Solidification of the Cu-35wt pct Fe Alloys with Liquid Separation

The microstructures of the Cu-35wt pct Fe alloys were investigated by melt-fluxing in combination with cyclic superheating and melt-spinning technique, respectively. Using the melt-fluxing with cyclic superheating technique, it was found that a complicated sub-microstructure formed in the separated minor phase, when the undercooling was 120 K (120 °C). The processes of the phase transformation from a liquid state to room temperature for undercooled Cu-35wt pct Fe alloys were discussed, in order to understand the solidification with metastable liquid separation. By means of melt-spinning technique, it was indicated that the microstructure of solidification for Cu-35wt pct Fe alloys could be refined due to the high cooling rate.     

The Structure and Kinetics of the Nanoscale Precipitation Processes in Al-1.0 wt pct Mg2Si-0.4 wt pct Mg-0.5 wt pct Ag Alloy

The influence of addition of 0.4 wt pct Mg on the precipitation sequence in the balanced Al-1.0 wt pct Mg₂Si bearing 0.5 wt pct Ag has been investigated during the continuous heating of the quenched alloy from the solid solution state. Differential scanning calorimetry (DSC) and high-resolution transmission electron microscopy techniques have been used. The DSC experiments showed that all processes occurred are thermally activated. The activation energies of the precipitation processes have been determined and hence the kinetics of these precipitates have been determined. The obtained results have shown that the existence of excess Mg inhibits the formation of the early stage clusters of solute-vacancy clusters. These clusters can be assisted by the binding energies between solute Si, Mg, and Ag atoms and the excess vacancies. On the other hand, excess Mg accelerates the precipitation of random, β′-phase and β-phase precipitates.     

Influence of Melt Superheating Treatment on the Cast Structure of Al–Sn Alloys

Russian Journal of Non-Ferrous Metals - In this article, the effect of melt superheating treatment (MST) for Al–Sn alloys has been studied. To determine the optimal superheat temperature, the...     

Analysis of the Deformation Behavior of Magnesium-Rare Earth Alloys Mg-2 pct Mn-1 pct Rare Earth and Mg-5 pct Y-4 pct Rare Earth by In Situ Energy-Dispersive X-ray Synchrotron Diffraction and Elasto-Plastic Self-Consistent Modeling

The deformation behavior of the Mg-RE alloys ME21 and WE54 was investigated. Although both alloys contain rare earth elements, which alter and weaken the texture, the flow curves of the alloys deviate significantly, especially in uniaxial compression test. Apart from the higher strength of the WE54 alloy, the compression flow curve does not exhibit the typical sigmoidal shape, which is associated with tension twinning. However, optical microscopy, X-ray texture measurements, and EBSD analysis reveal the activity of tension twinning. The combination of in situ energy-dispersive X-ray synchrotron diffraction and EPSC modeling was used to analyze these differences. The investigation reveals that twin propagation is decelerated in the WE54 alloy, which requires a change of the twinning scheme from the ‘finite initial fraction’ to the ‘continuity’ assumption. Furthermore, an enhanced activity of the 〈c+a〉 pyramidal slip system was observed in case of the WE54 alloy.     

Numerical Modeling of Macrosegregation in Binary Alloys Solidifying in the Presence of Electromagnetic Stirring

A model for predicting solidification and solute segregation of binary alloys undergoing electromagnetic stirring has been developed. A dual-zone formulation was employed to describe the velocity fields in the mushy region. The key feature of this model lies in its accounting for flow damping in the suspended particle region via turbulent interactions the crystallite surfaces. The damping force is given in terms of the turbulent kinetic energy, fraction solid, and the crystallite sphericity. The computed macrosegregation results for Al-4.5 pctCu alloy were validated against, and were found to agree with, experimental measurements. The effect of final grain size and frequency on segregation was also determined. This validated model represents a rigorous mathematical framework for describing the flow behavior and solute segregation in electromagnetically stirred melts.     

Influence of Forced/Natural Convection on Segregation During the Directional Solidification of Al-Based Binary Alloys

We analyzed the columnar solidification of a binary alloy under the influence of an electromagnetic forced convection of various types and investigated the influence of a rotating magnetic field on segregation during directional solidification of Al-Si alloy as well as the influence of a travelling magnetic field on segregation during solidification of Al-Ni alloy through directional solidification experiments and numerical modeling of macrosegregation. The numerical model is capable of predicting fluid flow, heat transfer, solute concentration field, and columnar solidification and takes into account the existence of a mushy zone. Fluid flows are created by both natural convection as well as electromagnetic body forces. Both the experiments and the numerical modeling, which were achieved in axisymmetric geometry, show that the forced-flow configuration changes the segregation pattern. The change is a result of the coupling between the liquid flow and the top of the mushy zone via the pressure distribution along the solidification front. In a forced flow, the pressure difference along the front drives a mush flow that transports the solute within the mushy region. The channel forms at the junction of two meridional vortices in the liquid zone where the fluid leaves the front. The latter phenomenon is observed for both the rotating magnetic field (RMF) and traveling magnetic field (TMF) cases. The liquid enrichment in the segregated channel is strong enough that the local solute concentration may reach the eutectic composition.     

The electrical conductivity of cast Al−Si alloys in the range 2 to 12.6 wt pct silicon

The changes in electrical conductivity of cast Al−Si alloys, in the range of 2 to 12.6 wt pct silicon due to strontium additions (0.03 wt pct) have been investigated and explained in terms of variations in microstructure. The strontium-containing alloys exhibited a higher conductivity than alloys with no strontium, and this conductivity difference increased as the silicon and magnesium contents were increased and the solidification rate was decreased. It has been demonstrated that this difference is due to changes in microstructural features of the eutectic silicon upon modification.     

Study on the Effect of Melt Convection on Phase Separation Structures in Undercooled CuCo Alloys Using an Electromagnetic Levitator Superimposed with a Static Magnetic Field

We studied the effect of melt convection on phase separation structures in undercooled Cu₈₀Co₂₀ alloys by using an electromagnetic levitator, where a static magnetic field was applied to control convection in the molten alloys. It was found that, when the static magnetic field was relatively small, dispersed structures with relatively fine Co-rich spheres distributed in the matrix of the Cu-rich phase were observed. However, a few large, coalesced Co-rich phases appeared in the Cu-rich matrix when the magnetic field exceeded a certain value, i.e., approximately 1.5 T in this study. The mean diameter of the droplet-shaped Co-rich phases distributed in the matrix of the Cu-rich phase increased gradually with the magnetic field and increased rapidly at approximately 1.5 T. Moreover, it was speculated from the result of periodic laser heating that the marked change in the phase separation structures at approximately 1.5 T might be due to a convective transition from turbulent flow to laminar flow in the molten sample, where the time variation of temperature in the lower part of the electromagnetically levitated molten sample was measured when the upper part of the sample was periodically heated.     

Effect of Multi-Scale Thermoelectric Magnetic Convection on Solidification Microstructure in Directionally Solidified Al-Si Alloys Under a Transverse Magnetic Field

The influence of a transverse magnetic field (B < 1 T) on the solidification structure in directionally solidified Al-Si alloys was investigated. Experimental results indicate that the magnetic field caused macrosegregation, dendrite refinement, and a decrease in the length of the mushy zone in both Al-7 wt pct Si alloy and Al-7 wt pct Si-1 wt pct Fe alloys. Moreover, the application of the magnetic field is capable of separating the Fe-rich intermetallic phases from Al-7 wt pct Si-1 wt pct Fe alloy. Thermoelectric magnetic convection (TEMC) was numerically simulated during the directional solidification of Al-Si alloys. The results reveal that the TEMC increases to a maximum ( \( u_{\rm{max} } \) ) when the magnetic field reaches a critical magnetic field strength ( \( B_{\rm{max} } \) ), and then decreases as the magnetic field strength increases further. The TEMC exhibits the multi-scales effects: the \( u_{\rm{max} } \) and \( B_{\rm{max} } \) values are different at various scales, with \( u_{\rm{max} } \) decreasing and \( B_{\rm{max} } \) increasing as the scale decreases. The modification of the solidification structure under the magnetic field should be attributed to the TEMC on the sample and dendrite scales.     

Observations on surface relief effects associated with the formation of θ(CuAl2) rods in Al-4.5 wt pct Cu

The surface relief associated with the formation of θ (CuAl₂) rods at 450° and 375°C on a prepolished surface of an Al-4.5 wt pct Cu alloy has been examined. Most rods show well-defined V-shaped depressions in the surface of a magnitude consistent with accommodation of the transformation volume change taking place normal to the specimen surface. A few rods show V-shaped relief extending upwards from the surface, which is tentatively considered in terms of the change in lattice dimensions between the matrix and precipitate, and hence a function of the rod habit direction and of the lattice orientation relationships. Prolonged annealing increases the magnitude of the relief. Re-solution of the precipitate results in an increase in downward relief at the rod site consistent with solution taking place by a net diffusion of (copper) atoms away from the precipitate sites. Formerly with the Department of Metallurgy and Institute of Materials Science, University of Connecticut, Storrs, Conn.     

Creep behavior of an Al-2. 0 wt pct Li alloy in the temperature range 300 °C to 500 °C

The elevated temperature deformation behavior of an Al-2. 0 wt pct Li alloy in the temperature range 300 °C to 500 °C was studied using constant extension-rate tension testing and constant true-stress creep testing under both isothermal and temperature cycling conditions. Optical microscopy and transmission electron microscopy (TEM) were employed to assess the effect of deformation on microstructure. The data showed that the stress exponent,n, has a value of about 5. 0 at temperatures above theα +δAlLi solvus (approximately 380 °C) and that subgrains form during plastic deformation. Models for dislocation-climb and dislocation-glide control of creep were analyzed for alloys deformed in the temperature range of stability of the terminal AlLi solid solution. A climb model was shown to describe closely the behavior of this material. Anomalous temperature dependence of the activation energy was observed in this same temperature range. This anomalous behavior was ascribed to unusual temperature dependence of either the Young’s modulus or the stacking fault energy, which may be associated, in turn, with a disorder-order transformation on cooling of the alloy. Formerly with the Materials Engineering Section. Department of Mechanical Engineering, Naval Postgraduate School.     

Influence of a Magnetic Field on Structure Formation during the Crystallization and Physicomechanical Properties of Aluminum Alloys

Russian Journal of Non-Ferrous Metals - The results of studying the structure and mechanical properties of A356.0 and A413.1 cast aluminum alloy subjected to a pulsed magnetic field of different...     

On the microstructure of rapidly solidified Al-15 wt pct mn ribbons: effect of annealing in the temperature range 300° to 600 °C

A study has been made of the relationship between the microstructures formed in rapidly solidified Al-15 wt pct Mn ribbons and the precipitation processes following annealing in the regime 300° to 600 °C from 15 minutes to 100 hours. The as-melt spun ribbons exhibit the icosahedral structure (in an Al-matrix), which is found to be stable up to ∼320 °C as deduced from electrical resistivity measurements. X-ray and TEM analyses reveal that decomposition of this phase is detectable after annealing at 400 °C/1 h, giving rise to Al₆Mn. At 450 °C, four phases, viz., the G, Al₆Mn, G’, and G″ intermetallic phases, are observed (in addition to Al), which result from the direct decomposition of the icosahedral phase. At higher temperatures, only the Al₆Mn phase + Al is observed. The effect of increase in annealing time is simply to coarsen the Al₆Mn particles. Our X-ray and electron diffraction data favor the model proposed by Shechtman and Blech for the icosahedral structure. The investigations were carried out using density and electrical resistivity measurements, differential thermal analysis, as well as X-ray diffraction and optical and transmission electron microscopy. Formerly Professeur Associè Formerly Postdoctoral Researcher     

Formation of a Intermediate Order in Metallic Glasses and a Long Order in Nanocrystalline Alloys with Allowance for the Character of Binding and the Transformation of the Short Order in a Melt

The results of studying the formation of short- and intermediate orders in amorphous alloys and a long-range order in nanocrystalline alloys in quenching of metallic melts show that the structures of metal atom groups in both states consist of both icosahedral and helicoidal packings, which coherently combine crystalline and noncrystalline coordinations due to fractional symmetry.     

<Emphasis Type="Italic">In Situ</Emphasis> Observation on Bubble Behavior of Solidifying Al-Ni Alloy Under the Interference of Intermetallic Compounds

The growth behavior of hydrogen bubbles and their interaction with intermetallic compounds during solidifying Al-Ni alloy were investigated by synchrotron radiation. The bubble growth can be divided as three stages, i.e., free growth, accelerated growth, and shrinkage. The free growth agrees well with stochastic model. The accelerated growth is attributed to the increase of hydrogen concentration and its gradient at the bubble–liquid interface caused by their contact. The negative hydrogen concentration gradient ahead of the bubble–liquid interface resulted in the bubble shrinkage. Also, the increasing gas pressure and decreasing Ni content promoted hydrogen dissolution.