Tribochemical equilibrium in mechanical alloying of metals

The structure of the product in mechanical alloying depends both on the elemental composition and the milling conditions. An increase of ball energy led to more pronounced crystallinity of the product. Mechanical alloying at small ball energy leads to the formation of amorphous alloys for Zr-Co and Cu-Ti systems. Demixing of Ti₃Cu₄ into crystalline TiCu and TiCu₄ and demixing of Zr₅₀Co₅₀ into Zr₃Co and ZrCo₂ was found. The results are explained on the basis of the concept of tribochemical equilibrium.     

Structure evolution in the Cu-Fe system during mechanical alloying

High-resolution electron microscopy was used to examine the structure evolution of Cu-60 at % Fe powder mixture during mechanical alloying. Fracture and refinement of particles, the lamellar structure formed by cold-welding, and nanocrystals, were all observed at atomic scale. The X-ray diffraction patterns show that the Bragg peaks from the b c c phase decrease obviously in intensity after 3 h milling and entirely disappear after 5 h milling. Lattice images of the products obtained after 3 h milling reveal that there are Nishiyama-Wasserman orientation relationships between the b c c and f c c phases, i.e. (001)//(110), [1 0]//[1 2] and [110]//[ 11] . It is likely that for a mechanically alloyed iron-rich powder mixture, ball milling induces a reverse martensitic transformation of b c c Fe(Cu) to f c c Fe(Cu) phase. The greatly extended f c c phase range is closely related to this transformation. After 5 h milling, nanocrystals with sizes about 10 nm are formed.     

Early and late mechanical alloying stages of the Pd-Si system

Four Pd-Si compositions (Pd/Si=2.5/1; 3/1; 4/1 and 5/1) were mechanically alloyed by a ball-milling technique. X-ray diffraction and fluorescence analysis were used to monitor the mechanical alloying (MA) process which was carried out in planetary and vibratory ball mills. The first step of the milling process consists in a very fine fragmentation of the silicon particles into the palladium matrix. After this early stage of milling, formation of the intermetallic line compound Pd₃Si can occur for the 2.5/1, 3/1 and 4/1 composition depending on the milling conditions and/or milling apparatus adopted: i.e. on the conditions of energy transfer experimentally realized. Subsequent milling indicates that amorphization probably occurs starting from the previously formed Pd₃Si compound. Long milling times, up to 56 h, promote a demixing process towards the parent elements for the 2.5/1 and 4/1 compositions. Thermal treatments of the long-term milled samples confirm the final products obtained from the near room-temperature solid state reaction induced by MA. For the Pd₅Si composition the conditions for Pd₃Si formation and subsequent amorphization were never reached.     

Nanocrystalline formation in immiscible Cu-Mo system subjected to mechanical alloying

The mechanical alloying process has been studied on the Cu-Mo system, the atomic pair of which is characterized by a positive heat of mixing of +19 kJ/mol. The EXAFS and X-ray diffraction measurements have been employed to analyze the structural changes taking place during milling. Two phases mixture of nanocrystalline fcc-Cu and bcc-Mo with a grain size of 10 nm has been formed by MA of Cu30Mo70 powders for 200 hours. The structural analysis based on the EXAFS spectra revealed that bcc and fcc crystal structure clearly do not change around Mo and Cu atoms up to 200 h of milling, respectively. Studies of the thermodynamical considerations by DSC analyses confirmed that the alloying does not occur even after 200 hours of MA in Cu-Mo system.     

Formation of nanocrystalline phases in the Cu-Zn system during mechanical alloying

The evolution of various nanocrystalline phases in the Cu-Zn system during the course of mechanical alloying of elemental powder blends, manifests a similar sequence of phase formation in all the compositions. Zinc-rich phases were always first to form, which can be attributed to the diversities of diffusivities and diffusion distances in the constituents. The extent of zinc in -phase becomes significant only after turns nanocrystalline. The crystallite size of reached a minimum (18 nm) near the - phase boundary, while the and phases showed quite coarse crystallite size due to their low melting points. Alloying was sluggish during dry milling compared to wet milling, or at lower milling speed, which demonstrates the effects of oxidation during milling and lower milling energy.     

Solid state amorphization of Mg-Zn-Ca system via mechanical alloying and characterization

Magnesium based bulk metallic glasses have attracted significant attention of researchers due to better mechanical and corrosion properties when compared to their crystalline counterparts especially for biomedical applications. Scaling up the part size and production volumes of such materials through liquid metallurgy route is challenging. In this work amorphous Ca₅Mg_60+xZn_35?x (X = 0, 3 and 7) alloys have been successfully synthesized through solid state amorphization using a high energy planetary ball mill. X-ray diffraction was used to identify the crystalline phases of the powder during reaction. Evolution of amorphous phase was analysed using a parameter involving the ratio of integral area of peaks to the integral area of background (I_PB) obtained from XRD patterns. Results showed reaction time increases with decreasing Zn content in Ca₅Mg_60+xZn_35?x (X = 0, 3 and 7) alloy to obtain maximum amorphous structure with a small amount of residual crystalline phase. Prolonged milling of these powders, to eliminate residual crystalline phases, resulted in the nucleation of Mg_102.08Zn_39.6 phase. The composition dependent characteristic temperatures and thermal stabilities were studied using differential scanning calorimetry.     

A study of nanocrystalline iron and aluminium metals and Fe3Al intermetallic by mechanical alloying

Pure iron and aluminium powders and a mixture of composition Fe₇₅Al₂₅ were treated mechanically in a high-energy mill for up to 40 h. X-ray diffraction and analytical transmission electron microscopy were coupled to elastic energy dissipation and dynamic Young's modulus measurements to study the structural transformation of the specimens induced by the mechanical treatment. A quantitative comparison between the structural behaviour of the pure elements and of the mixture was carried out. The role of the parameters such as the composition, the grain size and the activation energy during the process was examined in relation to the competing mechanisms of plastic deformation and recovery.     

Molecular dynamics investigation of mechanical mixing in mechanical alloying

Molecular dynamic simulation is exploited to obtain a deep insight of atomic scale mixing and amorphization mechanisms happening during mechanical mixing. Impact–relaxation cycles are performed to simulate the mechanical alloying process. The results obtained by structural analysis shows that the final structure obtained through simulation of mechanical alloying is in an amorphous state. This analysis reveals that amorphization occurs concurrently with the attainment of a perfectly mixed alloy. The results indicate diffusion and deformation are two important mechanisms for mixing during mechanical alloying. The rate of diffusion is controlled by the temperature and by the density of defects in the structure. Deformation enhances mixing directly by sliding atomic layers on each other and increases the number of defects in the structure. The results agree with mechanical alloying experiments described in the literature.     

Solid solubility extension of copper‐tin immiscible system during mechanical alloying

In this paper, copper‐5 wt.%‐tin (Cu‐5wt%Sn) powder mixture was mechanically alloyed in order to study the solid solubility extension during the alloying process. Nanocrystalline supersaturated solid solution has been prepared in this system by high energy ball milling. Based on the thermodynamic model, the Gibbs free energy change in this alloy system during the formation of solid solution is calculated to be positive, which means that there is no driving force to form solid solution in copper‐tin (Cu?Sn) immiscible system. It has been found that a large fraction of grain boundaries and a high density of dislocations play a significant role in the solid solubility extension of immiscible copper and tin.     

Synthesis of Mg-Ti alloy by mechanical alloying

Mg-based Mg-Ti binary alloys have been synthesized by mechanical alloying of Mg and Ti powder blends. It was found that mechanical alloying of Mg and Ti results in a nanocrystalline Mg-Ti alloy and an extended solubility of Ti in Mg, due to the favorable size factor and the isomorphous structure of Mg and Ti. In the case of Mg-20at.%Ti, about 12.5% Ti is dissolved in the Mg lattice when the mechanical alloying process reaches a stable state. The rest (about 7.5 at.%) remains as fine particles in the size of 50–150 nm in diameter. Dissolution of 12.5 at.% Ti in the Mg lattice causes a contraction of the unit cell volume from 0.0464 to 0.0442 nm³ and a decrease of the c/a ratio from 1.624 to 1.612 of the hexagonal structure. The supersaturated solid solution Mg-Ti alloy decomposes upon thermal annealing at temperatures above 200°C. Hydrogenation enhances the decomposition process at lower temperatures.     

Amorphization reaction during mechanical alloying: influence of the milling conditions

Amorphous Ni-Zr powders have been prepared by mechanical alloying of elemental crystalline powders. The glass-forming range has been determined in detail at different milling intensities. Depending on the milling conditions, at least partial crystallization of the formerly amorphous material can occur from 66 to 75 at% Ni, due to a temperature rise during milling at high intensity. In comparison with isothermal annealing experiments at various temperatures on completely amorphous powder, a relation between milling temperature and milling time is shown. This confirms the similarity of the amorphization process during mechanical alloying with the solid-state interdiffusion reaction in alternating crystalline multilayers.     

Characterization of Ti-HA composite fabricated by mechanical alloying

Titanium (Ti) and its alloys possess suitable mechanical characteristics for utilization in orthopedic implants. However, their poor integrity with native tissues is a major challenge in their clinical application. Composite structures of Ti and hydroxyapatite (HA) can be used to promote the bone ingrowth and integration of the implant with the surrounding tissue. Here, we report the fabrication of Ti-HA nanocomposite powders using a high energy planetary ball mill. We investigate the effects of fabrication parameters including HA content (10–30% w/w), milling time (20 and 50 h), and HA particle size (50 nm and 15 μm) on the characteristics of the fabricated composites. In particular, we determine the samples hardness, sintering density, surface roughness and topography for different conditions. The results show that the addition of HA to Ti decreases the sintering density and enhances the surface hardness. Also, we observe a direct relationship between HA concentration in the Ti matrix and the surface roughness.     

机械合金化制备SbSn金属间化合物的研究

用机械合金化法制备SbSn金属间化合物.使用X射线衍射仪、扫描电子显微镜、透射电镜和DSC差热分析方法对Sb、Sn混合粉末经不同工艺条件合成的产物进行了分析.结果表明:机械合金化法可合成Sb-Sn金属间化合物;随着机械合金化持续进行,合金化的粉末和晶粒不断细化,晶粒内部产生很大的晶格畸变,并且球磨产生的密度和缺陷使原子扩散加快.     

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.