Unidirectional solidification of ternary eutectic Al-Ni-Si alloys

Two monovariant alloys — type I (Al/6%Ni/0–2.5%Si) and type II (Al/12%Si/0–1.5%Ni) — and the ternary (Al-Ni-Si) eutectic have been studied. For the type I alloys particular attention was given to the effect of silicon concentration on the microstructural stability of the binary eutectic Al-Al₃Ni. Three distinct morphologies were observed, according to the solidification parameters. These were: well-aligned; aligned/cellular transitional; and cellular. The morphology of the silicon phase in the type II alloys was found to be very sensitive to any change in the solidification rate. The presence of very small amounts of nickel resulted in the formation of Al₃Ni; its growth was influenced by the kinetics and growth of the silicon phase. In the ternary eutectic, two of the three phases — Al₃Ni and Si — developed rod-like morphologies at the lowest solidification rates, with almost planar solid/liquid interfaces. As in the type II alloys, the silicon phase was sensitive to the solidification rate and it affected the growth of the Al₃Ni.     

Modeling of gas porosity and microstructure formation during dendritic and eutectic solidification of ternary Al-Si-Mg alloys

A two-dimensional(2-D)multi-component and multi-phase cellular automaton(CA)model coupled with the Calphad method and finite difference method(FDM)is proposed to simulate the gas pore for-mation and microstructures in solidification process of hypoeutectic Al-Si-Mg alloys.In this model,the pore growth,and dendritic and eutectic solidification are simulated using a CA technique.To achieve the equilibrium among multiple phases during ternary Al-based alloy solidification,the phase transition thermodynamics and kinetics are evaluated by adopting the Calphad method.The diffusion equations of hydrogen and two solutes are solved by FDM.The developed CA-FDM coupled model can be used for sim-ulating the evolution of gas microporosity and microstructures,involving dendrites and irregular binary and ternary eutectics,of ternary hypoeutectic Al-Si-Mg alloys.It has the capability of reproducing the interactions between the hydrogen microporosity formation and the growth of dendrites and eutectics,the competitive growth among the growing gas pores of different sizes,together with the time-evolving concentration fields of hydrogen and solutes.The simulated morphology of gas pore and microstructure has a good agreement with the experimental observation.The influences of the initial hydrogen concen-tration and cooling rate on the microporosity formation are investigated.It is found that the main portion of porosity formation occurs in the eutectic solidification stage through analyzing the profiles of porosity percentage and solid fraction varying with solidification time.The varying features of simulated porosity percentage,the maximum and average pores radii indicate that increasing initial hydrogen concentration promotes the formation of higher final porosity percentage and larger pores,while the size of gas pores will significantly reduce with increasing cooling rate,leading to a lower final porosity percentage.     

The solidification and mechanical properties of chill-cast AI-AI3IMi and AI-AI2Cu eutectic alloys

This paper describes the effect of chill-casting on the solidification behaviour and mechanical properties of the AI-AI₃Ni and AI-AI₂Cu eutectic alloys. Cellular microstructures were obtained by casting the eutectic alloys into preheated split-steel moulds mounted on either a water-cooled or plain copper chill, to promote growth along the length of the ingot and not radially from the mould wall. This produced the required cellular microstructure with good alignment of AI₃Ni fibres or AI₂Cu lamellae within the cells, with an interfibre/interlamellar spacing of 1 m. The experimental solidification results showed an increase in solidification rate with increasing distance from the chill associated with a decrease in interfibre/interlamellar spacing along the length of the solidifying ingot. There were no significant variations in the room-temperature tensile properties of the two chill-cast aluminium based eutectic alloys for the various casting conditions. Variations in solidification rate along the ingots for the different chill-casting conditions were not sufficient to affect the stress-strain behaviour of the chill-cast alloys. The room-temperature tensile behaviour of the chill-cast AI-AI₃Ni eutectic alloy was very similar to, and that of the AI-AI₂Cu eutectic alloy significantly different from, those obtained by Lawson and Kerr. The ultimate tensile strengths of the chill-cast eutectic alloys were not as high as those of the corresponding unidirectionally-solidified eutectic alloys prepared at a slow and constant solidification rate although the reasons for this were different for the two alloys. The ultimate tensile strength of the chill-cast AI-AI₃Ni eutectic alloy was found to be in reasonable agreement with that expected from the rule of mixtures for discontinuous fibre reinforcement.     

Controlled solidification of the eutectic LiF-MgF2

A two-phase model composite material of LiF and MgF₂ has been prepared by unidirectional solidification of the mixed eutectic fluoride. The microstructure has been examined by optical and electron microscopy. The morphology is that of regularly arranged rods of the MgF₂ phase (rutile structure type) in a continuous LiF matrix. This microstructure is restricted to high purity materials with a planar solid-liquid interface and progressively degrades into a cellular lamellar structure associated with constitutional supercooling. The crystallographic orientation relation between the phases for the regular morphology has been determined as: Growth axis //[001]MgF₂//[001]LiF, Preferred interfaces //(110)MgF₂//(100)LiF and (1¯10)MgF₂//(010)LiF.     

Modeling of free eutectic growth and competitive solidification in undercooled near-eutectic alloys based on in situ measurements

Free eutectic growth and its competition with single-phase growth in solidification of undercooled near-eutectic alloys are not yet fully understood. In this paper, the historical development of eutectic growth models was reviewed. The LZ model of free eutectic growth was evaluated using recent data of eutectic growth velocities in an undercooled Ni_81.3Sn_18.7 eutectic composition. An excellent agreement was achieved between the LZ model and the data. Crystal growth velocities in off-eutectic Ni₈₃Sn₁₇ and Ni₈₀Sn₂₀ compositions were measured using a high-speed camera technique. The present data of the off-eutectic compositions and the recent data of the eutectic composition were modeled using the LZ model and the LKT/BCT model of free dendritic growth. The modeling revealed that the competition between the free eutectic growth and the single-phase growth is controlled by the highest interface temperature criterion. A coupled zone of the α-Ni-Ni₃Sn eutectic was calculated using this criterion. The coupled zone agrees well with studies of solidified structures of undercooled samples.     

Rapid solidification of zirconium-based alloys

Zr based metal-metal binary and ternary alloys can be obtained in the amorphous state in very wide composition ranges. Several eutectic reactions and intermetallic compounds are present in these alloy systems which provide opportunities for examining the validity of different theories on glass formation. The amorphous phases in these alloys decompose by a variety of crystallization mechanisms. Instances of polymorphic, primary and eutectic crystallization have been encountered in these glasses. Zr-based metallic glasses possess excellent corrosion resistance and mechanical properties. In several studies their properties have been compared with that of their crystalline counterparts and interesting differences have emerged. In the solute lean Zr-based alloys very large freezing ranges are available for studying the liquid to solid transformation. It has been possible to study the formation of some of the low temperature phases directly from the liquid. This paper describes some of the aforementationed studies carried out on Zr-based amorphous and crystalline alloys.     

Quench modification of aluminium-silicon eutectic alloys

Directional solidification of aluminium-silicon eutectic alloys were carried out in order to investigate the mechanism by which the quench modification takes place. For this purpose a new type of Bridgman furnace was designed which can attain a high temperature gradient and a high interfacial growth velocity up to 1000 m^–1. It is established that the fibrous structure is the result of faceted-non-faceted growth of the coarse silicon particle at high solidification rate. It is observed that refining of the flake structure closely follows the characteristics of normal eutectic.     

The hardness of Al-Si eutectic alloys

The Vickers hardness values of Al-Si eutectic alloys, solidified unidirectionally at rates ranging from 2.8×10^–5 to 1 cm sec^–1, have been determined. These are compared with associated tensile and compressive properties. It is shown that there is no close correlation of hardness and strength over the entire range of growth rates although similar trends are seen between hardness and compressive yield strength. It is concluded that caution should be exercised when inferring strength from hardness data.     

Solidification process of nonfaceted--faceted eutectic alloys

Abstracts are not published in this journal     

SCN-Cam共晶合金的定向凝固研究

采用实时观测装置和定向凝固系统研究了SCN-Cam(Succinonitrile-wt%Camphor,wt%为质量分数)模型合金的凝固过程.实验结果表明,SCN-23.6%Cam共晶合金在常规条件下形成规则的共晶组织,共晶间距随界面推移速度的增大而减小;加入超声振动时,共晶合金生长出初生相;SCN-21%Cam亚共晶...     

Coupled eutectic growth in Al-Fe alloys

The conditions for fully eutectic growth in Al-Fe alloys at a temperature gradient of 20 K mm^–1 are reported for ranges of composition from 2.2 to 6.1 wt % Fe and of growth velocity from 0.03 to 10 mm sec^–1. All six main classes of growth structure (i.e. Al-Al₃Fe or Al-Al₆Fe eutectics either alone or together with primaryAl or Al₃Fe) were obtained, some of them reported for the first time for steady-state conditions. Observed concentration-dependences both of the limiting growth velocity for primary Al₃Fe and of the interphase spacing for the fully eutectic Al-Al₆Fe displacing it are in good agreement with theory. Hardness levels for the Al-Al₆Fe eutectic as a function of concentration are similar to those forAl dendritic structures grown in much thinner sections under splat-cooling conditions. The significance of some observed transitions in growth morphology for eutectic cells, Al₆Fe eutectic rods andAl dendrites is discussed.     

Coupled eutectic growth in Al-Fe alloys

The response to isothermal soaking at 773 to 913 K (0.83 to 0.99T _m) for holding times upto 1000 h is reported for Al-3 wt% Fe/metastable Al-Al₆Fe eutectic (10vol%Al₆Fe) directionally grown at 1.24 mm sec^–1. Breakdown is initiated by pinching-off and spherodization of Al₆Fe eutectic rods within eutectic cells and by growth of equilibrium Al₃Fe at grain boundaries and cell boundaries. Compared with equivalent Al-Al₃Ni, results indicate enhanced thermal stability of Al-Al₆Fe eutectic pending consumption by growing Al₃Fe. Hardness decreased with increased soaking time according to an Orowan relationship with Al₆Fe particle spacing.     

Directional solidification of Co-Cu-R permanent-magnet alloys

Co-Fe-Cu-Ce permanent-magnet alloys have been prepared by directional solidification using a modified Bridgman technique. Samples melted at low superheat temperatures (DeltaT sim 20degC above the melting point of about 1100°C) and solidified at moderate rates (∼2.3 cm/h) resulted in a reasonably homogeneous columnar grain structure with a preferred crystallographic orientation. Thecaxis is generally aligned within 15° of the growth axis. Increasing the speed of solidification led to a fine-grained structure with no texture, while decreasing the speed led to coarse columnar grains with erratic orientation. A large superheat temperature (DeltaT sim 300-400degC resulted in a reaction of the liquid with the alumina crucible wall and led to the formation of face-centered cubic Co-rich dendrites. With the modified Bridgman technique, oriented samples 8 cm long and 2.54 cm in diameter have been prepared with good magnetic properties. After annealing at 1000°C followed by aging at 400°C, a Co3.5Fe0.5CuCe alloy exhibited values ofiHc= 7000 Oe, Br= 6100 G, and(BH)_{max} = 9.2MG.Oe. A Co3.6Fe0.5Cu0.9Ce alloy exhibited values ofiHc= 6000 Oe,B_{r} = 6250G, aud(BH)_{max} = 9.5MG.Oe after similar treatment.     

Coarsening during solidification of aluminium-copper alloys

Coarsening of directionally solidified -phase dendrites and of particulate -phase/liquid mixtures was investigated in Al-4, 10 and 20 wt% Cu alloys, as a function of temperature, composition and presence or absence of forced convection. Isothermal dendritic coarsening in the absence of convection operated in two stages. In stage I the dendritic structure broke down through remelting into fragments which spheroidized quickly; in stage II the spherical particles coarsened slowly. The coarsening rate of the dendritic or particulate solid increased with temperature and copper dilution. Alloy inoculation with titanium slowed coarsening, yielding finer dendritic microstructures. The effect of turbulent flow on coarsening was manifested only for longer holding times. At higher impeller angular velocities the dendritic structure breaks down into fragments which spheroidize rapidly. At lower shear rates (below 650 rev min^–1) solid particles in solid-liquid mixtures coalesce into clusters, whereas at higher rates the clusters break up again into individual particles. A coarsening model was introduced which showed that coarsening is faster in the presence of forced convection, because of the resulting decrease in solute diffusion-boundary layer thickness.     

The directional solidification of Pb-Sn alloys

Directional solidification experiments have been carried out on different Pb-Sn alloys as a function of temperature gradient G, growth rate V and cooling rate GV. The specimens were solidified under steady state condition with a constant temperature gradient (50 °C/cm) at a wide range of growth rates ((10–400) × 10^–4 cm/s) and with a constant growth rate (17 × 10^–4 cm/s) at a wide range of temperature gradient (10–55 °C/cm). The primary dendrite arm spacing, ₁, and secondary dendrite arm spacing, ₂, were evaluated. This structure parameters were expressed as functions of G, V and GV by using the linear regression analysis. The results were in good agreement with the previous works.     

Amorphization of eutectic alloys by shock compression

The eutectic alloys Au–Si, Au–Ge, Al–Si and Al–Ge, known to be highly brittle under normal compression, were subjected to high-speed shock compression in order to induce amorphization. Plastic deformation was easily achieved in Au–Si, Al–Si and Al–Ge, yielding thin foils of these alloys. However, an amorphous phase was confirmed only in the case of Au–Si. Furthermore, a superstructure was observed in various locations in the Au–Si thin foil. Au–Ge was pulverized upon shock compression.     

Rapid eutectic growth of undercooled metallic alloys

On leave from the Northwestern Polytechnical University, People's Republic of China.     

Mechanical properties of the Sn-Zn eutectic alloys

The structure and mechanical properties of Sn-Zn unidirectionally frozen eutectic alloys have been examined over the growth range 5 to 4000mm h^–1. The structure is predominantly broken-lamellar below 750mm h^–1 but becomes increasingly fibrous at higher growth rates. The yield and ultimate strengths when tested in tension and compression were found to increase monotonically with growth rates up to 1000 mm h^–1 above which they assumed near constant values. This behaviour is attributed to some loss of axial growth at higher growth rates. The hardness measured on transverse sections increased over the entire growth rate range. Annealing at near eutectic temperatures followed by quenching increased the strength of alloys grown at less than 750 mm h^–1 and decreased that of those grown at higher rates. Similar behaviour was observed in selected Cd-Zn eutectic alloys. The increase in strength is attributed to solid solution hardening and the reduction to structural degradation during annealing. The Sn-rich matrix in this broken-lamellar eutectic appears to contribute significant strengthening to the composite.