Fast amorphization and crystallization in Al-Fe binary system by high-energy ball milling

Large amount of amorphous phase of Al-Fe binary system was obtained by MA of elemental powders using a high-energy ball mill at milling intensity of 150G (G is the gravitational acceleration). XRD, HRTEM and DSC were used to analyze the process of amorphization and crystallization. The time required achieving almost complete amorphous state is only 4.2 ks for Al-25 at.%Fe system and 3 ks for Al-30 at.%Fe system, respectively. The time of amorphous formation is very shorter than that of previous reports on Al-Fe binary system. Further milling causes rapid crystallization of the amorphous phase. By analysis of S(Q), the presence of a strong Al-Fe chemical short-range order in the amorphous matrix is suggested. Moreover, the superstructure of these Al-Fe clusters in the amorphous matrix is similar to the solid structure of Al₅Fe₂, and the clusters transform into the nucleus of Al₅Fe₂ intermetallic compound under the action of milling energy.     

Solid-state transformation in nanocrystalline Ti induced by ball milling

In the current study, we report for the first time a new Ti rhombohedral (trigonal) structure induced by HEBM and subsequent sintering. During ball milling of Ti powder, solid-state transformation does not only depend on the grain refinement but also on the successful deformation of the nano-sized crystallites due to high energy ball impacts. Thermal stability of Ti-nanocrystalline in FCC allotrope was investigated. Upon sintering, the unstable FCC restored back to the rhombohedral phase rather than to HCP. The appearance of HCP Ti after sintering could suggest that prolonged milling leads to dispersion of hard particles (HCP) into more ductile particles belonging to allotropic phases, and hence possibility of resurfacing on sintering.     

Metallothermic reduction of zinc sulfide induced by ball milling

Stoichiometric mixtures of ZnS + Al and ZnS + Mg were milled for different times in a planetary ball mill. The XRD traces of the as-milled samples showed the presence of zinc, MgZn₂, and MgS after 30-min milling in the ZnS–Mg system. The traces of MgZn₂ disappeared after 1-h milling and the reduction reaction seemed to have been completed after 5-h milling. The ZnS–Al system was somewhat different with only slight reduction to zinc after 1 h and ZnS peaks still present after 10 h of milling. Isothermal heating under argon atmosphere of 3-h-milled samples showed the presence of hexagonal ZnAl₂S₄ and mixtures of MgS and Zn_0.68Mg_0.32S in the ZnS–Al and ZnS–Mg systems, respectively. These results show that the reaction in the ZnS–Al system progressed gradually during milling. The decrease in the crystallite size of reactants materials (especially ZnS) during milling operation led to decrease in the formation temperature of hexagonal ZnAl₂S₄ phase and decrease in the transformation temperature of sphalerite (ZnS) to hexagonal wurtzite.     

Solid-state reactions in the Al-Fe system induced by ball milling of elemental powders

Elemental aluminium and iron powders have been mechanically alloyed in the atomic AlFe ratios of 11 and 13. The structural evolution of the samples was followed by X-ray diffraction and differential calorimetry. Extended milling caused the formation of an almost completely disordered b c c solid solution instead of the equilibrium B2 and DO₃ compounds. Upon heating to 700 °C the formation of the B2 phase was observed in the equiatomic samples and no significant variation of the long-range state of order in the iron-rich samples. Upon heating of samples pre-milled for short or intermediate times, at about 400 °C, the nucleation and growth of Al₅Fe₂ was observed which, upon further heating to 700 °C, transformed to the B2 phase or to the b c c solid solution, depending on the sample composition. These results suggest that the solid-state reactions proceed through diffusion of iron atoms in the aluminium layers, and that for an iron concentration in these layers below about 50 at%, the Al(Fe) solid solution could be in a metastable phase.     

Nanocrystals by high energy ball milling

It has been shown that nanometer-size grains can be induced in even brittl e intermetallic compounds by high energy ball milling. The large grain boundary area provided by these nanocrystallites can help provide, along with the disordering energy, the driving free energy for the crystalline-to-amorphous transformation. Examples were given for Nb₃Sn (complete amorphization), Ni₃Al (partial amorphization), and Ni₃Si (no amorphization).     

Carburising behaviour of low carbon steel with nanocrystalline surface layer induced by ball milling

Abstract

Ball milling, as a surface nanocrystallisation method, was employed to investigate the influence of severe plastic deformation on the carburisation treatment performed on low carbon steel. The results indicated an enhancement in the carburisation efficiency as a result of surface milling. This enhancement was attributed to the formation of a nanocrystalline layer in the surface of the treated samples. It was found that the main reasons for the accelerated kinetics of the carburisation process would be the considerable amounts of non-equilibrium defects and the finer austenite grains in the early and later stages of the treatment respectively, which facilitate the carbon diffusion.     

Fast diffusion in nanocrystalline ceramics prepared by ball milling

Nanocrystalline materials can show enhanced diffusivity compared to their microcrystalline counterparts due to the large fraction of atoms or ions located in interfacial regions. In the case of ceramics, resulting properties with potential applications are, e.g., fast ionic conductivity, high mechanical creep rate and increased catalytic activity. Different nanocrystalline ceramic materials were prepared by high-energy ball milling of coarse grained source materials. The samples were characterized by XRD, TEM, BET method and IR spectroscopy. These measurements show that the primary crystallites form larger agglomerates with internal interfaces and that the reduction of the crystallite size is accompanied by a structural degradation of the surface zone. An example is the partial amorphization observed for LiBO₂ by IR spectroscopy. The diffusivity and ion conductivity in these materials was studied by NMR relaxation, NMR line shape and impedance spectroscopies. It was possible to discriminate between highly mobile ions in the interfacial regions and immobile ions in the grains. In general diffusion in the nanocrystalline systems was found to be fast compared to that in the corresponding microcrystalline source materials.     

HREM observations of the synthesized process of nano-sized SiC by ball milling of Si and C mixed powders

The synthesis of nano-sized SiC through ball milling (BM) of elemental Si and graphite mixed powders at room temperature has been reported, and detailed reaction process has been characterized by high-resolution electron microscopy (HREM). High-resolution electron microscopy (HREM) observations presented herein suggest that amorphous graphite (a-graphite), amorphous silicon (a-Si) and nano-sized crystalline Si (c-Si) with many defects are produced during BM, which is prerequisite to the reaction. In some areas, SiC is synthesized through a diffusion of C atoms into the a-Si/and c-Si. In the former, a-Si(C) forms, and then mechanically-driven crystallization of the a-Si(C) occurs to form SiC. In the latter, C atoms directly replace Si atoms to form SiC with an orientation relationship of (111)_SiC//(111)_Si. In other areas, localized self-sustained reaction occurs to form slightly larger SiC grains.     

Microstructural evolution during combustion reaction between CuO and Al induced by high energy ball milling

The product of the combustion reaction between CuO and Al induced by high energy ball milling has been characterised by using X-ray diffractometry and scanning electron microscopy. It has been observed that the combustion reaction can be ignited very easily by the ball milling. The reaction product consists of polycrystalline Cu in bulk and particle forms and a large number of nanometer sized spherical Al2O3 particles attached to the surface of the Cu. It has been demonstrated that this microstructure is evolved through rapid solidification of Cu and Al2O3 melts and rapid condensation of Cu vapour. Cu and Al2O3 phases are separated in the reaction product. The reason for this is mainly attributed to the large difference in their density and the shaking force of the ball mill.     

Synthesis of Cu-W nanocomposite by high-energy ball milling

The Cu-W bulk nanocomposites of different compositions were successfully synthesized by high-energy ball milling of elemental powders. The nanocrystalline nature of the Cu-W composite powder is confirmed by X-ray diffraction analysis, transmission electron microscopy, and atomic force microscopy. The Cu-W nanocomposite powder could be sintered at 300-400 degrees C below the sintering temperature of the un-milled Cu-W powders. The Cu-W nanocomposites showed superior densification and hardness than that of un-milled Cu-W composites. The nanocomposites also have three times higher hardness to resistivity ratio in comparison to Oxygen free high conductivity copper.     

Synthesis by ball milling and characterization of Al-Zn alloys

Mechanical ball milling is a useful technique for systems with positive enthalpy of mixing. With this technique solubility of a solute in a solid solution can be enhanced. Al-Zn system has positive heat of mixing. High energy ball milling has been employed to produce four alloys of Al with 2.5 to 10 wt% Zn. Powders of Al (1–125 m) and Zn (0.7–5.0 m) were mixed together in the desired proportion and milled with a powder to ball weight ratio of 1:20. The size and shape of the particles of as-received and alloy powders were examined in a scanning electron microscope (SEM) while their microanalysis was performed by energy dispersive system (EDS) attached with SEM. It has been observed that 120 h of milling of the powders produced homogeneous alloys. X-ray diffraction (XRD) patterns confirm complete solubility up to 10 wt% Zn in Al. Using the quasi-chemical theory of binary solid solutions, the enthalpy of mixing of 10 wt% Zn in Al has been determined to be 276 cals/mol. It is shown that stress exerted by very high density of dislocations, generated by mechanical milling, plays a major role in the enhancement of solubility. Hardness has been measured and it increases with increasing solute content.     

球磨法制备高活性铝粉

为提高含铝炸药爆热性能,探索高活性金属铝粉的制备方法,采用立式球磨机对球形铝粉进行处理,研究球磨机的搅拌转速、球磨时间、助磨剂的配比对活性铝粉粒径、形貌、热性能的影响;利用扫描电子显微镜、激光粒度测试仪和同步热分析仪检测活性铝粉形貌、粒径及热分解特性;采用热分析参数法测定活性铝含量。结果表明:制备活性铝粉的最佳条件为搅拌转速1 100 r/min,研磨时间4 h,助磨剂占铝粉质量比4%;制备的活性铝粉粒径d_(50)为1.108μm,片状,活性铝质量分数由90.42%增加到98.42%;用于含铝炸药中,爆热值由6 805 kJ/kg增加到7 642 kJ/kg。     

Pressure induced amorphization in calcium phosphates

Polycrystalline hydroxylapatite (HA), hydrated tricalcium phosphate (HTCP), tricalcium phosphate ( TCP), dicalcium phosphate dihydrate (DCPD) and dicalcium phosphate anhydrous (DCPA) were subjected to various pressures upto 10 GPa and the retrieved materials were examined by XRD and FTIR. These compounds showed amorphization at pressures upto 10 GPa, the pressure being lowest for HTCP and DCPA. At intermediate pressures, the amorphous phase obtained was anisotropic. Significant changes in the infrared spectra were observed in all materials except DCPA. These changes are due to the lowering of site symmetry on amorphization.