Formation of amorphous Al-Cr alloys by mechanical alloying of elemental aluminium and chromium powders

Mechanical alloying (MA) of elemental aluminium and chromium powders has been performed using a conventional ball-mill. The MA process produces composite metal powders and homogeneously alloyed powders. During continuous heating at the rate of 0.33 K sec^–1 Al-15 at% Cr samples ball-milled for 800 and 1000 h showed two exothermal peaks. The first peak which appeared at the lower temperature corresponds to amorphization of the MA powders. It was confirmed by X-ray and transmission electron microscopy that the heattreated powders, quenched from a temperature just above the first peak, were amorphous phase. Amorphous Al-Cr alloys were formed using elemental powders by MA and subsequent heat treatment.     

Synthesis of nanocomposites and amorphous alloys by mechanical alloying

Mechanical alloying (MA) is a powder metallurgy processing technique that involves repeated cold welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Due to the specific advantages offered by this technique, MA was used to synthesize a variety of advanced materials. This article presents two specific examples of synthesis of nanocomposites containing a high volume fraction of the reinforcement phase in Al and TiAl matrices. It was possible to uniformly disperse 50 vol% of nanometric (50 nm) Al₂O₃ in Al and achieve high strength and modulus of elasticity. Similarly, it was possible to disperse 60 vol% of Ti₅Si₃ phase in the γ-TiAl intermetallic. Fully consolidated material showed superplastic behavior at 950 °C and a strain rate of 4 × 10⁻⁵ s⁻¹. Amorphous phases were produced by MA of blended elemental powder mixtures in several Fe-based compositions. From the systematic investigations carried out, it was possible to deduce the criteria for glass formation and understand the interesting phenomenon of mechanical crystallization. By conducting some controlled experiments, it was also possible to explain the mechanism of amorphization in these mechanically alloyed powder blends. Other examples of synthesis of advanced materials, e.g., photovoltaic materials and energetic materials, have also been briefly referred to. This article concludes with an indication of the topics that need special attention for further exploitation of these materials.     

机械合金化制备Nd60 Fe20 Al10 Co10非晶粉末的研究

利用机械合金化制备Nd60Fe20Al10Co10非晶粉末，采用X射线衍射(XRD)和振动样品磁强计(VSM)研究Nd60Fe20Al10Co10非晶的形成过程、磁性能变化及其与成分结构的关系。结果表明，90min后Al原子溶入Nd原子形成固溶体。球磨2h后出现少量非晶，20h后Co单质和Nd单质消失．组织为非晶相(含少量的α-Fe)。球磨100h最终得到非晶 少量的α-Fe纳米晶。球磨过程中，矫顽力随着合金中非晶的量增加而升高．球磨20h矫顽力达到43kA／m。Nd60Fe20Al10Co10合金具有硬磁性是由于非晶相的存在而造成的。     

Synthesis of metastable NiGe2 by mechanical alloying

Mechanical alloying of Ni–Ge elemental powder blends was carried out in a high-energy SPEX shaker mill to study phase evolution as a function of milling time. X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy techniques were employed to characterize the phases present in the milled powders. It was noted that a supersaturated solid solution formed in the early stages of milling containing up to about 12 at.% Ge. On continued milling, the equilibrium NiGe phase started to form at 5 h, and its amount in the powder increased with increasing milling time. On milling for about 60 h, the equilibrium intermetallic NiGe and Ge powder particles reacted to form the metastable NiGe₂ phase. Reasons for the formation of this metastable phase at room temperature and at atmospheric pressure, which is normally present at high temperatures and under high pressures, have been discussed.     

Characteristics of the mechanically-alloyed Ni60Ti40 amorphous powders during mechanical milling in different atmospheres

Mechanically alloyed Ni₆₀Ti₄₀ amorphous powders were further milled in argon, nitrogen and oxygen atmospheres. X-ray diffraction, differential scanning calorimetry and transmission electron microscopy analyses were made to investigate the structural changes during each process. No distinguishable structural changes during further milling of the amorphous powders in argon or nitrogen atmospheres were observed. However, in an oxygen atmosphere, the amorphous phase was crystallized into intermetallic compounds and was accompanied by the occurrence of titanium oxides in the initial stage. On further milling in an oxygen atmosphere, the intermetallic compounds decomposed and only the nickel saturation solution and titanium oxides remained. A thermodynamic explanation of the crystallization and decomposition has been proposed.     

Solid state synthesis of amorphous and/or nanocrystalline Al40Zr40Si20 alloy by mechanical alloying

Mechanical alloying of Al₄₀Zr₄₀Si₂₀ powder blend has been carried out in a high-energy shaker ball mill up to 50 h. Microstructural evolution at different stages of milling has been characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). During milling, elemental Zr seems to undergo a HCP→FCC polymorphic transformation that is closely related to grain refinement and plastic strain. On the other hand, nanocrystallization and mutual dissolution of the elemental blend lead to partial solid state amorphization and development of a composite microstructure comprising of varying proportion of an amorphous phase and nanocrystalline FCC-Zr based solid solution by appropriate hours of milling. In general, the present results compare well with that from our earlier studies on mechanical alloying of Al₄₀Nb₄₀Si₂₀ and mechanical milling of elemental Zr, respectively.     

Structural changes during mechanical alloying of elemental aluminium and molybdenum powders

Mechanical alloying of elemental aluminium and molybdenum powders was performed by conventional low-energy ball milling. Seven compositions (Al-3, 10, 17, 20, 27, 50 and 75 at% Mo), as well as elemental aluminium and molybdenum powders, were milled up to 1000 h. The structural changes during milling were followed by X-ray diffraction. In all cases milling produced refinement of the microstructure, and prepared powders were microcrystalline with grain sizes of the order of nanometres. A supersaturated solid solution of molybdenum in aluminium was formed. Only for the Al-75 at% Mo powders was a solution of aluminium in molybdenum observed. On leave from Institute of Technical sciences of Serbian Academy of sciences and Arts, Belgrade, Yugoslavia.     

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.     

The effect of Mn and B on the magnetic and structural properties of nanostructured Fe60Al40 alloys produced by mechanical alloying

The effect of Mn and B on the magnetic and structural properties of nanostructured samples of the Fe60Al40 system, prepared by mechanical alloying, was studied by 57Fe M?ssbauer spectrometry, X-ray diffraction and magnetic measurements. In the case of the Fe(60-x)Mn(x)Al40 system, 24 h milling time is required to achieve the BCC ternary phase. Different magnetic structures are observed according to the temperature and the Mn content for alloys milled during 48 h: ferromagnetic, antiferromagnetic, spin-glass, reentrant spin-glass and superparamagnetic behavior. They result from the bond randomness behaviour induced by the atomic disorder introduced by the MA process and from the competitive interactions of the Fe-Fe ferromagnetic interactions and the Mn-Mn and Fe-Mn antiferromagnetic interactions and finally the presence of Al atoms acting as dilutors. When B is added in the Fe60Al40 alloy and milled for 12 and 24 hours, two crystalline phases were found: a prevailing FeAl BCC phase and a Fe2B phase type. In addition, one observes an additional contribution attributed to grain boundaries which increases when both milling time and boron composition increase. Finally Mn and B were added to samples of the Fe60Al40 system prepared by mechanical alloying during 12 and 24 hours. Mn content was fixed to 10 at.% and B content varied between 0 and 20 at.%, substituting Al. X-ray patterns show two crystalline phases, the ternary FeMnAl BCC phase, and a (Fe,Mn)2B phase type. The relative proportion of the last phase increases when the B content increases, in addition to changes of the grain size and the lattice parameter. Such behavior was observed for both milling periods. On the other hand, the magnetic hyperfine field distributions show that both phases exhibit chemical disorder, and that the contribution attributed to the grain boundaries is less important when the B content increases. Coercive field values of about 10(2) Oe slightly increase with boron content. Comparison with previous results on FeAIB alloys shows that Mn promotes the structural stability of the nanostructured powders.     

Preparation of amorphous Fe-Zr alloys by mechanical alloying and melt spinning methods

Amorphous iron-zirconium alloys have been obtained by mechanical alloying (MA) starting from either pure elemental powders or from intermetallic compounds. X-ray diffraction was used in monitoring the amorphization process. Differences in the amorphization kinetics have been detected for the two different starting situations. The possibility of obtaining totally amorphous samples when starting from intermetallic compounds has been discussed. Amorphous iron-zirconium ribbons were also obtained by the classical melt-spinning (MS) method. A detailed structural comparison of the radial distribution functions led to the conclusion that the arrangement of the first neighbours is indistinguishable in the amorphous samples prepared by MA and MS methods.     

Forging compaction of amorphous Co67B33 powders obtained by mechanical alloying

An amorphous mechanically alloyed Co₆₇B₃₃ powder has been compacted by hot forging at 200, 300, 400, 500, 700°C applying a load up to 2 GPa in 2 seconds. The density approaches the theoretical value operating under inert gas at 700°C. The alloy structure changes from amorphous to crystalline. Hardness and wear resistance of the material forged at 700°C are superior to those of the starting amorphous alloy as well as to those of an alloy of identical composition obtained by conventional slow cooling of the melt.     

Synthesis of austenitic stainless steel powder alloys by mechanical alloying

Fe–18Cr–xNi (x = 8, 12, 13, 15, and 20 wt%) blended elemental powders were subjected to mechanical alloying in a high-energy SPEX shaker mill. The milled powders were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy and transmission electron microscopy techniques. It was shown that the sequence of phase formation in the Fe–18Cr–8Ni, Fe–18Cr–12Ni and Fe–18Cr–13Ni compositions was ferrite in the early stages of milling and then formation of austenite, which eventually transformed to stress-induced martensite on continued milling. The time for the formation of the austenite phase was shorter for the 12Ni and 13Ni powder blends than for the 8Ni powder. However, in the Fe–18Cr–15Ni and Fe–18Cr–20Ni compositions, the initial phase to form was ferrite and then a fully austenitic structure had formed on milling the powder for 10 h. No martensitic transformation occurred in this case on continued milling. The phase formation and microstructural features were confirmed by X-ray diffraction and transmission electron microscopy and diffraction techniques. A new metastable phase diagram was proposed outlining the stability of the austenite phase in ternary Fe–Cr–Ni alloys.     

Corrosion behaviour of amorphous Ti48Cu52, Ti50Cu50 and Ti60Ni40 alloys investigated by potentiodynamic polarization method

Potentiodynamic polarization studies were carried out on virgin specimens of amorphous alloys Ti₄₈Cu₅₂, Ti₅₀Cu₅₀ and Ti₆₀Ni₄₀ in 0.5 M HNO₃, 0.5 M H₂SO₄ and 0.5 M NaOH aqueous media at room temperature. The value of the corrosion current density (I_corr) was maximum for Ti₄₈Cu₅₂ alloy in all the three aqueous media as compared to the remaining two alloys. The value of I_corr for the alloy Ti₄₈Cu₅₂ was maximum (I_corr = 2.6 × 10^{- 5} A/cm²) in 0.5 M H₂SO₄ and minimum (I_corr = 3.5 × 10^{- 6} A/cm²) in 0.5 M NaOH aqueous solutions. In contrast, the alloy Ti₆₀Ni₄₀ exhibited the least corrosion current density in 0.5 M HNO₃ (I_corr = 40 × 10^{- 7}A/cm²) and in 0.5 M NaOH (I_corr = 5.5 × 10^{- 7} A/cm²) aqueous media as compared to those for Ti-Cu alloys, while its value in 0.5 M H₂SO₄ was comparable to that for Ti₄₀Cu₅₀. It is suggested that the alloy Ti₆₀Ni₄₀ is more corrosion resistant than the alloys Ti₄₈Cu₅₂ and Ti₅₀Cu₅₀ in all the three aqueous media.