₅)₄) as a precursor, we successfully synthesized silica coated Fe–Ni–Mo alloy flakes. These products were characterized by Scanning Electron Microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Furthermore, their electromagnetic parameters, complex permittivity and complex permeability within 2–12 GHz were measured. The results show that both and values of the silica coated flakes substantially decrease with the increase of coating times. Meanwhile, the μ′ values of silica coated samples are larger than those of the as-milled sample at the high frequencies. However, the maximum μ′ exhibits a decline while resonance frequency shifts to high values with the increase of coating times. The calculated reflection loss (RL) curve reveals that a proper thickness of coating can redound to improve the absorbing performances of silica coated Fe–Ni–Mo alloy flakes in high frequency range.     

Nitriding of Fe–18Cr–11Mn powders using mechanical alloying method through aerating nitrogen circularly

Mechanical alloying (MA) method was applied to nitride Fe–18Cr–11Mn stainless steel powders through aerating nitrogen circularly. Both the MA process and increasing nitrogen pressure circularly caused the expansion of nitrogen solubility in Fe. The microstructure of powders was affected by the milling time. The phase transformation of α-Fe to γ-Fe occurred during the MA process. The grain size of powder decreased, while the internal lattice strain increased by increasing the milling time and cycles of aerating nitrogen. The as-milled powder could obtain a fully austenitic structure after sintering and subsequent water quenching. The sintered samples had better density and higher microhardness when the powders milled for more time. The formation mechanism of nitriding of stainless steel powders using MA method in nitrogen was presented.     

Solidification microstructures selection of Fe–Cr–Ni and Fe–Ni alloys

The transition of solidified phases in Fe–Cr–Ni and Fe–Ni alloys was investigated from low to high growth rate ranges using a Bridgman type furnace, laser resolidification and casting into a substrate from superheated or undercooled melt. The ferrite–austenite regular eutectic growth, which is difficult to find in typical production conditions of stainless steels, was confirmed under low growth rate conditions. The transition velocity between eutectic and ferrite cell growth had a good agreement predicted by the phase selection criterion. Which of either ferrite or austenite is easier to form in the high growth range was discussed from the point of nucleation and growth. Metastable austenite formation in stable primary ferrite composition was mainly a result of growth competition between ferrite and austenite. For a binary Fe–Ni system, a planar metastable austenite in the steady state, simultaneous growth such as eutectic and banded growth between ferrite and austenite in an initial transient region are confirmed.     

Determination of mechanical properties of consolidated nanocrystalline iron–carbon alloys Fe–0.5%C and Fe–4.5%C by means of echography and acoustic microscopy

Iron and carbon powder were co-milled and consolidated. Investigation methods used echography and acoustic microscopy. Grain size, microstructure and mechanical properties were found to depend on milling, heating and alloy initial composition. Consolidated nanocrystalline iron hardness was found a lot lower than in formulated predictions according to results from milled powders using Petch-Hall law. This could be related to heating which reduces internal stresses and defects at grain boundaries. Observed alloys strengthening could be related to cementite precipitates or to synthesized carbide phases, and observed alloys elastic moduli reduction to residual-free carbon.     

Development of rolling textures in Cu–Ni alloys

Abstract

The development of texture during the cold rolling of Cu–12·5Ni and Cu–27Ni (wt-%) alloys has been studied using X-ray analysis and transmission electron microscopy (TEM). Pole figures and diffractometer intensity measurements from rolling sections confirm that the texture is of the ‘copper’ type, although the preferred orientation develops more slowly and is consequently less sharp than in the pure metal at equivalent strains. The microstructures were consistent with deformation by slip, no evidence of mechanical twinning being found despite the greater hardness of the alloys compared with copper. However, the presence of nickel in solid solution was found to alter the deformation sequence observed by TEM. Beyond 80% reduction (ε=2·0), the cell structure characteristic of deformed copper, both at low and high strains, was almost entirely replaced by an assembly of small, slightly elongated crystallites whose boundaries often lay at ~±35° to the rolling direction. Long microbands, associated with fine scale rippling in the optical microstructure, appeared after only ~90% reduction (ε=2·5), there being a much reduced tendency for such lamellae to group into transition bands than in copper. Compared with the pure metal, the macroscopic deformation of cupronickels thus proceeds more homogeneously, although larger orientation differences, e.g. of ~10;°, as measured by a precision convergent beam technique, existed between adjacent crystallites, adjacent microbands, and across crystallite/microband boundaries. Possible causes of these differences of behaviour in the alloys are discussed and related to the higher hardness and work hardening rates of Cu–Ni alloys.

MST/499