Papers by Keyword: Amorphisation

Abstract: This work studies the production of melt spun Fe₇₈Si₉B₁₃ ribbons with amorphous or nanocrystalline structure. The main objective is the preservation of the amorphous structure after obtaining powders by mechanical milling of the ribbons, as well as the study of the influence of the milling conditions on the size distribution and structure of the obtained powders. In order to obtain high quality amorphous ribbons, the wheel rotation speed, crucible-wheel distance, melt homogenization time, ejection pressure and the ejection temperature were optimized in the melt spinning process. Different mills were used for powder production, studying the size distribution, efficiency, and preservation of the amorphous character as a function of the milling time. Ribbons and powders were characterized by X-ray diffraction (XRD) and electron microscopy (SEM and TEM); laser diffraction was used for powder granulometry.

Abstract: The effect of milling time on the microstructure evolution and formation of amorphous phase of Ti₆₀Si₄₀ alloy produced by mechanical alloying (MA) has been investigated. Laser diffraction, Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and Differential Scanning Calorimetry (DSC) were employed to characterize the particle size, morphology and structure of mechanically alloyed Ti₆₀Si₄₀. When the milling time is increased to 20 h, the particle size decreases from 23.7 to 4.7 μm, the shape of the particles changes to spherical and the crystalline structure is transformed into an amorphous phase. The amorphous Ti₆₀Si₄₀ alloy is stable when heating up to 750oC. Above this temperature, the cold crystallization of the intermetallic compounds Ti₅Si₃ and/or Ti₅Si₄ begins.

Abstract: Mechanical alloying (MA) is a simple and versatile dry powder processing technique that has been used for the manufacture of both equilibrium and metastable phases of commercially useful and scientifically interesting materials. It owes its origin to an industry need to develop a nickel-based super alloy for gas turbine applications that had both oxide dispersion strengthening and precipitation hardening. This far-from equilibrium powder metallurgy processing technique involves fracturing, welding and re-welding of powder particles in a High Energy Ball Mill (HEBM). MA is an economically viable process with important technical advantages. Its utmost advantage is in the synthesis of novel alloys, e.g., alloying of ordinarily immiscible elements, that is not possible by any other technique. As MA is a completely solid-state processing technique, the limitations imposed by phase diagrams do not apply to it. The MA process is capable of producing different types of metastable effects in a variety of alloy systems. Some of the metastable effects achieved by MA are solid solution formation and amorphisation. MA has the possibility of producing superior and enhanced materials than those produces by conventional methods. In this work a review of MA and its present and potential applications for Ti-based materials are presented.

Abstract: Blended elemental powders with the nominal compositions (at%) of Ti50Al50, was mechanically alloyed in a planetary ball milling system for up to 100h, an amorphous Ti50Al50 phase was obtained in the process. The amorphization process as a function of time of milling was monitored by scanning electron microscope, X-ray diffraction and transmission electron microscope. It is shown that, as first, Al atoms diffuse into the host lattice of hexagonal Ti; subsequently, the milling accumulates a critical density of disorder that causes the Ti (Al) crystalline phase to collapse into an amorphous phase, it is suggested the grain size condition for formation of amorphous phase is 12nm. On the basis of thermodynamic models, the formation of the amorphous phase is discussed.

Abstract: A high dose of electron irradiation generates amorphous zones with critical vacancy concentrations in the pyrographite. The degree of disorder “” of amorphization of Graphite, natural graphite, pyrographite and polycrystal pyrographite are analyzed as a function of time “t”, and the amorphization kinetics under different voltages inside the HVTEM.

Abstract: In the present work, Al-Ni-Ce-C amorphous powders with different compositions have been prepared by a high energy ball milling technique. The amorphization behaviors of the Al-Ni-Ce-C powders have been studied using X-ray diffraction and transmission electron microscopy. The results show that alloy compositions have a great influence on the amorphization behavior of the Al-Ni-Ce-C powders and the optimum composition is Al₈₅Ni₁₀Ce₅+1 wt.% C for the formation of amorphous phase. The thermal stability of the as-milled powders has been investigated by differential scanning calorimetry. It has been found that the compositional effects on thermal stability are similar to those on the amorphization of Al-Ni-Ce-C. In addition, the more addition of carbon decreases both glass forming ability and thermal stability of the Al-Ni-Ce-C system under ball milling conditions. Furthermore, comparison has been carried out between Al₈₅Ni₁₀Ce₅+1 wt.% C amorphous powders and Al₈₅Ni₁₀Ce₅ glassy ribbons with respect to thermal stability.

Abstract: The results presented here concern the NiTi alloy subjected to plastic deformation by compression combined with reversion oscillating torsion. The compression rate was 0.05 mm/s and the torsion frequency and angle were 1Hz and ± 3o, respectively. The maximal strain obtained was c = 6.20. The structure of the deformed samples was studied with the use of X-ray phase analysis and TEM observations. It was found that the structure consists of a mixture of highly deformed B2 parent phase and B19’ martensite. The strain distribution after the applied plastic deformation was not uniform, the highest strain region was in the middle of the cylinder sample. In these regions small amount of the Ni2Ti phase was indentified. The TEM studies revealed some amorphous areas in the most strained region of the samples.

Abstract: We investigate the structural and electrical properties of polycrystalline 3C-SiC obtained from P ion implanted 4H-SiC with the box-shaped doping profile (NP: 6 x 1020/cm3, thickness: 400 nm, ion dose: 1.6 x 1016/cm2, room temperature). RBS measurement reveals that the highly defective region is formed by P ion implantation, which remains even after annealing at 1700 oC. X-TEM observation shows the P ion induced amorphous layer is recrystallized to twinned-3C-SiC. After annealing at 1300 oC, a sheet resistance of 950 /sq. and sheet carrier concentration of 1 x 1015/cm2 was obtained. By increasing the annealing temperature from 1500 to 1700 oC, the sheet resistance was drastically decreased to about 200 /sq., while there was a small change in the sheet carrier concentration. For the sample annealed at 1700 oC, the electrical activity of the P impurity was estimated to be about 10 % which is comparable to the case of hot implanted sample.

Abstract: Single-component, self-setting and injectable calcium phosphate cement (CPC) based on amorphization process of dicalcium phosphate dehydrate (DCPD) is proposed. After preparation of DCPD by wet chemistry, the material was dry milled in an Attritor high energy process (at 400 RPM) during 20 minutes. Experiments were also conducted using a regular ball milling process at 15 and 30h residence time. Amorphization of DCPD confirmed using FTIR, XRD and 31 P solid-states NMR (cross-polarization and decoupling). Upon hydration of amorphized DCPD powder with saline (0.55 ml/g), putty-like consistency produced. The paste hardened in 30 minutes at 37°C and reached a compressive strength of about 20 MPa. The final product was a low crystalline calcium deficient apatite, similar to the composition and structure of bone mineral.

Abstract: Recent studies indicate that the existing criteria for amorphous phase formation are not precise. This study emphasizes that the extent of deviation from the linear relationship of the chemical contribution to enthalpy of the solid solution with the enthalpy difference between the amorphous phase and the constituent elements is a measure of tendency to bypass amorphous phase formation. Mechanical alloying experiments on a number of compositions in Al-Ni-Ti system showed that the compositions, not following the aforesaid relation, yield partial or no amorphization.