Mechanical alloying—a novel synthesis route for amorphous phases

Mechanical alloying (MA) pioneered by Benjamin is a technique for the extension of solid solubility in systems where the equilibrium solid solubility is limited. This technique has, in recent years, emerged as a novel alternate route for rapid solidification processing (RSP) for the production of metastable crystalline, quasicrystalline, amorphous phases and nanocrystalline materials. The glass-forming composition range (GFR), in general, is found to be much wider in case of MA in comparison with RSP. The amorphous powders produced by MA can be compacted to bulk shapes and sizes and can be used as precursors to obtain high strength materials. This paper reports the work done on solid state amorphization by MA in Ti-Ni-Cu and Al-Ti systems where a wide GFR has been obtained. Al-Ti is a classic case where no glass formation has been observed by RSP, while a GFR of 25–90 at.% Ti has been obtained in this system, thus demonstrating the superiority of MA over RSP. The free energy calculations made to explain GFR are also presented.     

Structure of nanocomposites of Al-Fe alloys prepared by mechanical alloying and rapid solidification processing

Structures of Al-based nanocomposites of Al-Fe alloys prepared by mechanical alloying (MA) and subsequent annealing are compared with those obtained by rapid solidification processing (RSP). MA produced only supersaturated solid solution of Fe in Al up to 10 at.% Fe, while for higher Fe content up to 20 at.% the nonequilibrium intermetallic Al₅Fe₂ appeared. Subsequent annealing at 673 K resulted in more Al₅Fe₂ formation with very little coarsening. The equilibrium intermetallics, Al₃Fe (Al₁₃Fe₄), was not observed even at this temperature. In contrast, ribbons of similar composition produced by RSP formed fine cellular or dendritic structure with nanosized dispersoids of possibly a nano-quasicrystalline phase and amorphous phase along with α-Al depending on the Fe content in the alloys. This difference in the product structure can be attributed to the difference in alloying mechanisms in MA and RSP.     

Mechanical alloying and milling

Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.     

RSPAl—Li—Cu—Mg—Zr合金的显微组织研究

通过透射电镜组织观察及动态拉伸试验，研究了一种ＲＳＰＡｌ－Ｌｉ合金的显微组织及其在合金中所起的作用。结果表明，快速凝固方法显著细化了Ａｌ－Ｌｉ合金的晶粒，可起到细晶强化和缩短性变形时位错共面滑移距离的作用。δ’（Ａｌ３Ｌｉ）相是合金中的主要强化相，晶粒细化对δ’相的析出没有影响。β’（Ａｌ３Ｚｒ）相在合金的热加工过程中形成，可起到阻止晶粒长大和分散共面滑移的作用。合金易于形成晶界无析出带易成连续的     

Rapid solidification processing

It has been over eight years since the subject of rapid solidification processing (RSP) has been given a general review. The early reviews of RSP were undertaken before the very rapid expansion of the late 1970s and early 1980s and the most recent general review was published in 1984, at approximately the height of RSP activity in the USA. More recent specialized reviews have been prepared for magnesium, titanium and aluminium alloys. The expansion of research and application testing resulted from the first major attempts to employ RSP on a large scale under both industrial and government sponsorship. The objective here is to develop the historical background of RSP, and then to update the present theoretical base for this technology. Examples of recent work on the rapid solidification of metals and ceramics will be given, along with examples of applications. Finally, an assessment of the future directions and prospects for RSP will be presented.     

Microstructures and phase selection in rapidly solidified Zn-Mg alloys

The Zn-Mg system has potential glass-forming ability, and therefore studies were made of rapidly solidified zinc-based Zn-Mg alloys containing up to 6 wt% Mg. These alloys exhibited interesting eutectic phase selections and structural transitions across the ribbon thickness which are represented on a microstructure selection diagram for rapid solidification conditions. Although rapid solidification is known in many cases to produce metastable phases, in this case the equilibrium eutectic mixtures of Zn-Mg₂Zn₁₁ are observed after rapid solidification, whereas the metastable eutectic mixture Zn-MgZn₂ forms under normal solidification conditions. However, in the melt-spun Zn-Mg alloy which is exactly at eutectic composition, three different structures are observed across the ribbon thickness. These three structures do not exist simultaneously in the same region, but structural transitions occur as the thickness increases from the wheel side to the free side. Eutectic and hypereutectic alloys show a tendency to form a metallic glass. In these alloys a critical growth velocity exists beyond which eutectic solidification is not possible, suggesting a possible transition from eutectic solidification to amorphous phase formation. The eutectic phase selection and the extent to which a specific microstructure is present depends on the variation in growth rate and solid-liquid interface stability during rapid solidification.[/p]     

Solidification and melting of high temperature materials: in situ observations by synchrotron radiation

Until recently understanding the solidification behavior of high temperature materials, including many intermetallic systems, required evaluation of a great number of individual solidification experimental results. An additional challenge was the reactivity of metallic melts at elevated temperatures. Alternative methods for in situ observation of solidification processes using the high-energy synchrotron X-ray diffraction, which came up in the last decade, are reviewed in the present work. Here, solidifying phases and transformation sequences are directly related to their X-ray diffraction pattern, which avoids any confusion caused by subsequent phase transformations especially in complex systems. By containerless processing with aerodynamic, electrostatic and electromagnetic levitation methods, adapted to the application at the synchrotron beamline, contamination of the melt with impurities is avoided, which can corrupt the results of solidification studies by conventional methods. To date, the majority of the studies is focused on metastable phase formation and the structure of undercooled melts. Current efforts on liquid–solid phase transformations under conditions close to the equilibrium, which provide a great potential for acquisition of phase diagram data of refractory and reactive alloys, are also addressed.     

Metastable phases in amorphous metals with emphasis on Fe3B

The amorphous metallic state may be obtained by a number of methods such as vapour deposition, ion-beam implantation and mixing, and by rapid solidification of liquid metals. New metastable phases may be derived from the amorphous state. This paper reviews the type and nature of some of these metastable phases. In particular, it will be shown that the metastable bc tetragonal Fe₃B phase may be related to the presence of simple unit cells in the amorphous phase. Fe₃B may also be found in a conventionally cast material.     

Rapid solidification of steels

Solidification at very high rates of cooling results in considerable refinement in the microstructure of alloys. It enables the formation of extremely fine grains, extends solid solubilities and leads to the nucleation of many metastable crystalline phases. We discuss briefly the methods of rapid solidification and their impact on microstructure and structure of alloys. Each of the aspects of structure development is illustrated with examples from steels.     

Principles of equal-channel angular pressing as a processing tool for grain refinement

During the last decade, equal-channel angular pressing (ECAP) has emerged as a widely-known procedure for the fabrication of ultrafine-grained metals and alloys. This review examines recent developments related to the use of ECAP for grain refinement including modifying conventional ECAP to increase the process efficiency and techniques for up-scaling the procedure and for the processing of hard-to-deform materials. Special attention is given to the basic principles of ECAP processing including the strain imposed in ECAP, the slip systems and shearing patterns associated with ECAP and the major experimental factors that influence ECAP including the die geometry and pressing regimes. It is demonstrated that all of these fundamental and experimental parameters play an essential role in microstructural refinement during the pressing operation. Attention is directed to the significant features of the microstructures produced by ECAP in single crystals, polycrystalline materials with both a single phase and multi-phases, and metal-matrix composites. It is shown that the formation of ultrafine grains in metals and alloys underlies a very significant enhancement in their mechanical and functional properties. Nevertheless, it is demonstrated also that, in order to achieve advanced properties after processing by ECAP, it is necessary to control a wide range of microstructural parameters including the grain boundary misorientations, the crystallographic texture and the distributions of any second phases. Significant progress has been made in the development of ECAP in recent years, thereby suggesting there are excellent prospects for the future successful incorporation of the ECAP process into commercial manufacturing operations.     

Non-equilibrium synthesis of new materials

Over the last few decades it has become apparent that metallic materials produced by conventional casting processes have reached their limit. Some improvements have been obtained by rapidly cooling bulk alloys but major increases have required the development of rapid solidification processes. However, more radical approaches, such as mechanical alloying (MA) and physical vapour deposition (PVD), are also being explored and these are the subject of this paper. This paper describes the physical basis behind the development of MA and PVD in the Structural Materials Centre and presents the results of some materials development programmes on aluminium, magnesium and titanium based materials.     

Fabrication of Cu rich bulk metallic glass composites via solidification method

Abstract

Cu based bulk metallic glasses and composites with tiny crystalline phases embedded in metallic glass matrix have been successfully fabricated by solidification technique in the present work. The formation of crystalline phases and structure inhomogeneity in bulk metallic glasses was characterised. Al is used as the minor alloying element to partly substitute Cu element in 61Cu–34Zr–5Ti. The results show that quarternary 60Cu–34Zr–5Ti–1Al alloy exhibits monoamorphous feature, and 56Cu–34Zr–5Ti–5Al alloy has a few crystalline peaks superimposed on a broad diffraction peak, suggesting that a composite structure forms in certain solidification conditions. To further identify the microstructure of the as cast rod, all samples were characterised by scanning electron microscopy (SEM). Small size phases are found in 2 mm diameter 56Cu–34Zr–5Ti–5Al rod, which has larger plastic deformation. The composition of those crystalline phases is also investigated. All results indicate that the presence of certain phases in metallic matrix benefits the mechanical properties of the as cast bulk metallic glasses.     

Influence of solidification variables on the microporosity formation on Al–Cu (4.5 wt%) alloy with axial heat processing

The effects of solidification rate, hydrogen concentration, and level of convection on porosity formation in Al–Cu (4.5 wt%) alloys were investigated using Axial Heat Processing (AHP). This processing technique is similar to the conventional directional solidification (DS) technique, except that it utilizes a graphite baffle immersed near the solidification interface to control the shape of the interface and impart an axial temperature gradient. It was found that the samples produced by AHP contained 20–40% less microporosity than similar samples produced by conventional DS. The reduction was also more pronounced with decreasing a cooling rate and increasing an initial hydrogen concentration in the melt. These differences are attributed to the solute accumulation that is due to the confinement of the liquid below the baffle and the concomitant reduction in the convection level near the interface.     

Mechanism for metastable phase formation in undercooled Ti–Al peritectic alloys

Melt undercooling technique has been proved to be a powerful tool to investigate the formation of metastable phase in rapid solidification processing. Here, the recently observed phase evolution behaviour of the near equal atomic percent Ti–Al alloys was analyzed, as a function of melt undercooling, by thermodynamic and kinetic calculation. It was found that the formation of metastable phase is lightly predicted by the chemical Gibbs energy difference among competing phase but strongly controlled by kinetic effects arising from the competition of nucleation. The transient nucleation theory, with a consideration of incubation time, was proved to be an effective tool to explain the metastable phase formation in the undercooled near equal atomic percent Ti–Al alloys.     

Calculation of cooling and heating rates and transformation curves for the preparation of metallic glasses

A Fortran program is described for calculating and plotting time-temperature-crystallization curves during rapid heating and cooling by, for example, laser glazing, electron beam melting, melt-spinning or other techniques for the production of metallic glasses and metastable phases. Experiments were carried out to demonstrate the applicability of the program. In the case of Fe-B alloys produced by melt-spinning the behaviour during continuous cooling and isothermal annealing could be predicted. In the case of Ni-Nb alloys the program was used to establish the experimental conditions for the formation of metallic glass by electron beam melting. The program was also used to determine the conditions necessary for glass formation by laser melting a surface layer of boron alloyed to the surface of an Fe-Cr substrate.     

Phase stabilization of plasma-sprayed alumina coatings by spraying mechanically blended alumina–chromia powders

A study of phase stabilization is performed on atmospheric plasma-sprayed alumina–chromia coating applied on low-carbon steel substrates. Alumina–chromia mixture with varying amounts of chromia content (1?wt.% to 6?wt.%) is used for this purpose. X-ray diffraction (XRD) analysis is carried out to investigate the different phase compositions of the coatings. Quantification of the different phases present in the coatings is performed by subsequent use of Rietveld refinement method. Surface morphology, microhardness, and wear behavior of the different coatings are also observed. Rietveld analyses performed on coatings ensure the stabilization of metastable phases present in the alumina coatings by chromia addition. Significant increase of the α -alumina and (Al_xCr_1-x)₂O₃ content is observed in alumina coatings with 4?wt.% chromia content. This is supported by the improved hardness and wear-resistant properties of the alumina coatings containing 4?wt.% chromia. The minimum surface roughness of the coating is also observed for alumina coatings with 4?wt.% chromia content among all the alumina coatings with different chromia content. The formation of alumina–chromia solid solution and solidification of α-alumina are found to be the reasons behind the enhancement in mechanical properties of the coating.     

The effect of pressure on metastable phase formation in the undercooled Bi-Sn system

A metastable phase was produced by the solidification of highly undercooled Bi-48.6 at% Sn alloy droplet samples. During heating the metastable phase was observed to melt at 116° C at ambient pressure. The onset of the metastable endotherm was found to increase with increasing pressure, while the liquidus and eutectic temperature for the structure stable at ambient pressure decreased with increasing pressure. Based on the pressure dependence of the melting trend, the metastable phase will be stable at the expense of the stable ambient pressure structure under high hydrostatic pressure conditions (above 1 GPa). Both microstructural observations and X-ray examinations at ambient pressure revealed that the metastable phase was present in droplet samples and that the X-ray diffraction pattern was close to that of the high-pressure stable phase previously reported as a rhombic cell. High-pressure thermal analysis has also allowed for identification of the effect of pressure in promoting favourable formation kinetics and the kinetic transition from the equilibrium phases to the metastable phase at high undercooling.     

On the Formability of Some Metastable Alloy Phases

Pattern recognition and neural network methods have been used to investigate the formability of metastable alloy phases, It has been found that some chemi cal bond parameters Such as valence electron number, electronegativity and metallic radii of cor-nponent elements are the dominating fac tors affecting metastable alloy phase formation. Some semi-empirical rules found in this way may be useful for the construction of expert system for materials design.     

Isothermal melt processed Bi-2212/Ag tapes containing MgO and Al2O3 additions

The development of the superconducting phase in melt-processed Bi-2212 tapes depends on controlling the development of solid phases in the partial melt and the conversion of the partial melt into a highly-aligned superconducting phase. MgO and Al₂O₃ additions have been used with isothermal melt processing (IMP) for the grain refinement of phases in the partial melt and the prevention of secondary phase growth during solidification. Controlled solidification with reduced oxidation rates also significantly increased the transport properties of the tapes with or without the oxide additions. The optimal oxidation rate was found to depend upon the processing temperature. Critical current densities in excess of 100 kA cm^–2 were obtained in tapes melt-processed below 800 °C.