Effect of Cooling Rate on the Microstructure of Semi-Solid Al–25Si–2Fe Alloy During Electromagnetic Stirring

The effect of cooling rate on the microstructure of semi-solid Al–25Si–2Fe alloy was investigated during electromagnetic stirring. It was found that as the cooling rate was increased from 7 to 21 °C/min, the equivalent diameter of the primary Si particles decreased from 70 ± 5 to 25 ± 2 μm. The primary Si particles form a fine blocky structure when the cooling rate is 21 °C/min. When the cooling rate is increased to 30 and 33 °C/min, the primary Si particles coarsen and adopt plate or other irregular shapes. As the melt cools to 690 °C, Fe inter-metallic phases present different morphologies at different cooling rates during EMS. These phases in the Al–25Si–2Fe alloy are mainly in the form of δ-Al₄FeSi₂ at higher cooling rates.     

Solidification characteristics of the Al-8.3Fe-0.8V-0.9Si alloy

Studies of solidification behavior have been conducted on cast Al-Fe-V-Si alloys. The first phase to precipitate during solidification of an Al-8.3Fe-0.8V-0.9Si alloy is Al₃Fe(V,Si), which is isostructural with the Al₃Fe phase. Thereafter, the solidification proceeds through several invariant reactions. The final invariant reaction is associated with a pronounced arrest. The temperature of this arrest is a function of the cooling rate and modification treatment, with magnesium added as an Al-20 pct Mg or Ni-20 pct Mg master alloy. The coarse iron aluminide precipitates in a slow-cooled (>1 °C/s) cast structure transform to a ten-armed, star-like morphology upon chill casting the melt (cooling rate >10 °C/s) from 900 °C or upon water quenching from above 800 °C. Treatment with magnesium refines the morphology, size, and distribution of iron aluminide precipitates in slow-cooled alloys.     

Solidification parameters,microstructures, and mechanical properties of rapidly solidified iron-based alloys

Cooling rate measurements were carried out using a computer controlled melt spinning unit for the production of rapidly solidified Fe 6.3 wt.% Si and Fe 3.2 wt.% C melt spun ribbons employing a wide range of process parameters. The cooling rates are mainly a function of the ribbon thickness, and are independent of the alloy composition and wheel material. The resulting microstructures have been characterized by light optical and electron microscopy (SEM and TEM) investigations and were found to be influenced by the cooling conditions during and after solidification. Grain sizes and secondary dendrite arm spacings are related to the cooling rates by means of exponential relationships. In addition to this, rapidly solidified eutectic Fe 4.2 wt. %C alloy powder was produced by argon melt-atomization. Powder particles of 20 μm to 80 μm size solidified microcrystalline and exhibit cementite, metastable γ-phase, and martensite. The cementite matrix is of dendritic structure. After consolidating the powder by hot pressing below A, the microstructure changes to the fine equiaxed grains containing about 66 Vol.% Fe₃C and a dispersoid of ferrite ≤1 μm within the cementite matrix. This material exhibits high tensile strength and wear resistance at room temperature. At elevated temperatures in the region between 650°C and 750°C, and at strain rates of ε ? 10^?4s^?1 the fine grained ceramic-like material reveals superplastic behaviour.     

Phase Selection and Microstructure Evolution in Nonequilibrium Solidification of Fe40Ni40B20 Alloy

Phase selection and microstructure evolution in nonequilibrium solidification of ternary eutectic Fe₄₀Ni₄₀B₂₀ alloy have been studied. It is shown that γ-(Fe, Ni) and (Fe, Ni)₃B prevail in all the as-solidified samples. No metastable phase has been found in the deeply undercooled samples. This is explained as resulting from the size effect of undercooled solidification. At small and medium undercoolings, the dendrite γ-(Fe, Ni) appears as the leading phase. This is ascribed to the existence of the skewed coupled growth zone in FeNiB alloy. With increasing undercooling, the amount of dendrites first increases and then decreases, accompanied by a transition from regular eutectic to anomalous eutectic. The formation mechanisms of the anomalous eutectics are discussed. Two kinds of microstructure refinement are found with increasing undercooling in a natural or water cooling condition. However, for melts with the same undercooling, the as-solidified microstructure refines first, and then coarsens with an increasing cooling rate. The experimental results show that the nanostructure eutectic cell has been obtained in the case of Ga-In alloy bath cooling with an initial melt undercooling of approximately 50 K (50 °C).     

Enrichment of Fe‐Containing Phases and Recovery of Iron and Its Oxides by Magnetic Separation from BOF Slags

Steel slag normally contains a large amount of iron and its oxides. Therefore, it is a potential renewable resource in case of inadequate iron ore supply. To recover the metals from steel slag, two types of BOF slags were remelted at 1873 K. The liquid slags were cooled using four types of cooling conditions, namely, water granulation, splashing, air cooling, and furnace cooling, to investigate the influence of cooling rate on mineral components, especially the enrichment behavior of Fe‐containing minerals. Subsequently, wet magnetic separation was conducted to examine the relations between iron recovery ratio and cooling conditions. The results show that the slags under the four cooling conditions mainly contained dicalcium silicate, RO phase, Fe_tO, 2CaO(Fe,Al)₂O₃, and calcium ferrite. However, tricalcium silicate, 12CaO·7Al₂O₃, M‐A spinel, and free CaO and MgO were occasionally observed. The amount of glass matrix decreased, the Fe‐containing minerals increased, and the minerals more fully crystallized when the cooling rate of the liquid slag was decreased. From granulation to furnace cooling of the slags, the iron content in the recovered concentrate and the iron recovery ratio both increased. This result is in agreement with the findings on phase transformation through SEM analysis.     

Intermetallic phases formed during DC-casting

The Al-Fe and Al-Fe-Si particles formed during DC-casting of an Al-0.25 wt pct Fe-0.13 wt pct Si alloy have been examined. The particles were analyzed by transmission electron microscopy (TEM) and energy dispersive spectroscopy of X-rays (EDS). Crystal faults were studied by high resolution electron microscopy (HREM). Samples for electron microscopy were taken at various positions in the ingot,i.e., with different local cooling rates during solidification. At a cooling rate of 6 to 8 K/s the dominating phases were bcc α-AlFeSi and bct Al _m Fe. The space group of bcc α-AlFeSi was verified to be Im3. Superstructure reflections from Al _m Fe were caused by faults on {110}-planes. At a cooling rate of 1 K/s the dominating phases were monoclinic Al₃Fe and the incommensurate structure Al _x Fe. In Al₃Fe, stacking faults on {001} were frequently observed. The structure of Al _x Fe is probably related to Al₆Fe. Some amounts of other phases were detected. For EDS-analysis, extracted particles mounted on holey carbon films were examined. Extracted particles were obtained by dissolving aluminum samples in butanol. Accurate compositions of various Al-Fe-Si phases were determined by EDS-analysis of extracted crystals.     

Influence of melt temperature on Rapid solidification of Al-Y and Al-Si alloy system

Solidification behavior of Al-Y and Al-Si were investigated for different melt temperatures. Levitation casting technique was used to achieve a cooling rate of around ～2000K/s during the experiment. Light optical microscopy (LOM) and Scanning electron microscopy (SEM) were used to analyze the samples. An energy dispersive system (EDS) analysis of SEM was performed on the samples to identify the phases. Plate like structure of Al₈Y₃ primary phase was observed at low melt temperature with small percentage of peritectic transformation of Al₈Y₃ and liquid melt into Al₉Y₂. A pre-dentritic star like crystal of Al₃Y was observed in a fine eutectic matrix at very high melt temperature. Amount and number of primary Si crystals formed in a unit area during the solidification increases as the melt temperature increases. It is believed the melt temperature affects the solidification pattern by changing chemical short range order.     

Intermetallic phases formed during DC-casting of an Al−0.25 Wt Pct Fe−0.13 Wt Pct Si alloy

The Al−Fe and Al−Fe−Si particles formed during DC-casting of an Al-0.25 wt pct Fe-0.13 wt pct Si alloy have been examined. The particles were analyzed by transmission electron microscopy (TEM) and energy dispersive spectroscopy of X-rays (EDS). Crystal faults were studied by high resolution electron microscopy (HREM). Samples for electron microscopy were taken at various positions in the ingot,i.e., with different local cooling rates during solidification. At a cooling rate of 6 to 8 K/s the dominating phases were bcc α-AlFeSi and bct Al _m Fe. The space group of bcc α-AlFeSi was verified to be Im3. Superstructure reflections from Al _m Fe were caused by faults on {110}-planes. At a cooling rate of 1 K/s the dominating phases were monoclinic Al₃Fe and the incommensurate structure Al _x Fe. In Al₃Fe, stacking faults on {001} were frequently observed. The structure of Al _x Fe is probably related to Al₆Fe. Some amounts of other phases were detected. For EDS-analysis, extracted particles mounted on holey carbon films were examined. Extracted particles were obtained by dissolving aluminum samples in butanol. Accurate compositions of various Al−Fe−Si phases were determined by EDS-analysis of extracted crystals.     

金属间化合物Al3Fe熔体结构的计算机模拟

本文利用ＴＢ模型对Ａｌ３Ｆｅ熔体结构进行了分子动力学计算机模拟，详细考察了在快速弟固条件下Ａｌ３Ｆｅ熔体结构的温度变化特征，计算了不同温度下的偶分布函数，分析了在快速凝固条件下Ａｌ３Ｆｅ合金熔体的结构特点。     

Gas Atomization of Amorphous Aluminum Powder: Part II. Experimental Investigation

The optimal processing parameters that are required to atomize amorphous Al were established on the basis of numerical simulations in part I of this study. In this part II, the characterization of cooling rate experienced by gas-atomized, Al-based amorphous powders was studied via experiments. An experimental investigation was implemented to validate the numerical predictions reported in part I of this study. The cooling rate experienced by the powders, for example, was experimentally determined on the basis of dendrite arm spacing correlations, and the results were compared with the numerical predictions. The experimental studies were completed using commercial Al 2024 as a baseline material and Al₉₀Gd₇Ni₂Fe₁ metallic glass (MG). The results showed that the cooling rate of droplets increases with decreasing particle size, with an increasing proportion of helium in the atomization gas and with increasing melt superheat. The experimental results reported in this article suggest good agreement between experiments and numerical simulations.     

Computer simulation of the crystal morphology and growth rate during ultrarapid cooling of an Fe-B Melt

The solidification of a binary Fe-B melt is studied by computer simulation with regard for the temperature dependence of the diffusion coefficient and the possibility of nonequilibrium alloying-component entrapment (i.e., the dependence of the distribution coefficient on the ratio of the solidification rate V to the diffusion rate V _D). The effect of high cooling rates of the melt on the dendritic morphology is analyzed. The dependence of the dendrite-tip growth rate on the melt supercooling is obtained. At large supercoolings, a morphological transition is shown to occur; it is related to the change from a diffusion to a diffusionless dendrite growth mode.     

The kinetics of oxidation of iron-silicon and iron-aluminum alloy melts by pure oxygen or oxygen-bearing gases

The kinetics of oxidation of Fe−Si and Fe−Al melts by pure oxygen, and that of pure Fe by He−O₂, N₂−O₂, or Ar−O₂ mixtures have been investigated by a modified Sieverts' method at 1600°C. Considerable decrease in the oxidation rate has been observed for the alloy melts containing a few percent of Si or Al since formation of a silica- or alumina-rich oxide layer on the melts prevents further progress of the exothermic chemical reaction. The oxidation rate for melts high in Al has been considered to be limited by the diffusion of ions through the oxide layer. Addition of diluents to O₂ markedly and continuously decreases the oxidation rate of a pure Fe melt. The latter rate has been show to be controlled by the diffusion of O₂ across the gaseous boundaries at gas/melt interfaces.     

Interaction of exogenous refractory nanophases with antimony dissolved in liquid iron

The heterophase interaction of Al₂O₃ refractory nanoparticles with a surfactant impurity (antimony) in the Fe–Sb (0.095 wt %)–O (0.008 wt %) system is studied. It is shown that the introduction of 0.06–0.18 wt % Al₂O₃ nanoparticles (25–83 nm) into a melt during isothermal holding for up to 1200 s leads to a decrease in the antimony content: the maximum degree of antimony removal is 26 rel %. The sessile drop method is used to investigate the surface tension and the density of Fe, Fe–Sb, and Fe–Sb–Al₂O₃ melts. The polytherms of the surface tension of these melts have a linear character, the removal of antimony from the Fe–Sb–Al₂O₃ melts depends on the time of melting in a vacuum induction furnace, and the experimental results obtained reveal the kinetic laws of the structure formation in the surface layers of the melts. The determined melt densities demonstrate that the introduction of antimony into the Fe–O melt causes an increase in its compression by 47 rel %. The structure of the Fe–Sb–O melt after the introduction of Al₂O₃ nanoparticles depends on the time of melting in a vacuum induction furnace.     

Heat Transfer and Cooling Rate Model of Flow Melt on Vibration Wall

In this paper, a model of heat transfer of melt flow on a vibration wall has been established. Calculation results show that the temperature boundary layer thickness decreases with the increase of vibration frequency and amplitude. As the vibration frequency and amplitude increase, the heat transfer coefficient between alloy melt and slope and between cooling water and slope increase. Cooling rate of melt can reach 400-600 K/s which belong to the sub-rapid solidification regime. The heat transfer mode doesn’t change during the flow process, so vibration not only strengthens the cooling rate of melt, but also stabilizes heat transfer between melt and slope. The grain size of solidification microstructure decreases with increasing vibration intensity, which indicates that vibration increases cooling rate and accelerates nucleation rate. So, the established model agrees with verification experiment, and can relatively well explain the heat transfer and cooling rate of melt flow on vibration plate.     

Iron intermetallic phases in the Al corner of the Al-Si-Fe system

The iron intermetallics observed in six dilute Al-Si-Fe alloys were studied using thermal analysis, optical microscopy, and image, scanning electron microscopy/energy dispersive X-ray, and electron probe microanalysis/wavelength dispersive spectroscopy (EPMA/WDS) analyses. The alloys were solidified in two different molds, a preheated graphite mold (600°C) and a cylindrical metallic mold (at room temperature), to obtain slow (}0.2 °C/s) and rapid (}15 °C/s) cooling rates. The results show that the volume fraction of iron intermetallics obtained increases with the increase in the amount of Fe and Si added, as well as with the decrease in cooling rate. The low cooling rate produces larger-sized intermetallics, whereas the high cooling rate results in a higher density of intermetallics. Iron addition alone is more effective than either Si or Fe+Si additions in producing intermetallics. The alloy composition and cooling rate control the stability of the intermetallic phases: binary Al-Fe phases predominate at low cooling rates and a high Fe:Si ratio; the β-Al₅FeSi phase is dominant at a high Si content and low cooling rate; the α-iron intermetallics (e.g., α-Al₈Fe₂Si) exist between these two; while Si-rich ternary phases such as the δ-iron Al₄FeSi₂ intermetallic are stabilized at high cooling rates and Si contents of 0.9 wt pct and higher. Calculations of the solidification paths representing segregations of Fe and Si to the liquid using the Scheil equation did not conform to the actual solidification paths, due to the fact that solid diffusion is not taken into account in the equation. The theoretical models of Brody and Flemings^[44] and Clyne and Kurz^[45] also fail to explain the observed departure from the Scheil behavior, because these models give less weight to the effect of solid back-diffusion. An adjusted 500°C metastable isothermal section of the Al-Si-Fe phase diagram has been proposed (in place of the equilibrium one), which correctly predicts the intermetallic phases that occur in this part of the system at low cooling rates (}0.2 °C/s).     

Diary

Abstract

The addition of amorphous Fe–Si–B particles to Fe powder increases the shrinkage of sintered components resulting in higher densification rates. Consequently, several research groups worldwide have studied the properties of such systems in an attempt to produce superior structural alloys. In the present work, Fe₇₅Si₁₀B₁₅ ribbons obtained by melt spinning were milled in a high energy Spex mill for times varying from 2 to 32 h. The resulting powders were characterised by differential thermal analysis and X-ray diffraction. The results showed that the amorphous characteristics of the ribbons persisted after the milling process. Next, samples consisting of a mixture of Fe powder and 4 wt-% milled amorphous phase were uniaxially pressed and sintered following a series of thermal cycles. High temperature microstructures were obtained for compacts subjected to rapid cooling from the sintering temperature. The results of scanning electron microscopy and energy dispersive spectroscopy revealed substantial precipitation of fine Fe₂B particles before α → γ allotropic transformation. In addition, an oxide phase was observed in the interface between Fe and the additive particles. Preliminary analysis suggested that the oxide particles can be easily reduced by adding small amounts of carbon to the system. PM/0765     

Modelling and simulation of twin-roll casting of bulk metallic glasses

As an economic and direct route to continuous thin strip production from the melt, twin roll casting (TRC) has been established as an effective process for aluminium alloys. Its adaptation to casting of bulk amorphous alloy strip necessitates matching of the thermal and mechanical behaviour of the cooling multi-component melt to the requirements (especially cooling rate, and strip exit temperature and thermal gradient) of vitrification. Using a dedicated control volume numerical model of TRC, simulation of the casting of 2 mm thick Vit 1 (Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5) alloy strip shows that the acceptable casting speeds are in the range 2.5 to 3.5 cm/s. The effects of varying strip thickness and strip-roll heat transfer coefficient (HTC) on this casting window are assessed. The differences between modelling of conventional alloy solidification and metallic glass formation are presented.     

Iron intermetallic phases in the Al corner of the Al-Si-Fe system

The iron intermetallics observed in six dilute Al-Si-Fe alloys were studied using thermal analysis, optical microscopy, and image, scanning electron microscopy/energy dispersive X-ray, and electron probe microanalysis/wavelength dispersive spectroscopy (EPMA/WDS) analyses. The alloys were solidified in two different molds, a preheated graphite mold (600 °C) and a cylindrical metallic mold (at room temperature), to obtain slow (∼0.2 °C/s) and rapid (∼15 °C/s) cooling rates. The results show that the volume fraction of iron intermetallics obtained increases with the increase in the amount of Fe and Si added, as well as with the decrease in cooling rate. The low cooling rate produces larger-sized intermetallics, whereas the high cooling rate results in a higher density of intermetallics. Iron addition alone is more effective than either Si or Fe+Si additions in producing intermetallics. The alloy composition and cooling rate control the stability of the intermetallic phases: binary Al-Fe phases predominate at low cooling rates and a high Fe:Si ratio; the β-Al₅FeSi phase is dominant at a high Si content and low cooling rate; the α-iron intermetallics (e.g., α-Al₈Fe₂Si) exist between these two; while Si-rich ternary phases such as the δ-iron Al₄FeSi₂ intermetallic are stabilized at high cooling rates and Si contents of 0.9 wt pct and higher. Calculations of the solidification paths representing segregations of Fe and Si to the liquid using the Scheil equation did not conform to the actual solidification paths, due to the fact that solid diffusion is not taken into account in the equation. The theoretical models of Brody and Flemings^[44] and Clyne and Kurz^[45] also fail to explain the observed departure from the Scheil behavior, because these models give less weight to the effect of solid back-diffusion. An adjusted 500 °C metastable isothermal section of the Al-Si-Fe phase diagram has been proposed (in place of the equilibrium one), which correctly predicts the intermetallic phases that occur in this part of the system at low cooling rates (∼0.2 °C/s).