The ball milling induced transformation of alpha-Fe2O3 powder in air and oxygen atmosphere

The mechanochemical treatment of alpha-Fe2O3 powder was done concurrently in air and oxygen atmospheres using a conventional planetary ball mill. The influence of the duration of milling and of the balls-to-powder mass ratio on the transformation of alpha-Fe2O3 was investigated. Under appropriate milling conditions, alpha-Fe2O3, completely transforms to Fe3O4, and for prolonged milling to the Fe-1 _ O-x phase, either in air or oxygen atmosphere. Owing to the higher oxygen pressure, the start of the reaction in oxygen is delayed by similar to 1 h in comparison with the reaction in air. The reverse mechanochemical reaction Fe-1 . O-- GT Fe3O4-- GT , alpha-Fe2O3 takes place under proper oxygen atmosphere. The oxygen partial pressure is the critical parameter responsible for the mechanochemical reactions. The balls-to-powder mass ratio has a considerable influence on the kinetics of mechanochemical reactions. Below the threshold value the reaction does not proceed or proceeds very slowly. Plausibly, three phenomena govern mechanochemical reactions: (i) the generation of highly energetic and localized sites of a short lifetime at the moment of impact; (ii) the adsorption of oxygen at atomically dean surfaces created by particle fracture; and (iii) the change of activities of the constituent phases arising from a very distorted (nanocrystalline) structure. (C) 1999 Elsevier Science S.A. All rights reserved

The influence of mechanochemical treatment of the Bi2O3-ZrO2 system on the structural and dielectric properties of the sintered ceramics

A powder mixture of alpha-Bi2O3 and ZrO2, both monoclinic, in the molar ratio 2:3, was mechanochemically treated in a planetary ball mill in an air atmosphere for up to 20 h, using steel vial and hardened-steel balls as the milling medium. Mechanochemical reaction leads to the gradual formation of an amorphous phase. After 5 h of milling the starting alpha-Bi2O3 and ZrO2 were transformed fully into a non-crystalline phase. After milling for various times the powders were compacted by pressing and isothermal sintering. The pressed and sintered densities depended on the milling time. Depending on the duration of the mechanochemical treatment and sintering temperature, the phases: gamma-Bi-12(ZrxFe1-x)O-20; Bi(ZrxFe1-x)O-3 and Bi-2(ZrxFe1-x)(4)O-9 were obtained by reactive sintering, whereby the Fe originates from vial and ball debris. The dielectric permittivity of the sintered samples significantly depends on the milling time. Samples milled for 10 and 15 h and subsequently sintered at 800degreesC for 24 h exhibit a hysteresis dependence of the dielectric shift (in altering electric fields higher than 10 kV/cm at room temperature), confirming that the synthesized materials possess ferroelectric properties. (C) 2004 Kluwer Academic Publishers

The ball milling induced transformation of alpha-Fe2O3 powder in air and oxygen atmosphere

The mechanochemical treatment of alpha-Fe2O3 powder was done concurrently in air and oxygen atmospheres using a conventional planetary ball mill. The influence of the duration of milling and of the balls-to-powder mass ratio on the transformation of alpha-Fe2O3 was investigated. Under appropriate milling conditions, alpha-Fe2O3, completely transforms to Fe3O4, and for prolonged milling to the Fe-1 _ O-x phase, either in air or oxygen atmosphere. Owing to the higher oxygen pressure, the start of the reaction in oxygen is delayed by similar to 1 h in comparison with the reaction in air. The reverse mechanochemical reaction Fe-1 . O-- GT Fe3O4-- GT , alpha-Fe2O3 takes place under proper oxygen atmosphere. The oxygen partial pressure is the critical parameter responsible for the mechanochemical reactions. The balls-to-powder mass ratio has a considerable influence on the kinetics of mechanochemical reactions. Below the threshold value the reaction does not proceed or proceeds very slowly. Plausibly, three phenomena govern mechanochemical reactions: (i) the generation of highly energetic and localized sites of a short lifetime at the moment of impact; (ii) the adsorption of oxygen at atomically dean surfaces created by particle fracture; and (iii) the change of activities of the constituent phases arising from a very distorted (nanocrystalline) structure. (C) 1999 Elsevier Science S.A. All rights reserved

A comparative study of microstructure, mechanical and fracture properties of Ni3Al-based intermetallics produced by powder metallurgy and standard melting and casting processes

A comparative study of the microstructure, mechanical properties and fractography of Ni3Al macro- and microalloyed intermetallics produced by powder metallurgy (PM) and standard vacuum melting and casting processes has been carried out. Non-porous PM compacts were obtained by vacuum hot pressing of powders produced either by gas atomization or by a rotating electrode process. All materials showed a positive temperature dependence of the compression yield strength. The maximum strength was attained between 600 and 700 degrees C, then the decrease occurs. With increase in temperature the ductility of all materials slightly decreased to a minimum and then abruptly increased. Values of mechanical properties of PM compacts were higher than those of as-cast material. There is a correlation between the fracture morphology and the ductility of Ni3Al, i.e. the higher ductility corresponds to transgranular fracture, while the minimum ductility is a consequence of intergranular fracture

Structure, properties and application of Ni3Al aluminides

Intermetallic compounds have long been recognized as potentially useful structural materials, Ni3Al aluminides possess excellent oxidation resistance at high temperatures as well as good high-temperature strength, This paper reviews the progress which has been made during the last few years in the research and development of Ni3Al aluminides. Comparative results on microstructural and mechanical properties (compression) of Ni3Al-base aluminide produced by powder metallurgy (PM), vacuum melting and casting processes have been reported. Fine-grained PM aluminide exhibits the higher strength than the coarse-grained as-cast material. Ductility is not significantly affected by the grain size.Advanced Materials for High Technology Applications, 1st Yugoslavian Advanced Materials Conference - Advanced Materials 95 on Challenges of Tomorrow, Sep 18-22, 1995, Herceg Novi, Yugoslavi

Mechanochemical treatment of alpha-Fe2O3 powder in air atmosphere

Powder of alpha-Fe2O3 was mechanochemically treated in a planetary ball mill in an air atmosphere. Structural changes were followed by X-ray diffraction analysis, magnetization measurements and differential scanning calorimetry after various milling times. It was found that complete transformation of alpha-Fe2O3 to Fe3O4 is possible during milling in an air atmosphere under appropriate milling conditions. Presumably, the decrease in the oxygen partial pressure during milling has a critical influence on promoting the dissociation of alpha-Fe2O3. Before nucleation of the Fe3O4 phase, the crystallites of the alpha-Fe2O3 phase are reduced to a minimal size accompanied by the introduction of atomic-level strain. Local modeling of a collision event, coupled with a classical thermodynamic assessment of the Fe2O3-Fe3O4 system, were used to rationalize the experimental results. It is proposed that the mechanochemical reactions proceed at the moment of impact by a process of energization and freezing of highly localized sites of a short lifetime. Excitation on a time scale of similar to 10(-5) s corresponds to a temperature rise of the order of (1-2) x 10(3) K. Decay of the excited state occurs rapidly at a mean cooling rate higher than 10(6) K s(-1). (C) 1998 Elsevier Science S.A. All rights reserved

Titanium carbide reinforcement in iron matrix through carbothermal reduction of mechanically milled hematite and anatase

This study investigated the influence of the duration of milling on the formation of TiC-reinforced iron composite through carbothermal reduction of a hematite and anatase mixture. Mixtures of hematite, anatase, and graphite powders were mechanically activated in a planetary ball mill in an argon atmosphere with different milling times (0 to 60 hours). X-ray diffraction showed that with increasing milling time, the crystallite size of the hematite decreased to nanometer range, accompanied by an increment in internal strain. Prolonging the milling process increased dislocation density of the as-milled powder. The as-milled powder was consolidated by cold pressing under 100 MPa and sintered in vacuum at 1373 K (1100 °C). High temperature during sintering resulted in the formation of iron and titanium carbide phases as confirmed by X-ray diffraction, scanning electron microscope, and energy dispersive X-ray analysis. Without mechanically activated milling, the reaction forming TiC did not occur during sintering at 1373 K (1100 °C), indicating a reduction in reaction temperature promoted by mechanical milling. An increase in milling time resulted in an increase in sintered density and hardness due to the fineness of the composite powder, together with complete TiC and iron phase formation