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.     

Partitioning of titanium during solidification of aluminium alloys

Abstract

The purpose of the present work is to compare the segregation behaviour of Ti solute in low solute alloys, such as nominally pure Al, and high solute alloys typical of Al–Si casting alloys. Microprobe measurements of Ti segregation within grains show that, under the solidification conditions employed, the measured partition coefficient of Ti in pure Al is 6.7 compared with the phase diagram prediction of ～7.5. Similar measurements in an Al–7Si–0.3Mg alloy resulted in a partition coefficient of 3.2. Based on the measured partition coefficients, this translates into a reduction in the growth restriction factor (a measure of the segregating power of solute elements) from 8.75 K in pure Al to 3.36 K in Al–7Si–0.3Mg with an addition of 0.05 wt-%Ti. This may explain why Ti in solution is a less effective grain refiner in casting alloys than in low solute content wrought alloys. In addition, the measured Ti segregation profiles were compared with predicted profiles based on the Scheil or similar solidification relationships. It was found that the predictions of the Scheil equation produced a good fit with the measured Ti segregation profiles once it was adapted for the geometrical nature of dendritic solidification.     

Thermal analysis of copper-tin alloys during rapid solidification

A series of solidification experiments using a mirror furnace and a levitation technique were performed on different Cu-Sn alloys. Cooling curves during solidification were registered using a thermocouple of type K connected to a data acquisition system. The undercooling, cooling rates of the liquid and of the solid state, solidification times and temperatures were evaluated from the curves. The samples were found to solidify far below the liquidus temperature. The cooling curves for different samples and alloys were simulated using a FEM solidification program. The heat transfer coefficient, heat of fusion and specific heat were evaluated. It was found that the calculated values of the heat of fusion were much lower than the tabulated ones. The calculated values of the specific heat in the solid state were also found to be much higher than those quoted in the literature, especially for the mirror furnace experiments. The effect of rapid cooling on the thermodynamic state and the solidification process of the alloys has been evaluated. The effect of cooling rate on the formation and condensation of vacancies is discussed. It is proposed that a large number of vacancies form during rapid solidification and that they condense during and after the solidification. The influence of these defects on the thermodynamics and solidification of the alloys has been evaluated.     

Partition of solute elements during solidification of iron-carbon-chromium alloys

The partition coefficients of chromium between austenite and liquid iron,k _Cr ^A/L , were determined from the experiment of rapid cooling of iron-carbon hypo-eutectic alloys containing a small amount of chromium from coexisting solid—liquid states; the partition coefficients between eutectic and its liquid,k _0,Cr, andk _0,Cr, for the stable and metastable eutectic solidifications were obtained from the zone-melting experiment of iron—carbon eutectic alloys containing a small amount of chromium. Chromium was rejected to liquid iron on the crystallization of primary austenite,k _Cr ^A/L <1. On the eutectic solidification, chromium was enriched in eutectic liquid for the stable system,k _0,Cr<1, and was conversely diluted in the liquid for the metastable system,k _0,Cr>1. The relationship between effective and equilibrium partition coefficients given by Burtonet al. was observed for the results of the zone melting experiment and, from the relationship, the thickness of boundary layer in the liquid ahead of the solid—liquid interface was found to be 0.17 mm for the stable system and 0.11 mm for the metastable system. Thermodynamic calculation of the partition coefficients of chromium and carbon proved to represent the observed partition coefficients well.     

Modelling of convection during solidification of metal and alloys

The role of convection during solidification is studied with the help of a mathematical model. The effect of various mush models on convection and consequent macrosegregation is examined with the help of numerical simulations. The predicted macrosegregation profiles are compared with published experimental data. Subsequently, the importance of proper auxiliary relationship for thermo-solutal coupling in the mushy region is highlighted through some careful numerical simulations. Finally, the role of material parameters on double-diffusive convection is illustrated through comparative study of solidification of aqueous ammonium chloride, iron-carbon and lead-tin binary systems. Important results of these studies are presented and discussed.     

An investigation of the micromechanisms of fatigue crack growth in structural gas turbine engine alloys

This paper presents an overview of fatigue fracture modes in selected structural alloys employed in gas turbine engines. These include the mechanisms of fatigue crack growth in the near-threshold, Paris and high-K regimes obtained from Ti-6Al-4V, Inconel 718 and PWA 1472 (a single crystal nickel-based superalloy of similar chemical composition to Inconel 718). Fatigue fracture modes in these materials are shown to be strong functions of the stress intensity factor range, K, and the maximum stress intensity factor, K _max. Fatigue mechanism maps are also presented to show the parametric ranges of K and K _max corresponding to the different fatigue fracture modes.     

Numerical analysis of constitutional supercooling during directional solidification of alloys

Some limitations of Tiller’s morphological stability criterion are discussed in the present study. This criterion assumes a purely diffusive regime in the melt as well as a planar solid–liquid interface and a constant solidification rate. But experimental works in agreement with previous numerical modeling have shown a significant decrease of the growth rate and a variable interface curvature during the concentrated semiconductor alloys solidification. The mathematical expression of the morphological stability criterion was derived by using Tiller’s equation, predicting the solute distribution in the liquid. The numerical computations performed in this study show a significant disagreement between the numerical results and Tiller’s formula. Numerical modeling conducted in conditions when the supercooling should occur, show that the Tiller’s stability criterion cannot predict the moment of interface destabilization. The interface destabilization is numerically observed when some fluctuations appear in the liquid solutal profiles and cause the appearance of a supercooled zone inside the liquid at small distance from the interface. The present numerical results are not in contradiction with the basic elements of the classical constitutional supercooling theory, providing only that the stability criterion cannot predict the moment of the interface destabilization.     

Dendrite coarsening during solidification of hypo- and hyper-eutectic Al-Cu alloys

Dendrite coarsening during cooling at a constant rate was compared at various stages of solidification with that during isothermal holding for Al-Cu alloys of hypo- and hypereutectic compositions. For each specimen, the undercooling for the initial dendrite formation and the time elapsed after it were measured directly. The dendrite arm spacing was shown to be determined solely by the latter, and the dendrite structure was therefore coarsening-controlled from the early stage of solidification. The rate of coarsening in terms of the dendrite arm spacing during solidification at a constant cooling rate was same as that during isothermal holding in all the alloys tested. Numerical values of the fractional rate of solidification were evaluated for the hypo-eutectic compositions and the results show that the rate of dendrite coarsening does not depend on the fractional rate of solidification. Aluminium dendrites show structural coarsening with progressive solidification in the same way as during isothermal holding. CuAl₂ dendrites show curved boundaries after isothermal holding whereas those cooled at a constant rate are faceted.     

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.     

结构不均匀TiNi合金中的马氏体相变特征

通过设计试样原始表面形状和冷轧的方法获得了一种原位内生形状记忆合金复合材料。利用设计试样原始表面形状的方法，将具有不同位错密度的宏观区域引入形状记忆合金内部，则整个材料就获得了复合材料的特征。结果表明在第一次加热过程中正弦试样DSC曲线上出现双峰，分别对应于设计试样内部的不同变形区域，且由于位错织构和马氏体变体界面的作用，使得逆转变温度扩展到一个较大的温度范围。所设计的材料具体可以应用于所谓的广域温度记忆效应和类因瓦效应领域。所有上述现象表明为控制材料的热特性而进行适当的位错织构设计的方法是可行的。     

Microstructural development during transient directional solidification of hypermonotectic Al–Bi alloys

Directional unsteady-state solidification experiments were performed with hypermonotectic Al–5.0 wt%Bi and 7.0 wt%Bi alloys. Thermal parameters such as the growth rate (v) and the thermal gradient (G) were experimentally determined by cooling curves recorded along the casting length. The predominant Bi-rich phase was characterized by droplets embedded in the aluminum matrix. Both the interphase spacing (λ) and the Bi-rich particles diameter (d) were measured along the casting length. These microstructural features were correlated to the solidification thermal parameters: growth rate, cooling rate and thermal gradient. An experimental law expressing λ as a function of both G and v was found to better represent the growth of hypermonotectic Al–Bi alloys. Moreover, it was found that the interphase spacing decreases with increasing alloy bismuth content.     

Kinetic aspects of the solidification of aluminium-zinc alloys

A method for determination of the kinetics of linear and volumetric dendritic growth on the basis of a model of dendritic solidification process referred to one mole of alloy has been presented. The model parameters have been derived using Krupkowski's equation of dendritic solidification for the case of Al-40 wt% Zn alloy solidified at a definite rate. The constructed kinetics curves have been analysed using the Johnson—Mehl relation and compared with that of Chalmers. The results of calculation concerning the maximum microsegregation have made it possible to define the linear and volumetric dendritic growth rates. The relationships between these rates and the experimental cooling rate of ingot have been discussed.     

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.     

Unidirectional solidification of Ni-Mo-Al eutectic alloys

The composition triangle of the Ni-Mo-Al ternary system contains a monovariant trough between theγ′ (Ni₃Al)-α(Mo) pseudobinary eutectic and a guessedγ(Ni)-γ′ (Ni₃Al)-α(Mo) ternary eutectic. Alloys with compositions on this trough were directionally solidified at various growth rates. The microstructure of the alloys consists of fine Mo fibres of rectangular cross-section in an Ni₃Al/Ni matrix. The determined crystallographic relationship does not correspond to an interface of low lattice mismatch. The eutectic trough strongly extends towards increasing Mo contents so that the volume fraction of the Mo fibres varies between 18 and 25% dependent on composition. It was not possible to determine the position of a ternary eutecticγ-γ′-α because the liquidus temperatures along the trough are almost equal and the distribution of the three phases is strongly dependent on the growth rate.     

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.     

Phase transformation occurring during the heat-treatment of magnesium rare earth alloys

This work discusses the phase transformations occurring and mechanical properties of a cast and extruded Mg-5%Y-4%Gd alloy, 90 and 20 mm in diameter bar, after various heat treatments. The influence of quenching temperature, artificial and natural ageing on microstructure, microhardness and mechanical properties is presented. It is demonstrated that the alloy strengthens during natural ageing and initially recovers during artificial ageing. An understanding of the alloy behavior leads to optimization of the heat-treatment procedures; the best combination of mechanical properties is obtained after artificial aging at 200° for 3 days.     

On the solidification microstructure of copper-rich niobium alloys

The microstructure of copper-rich niobium alloys has been studied as a function of cooling rate. The main alloy composition investigated was Cu-7 wt% Nb. The cooling rate was varied from 40° C sec^–1 to approximately 7×10^4° C sec^–1. The microstructure as observed in the scanning electron microscope changes from well-dispersed niobium spheres for fast-cooling to an heterogeneous distribution of niobium flowers for slow-cooling.     

Oscillatory instabilities of the liquid and mushy layers during solidification of alloys under rotational constraint

Summary Linear flow instabilities due to oscillatory disturbances of the liquid and mushy regions during solidification of binary alloys are investigated under a rotational constraint where the rotation axis is inclined to gravity vector. Results of stability analyses and numerical computations for a preferred centrifugal mode of general oscillatory disturbances at zero and non-zero rotation rates are determined which provide information about the preference of oscillatory flow and its role on the solidification system as modified by the rotational effects. The main results are due to a preferred oscillatory mode of convection which is more significant for non-zero rotation case and is restricted mostly to the mushy region. The preferred oscillatory mode of convection is a traveling wave in the presence of rotation, but it is a standing wave in the absence of rotation. The results for different Prandtl numbers indicate that the freckles formation tendency for metallic alloys is less than that for aqueous solutions. Freckles are imperfections that reduce the quality of the solidified materials.