Immiscible Metal Systems Research Articles

Mechanical Alloying of Immiscible Metallic Systems: Process, Microstructure, and Mechanism

Immiscible metallic systems provide new opportunities for tailoring material properties due to their unique microstructure. However, they are immiscible in the liquid state, or separate into incompatible liquid phase during the cooling process, making it difficult to obtain their alloys or phases. Mechanical alloying is a solid‐state processing route, whose basic principle is to utilize mechanical impact to make materials suffer cyclic deforming, fracturing, and welding. It reduces the energy barrier and provides pathways of atom diffusion, achieving atomic‐level alloying in solid‐state. Notably, mechanical alloying does not undergo the cooling process from liquid to solid phase, which can avoid the solute separation. This article discusses the mechanical alloying process, microstructure, and mechanism of immiscible metallic systems. It focuses on the alloying principles, including solid solutions, intermetallic compounds, nanocrystalline or amorphous alloys. In addition, the effects of process parameters, including rotation speed, milling time, and ball‐to‐powder weight ratio, on microstructure and phase constitution are comprehensively reviewed. Some useful suggestions regarding the preparation of immiscible metallic systems are also put forward for future works.

The physicochemical features of binary immiscible metal systems in a new method of synthesis and crystallization

Features of the structure of phase diagrams of immiscible binary metal systems for synthesis and crystallization of various compounds by a new method are considered: temperature and concentration limits of immiscibility, ratio of densities of system components, use of mixed solvents, and processes of hardening of pure immiscible systems. An example of obtaining crystals of refractory compounds is given.

Binary immiscible metal systems for preparation of borides

The binary systems with the immiscibility in the liquid state are considered as a tool for synthesis and crystallization of borides. New preparation method using two solvents (immiscibility gap method) is compared with the conventional solution-melt method using one solvent (flux method). The appropriate systems are analyzed with respect to the temperature and concentration limits of the immiscibility gap, density ratio of solvents and reagents, interaction of reagents, formation and crystallization of compounds, and allocation of products in the metallic matrix.

A Calphad-compatible method to calculate liquid/liquid interfacial energies in immiscible metallic systems

For the calculation of liquid/liquid interfacial energies in monotectic metallic alloys only simplified models are known, which suppose the validity of the regular solution model for the liquid solution. In the present paper a Calphad-compatible method is developed to calculate the liquid/liquid interfacial energies in binary, ternary and multicomponent liquid monotectic alloys. The method can be considered as the extension of the Butler equation to the liquid/liquid interfaces. The first order interfacial phase transition has been predicted at a certain critical temperature, which is about 18% of the bulk critical temperature. Below this critical temperature the compositions of the two sides of the liquid/liquid interface are different, while above this critical temperature the compositions at the two sides of the interface are identical. The latter case is valid for the majority of liquid metallic systems above their monotectic temperatures. The method has been validated against the experimentally measured liquid/liquid interfacial energies in the Ga–Pb and Al–Bi systems. The method is found to be very sensitive to the correctness of the Calphad-assessment of the partial excess Gibbs energies of the components in the liquid solution. The excess Gibbs energy function of the liquid Al–Bi solution was reassessed using our recent exponential temperature dependence of the interaction energies.

Nanosized mechanocomposites and solid solution in immiscible metal systems

Mechanical alloying is the most perspective method for preparation of nanosized mechanocomposites in non-mixed liquid and solid metals. It is known that plastic metals get fragile in the presence of the liquid metal phase, and this phase spreads over the surface of solid metal due to good wettability. Temperature of milling bodies can rise by several hundred degrees in the high-energy planetary ball mill. Low melting metals, such as gallium, indium, tin, and bismuth can melt on the surface of the balls. So, a pair composed of liquid and solid metal can form in an activator. In such cases, it can be assumed that mechanical activation of metal systems with positive mixing enthalpies, can allow forming a morphologically metastable structure with unusually high concentration of inter-phase boundaries due to easy-melting component spreading along the boundaries of particles formed by comminution of the solid component. Nanosized mechanocomposites were prepared for Cu-Bi and Fe-Bi systems. The content of Bi in these systems must be lower than 3 at. (10 wt.)%. The obtained mechanocomposites represent nanosized particles of copper (or iron) coated with a layer of bismuth in 2-3 atoms. Mechanical activation of the formed composites leads to the formation of supersaturated solid solutions.

Transmission Electron Microscopy Studies of Mechanical Alloying in the Immiscible Fe<sub>2</sub>O<sub>3</sub>-SnO<sub>2</sub> System

Microstructural development and nanoscale compositional variations in mechanically alloyed Fe 2 O 3 -SnO 2 powders have been examined by transmission electron microscopy and energy dispersive X-ray spectrometry The mean grain size was found to stabilize around 10 nm after 19 h milling time, in close agreement with that estimated from X-ray diffraction line broadening measurements, whereas dissolution of SnO 2 grains was incomplete even after 110 h Isolated grains with the SnO 2 cassiterite structure, of diameter >10 nm, persisted up to the maximum milling time These observations are discussed in relation to previous measurements in the same system by X-ray diffraction and Mossbauer spectroscopy, which suggested that alloying on the atomic scale occurred after 110 h milling The present studies confirm that the amount of Sn dissolved in the Fe 2 O 3 hematite lattice increases with longer milling times, indicating that a supersaturated solid solution is formed, but that mixing may be locally inhomogeneous at the atomic level. Similar conclusions have been reported for studies of mechanical alloying in immiscible metallic systems The tendency for SnO 2 grains above a certain critical size to remain undissolved, while smaller grains can more easily enter into solid solution with Fe 2 O 3 , is consistent with the expected behaviour due to the increased chemical contribution to the interfacial energy with decreasing grain size Mossbauer results also showed that some of the SnO 2 had not reacted with Fe 2 O 3 after 110 h milling.

Characterization of CoCu mechanical alloys by linear sweep voltammetry

The mechanical alloying process has shown that it is possible to obtain supersaturated solid solutions at any composition in immiscible metallic systems by ball milling of elemental powder mixtures. The mechanically alloyed CoCu powders are chemically homogeneous supersaturated solid solutions and exhibit crystallite sizes of a nanometer order. In the present work, the mechanically alloyed CoCu powders have been characterized by linear sweep voltammetry in 0.5 M NaOH and 0.15 M Na 2B 4O 7 aqueous solutions. The electrochemical behavior has been related to the characteristics of enhanced solubility found in CoCu mechanical alloys. In order to perform a systematic study of this system, the voltammograms of pure Cu, pure Co, CoCu original powder mixtures and CoCu alloys, obtained by melting and casting, were also analyzed for comparison. The results showed that the voltammograms of mechanically alloyed CoCu powders presented peaks of dissolution at potentials intermediate between those exhibited in pure Cu and Co samples. Moreover, a third dissolution peak occurred at more noble potentials in mechanical alloys, but not in as-cast alloys. This peak is associated with the formation of the solid solution, in mechanical alloys. This suggests that both elements are dissolved simultaneously, which confirms that CoCu mechanical alloys are true supersaturated solid solutions. Additionally, it was found that the MA CoCu alloys showed corrosion rates higher than those in as-cast alloys, which is attributed to the high value of stored energy presented in MA alloys as a result of the severe plastic deformation during ball milling.

The effects of non-equilibrium processing in the development of copper alloys

Two non-equilibrium processes, rapid solidification (RS) and mechanical alloying (MA), have been used to synthesize and control highly refined microstructures in liquid immiscible metal systems and limited solubility systems at high volume fractions of the immiscible constituent. Studies of AlIn, CuCr, CuW and CuPbSn microcomposites suggest that they possess technologically important mechanical and physical properties and notable thermal stability when the gross chemical segregation that occurs during conventional processing can be avoided. The effects of RS and MA on several copper alloy systems are presented. The concept of phase redistribution processing (PRP) is also discussed. PRP is the sequential application of RS and MA to achieve substantial microstructural refinement and redistribution of phases in systems displaying segregation or coarse or localized precipitation and dispersions. The effectiveness of PRP is demonstrated by its application to a CuPbSn automotive bearing alloy. The examples presented in this paper suggest the wide applicability of RS, MA and PRP to previously intractable alloy systems to create useful new alloys and new forms of conventional alloys.