Synthesis and structural characterization of nanocrystalline (Ni,Fe)3Al intermetallic compound prepared by mechanical alloying

Synthesis of (Ni, Fe)₃Al intermetallic compound by mechanical alloying (MA) of Ni, Fe and Al elemental powder mixtures with composition Ni₅₀Fe₂₅Al₂₅ was successfully investigated. The effects of Fe-substitution in Ni₃Al alloy on mechanical alloying process and on the final products were investigated. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometry, scanning electron microscopy and microhardness measurements. At the early stages, mechanical alloying resulted in a Ni (Al, Fe) solid solution with a layered nanocrystalline structure consisting of cold welded Ni, Al and Fe layers. By continued milling, this structure transformed to the disordered (Ni, Fe)₃Al intermetallic compound which increased the degree of L1₂ ordering upon heating. In comparison to Ni–Al system, Ni (Al, Fe) solid solution formed at longer milling times. Meanwhile, the substitution of Fe in Ni₃Al alloy delayed the formation of Ni (Al, Fe) solid solution and (Ni, Fe)₃Al intermetallic compound. The microhardness for (Ni, Fe)₃Al phase produced after 80 h milling was measured to be about 1170HV which is due to formation of nanocrystalline (Ni, Fe)₃Al intermetallic compound.     

Fast amorphization and crystallization in Al-Fe binary system by high-energy ball milling

Large amount of amorphous phase of Al-Fe binary system was obtained by MA of elemental powders using a high-energy ball mill at milling intensity of 150G (G is the gravitational acceleration). XRD, HRTEM and DSC were used to analyze the process of amorphization and crystallization. The time required achieving almost complete amorphous state is only 4.2 ks for Al-25 at.%Fe system and 3 ks for Al-30 at.%Fe system, respectively. The time of amorphous formation is very shorter than that of previous reports on Al-Fe binary system. Further milling causes rapid crystallization of the amorphous phase. By analysis of S(Q), the presence of a strong Al-Fe chemical short-range order in the amorphous matrix is suggested. Moreover, the superstructure of these Al-Fe clusters in the amorphous matrix is similar to the solid structure of Al₅Fe₂, and the clusters transform into the nucleus of Al₅Fe₂ intermetallic compound under the action of milling energy.     

Synthesis of nanocrystalline alloys and intermetallics by mechanical alloying

Nanocrystalline Al₃Ni, NiAl and Ni₃Al phases in Ni-Al system and theα, β, γ, ɛ and deformation induced martensite in Cu-Zn system have been synthesized by mechanical alloying (MA) of elemental blends in a planetary mill. Al₃Ni and NiAl were always ordered, while Ni₃Al was disordered in the milled condition. MA results in large extension of the NiAl and Ni₃Al phase fields, particularly towards Al-rich compositions. Al₃Ni, a line compound under equilibrium conditions, could be synthesized at nonstoichiometric compositions as well by MA. The phases obtained after prolonged milling (30 h) appear to be insensitive to the starting material for any given composition > 25 at.% Ni. The crystallite size was finest (∼ 6 nm) when NiAl and Ni₃Al phases coexisted after prolonged milling. In contrast, in all Cu-Zn blends containing 15 to 85 at.% Zn, the Zn-rich phases were first to form, and the final crystallite sizes were coarser (15–80 nm). Two different modes of alloying have been identified. In case of NiAl and Al₃Ni, where the ball milled product is ordered, as well as, the heat of formation (ΔH _f) is large (> 120 kJ/mol), a rapid discontinuous mode of alloying accompanied with an additive increase in crystallite size is detected. In all other cases, irrespective of the magnitude of ΔH _f, a gradual diffusive mode of intermixing during milling seems to be the underlying mechanism of alloying.     

Formation of a Al50Fe50 solid solution by mechanical alloying

This work investigates the alloying reaction undergone by Al₅₀Fe₅₀ powder mixtures submitted to mechanical processing by ball milling. The transformation kinetics was studied by quantitative X-ray diffraction. Experimental evidences indicate that Al gradually dissolves in Fe, finally forming a crystalline solid solution. A phenomenological model was developed to describe the observed kinetics with reference to the number of collisions and to the fraction of powder effectively processed at individual collisions. It is shown that only about 5 μg of powders are involved in the Al dissolution processes at collision. It is also shown that a Al₃₀Fe₇₀ solid solution already forms at the first impact via local dissolution processes.     

Effect of annealing on microstructure and mechanical properties of bulk nanocrystalline Fe3Al alloy with 5 wt.% Cu prepared by aluminothermic reaction

Microstructure and mechanical properties of bulk nanocrystalline Fe₃Al based alloy with 5 wt.% Cu prepared by aluminothermic reaction before and after annealed at 873, 1073 and 1273 K for 8 h were investigated. Microstructures of the alloy before and after the annealing consisted of a Fe-Al-Cu matrix, a little Al₂O₃ sphere and Fe₃AlC_x fiber phases. The matrix of the alloy before the annealing was composed a nanocrystalline phase with disordered bcc crystal structure and a little amorphous phase. The amorphous phase disappeared after the annealing and Fe₃Al phase with ordered DO₃ structure appeared in the alloy after annealed at 1073 and 1273 K in the matrix of the alloy. Size of the Fe₃AlC_x fiber phase increased with the annealing temperature. The alloy after the annealing had better plasticity, higher yield strength than that of the alloy before the annealing, and the alloy after annealed at 1273 K had the highest yield strength.     

Solid state amorphization transformation in the mechanically alloyed Fe27.9Nb2.2B69.9 powders

Mössbauer spectrometry and Rietveld analysis of X-ray diffraction patterns were used to follow the solid state amorphization transformation during the milling process of the Fe_27.9Nb_2.2B_69.9 powders. The reaction between elemental Fe, Nb and B powders leads to the formation of the Nb(B) and Fe(B) solid solutions after 1 and 10 h of milling, respectively. A mixture of α-Fe, Nb(B) and highly disordered Fe(Nb, B) solid solution is found after 25 h of milling. An amorphous structure is obtained on further milling time (100 h). From the Mössbauer spectrometry results, it is observed that the total mixing of the elemental powders, at the atomic level, is achieved after 50 h of milling and a stationary state corresponding to a full paramagnetic amorphous phase is reached after 100 h of milling. The amorphization process can be described by an Avrami parameter close to n = 1.     

Phase transformations of Ni-15 wt.% B powders during mechanical alloying and annealing

The formation of Ni-B binary intermetallic compounds was investigated by mechanical alloying (MA) of the Ni-15 wt.% B (≈ Ni-48 at.% B) powder mixture and subsequent heat treatment. It was found that an interstitial Ni(B) solid solution was formed at the early stage of milling, followed by the formation of Ni₃B intermetallic compound after 25 h of milling. On further milling, the Ni₃B transformed to Ni₂B and o-Ni₄B₃ (orthorhombic). Phase transformation during heating of Ni(B) solid solution phase up to 800 °C could be represented by Ni(B) → Ni₃B → Ni₂B. Other intermetallics can be formed by heat treatment of Ni(B) solid solution at temperatures above 800 °C.     

Effects of milling intensity on amorphisation of Al–Fe mixed powders by mechanical alloying

Abstract

The effects of milling intensity on the amorphisation of Al–Fe powder mixtures containing 10, 15, 20, and 25 at.-% Fe, during mechanical alloying by a high energy planetary ball mill, have been investigated. Two levels of milling intensity, 80g and 150g (where g is acceleration due to gravity), were adopted. Two different pathways of amorphisation reaction were found; Al–Fe solid solution → amorphous phase; and Al–Fe solid solution → formation of Al₅Fe₂ phase → amorphous phase. The former reaction was observed in Al–10, –15, –20, and –25 at.-%Fe compositions at the lower milling intensity and in Al–10 at.-%Fe at the higher milling intensity. The latter reaction was observed in Al–15, –20, and –25 at.-%Fe compositions at the higher milling intensity. The two pathways of amorphisation reaction are explained mainly by a kinetics approach.     

Microstructure and martensitic transformation in Si-coated TiNi powders prepared by ball milling

Si was coated on the surface of Ti–49Ni (at%) alloy powders by ball milling in order to improve the electrochemical properties of the Si electrodes of secondary Li ion batteries and then the microstructure and martensitic transformation behavior were investigated by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Ti–Ni powders coated with Si were fabricated successfully by ball milling. As-milled powders consisted of highly deformed Ti–Ni powders with the B2 phase and amorphous Si layers. The thickness of the Si layer coated on the surface of the Ti–Ni powders increased from 3–5 μm to 10–15 μm by extending the milling time from 3 h to 48 h. However, severe contamination from the grinding media, ZrO₂ occurred when the ball milling time was as long as 48 h. By heating as-milled powders to various temperatures in the range of 673–873 K, the highly deformed Ti–Ni powders were recovered and Ti₄Ni₄Si₇ was formed. Two-stage B2–R–B19′ transformation occurred when as-milled Si-coated Ti–49Ni alloy powders were heated to temperatures below 873 K, above this temperature one-stage B2–B19′ transformation occurred.     

Phase equilibria of the Co-Ni-Zr system at 1198 K

The isothermal section of the phase diagram of the Co-Ni-Zr ternary system at 1198 K has been investigated by means of diffusion triple and electron microprobe analysis. This isothermal section consists of six three-phase regions. The homogeneity ranges of Co₂₃Zr₆, Co₂Zr, CoZr, Ni₅Zr, Ni₇Zr₂ and NiZr are about 16.7%-18.8 at.%Zr, 28.6%-32.3 at.%Zr, 49.2%-51.7 at.%Zr, 16.0%-18.7 at.%Zr, 22.3%-24.1 at.%Zr and 49.8%-52.1 at.%Zr, respectively; CoZr₂ and NiZr₂ are nearly linear compounds. The largest solubilities of Ni in Co₂₃Zr₆, Co₂Zr and CoZr are about 4.9 at.%Ni, 5.4 at.%Ni and 4.5 at.%Ni, respectively. CoZr₂ and NiZr₂ form a continuous solid solution (Co,Ni)Zr₂. No ternary compound is observed.     

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.     

Characterization of Al/Ni multilayers and their application in diffusion bonding of TiAl to TiC cermet

The Al/Ni multilayers were characterized and diffusion bonding of TiAl intermetallics to TiC cermets was carried out using the multilayers. The microstructure of Al/Ni multilayers and TiAl/TiC cermet joint was investigated. The layered structures consisting of a Ni₃(AlTi) layer, a Ni₂AlTi layer, a (Ni,Al,Ti) layer and a Ni diffusion layer were observed from the interlayer to the TiAl substrate. Only one AlNi₃ layer formed at the multilayer/TiC cermet interface. The reaction behaviour of Al/Ni multilayers was characterized by means of differential scanning calorimeter (DSC) and X-ray diffraction. The initial exothermic peak of the DSC curve was formed due to the formation of Al₃Ni and Al₃Ni₂ phases. The reaction sequence of the Al/Ni multilayers was Al₃Ni → Al₃Ni₂ → AlNi → AlNi₃ and the final products were AlNi and AlNi₃ phases. The shear strength of the joint was tested and the experimental results suggested that the application of Al/Ni multilayers improved the joining quality.     

Composition,structure and crystallite size of raney catalysts proceeding from several NiAl and FeAl Intermetallic phases

The intermetallic phases NiAl₃, Ni₂Al₃, (Ni_1?xFe_x)₂Al₃, FeAl₃ and Fe₂Al₅, are obtained as single phase, then they are used to prepare Raney catalysts. Composition, structure and crystallite size of Ni, Fe and (NiFe) catalysts are determined by chemical analysis and X-Ray diffraction. For Raney Ni, the composition results show the residual Al content to be higher in the catalysts proceeding from the intermetallics with the lower Al content. For the Raney (Ni,Fe) catalyst, the Fe/Ni ratio is of the same order than in the (Ni_1?xFe_x)₂Al₃ master alloy. From the structure results, a second phase, likely NiAl, is found beside fcc Ni in the catalysts with high residual Al content. The crystallite size of the various catalysts is in the range 3–15 nm; the surface area obeys a linear relationship versus the reciprocal of crystallite size.     

Interface microstructure of diffusion bonded Ni3Al intermetallic alloy and austenitic stainless steel

The diffusion bonding of a Ni₃Al intermetallic alloy to an austenitic stainless steel has been carried out at temperatures 950, 1000 and 1050 °C. The influence of bonding temperature on the microstructural development and hardness across the joint region has been determined. The microvoids in the interface have been found to decrease with increasing bonding temperature. The intermetallic phase Al₃Ni has been detected at the Ni₃Al side of the diffusion couple. Diffusion of Cr and Fe from the stainless steel to the Ni₃Al alloy has been observed.     

Metastable phases formation induced by mechanical alloying

Elemental aluminium, titanium and iron powders with compositions of Al₉₀Ti₁₀, Al₅₅Ti₄₅, Al₆₅Ti₂₅Fe₁₀, respectively, were mechanically alloyed in a planetary ball mill. The sequence of phase formation was characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). Various metastable phases were experimentally observed: supersaturated solid solution Al(Ti) for Al₉₀Ti₁₀, amorphous phase and L1₂-Al₃Ti compound for Al₅₅Ti₄₅, amorphous phase and supersaturated solid solution Al(Ti,Fe) for Al₆₅Ti₂₅Fe₁₀, and an fcc crystalline phase was inevitably found in those alloys. The formation of the fcc crystalline phase has been critically assessed. The results suggest that the fcc crystalline phase seems to be metastable and it cannot be solely attributed to the contamination from the milling atmosphere underthe present experimental conditions.     

Formation and microstructure of quasicrystals in suction cast Al‐6 wt.% Mn alloys with additions of nickel and iron elements

The formation and microstructure of quasicrystals in suction cast Al‐6 wt.% Mn‐2 wt.% TM (TM = Ni, Fe) alloys were investigated by transmission electron microscopy, scanning electron microscopy, energy dispersive spectrometry, and X‐ray diffraction. The suction cast Al‐6 wt.% Mn‐2 wt.% Ni alloy consists of a single decagonal phase of Al₅₆Mn₁₁Ni₂, whereas the Al‐6 wt.% Mn alloy with 2 wt.% iron addition comprises a primitive icosahedral phase and a decagonal phase of Al₄₀Mn₇Fe₂. Thus, the addition of nickel or iron favors quasicrystal formation in the suction cast Al‐6 wt.% Mn alloys. Based on a 4 : 1 matching ratio of aluminum atoms to heavier atoms, the approximate electron to atom ratio is 1.85 in two decagonal phases of Al₅₆Mn₁₁Ni₂ and Al₄₀Mn₇Fe₂. Various morphologies of quasicrystals with a size of more than 5 μm were observed in the microstructure of suction cast Al‐6 wt.% Mn‐2 wt.% TM (TM = Ni, Fe) alloys. The decagonal Al₄₀Mn₇Fe₂ phase nucleates epitaxially and grows on the icosahedral phase.     

Tribological properties of Al6061–Al2O3 nanocomposite prepared by milling and hot pressing

Tribological properties of bulk Al6061–Al₂O₃ nanocomposite prepared by mechanical milling and hot pressing were investigated. Al6061 chips were milled for 30 h to achieve a homogenous nanostructured powder. A 3 vol.% Al₂O₃ nanoparticles (∼30 nm) were added to the Al6061 after 15 and 30 h from the beginning of milling. The milling times with Al₂O₃ in these two samples were then 15 h and 30 min, respectively. Additionally, 3 vol.% Al₂O₃ (1 μm and 60 μm) was added to the Al6061 after 15 h of milling; where, the micron size Al₂O₃ in these two samples, was milled 15 h with the matrix. Hot pressing of milled samples was executed at 400 °C under 128 MPa pressure in a uniaxial die. The hot pressed samples were characterized by micro-hardness test, bulk density measurements, pin on disc wear test, and finally scanning electron microscopy observations. Fifteen hour-milled nanocomposite with nanoscale Al₂O₃, showed improvement in wear resistance and bulk density compared with that of 30 min-milled nanocomposites due to better dispersion of Al₂O₃ nanoparticles, improved surface quality of nanocomposite particles before pressing and more grain refinement of Al matrix. Moreover, increasing the reinforcement size increased the wear rate because of reduction in relative density, hardness and inter-particle spacing.     

Study on the electrical conductivity and oxygen diffusion of oxide-ion conductors La2Mo2−xTxO9−δ (T = Al, Fe, Mn, Nb, V)

In this paper, the influence of cationic substitutions at Mo site with Al³⁺, Fe³⁺, Mn⁴⁺, Nb⁵⁺ and V⁵⁺ ions on the structure, oxygen ion diffusion and electrical properties in La₂Mo₂O₉ oxide-ion conductors have been investigated by X-ray diffraction method, dielectric relaxation technique and direct current conductivity measurement. Except for V⁵⁺ substitution all of these substitutions up to 5% cannot suppress the phase transition in La₂Mo₂O₉. In the dielectric measurement, one prominent relaxation peak is observed in temperature spectrum as well as in frequency spectrum, which is associated with the short-distance diffusion of oxygen vacancies. The activation energy for oxygen ion diffusion is deduced as in the range of 1-1.1 eV for Al, Fe, Mn and Nb doped samples and 1.4-1.5 eV for V doped samples. All substituted samples have a higher conductivity than the un-doped compound. In the Al, Fe, Mn and Nb substituted materials the phase transition is not suppressed; however, K substitution at the La site can completely suppress the transition and maintains high conductivity at low temperature.     

Structure study of Bi4Ti3O12 produced via mechanochemically assisted synthesis

Nanosized bismuth titanate was prepared via high-energy ball milling process through mechanically assisted synthesis directly from their oxide mixture of Bi₂O₃ and TiO₂. Only Bi₄Ti₃O₁₂ phase was formed after 3 h of milling time. The excess of 3 wt% Bi₂O₃ added in the initial mixture before milling does not improve significantly the formation of Bi₄Ti₃O₁₂ phase comparing to stoichiometric mixture. The formed phase was amorphized independently of the milling time. The Rietveld analysis was adopted to determine the crystal structure symmetry, amount of amorphous phase, crystallite size and microstrains. With increasing the milling time from 3 to 12 h, the particle size of formed Bi₄Ti₃O₁₂ did not reduced significantly. That was confirmed by SEM and TEM analysis. The particle size was less than 20 nm and show strong tendency to agglomeration. The electron diffraction pattern indicates that Bi₄Ti₃O₁₂ crystalline powder is embedded in an amorphous phase of bismuth titanate. Phase composition and atom ratio in BIT ceramics were determined by X-ray diffraction and EDS analysis.     

The magnetoelectric perovskite Sr3Fe2TeO9: An insight from magnetic measurements and neutron powder diffraction

A study of the crystallographic and magnetic structures of the double perovskite Sr₃Fe₂TeO₉ has been carried out on a polycrystalline sample using neutron powder diffraction (NPD) data between 10 and 650 K. An analysis of the NPD patterns at room temperature has shown that this compound crystallises in the tetragonal space group I4/m with a = 5.5614(7) Å and c = 7.867(1) Å and has a partially ordered arrangement of Fe and Te at the B-sites. The compound undergoes an I4/m → Fm-3m improper ferroelectric phase transition near 460 K. A low-temperature ferrimagnetic ordering (below T_N = 260 K) has been followed from the magnetisation measurements and sequential NPD data analysis. In good agreement with magnetic measurements the ferrimagnetic structure with very weak magnetisation is defined by the propagation vector k = (0, 0, 0). In addition to the obtained experimental results on magnetic and electric properties some aspects of magnetoelectricity in this perovskite are also discussed and compared with those of another quaternary oxide Sr₃Fe₂B⁶⁺O₉.