XAFS Study on Solid State Amorphization of Alloys by Mechanical Alloying

Structural evolution of alloys by ball milling during solid state amorphization were studied by means of XAFS technique. The first one is amorphization process of Fe and B powder mixtures by mechanical alloying (MA), and the second one is amorphization process of ordered B2 CoZr intermetallic compound by mechanical milling (MM). The mixing process of Fe and B and disintegration process of ordered B2 CoZr intermetallic compound crystal were observed clearly in atomic level by XAFS method. The micro mechanism of amorphization process of alloy by ball milling was discussed.

AMORPHIZATION TRANSFORMATION BY MECHANICAL ALLOYING IN THE Mo-Si SYSTEM

Starting from elemental powders, complete MoSi2 powder forms abruptly between 3.5and 4 h during mechanical alloying (MA) of the Mo-66 at.% Si powders. Continuous milling of this MoSi2 Phase leads to a nanocrystalline powder and amorphizationtransformation takes place after 100 h milling. Howeven MA of the Mo-37.5 at.%Si powders does not result in the formation of the Mo5Si3 crystalline phase, but the formation of a Mo(Si) supersaturated solid solution (SSS) and a completely amorphots phase after 5 h and 70 h milling, respectively. The free energy of the Mo-Sisystem has been calculated and it has been found that there is no driving force for the amorphization reaction under normal conditions. The amorphization by MA of the Mo-Si system is attributed to a solid-state amorphization reaction in which defects and a very fine grain size induced during milling process may raise the free energy of the crystalline intermetallic phase (for MoSi2) or the Mo(Si) supersaturated solid solution (for Mo5Si3) above that of the amorphous phase.

FORMATION OF AMORPHOUS Ni-Ti ALLOY BY MECHANICAL ALLOYING

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STUDY OF AMORPHOUS Fe-Zr-B-(Cu) ALLOYS BY MECHANICAL ALLOYING

The technique of mechanical alloying(MA) for preparing amorphous Fe-Zr-B-(Cu) alloys has been studied The structure of samples was analysed by X-ray diffractometry, electron microscopy and Mssbauer spectroscopy.The results show that Fe78Zr10B12 sample of a nearly complete amorphous phase may be prepared but in Fe91Zr7B2 sample metastable b c c phase was obtained Using the same procedure a mixture of amorphous phase and α-Fe(Zr, B) was generated in Fe85Zr7B8. Furthermore,the results of Mssbauer reveal that there are two kinds of short-range ordered phases in the sample. While adding Iat% Cu in Fe78Zr10B12 or Fe85Zr7B8 sample, it is found that the hyperfine field of amorphous phases decreases significantly.

Electrochemical Hydrogen Storage Performance of the Nanocrystalline and Amorphous Pr-Mg-Ni-based Alloys Synthesized by Mechanical Milling

The PrMg12-type composite alloy of PrMg_(11)Ni + x wt% Ni (x=100,200) with an amorphous and nanocrystalline microstructure were synthesized through the mechanical milling.Effects of milling duration and Ni content on the microstructures and electrochemical hydrogen storage performances of the ball-milled alloys were methodically studied.The ball-milled alloys obtain the optimum discharge capacities at the first cycle.Increasing Ni content dramatically enhances the electrochemical property of alloys.Milling time varying may obviously impact the electrochemical performance of these alloys.The discharge capacities show a significant upward trend with milling duration prolonging,but milling for a longer time more than 40 h induces a slight decrease in the discharge capacity of the x=200 alloy.As milling duration increases,the cycle stability clearly lowers,while it first declines and then augments under the same condition for the x=200 alloy.The high-rate discharge abilities of the ball-milled alloys show the optimum values with milling time varying.

Structure and Magnetic Property of Amorphous Fe-Ni Particles Prepared by Mechanical Alloying

The amorphous Fe 67.5 Ni 32.5 alloy particles have been prepared by mechanical alloying (MA) process of elemental powders. The crystallization is observed at temperature around 405°C by means of differential thermal analyzer. It is found that the coercivity H C and the effective magnetic anisotropy value of amorphous phase are not far from that of crystal phase. The magnetization behavior of amorphous and nanocrystalline samples prepared by mechanical alloying and milling process are quite similar, they obey 1/H law at middle high field and 1/H 2 at higher field. The 1/H field dependence is attributed to a large number of dislocations created during MA process. The studies on the 1/H 2 field dependence reveals that magnetization of Fe Ni ultrafine particles are governed predominantly by internal stress fields induced by MA process.

Mechanical alloying characteristic and thermal stability of Al_(78)Fe_(20)Zr_2 amorphous powders

The powders of pure Al, Fe, and Zr for preparing Al78Fe20Zr2 were subject to a high-energy planetary ball milling.The microstructure evolution of the mixtures at the different intervals of milling was characterized by X-ray diffraction（XRD）, transmission electron microscopy（TEM） and differential scanning calorimetry（DSC）.It was found that a nearly complete amorphization could be achieved in the mixtures after ball milling for 23 h.Further ball milling led to the crystallization of the amorphous powders.A long time ball milling, e.g., 160 h, led to a complete crystallization of the amorphous powders and the formation of Al3Zr and Al13Fe4.The crystallization products caused by ball milling are almost the same as that produced by isothermal annealing of the amorphous powders in vacuum at 800 K for 1 h.

Formation and mechanical properties of ductile Fe-based amorphous alloys using a cast iron with minor addition of B and Al

Fe-based amorphous alloys with ductility were synthesized using the commercial cast iron QT50 (denoted as QT) with the combining minor addition of B and Al by single roller melt-spinning. The melt-spun (QT1-xBx)99Al1 (x is from 0.006wt% to 0.01wt%) amorphous alloys exhibit onset crystallization temperatures and Curie temperatures of 759-780 and 629-642 K respectively, and whi- ch increase with B content. The amorphous ribbons are ductile and can be bent 180° without breaking. With the increase in B content from 0.006wt% to 0.01wt%, the Vickers microhardness of the amorphous alloys increases from Hv 830 to Hv 1110. The effects of the additional B and Al elements on the glass forming ability and mechanical properties were also discussed.

AMORPHOUS ALLOY Al-Y-Ni FORMED BY MECHANICAL ALLOYING

The amorphization transformation of Al-Y-Ni system by mechanical alloying was inves- tigated in both thermodynamic and kinetic aspects.The amorphous powder of Al-rich composition Al_(80)Y_(15)Ni_5 was formed after 70 h milling.The process of the amorphiza- tion was monitored by X-ray diffraction.The dependence of the amorphization on the milling intensity was also discussed.The results show that over milling with extra-high intensity has an unfavourable effect on amorphization of the system.

SYNTHESIS OF Ni-Mo ALLOYS BY MECHANICAL ALLOYING

Ni_(100-x)Mo_x(x=20,30,38)alloys have been synthesized by mechanical alloying.The structure evolution of powders in mechanical alloying process has been monitored by X-ray diffraction,scanning electron microscopy and transmission electron microscopy analyses.The results show that the Ni_(62)Mo_(38)sample becomes partially amorphous after high energy ball milling,while the Ni_(80)Mo_(20)and Ni_(70)Mo_(30)become non-equilibrium nanocrystals.

Amorphization and magnetic properties of Fe_(62)Nb_(38) mechanically alloyed powders

The amorphization and magnetic properties of Fe_(62)Nb_(38) mechanicallyalloyed powders were investigated. In the initial mechanical alloying processes, the latticestructure of pure Fe is destroyed due to the cold-welding and fracturing, accompanying the reductionof ferromagnetic properties. The M_S value of Fe_(62)Nb_(38) powders with ball-milling time t = 6 his only 48.1 A·m^2/kg. With prolongating of mechanical alloying processes, a solid stateamorphization reaction (SSAR) takes place and the Fe-Nb ferromagnetic amorphous phase is formed.With the milling time increasing from 6 to 18 h, the saturation magnetization of Fe_(62)Nb_(38)powders increases with enhancement of the proportion of ferromagnetic amorphous phase in milledpowders. The M_S value of the Fe_(62)Nb_(38) amorphous powders is 98 A·m^2/kg, which is very closeto the value estimated from dilute model. However, the Curie temperature of the Fe_(62)Nb_(38)amorphous phase is only 206℃, which is much smaller than that of the pure Fe. This implies that theexchange interaction between Fe atoms in amorphous alloyed Fe_(62)Nb_(38) becomes weak due to theNb dilution. Investigation shows that the variation of magnetic properties of milled powders is oneof important tools for describing the amorphization by mechanical alloying.

PREPARATION OF AMORPHOUS Mo_5Si_3 BY MECHANICAL ALLOYING AND THE CRYSTALLIZATION

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FORMATION OF AMORPHOUS Al_(80)Fe_(20)BY MECHANICAL ALLOYING

Amorphous Al_(80)Fe_(20) powder which can hardly be obtained using conventional rapid quenching technique was directly prepared from crystalline Fe and A1 powders by means of mechanical alloying process.The amorphization process was examined by X-ray analysis and Mssbauer spectroscopy,and the thermal behaviour was studied by DSC.The results show that the rate of amorphization is controlled by the diffusion reaction at the interfaces and changes with the milling time.The effect of milling conditions on amorphization by mechanical alloying was al- so investigated.

Synthesis of Mg-Cu-Gd amorphous alloy by mechanical alloying and its thermal stability

Mg65Cu25Gd10 amorphous alloy powders were synthesized by mechanical alloying (MA). The most suitable process is that the milling velocity is 600 r/min and the mass ratio of ball to powder is 20:1, so the shortest forming time of the amorphous is about 40 h. The thermal stability of the powders was analyzed by differential scanning calorimetry (DSC). The glass transition temperature, (Tg), onset temperature of crystallization(Tx), width of supercooled liquid region, were determined to be 444.8, 531.1 and 86.3 K, respectively. Contrasting them with the parameters of samples produced by traditional copper-mold casting, it is found that the thermal stability of the former is better than that of the latter. At the same time, the crystallization kinetics of those amorphous powders was researched by Kissinger equation and the apparent activation energy at Tg and Tx was calculated.

Effect of consolidation parameters on mechanical properties of Cu-based bulk amorphous alloy consolidated by hot pressing

Cu50Zr40Ti10 bulk amorphous alloys were fabricated by hot pressing gas-atomized Cu50Zr40Ti10 amorphous powder under different consolidation conditions without vacuum and inert gas protection. The consolidation conditions of the Cu50Zr40Ti10 amorphous powder were investigated based on an L9（34） orthogonal design. The compression strength and strain limit of the Cu50Zr40Ti10 bulk amorphous alloys can reach up to 1090.4 MPa and 11.9 %, respectively. The consolidation pressure significantly influences the strain limit and compression strength of the compact. But the mechanical properties are not significantly influenced by the consolidation temperature. In addition, the preforming pressure significantly influences not the compression strength but the strain limit. The optimum consolidation condition for the Cu50Zr40Ti10 amorphous powder is first precompacted under the pressure of 150 MPa, and then consolidated under the pressure of 450 MPa and the temperature of 380 °C.

Effect of mechanical alloying and sintering process on microstructure and mechanical properties of Al-Ni-Y-Co-La alloy

Al86Ni7Y4.5Co1La1.5 （mole fraction, %） alloy powder was produced by argon gas atomization process. After high-energy ball milling, the powder was consolidated by vacuum hot press sintering and spark plasma sintering （SPS） under different process conditions. The microstructure and morphology of the powder and consolidated bulk sample were examined by X-ray diffraction （XRD）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. It is shown that amorphous phase appears when ball milling time is more than 100 h, and the bulk sample consolidated by SPS can maintain amorphous/ nanocrystalline microstructure but has lower relative density. A compressive strength of 650 MPa of Al86Ni7Y4.5Co1La1.5 nanostructured samples is achieved by vacuum hot extrusion （VHE）.

Effects of Ta addition on the microstructure and mechanical properties of Ti_(40)Zr_(25)Ni_8Cu_9Be_(18) amorphous alloy

The effects of Ta addition on the microstructure and mechanical properties of Ti40Zr25Ni8Cu9Be18 bulk amorphous alloy were investigated by using X-ray diffraction (XRD), transmission electron microscopy (TEM), scan electron microscopy (SEM) and compressive testing. As a result, the addition of Ta (0-8at%) prompted the successive precipitation of quasicrystalline phase, CuTi2 phase and bcc β-Ti solid solution. Additionally, the addition of less Ta content (3at%-5at%) led to the formation of amorphous ma- trix/nanoquasicrystal/CuTi2 complex phase structure; and nanoquasicrystals, as reinforcement precipitates, improved the fracture strength of Ti-Zr-Ni-Cu-Be-Ta alloys, which led to the high compressive fracture strength 1856 MPa of Ta5 alloy. With increasing Ta content (5at%-8at%), although the ductile dendritic β-Ti solid solution was precipitated, the strength and plasticity decreased to a great extent resulting from the growth of quasicrystalline phase and CuTi2 phase.

Phase Development and Crystallization Kinetics of NiTi Prepared by Mechanical Alloying

NiTi alloy is produced by mechanical alloying(MA). It becomes amorphous after milling for enough time, such as 100 h in this paper. DSC measurement shows that the crystallization temperature is 676 K for the amorphous powder. Activation energy of crystallization is 199.98 kJ/mol for MA powder, which is lower than that of amorphous prepared by magnetron sputtering. Avrami parameter of crystallization is 1.07.

Formation mechanism of amorphous Ni-Fe-P alloys by electrodeposition

The formation mechanism of the amorphous Ni-Fe-P coating was studied by analysis of the forming thermodynamics, dynamics, and crystallography of the amorphous alloy. The results show that, in the initial stage of deposition a thin 'crystal epitaxial growth' layer first forms, and then transforms to amorphous gradually. The cross section in Ni-Fe-P coatings by electrolytic etching exhibits a banded structure of alternate dark and light bands. It is proposed that the banded structure is caused by a change in the P content with thickness,which is due to alternated depletion and enrichment of [OH-] in the diffusion layer resulting from the generation and evolution of hydrogen gas. The amorphous Ni-Fe-P coating will be formed in proper composition, high nucleation rate and strongly hindered growth of the crystal nucleus. Amorphous Ni-Fe-P alloys form as islands, and grow up by layer.

Wear mechanism of electrodeposited amorphous Ni-Fe-P alloys

The wear mechanism of amorphous Ni-Fe-P coating was discussed. The wear resistance of the amorphous Ni-Fe-P coatings was tested on a Timken wear apparatus, and the wear track of the amorphous Ni-Fe-P coatings as-deposited and heated at various temperatures was observed by SEM. The results show that the wear resistance reaches a maximum value at NaHPO2·H2O concentration of 5 g/L, and heating at 400 ℃. The wear mode of the coating will change with the heating temperature increasing from pitting+plowing at 200 ℃ to pitting at 400 ℃, and to plowing at 600 ℃. The pits on the worn surface of the amorphous Ni-Fe-P coating result from the tribo-fatigue fracture. The cracks of spalling initiate at pits and propagate at certain angle with the sliding direction on surface, and then extend into sub-surface along the poor P layers or the interface between layers. Finally under repeated action of the stress in the rubbing process the cracks meet and the debris forms. The generation of the pits and spalling is related with the internal stress, brittleness and layer structure of the amorphous Ni-Fe-P coating.