Early Stages Of Mechanical Alloying Research Articles

A mapping approach to pattern formation in the early stages of mechanical alloying

ABSTRACTThe present work focuses on the refinement of the lamellar structure in composite Ag-Cu particles during the early stages of mechanical processing. We combine the statistical description of the kinetics of processes activated by ball milling with the mathematical description of advection cycles induced by plastic deformation. The model curve satisfactorily best fits the experimental data obtained by suitably designed experiments.

Mechanical Alloying of Fe-C System Based Alloy

This paper reviews the research of the author’s group regarding mechanical alloying (MA) of Fe-C system based alloy over the last ten years. Powder mixtures of Fe-xC-yM (x and y=0~6 mass %, M: additive element such as Mn, Si, Cr) were mechanically alloyed (MAed) by using a planetary ball mill under Ar gas atmosphere for various time up to 100 h. The MAed powders then were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential thermal analysis (DTA). First, effect of MA time on powders particles was described. Effects of other MA conditions such as temperature, rotation speed and additive elements were then discussed in detail. In the Fe-C system based alloys, amorphization of graphite powders occur first during the early stage of MA and super-saturated iron solid solution is then formed and at the same time, the crystallite size becomes smaller. The carbon content reaches its limitation (about 1.30 at %). The condition of MA gave an effect to the alloying process.

Thermodynamic analysis of nanostructured (Fe,Cr)3Al formation during mechanical alloying

Nanostructured (Fe,Cr)3Al compound was produced by mechanical alloying (MA) of the Fe50Cr25Al25 elemental powder mixture, and the effect of Cr addition on the formation of solid solution in the ternary Fe–Cr–Al system was investigated. The structural and phase changes of powder particles during MA were studied using X-ray diffractometry and transmission electron microscopy. The results showed that at early stages of MA, a BCC-Fe(Cr,Al) solid solution was formed, which transformed to the disordered (Fe,Cr)3Al compound by further milling. The thermodynamic analysis based on Miedema model showed that for all binary Fe–Al, Fe–Cr and Cr–Al systems and ternary Fe–Cr–Al system, the formation of solid solution is thermodynamically favourable for all compositions. It was also demonstrated that for the ternary Fe50Cr25Al25 alloy, more driving force is available to form solid solution compared with the Fe75Al25 binary alloy. The results of thermodynamic analysis were in agreement with the MA results.

Phase evolution during early stages of mechanical alloying of Cu–13 wt.% Al powder mixtures in a high-energy ball mill

We report the phase and microstructure evolution of the Cu–13wt.% Al mixture during treatment in a high-energy planetary ball mill with a particular focus on the early stages of mechanical alloying. Several characterization techniques, including X-ray diffraction phase analysis, nuclear magnetic resonance spectroscopy, differential dissolution, thermal analysis, and electron microscopy/elemental analysis, have been combined to study the evolution of the phase composition of the mechanically alloyed powders and describe the microstructure of the multi-phase products of mechanical alloying at different length scales. The following reaction sequence has been confirmed: Cu+Al→CuAl2(+Сu)→Cu9Al4+(Cu)→Cu(Al). The phase evolution was accompanied by the microstructure changes, the layered structure of the powder agglomerates disappearing with milling time. This scheme is further complicated by the processes of copper oxidation, reduction of copper oxides by metallic aluminum, and by variation of the stoichiometry of Cu(Al) solid solutions with milling time. Substantial amounts of X-ray amorphous phases were detected as well. Differential dissolution technique has revealed that a high content of aluminum in the Cu(Al) solid solution-based powders is due to the presence of Al-rich phases distributed between the Cu(Al) crystallites.

Cementite formation in copper matrix under mechanical activation using carbon media

We studied the structure and phase transformations of nanocrystalline alloys prepared with mechanical alloying (MA) and annealing by milling copper and iron powders with graphite and xylene. At an early stage of MA, a supersaturated solid solution of iron is formed in copper, regardless of the carbon-precursor type used. In the case of graphite, the formation of iron carbides occurs at a later stage of milling. MA in xylene results in an insignificant amount of carbon phases. Heat treatment leads to the formation of nanocrystalline copper composites with 30 vol.% Fe3C in the two cases of using graphite and xylene. The grain size (30 nm) of the annealed (800 °C) Cu + Fe3C composite produced by MA with xylene is five times less than that of the annealed sample produced with graphite.

Mesostructural refinement in the early stages of mechanical alloying

This study focuses on the early stages of the mechanical alloying of 22 different binary mixtures. In each case, the number of alloyed particles increases linearly with the number of collisions, and the volume of powder effectively processed during a single collision is ∼9.0 × 10−3 mm3. The alloyed particles exhibit a heterogeneous mesostructure with characteristic lengths decreasing exponentially with the number of collisions. The rate of mesostructural refinement is inversely proportional to the difference between the hardnesses of the individual metals.

Study on solid state reactions of nanocrystalline Cu–Ge alloys upon mechanical alloying and annealing

The structural evolution of Cu–12 wt·%Ge (∼Cu–11 at·%Ge) alloy processed by means of mechanical alloying (MA) with subsequent heat treatment was studied using X-ray diffraction profiles, scanning electron microscopy, transmission electron microscopy (TEM) and high resolution TEM observations as well as differential thermal analysis(DTA). The fcc Cu(Ge) solid solution (α) was produced at early stages of MA and amorphised upon further milling. This was followed by the formation of ζ−Cu5Ge intermetallic nanocrystals after 40 h of milling. The subsequent annealing process led to the nanocrystallisation of the amorphous powder. The X-ray crystallite size data showed the exceptional thermal stability of the alloy prepared. The mechanism of the stability was also investigated. In addition, a thermodynamic analysis based on Miedema’s semiempirical model was carried out for the evaluation of the experimental results and a good agreement was found between them.

Effects of Ni addition on the microstructure and properties of nanostructured copper–germanium alloys

The effects of Ni addition on the structure and various phase transitions of nanocrystalline Cu–Ge alloys were investigated for the first time. Cu84Ge16−xNix (x = 0,1,3 and 5 wt.%) alloys were mechanically alloyed at room temperature and evaluated by means of X-ray diffraction, differential thermal analysis, thermal gravimetry, and scanning and transmission electron microscopy observations. Nanocrystalline α−Cu(Ge) solid solution was observed to form during the early stages of mechanical alloying. After prolonged milling times, it was partially transformed into the ζ−Cu5Ge intermetallic compound with an hcp structure. Increased quantities of Ni in the alloys not only delayed the formation of the intermetallic phase upon mechanical alloying and the nanocrystallization transition during annealing but also decreased the crystallite and particle size, which gave rise to nanocrystalline alloys with a high thermal stability at temperatures near their melting points. The mechanism leading to the high stability achieved was also investigated and described.

Thermodynamic analysis of solid solution formation in the nanocrystalline Fe–Ti–Al ternary system during mechanical alloying

The nanocrystalline (Fe,Ti)3Al intermetallic compound was synthesized by mechanical alloying (MA) of elemental powders with the composition of Fe50Al25Ti25. The structural changes of powder particles during MA were studied by X-ray diffractometry. It was found that at early stages of MA, a nanocrystalline Fe(Ti, Al) solid solution is formed. This structure was transformed to disordered (Fe,Ti)3Al intermetallic compound at longer milling times. Thermodynamic analysis of this system revealed that in this ternary system, the least thermodynamic driving force for solid solution formation occurred for those compositions that contained 90% of one element. Fe substitution by Ti in Fe3Al compound decreased the Gibbs free energy change, resulting in a more stable structure.

The production of nanocrystalline cobalt titanide intermetallic compound via mechanical alloying

A nanocrystalline cobalt titanide (CoTi) intermetallic compound has been produced by mechanical alloying (MA) of the elemental powders. In all MA experiments, the ball-to-powder weight ratio was 10:1. The phase evolution and microstructural changes of the powders during mechanical alloying were investigated by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy and microhardness measurements. The results showed that after an optimum mechanical alloying time of 12 h, a CoTi intermetallic phase with the crystallite size of less than 10 nm, the particle size of about 10 μm and the lattice strain of 1.1% was achieved. The study of the mechanism for the formation of the CoTi phase revealed that a composite structure with coarse lamellar-like components was formed with the dissolution of Ti in Co in the early stages of mechanical alloying. The resulting solid solution ultimately provided for the formation of nanocrystalline CoTi phase with the high microhardness value of about 580 Hv.

Nb Doping Effects on Formation Mechanism and Structural Characteristics of Nanostructured Ti5Si3

Nanocrystalline (Ti,Nb)5Si3 intermetallic compound was successfully synthesized by mechanical alloying of Ti62.5-xNbxSi37.5 (X = 0, 10, 20) powder mixtures. The structural changes of powder particles during mechanical alloying (MA) were studied by X-ray diffraction, scanning electron microscopy, and microhardness measurements. It was found that a microstructure containing Ti/Nb/Si particles was formed in the early stages of MA followed by dissolution of Nb in Ti and gradual formation of disordered (Ti,Nb)5Si3 intermetallic compound having a nanograin structure. Annealing increased the D88 ordering in all samples. It was found that the long range order (LRO) parameter of intermetallic compound decreased with increasing Nb content. Also, after 45 h of MA, the microhardness of Ti52.5Nb10Si37.5 powder mixture increased to 1325 HV. It was also increased continually by increasing the Nb content of the compound.

Structural evolutions of the Fe–40Al–5Cr powders during mechanical alloying and subsequent heat treatment

Nanocrystalline Fe–40Al–5Cr powders were synthesized by mechanical alloying (MA) of the Fe, Al elemental powders and the Fe–20Cr prealloyed powders. Structural evolutions, morphologies, composition homogenization and ordering transformation of the Fe–40Al–5Cr powders during MA and subsequently heat treatment were studied. In the early stage of MA (<15 h), solid solutions of Fe(Al) and Fe–Cr(Al) form through Al diffusing into Fe and Fe–20Cr, respectively. Layered Fe(Al)/Fe–Cr(Al) laminae are the typical structure of the milled powders. On prolonging the milling time from 15 to 70 h, the layered laminae disappear, and the milled powders turn to be dense, fine and spherical. Meanwhile, the composition homogenization of the milled powders is also gradually accomplished. After the Fe–40Al–5Cr powders milled for 70 h, nanocrystalline Fe(Al, Cr) powders form with a homogeneous composition. During heating the milled powders from room temperature to 600 °C, the ordering transformation from Fe(Al, Cr) to B2-FeAl(Cr) occurs. The Cr content in the Fe–40Al–5Cr powders was found to hinder the composition homogenization and ordering transformation of the powders during MA and subsequent heat treatment.

Preparation and properties of FeZrB amorphous-nanocrystalline soft magnetic alloy

Fe68Zr20B12 amorphous alloy was prepared by mechanical alloying(MA) method and annealed at different temperatures. Microstructures and magnetic properties of Fe68Zr20B12 alloys as-milled and annealed at 693, 843, 943 and 993 K were studied. The raw powders(Fe, Zr, B) formed b. c. c. α-Fe solid solution at early stages of MA and then transformed into amorphous alloy. Grain size(D) of Fe68Zr20B12 alloys increases with increasing annealing temperature and keeps at nanometer level. The specific saturation magnetization(σs) increases with increasing annealing temperature from 300 K to 943 K, and then decreases with annealing temperature at 993 K because of the precipitation of Fe3B.

Iron Aluminide Coating of Structural Steel by Mechanical Alloying Followed by Annealing

Iron aluminide coating of structural steel by mechanical alloying (MA) followed by annealing was examined. As starting materials, stabilized Al powders containing fat, fat-free Al powders and pure Fe powders were used. Stabilized Al-25 at% Fe powder deposited on the steel substrate from the early stage of MA and formed the coating layer. The mixing and the refinement of Al rich and Fe rich phases in the coating layer were promoted by the suppression of cold welding due to the presence of fat in the coating layer, resulting in the formation of coating layer with fine and homogeneous microstructure. In annealing for 7.2 ks after MA, Fe2Al5 was formed at 673 K, Fe2Al5 and FeAl were formed at 873 K and Fe2Al5, FeAl and Fe3Al were formed at1073 K, respectively. Thus, the formation temperature of FeAl was lower than that of the conventional hot dip coating or pack cementation. This is because interdiffusion between the steel substrate and the coating layer is enhanced by the refinement of microstructure of coating layer due to MA. By coupling annealing to MA, it is possible to improve embrittlement of iron aluminide coating layer.

Effect of niobium and aluminium additions in TiAl prealloyed powders during high-energy ball milling

Mechanical alloying (MA) of TiAl/TiAl– xNb– yAl–1Cr ( x = 4, 8 and 12, y = 2.5, 4 and 6.5 in at.%) was carried out with optimized ball milling parameters. MA of prealloyed TiAl powder resulted nanocrystalline structure at early stages of MA and sustained up to 50 h. TiAl–Nb–Al–Cr systems exhibited amorphous structure in lower Nb and Al content and formation of Nb 2Al nanocrystalline compound with increasing Nb and Al additions. Stability of TiAl covalent bonded intermetallic compound was weakened by dissolution of Nb in the matrix and resulted amorphous structure. Formation of brittle Nb 2Al and presence of TiAl compounds favoured fracturing during MA which resulted particle refinement to submicron size and spherical morphology. DTA scans exhibit no phase transformation in nanocrystalline TiAl and nanointermetallic composites, but amorphous powder exhibits a sharp exothermic peak.

Synthesis and characterization of nanocrystalline NiTi intermetallic by mechanical alloying

Mechanical alloying (MA) has been used to produce NiTi intermetallic with nanocrystalline structure from the elemental powders. The product was characterized using X-ray diffraction, scanning electron microscopy and microhardness measurements. The results showed that disordered B2–NiTi phase can be obtained with grain size of 25 nm, particle size of 15 μm, lattice distortion and high microhardness of 1.2% and 922 HV, respectively. The mechanism of NiTi formation was investigated. In the early stages of MA, a composite lamellar structure of components is formed with the dissolution of Ti in Ni at the same time. The resulting solid solution finally leads to the formation of nanocrystalline disordered B2–NiTi phase. Annealing of the milled powder at 1173 K leads to grain growth, decrease of microhardness and transformation of disordered structure to ordered NiTi, with long-range order of 0.94. A small amount of NiTi 2 and Ni 3Ti phases was also detected.

Mechanical alloying of graphite and magnesium powders, and their hydrogenation

Graphite and elemental magnesium (Mg) powders, whose chemical compositions were C 100− x Mg x (0 ≤ x ≤ 40 at.%), were mechanically alloyed (milled) in an argon gas atmosphere, and these powders were loaded with hydrogen in a high-pressure vessel at a temperature of 308 K. The initial hydrogen gas pressure for hydrogenation experiment was 4 MPa. The periodicity of the c-axis of the graphite crystal structure was destroyed completely for C 100 powders and partly for the powder containing Mg at an early stage of mechanical alloying (MA), indicating the formation of a turbostratic structure (graphene) for all powders. Although elemental Mg still remained in Mg-rich powders (C 60Mg 40) even after longer MA (80 h), other powders reached to mostly the turbostratic structure with no sign of crystalline Mg. The maximum hydrogen concentration levels after hydrogenation for C 100 powder were 0.1 wt.% after MA for 25 h and 0.4 wt.% after MA for 80 h, suggesting that nanostructured graphite uptakes more hydrogen. On the other hand, hydrogen concentration level for C 90Mg 10 powders was less than 0.1 wt.% after MA for 15 h, but it reached to about 1 wt.% after MA for 25 h, and dropped down slightly from 1 wt.% after MA for 80 h. Furthermore, hydrogen absorption occurred smoothly and quickly for C 90Mg 10 powders, although longer induction time was required for C 100 powders. Further addition of Mg to graphite reduced the maximum hydrogen concentration level in the powders, and the powders containing more than 30 at.% Mg did not absorb hydrogen at all at 308 K.

Amorphization behavior of mechanical alloyed CoTa alloy powders prepared by ball milling

Amorphization behavior of Co x -Ta 100- x powders by mechanically alloying mixtures of pure crystalline Co and Ta powder with a high-energy shaker ball mill was studied. The mechanically alloyed powders were amorphous for the composition range between Co 10Ta 90 and Co 80Ta 20. This range is larger than for amorphous alloys prepared by the r.f. sputtering process and equal to that of electron-gun-deposition method. A supersaturated cobalt solid solution formed for Co-rich composition. Moreover, the thermal stability was investigated by differential thermal analysis. The crystallization temperature of amorphous Co-Ta powders was found to be proportional to Ta content and higher than those of amorphous alloys produced by thin-film techniques. The high values of activation energy of amorphous Co-Ta powders were observed near the eutectic compositions. Experimental results indicated that the amorphization rate in the early stage of mechanical alloying was faster for the middle compositions than for Co- and Ta-rich compositions.

Microstructural evolution of Ni 40Zr 60 alloy during early stage of mechanical alloying of intermetallic compounds NiZr 2 and Ni 11Zr 9

The microstructural change of Ni 40Zr 60 alloy during mechanical alloying of mixtures of the intermetallic compounds NiZr 2 and Ni 11Zr 9 has been studied by transmission electron microscopy. A specific “cauliflower” phase was formed during early stage of mechanical alloying process. It is suggested that the solid state reaction between intermetallic compounds NiZr 2 and Ni 11Zr 9 is not the only origin for the formation of the “cauliflower” phase.

Ａｌ‐Ｃｕ‐Ｔｉ混合粉末の機械的合金化

Mixture of aluminum, copper and titanium powders was mechanically alloyed and then the powder was heat-treated at various temperature. A change of structure of the powder with milling period was investigated by using XRD, DTA, SEM and EPMA. In the early stage of mechanical alloying, particles of Cu and Ti are deformed and dispersed in Al matrix, but degree of deformation of Ti particle is smaller than that of Cu particle.Particles of Cu and Ti become finer with the increasing of milling period, and they disperse homogeneously as the milling period reaches to 72.0ks. Intermetallic Al2Cu phase was observed in the powder mechanically alloyed for 21.6ks. In the powder heattreated at 773K intermetallic Al3Ti and AlCu phases were observed. The forming temperature of Al3Ti and AlCu phase decrease with the increasing of milling period.Intermetallic AlTi phase was observed at low temperature in the powder mechanically alloyed for 72.0ks.