Microstructure and Mechanical Properties of Rapidly Solidified Hypereutectic Al–Si and Al–Si–Fe Alloys

The microstructure and mechanical properties of rapidly solidified Al–18 wt% Si and Al–18 wt% Si–5 wt% Fe alloys were investigated by a combination of optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction, tensile testing, and wear testing. The centrifugally atomized binary alloy powder consisted of the -Al (slightly supersaturated with Si) and Si phases and the ternary alloy powder consisted of the -Al (slightly supersaturated with Si), silicon, and needle-like metastable Al–Fe–Si intermetallic phases. During extrusion the metastable -Al₄FeSi₂ phase in the as-solidified ternary alloy transformed to the equilibrium -Al₅FeSi phase. The tensile strength of both the binary and the ternary alloys decreased with a high-temperature exposure, but a significant fraction of the strength was retained up to 573 K. The specific wear gradually increased with increasing sliding speed but decreased with the addition of 5 wt% Fe to the Al–18 wt% Si alloy. The wear resistance improved with annealing due to coarsening of the silicon particles.     

Liquid–liquid phase equilibria,density difference,and interfacial tension in the Al–Bi–Si monotectic system

We have performed thermodynamic calculation of the phase equilibria in the ternary monotectic system Al–Bi–Si. The liquid–liquid miscibility gap in the Al–Bi–Si system extends over almost the entire concentration triangle. The thermal analysis data for (Al_0.345Bi_0.655)_100−x Si _x alloys (x = 2.5, 5, 7.5, and 10 wt%) excellently agree with the calculated phase diagram. The experimental density difference of the coexisting liquid phases shows a good agreement with the density difference calculated in the approximation of ideal solution using the densities of pure elements and the compositions of L′ and L′′ from the thermodynamic calculation. The liquid–liquid interfacial tension in the (Al_0.345Bi_0.655)_100−x Si _x liquid alloys increases with Si content. The experimental temperature dependence of the interfacial tension is well described by the power low in reduced temperature (T _C–T) at approach of the critical temperature with the exponent μ = 1.3, which is close to the value predicted by the renormalization group theory of critical behavior.     

Thermomigration Kinetics in the Si–Al–Ga and Si–Al–Sn Systems

The thermodynamics and kinetics of the thermomigration of molten zones based on Al–Ga and Al–Ga melts in the preparation of silicon epilayers have been studied in detail. We have determined the threshold thermomigration temperature for zones of various compositions. The migration onset temperature has been shown to increase monotonically with increasing Ga or Sn concentration in the liquid phase. The thermomigration rate of Si–Al–Ga zones decreases with increasing gallium concentration at temperatures below 1473 K and increases at higher temperatures. The thermomigration rate of Si–Al–Sn zones decreases with increasing Sn concentration over the entire temperature range studied. No chemical compounds have been detected in the Si–Al–Ga or Si–Al–Sn system, which simplifies the use of the thermomigration method in these systems.     

Effects of Si and Cu contents on grain size of Al–Si–Cu alloys

The influence of the silicon and copper contents on the grain size of high-purity Al–Si, Al–Cu, and Al–Si–Cu alloys was investigated. In the Al–Si alloys, a poisoning effect was observed and a poor correlation between the grain size and growth restriction factor was obtained. A possible cause of the poisoning effect in these alloys is the formation of a TiSi₂ monolayer on the particles acting as nucleation sites or another poisoning mechanism not associated with TiSi₂ phase formation. In the Al–Cu alloys, a good correlation between the grain size and growth restriction factor was found, whereas in the Al–Si–Cu alloys, the correlation between these two parameters was inferior.     

Corrosion resistance of directionally solidified Al–6Cu–1Si and Al–8Cu–3Si alloys castings

The aim of this article is to compare the electrochemical corrosion resistance of two as-cast Al–6 wt.% Cu–1 wt.% Si and Al–8 wt.% Cu–3 wt.% Si alloys considering both the solutes macrosegregation profiles and the scale of the microstructure dendritic arrays. A water-cooled unidirectional solidification system was used to obtain the as-cast samples. Electrochemical impedance spectroscopy (EIS) and potentiodynamic anodic polarization techniques were used to analyze the corrosion resistance in a 0.5 M NaCl solution at 25 °C. It was found that the Al–8Cu–3Si alloy has better electrochemical corrosion resistance than the Al–6Cu–1Si alloy for any position along the casting length. At the castings regions where the Cu inverse profile prevailed (up to about 10 mm from the castings surface) the corrosion current density decreased up to 2.5 times with the decrease in the secondary dendrite arm spacing.     

Corrosion Resistance of Alloys of the Al–Cu–Fe–(Si,B) System in Mineralized Saline and Acid Solutions

Materials Science - We study the influence of alloying with 4–7 at.% Si and/or 1–3 at.% B on the corrosion behavior of Al– Cu–Fe quasicrystalline alloys in the solutions of...     

Incipient melting of Al5Mg8Si6Cu2 and Al2Cu intermetallics in unmodified and strontium-modified Al–Si–Cu–Mg (319) alloys during solution heat treatment

The present work was performed on seven alloys containing in common Al–6.5 wt%Si–3.5 wt%Cu, with magnesium in the range 0.04–0.45 wt%, and strontium in the range 0–300 p.p.m. The alloys were cast in the form of tensile test bars, solution heat treated in the temperature range 480–540°C for times up to 24 h. Two types of solution heat treatment were applied: (i) single-stage, where the test bars were solution treated at a certain temperature for 12 h prior to quenching in hot water (60°C); (ii) two-stage, where the test bars were solution treated for 12 h/510°C+12 h/T°C (T=510, 520, 530, 540°C), followed by quenching in hot water. In the low-magnesium alloys (i.e. with Mg0.04 wt%), melting of the Al2Cu phase commenced at 540°C. Increasing the magnesium content to 0.5 wt% reduced the incipient melting temperature of the Al5Mg8Si6Cu2 phase to 505°C. The mechanism of incipient melting and its effect on the tensile properties have been discussed in detail. © 1998 Chapman & Hall     

Viscosity of Industrially Important Zn–Al Alloys Part II: Alloys with Higher Contents of Al and Si

The viscosity of Zn–Al alloys melts, with industrial interest, was measured for temperatures between 693 K and 915 K, with an oscillating cup viscometer, and estimated expanded uncertainties between 3 and 5?%, depending on the alloy. The influence of minor components, such as Si, Mg and Ce?+?La, on the viscosity of the alloys is discussed. An increase in the amount of Mg triggers complex melt/solidification processes while the addition of Ce and La renders alloys viscosity almost temperature independent. Furthermore, increases in Al and Si contents decrease melts viscosity and lead to an Arrhenius type behavior. This paper complements a previous study describing the viscosity of Zn–Al alloys with quasi-eutectic compositions.     

Imaging of Structure Evolution in Solidifying Al–Bi Immiscible Alloys by Synchrotron Radiography

Using synchrotron X-ray imaging technique,the segregation evolution in solidifying Al–10 wt% Bi immiscible alloys was investigated at different cooling rates.Irrespective of the cooling rate,most of the Bi solute appeared at the upper part of the sample after solidification.The reason for this Bi enrichment phenomenon is different for different cooling rates.Besides Marangoni motion,positive segregation,which has rarely been noticed before,can also make Bi solute transfer to the hot top zone.It is also found that,bubbles(or pores) appear in solidifying Al–10 wt% Bi alloys,and the number of bubbles(or pores) increases with the increase of the cooling rate,while the size of the bubbles(or pores) decreases.     

Decomposition of supersaturated solid solutions in Al–Cu–Mg–Si alloys

The Al–Cu–Mg–Si alloying system is a base for a diverse group of commercial alloys which acquire their properties after quenching and aging. Therefore, the knowledge of the phase composition of hardening precipitates and the conditions under which they are formed is very important. ast reference data were analyzed along with experimental results and calculations of phase equilibria. Different alloys were compared based on the composition of the supersaturated solid solution. It is shown that the phase composition of aging products in alloys with Mg : Si > 1 agrees well with the equilibrium phase composition at a temperature of annealing. However, the sequence of precipitation in the alloys with Mg : Si < 1 is more complicated. The hardening in these alloys occurs with precipitation of the and phases and their precursors. The former phase may contain copper and later transforms either to and (Mg₂Si) or to Q phase depending on the amount of copper and annealing temperature.     

Simultaneously Improved Bendability and Strength of Al–Mg–Si–Cu–Zn Alloys by Controlling the Formation and Evolution of Primary Fe-Rich Phase

In present work, the formation, evolution, and distribution of the primary Fe-rich phase in an Al–Mg–Si–Cu–Zn–Fe–Mn alloy are coupling controlled by ultrasonic melt treatment (USMT) and thermomechanical processing (TMP). It is shown in the results that the size of grains and Fe-rich phase in the as-cast state can be greatly reduced by the applied optimum USMT at 680 °C. Additionally, the transformation rate of β-Fe-rich phase to α-Fe-rich phase can be also enhanced. After the coupling control of USMT and TMP, the number density and distribution uniformity of multiscale Fe-rich particles can be greatly increased or improved, which contributes to the fine-grained recrystallization microstructure and weakened texture. Finally, compared with the 6xxx series Al alloys (such as AA6016 and AA6111), the alloy sheet in the pre-aging state exhibits substantially improved bendability and strength (the plastic strain ratio and tensile strength are 0.67 and 304 MPa, respectively). The effect of USMT on the formation and transformation of primary Fe-rich phase and the mechanisms of improved bendability and strength are deeply discussed.     

The fatigue damage development in a cast Al–Si–Cu alloy

The experimental identification of fatigue damage mechanisms and evaluation of their development rate, based on changes in material respond on cycle loading, has been presented in the work. The research has been conducted on hyper-eutectic cast alloy AlSi8Cu3. The microstructure and fracture analyses were performed. The high cycle fatigue tests were conducted with frequency of 20 Hz under constant nominal stress amplitude with monitoring the strain response of material during the test. The ratcheting was found as the main mechanism of the fatigue damage. It was established that the linear fatigue accumulation law should not be used for fatigue life prediction in case of the tested cast aluminum alloy.     

Growth direction and Si alloying affecting directionally solidified structures of Al–Cu–Si alloys

The roles of growth direction and Si content on the columnar/equiaxed transition and on dendritic spacings of Al–Cu–Si alloys still remain as an open field to be studied. In the present investigation, Al–6 wt-%Cu–4 wt-%Si and Al–6 wt-%Cu alloys were directionally solidified upwards and horizontally under transient heat flow conditions. The experimental results include tip growth rate and cooling rates, optical microscopy, scanning electron microscopy energy dispersive spectrometry and dendrite arm spacings. It was found that silicon alloying contributes to significant refinement of primary/secondary dendritic spacings for the upward configuration as compared with corresponding results of the horizontal growth. Experimental growth laws are proposed, and the effects of the presence/absence of solutal convection in both growth directions are discussed.     

The segregation of copper and silicon in Al–Si–Cu alloy during electromagnetic centrifugal solidification

The present paper investigates the segregation of copper and silicon in an Al–1wt%Cu–1wt%Si alloy solidified under the co-action of centrifugal and electromagnetic forces. The reasons for the solute segregation and the effect of electromagnetic force on segregation are discussed. Tubular samples cut from the solidified alloy are analyzed, the results showing that the segregation of copper and silicon occurs along the normal direction of the samples and that the electromagnetic field has a remarkable influence on the segregation of both copper and silicon. As the exciting current increases, the segregation of copper decreases, while the segregation of silicon first increases and then decreases. The migration of solute atoms in the melt depends not only on the density difference between the solute and aluminum atoms, but also on the strength of the electromagnetic force. The magnetic force changes the rotation velocity of the melt, reduces the migration velocity of copper and causes the reduction of copper segregation. Because of the difference of the electrical conductivity between the solute and the aluminum melt, the reductions of velocity are not equal.     

Microstructure Evolution of Cu-Pb Monotectic Alloys Processed in Drop Tube

Rapid solidification of Cu-Pb monotectic alloys has been accomplished during free fall in a 3m drop tube.Both macrosegregated and uniformly dispersed structures are observed in Cu-40wt pct Pb alloy droplets,whereas droplets of composition Cu-64wt pct Pb exhibit only macrosegregation morphologies.The microstructures are strongly dependent on droplet size.The higher undercooling tends to facilitate liquid phase separation and results in more extensive macrosegregation in smaller droplets.There exists a pronounced tendency for the Pb-rich liquid to occupy the surface of the droplets of both compositions,resulting from the quite lower surface tension of the Pb-rich phase and cauing a Pb-rich layer at the surface of the solidified droplet.The nucleation of monotectic cells in the Cu-40 wt pct Pb droplets with dispersed structures preferentially occurs at the droplet surface.A single nucleation event takes place more frequently as droplet size is reduced.