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.     

Corrosion resistance of Al–Li alloys

Abstract

Experiments designed to investigate the corrosion resistance of several lithium-containing aluminium alloys are described. Intergranular corrosion was investigated using the NAWL test, exfoliation corrosion using the EXCO test, and stress corrosion using the C-ring test. It was found that intergranular corrosion varied with extrusion parameters and was more severe in copper-containing alloys. Alloys containing less than 0·4% Cu were not susceptible to stress corrosion. In alloys that failed, susceptibility increased as the aging treatment was extended from the underaged to the peak aged temper and also with increasing copper content. The addition of copper to ternary Al–Mg–Li alloys also increased the exfoliation corrosion attack.

MST/494     

Microstructure and mechanical properties of directionally solidified Al–Si eutectic alloys with and without antimony

Abstract

Hardness H, interjlake spacing λ, and tensile properties are reported for Al–12·7Si and Al–12·7Si–0·2Sb (all wt-%) eutectic alloys directionally solidified at growth velocities of up to 250 μm s^?1 and under temperature gradients in the liquid of up to 12·9 K mm^?1. The hardness is related to interflake spacing by the equation H=H^o+Kλ^?0·2, where H^o is the initial hardness of the alloy. This behaviour contradicts previous results, which suggest that a Hall–Petch relationship is followed. The tensile properties are shown to follow similar behaviour, confirming that hardness shows the same dependence as proof stress on interflake spacing. However, the nature of the relationship depends on the Si morphology and caution should be exercised in using hardness or interflake spacing to indicate proof stress.

MST/1585     

Microstructure and mechanical properties of directionally solidified high-Nb containing Ti–Al alloys

Two high-Nb containing Ti–Al alloys, Ti–16Al–8Nb and Ti–16Al–8Nb–1Sn were fabricated using directional solidification. Their microstructures and mechanical properties at both room and high temperatures were studied. Results showed that the addition of 1% Sn promoted the formation of laths and contributed remarkably to the enhancement in room-temperature strength and high temperature ductility of Ti–Al alloy. The alloys exhibited the feature of quasi-cleavage fracture at room temperature and they experienced significant plastic deformation at high temperatures.     

Eutectic spacing and faults of directionally solidified Al–Al3Ni eutectic

The Al–Al₃Ni eutectic was directionally solidified at a thermal gradient of 4.5 K/mm in a vacuum Bridgman–type furnace in order to study eutectic spacing selection criterion.The microstructure was examined in transverse and longitudinal sections and the interrod spacings were measured at different growth velocity. It has been shown that the interrod spacing is not unique and displays a limited range for rodlike Al–Al₃Ni eutectic alloy. The initial growth velocities are not responsible for the eutectic spacing range, while such faults as branching, endingand diameter change have a significant influence on the eutectic spacing adjustment.     

Eutectic spacing and faults of directionally solidified Al–Al3Ni eutectic

The Al–Al₃Ni eutectic was directionally solidified at a thermal gradient of 4.5 K/mm in a vacuum Bridgman-type furnace in order to study eutectic spacing selection criterion. The microstructure was examined in transverse and longitudinal sections and the interrod spacings were measured at different growth velocity. It has been shown that the interrod spacing is not unique and displays a limited range for rodlike Al–Al₃Ni eutectic alloy. The initial growth velocities are not responsible for the eutectic spacing range, while such faults as branching, ending and diameter change have a significant influence on the eutectic spacing adjustment.     

Microstructural characteristics of directionally solidified Ni–43Ti–7Al alloys

Abstract

Ni–43Ti–7Al (at-%) alloy was directionally solidified at different withdrawal rates (2, 20 and 100 μm s^?1) and a constant temperature of 1550°C by liquid metal cooling method. Results show that as the withdrawal rate decreases from 100 to 2 μm s^?1, the cellular arm spacing increases from 39·5 to 126 μm, the size of Ti₂Ni and the stability of the liquid/solid interface also increase, while the volume fraction of Ti₂Ni decreases from 3·1 to 0·9%. Moreover, microstructural analysis reveals that a NiTi+Ti₂Ni anomalous eutectic structure is formed in intercellular regions of directionally solidified samples withdrawn at 20 and 100 μm s^?1. However, in the sample withdrawn at 2 μm s^?1, Ti₂Ni phases represent strip and liquid droplet morphologies in the intercellular region. Finally, the possible explanation to the change of microstructure is discussed.     

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.     

Effect of magnesium content on thermal and structural parameters of Al–Mg alloys directionally solidified

In this study, the influence of magnesium content on thermal and structural parameters during the unsteady-state unidirectional solidification of Al–Mg alloys is analyzed. Using a special device, Al–Mg alloys containing 5, 10, and 15 wt% Mg were submitted to unidirectional solidification. Using a data acquisition system, the temperature variations along the casting during solidification were measured. From these results, the variations of solidification parameters as growth rate of dendrite tips, thermal gradient, cooling rate, and local solidification time were determined. The variation of global heat transfer coefficient at metal/mould interface was estimated through the adjustment of experimental temperature variation close to the interface and numerical predictions. Primary and secondary dendrite arms spacing variations during solidification were measured by optical microscopy. From these results, comparative analysis were developed to determine the influence of magnesium content.     

Density and viscosity of ternary Al–Cu–Si liquid alloys

Densities and viscosities of ternary Al–Cu–Si liquid alloys have been investigated over a wide temperature and composition range. Density was measured using electromagnetic levitation as a container-less technique, while viscosity was measured by means of a high-temperature oscillating cup viscometer. In this ternary system, binary interaction parameters as well as a third (ternary) interaction parameter need to be taken into account for the excess volume to describe the liquid densities. The temperature dependences of the viscosities are well described by the Arrhenius law. A maximum of the activation energy of viscous flow is found in that compositional range in which intermetallic phases exist in the solid state.     

Microstructure evolution of directionally solidified Nb–Si alloy

Abstract

By taking the method of liquid–metal cooled directional solidification, alloys with a nominal composition of Nb–14Si–24Ti–10Cr–2Al–2Hf (at-%) were prepared under different conditions. Alloys were initially directional solidified with different withdrawal rates (R?=?1·2, 6, 18 mm min^?1) at 1750°C and subsequently heat treated at 1450°C for 10 h. These processes aimed to investigate the microstructure of directionally solidified (DS) and heat treated (HT) alloys by XRD, SEM, and EDS. The microstructure of DS alloy was composed of (Nb,Ti)_SS, (Nb,Ti)₅Si₃, and Laves phase Cr₂Nb, and the former two components formed (Nb,Ti)_SS+(Nb,Ti)₅Si₃ eutectics. In addition, (Nb,Ti)₅Si₃ laths only presented in DS1·2 alloy. With the increasing withdrawal rates, the microstructure of alloy altered from hypereutectic into pseudo-eutectic, accompanied with the eutectic morphology transformation from petaloid into coupled. Also, the dimension of constituent phases reduced. However, after heat treatment, the constituent phases did not change. The petaloid morphology of eutectics in DS specimens disappeared and coupled eutectic transferred into network. The block or needle-like Cr₂Nb gathered along the boundary between (Nb,Ti)₅Si₃ and (Nb,Ti)_SS, and the overall alloy composition became homogenisation.     

Calculations of electrical resistivity for Al–Cu and Al–Mg–Si alloys

Abstract

A system for thermodynamical calculations (Thermo-Calc) was used to derive the solid solubility of the alloying elements in commercial Al–Cu and Al–Mg–Si alloys. The electrical resistivity was then calculated using a model developed by the authors based on the Matthiessen's rule. The calculated resistivity agreed with the observed resistivity within ±2.5 nΩ m for the Al–Mg–Si alloys and ±2 nΩ m for the Al–Cu alloys, except for Al–Mg–Si alloys containing boron or chromium and Al–Cu alloys with special compositions.     

Damping behaviour and mechanical properties of rapidly solidified Al–Fe–Mo–Si/Al alloys

In order to develop a new high damping aluminium alloy with strength and toughness for advanced aircraft structure application, rapidly solidified (RS) Al–Fe–Mo–Si/Al alloys were synthesized. The damping behaviour, mechanical properties and microstructures of the alloys were studied. Results showed that the damping capacities of RS Al–Fe–Mo–Si/10–15% Al alloys are stable between 7.0–10.0×10-3 at room temperature, which almost reach the high damping threshold, 10.0×10-3. At lower frequency (0.1–10 Hz) the damping capacity is decidely frequency and temperture dependent above 50°C, with lowest frequency and highest temperature resulting in the highest less factor. It was noted that mechanical properties of the Al–Fe–Mo–Si/10–15% Al alloys are both excellent at room temperature (b=536–564 MPa, =7.2–11.4%) and at elevated temperature (250°C: b=295–324 MPa). Analysis of microstructures reveal that the damping capacity arises from deformation of the pure Al areas, and strength at elevated temperature from the dispersion strengthening of intermetallic phase. © 1998 Chapman & Hall     

Microstructures and mechanical properties of directionally solidified Ti–45Al–8Nb–(W,B, Y) alloys

Intermetallic Ti–45Al–8Nb–(W, B, Y) (at.%) alloys were directionally solidified at growth rates of 10–400 μm/s with a Bridgeman type apparatus. Microstructures and room temperature (RT) mechanical properties of the directionally solidified (DS) alloys were investigated. The microstructures with different segregation morphologies were observed at different growth rates. Fully lamellar (FL) microstructure evolves into a massive microstructure when the growth rate is up to 100 μm/s. Both the width of columnar grain and the interlamellar spacing decrease with increasing growth rate. Compressive properties were not proportional to the growth rates but closely related to the segregation morphologies. Only the DS alloy with columnar pattern of Al-segregation had tensile ductility. A better RT tensile plastic elongation level of 2% and yield strength 475 MPa were obtained at growth rate of 10 μm/s. Cracks propagated in transgranular mode predominantly. Larger elongated B2 particles produced in the interdendritic regions were detrimental to the tensile ductility of the DS alloy.     

Impact toughness and fractography of Al–Si–Cu–Mg base alloys

Critical automotive applications using heat-treatable alloys are designed for high impact toughness which can be improved using a specified heat treatment. The alloy toughness and fracture behavior are influenced by the alloy composition and the solidification conditions applied. The mechanical properties of alloys containing Cu and Mg can also be enhanced through heat treatment. The present study was undertaken to investigate the effects of Mg content, aging and cooling rate on the impact toughness and fractography of both non-modified and Sr-modified Al–Si–Cu–Mg base alloys. Castings were prepared from both experimental and industrial 319 alloy melts containing 0–0.6wt% Mg. Test bars were cast in two different cooling rate molds, a star-like permanent mold and an L-shaped permanent mold, with dendrite arm spacing (DAS) values of 24 and 50 μm, respectively. Test bars were aged at 180 °C and 220 °C for 2–48 h. Charpy Impact test was used to provide the impact energy. It was observed that high cooling rates improve the impact toughness whereas the presence of Cu significantly lowers the impact properties which are determined mainly by the Al₂Cu phase and not by the eutectic Si particles. The addition of Mg and Sr were also seen to decrease the impact toughness. The crack initiation energy in these alloys is greater than the crack propagation energy, reflecting the high ductility of Al–Si–Cu–Mg base alloys.     

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     

Effect of heterogeneous deformation on precipitate distribution in Al–3·7Cu and Al–3·8Cu–1·8Mg alloys

Abstract

Cold rolling and hot rolling of solution treated Al–3·7Cu and Al–3·8Cu–1·8Mg alloys were performed to test the effect of inhomogeneous precipitation in shear bands during following aging of the material. The most effective inhomogeneous distribution of particles was observed for samples hot deformed and aged within the temperature range 473–573 K. It was noted that magnesium addition intensifies coarse shear bands development and following coarsening of particles within sheared area.     

The electrochemical properties of melt-spun Al–Si–Cu alloys

Melt spinning was used to prepare Al_75−XSi₂₅Cu_X (X = 1, 4, 7, 10 mol%) alloy anode materials for lithium-ion batteries. A metastable supersaturated solid solution of Si and Cu in fcc-Al, α-Si and Al₂Cu co-existed in the alloys. Nano-scaled α-Al grains, as the matrix, formed in the as-quenched ribbons. The Al₇₄Si₂₅Cu₁ and Al₇₁Si₂₅Cu₄ anodes exhibited initial discharge specific capacities of 1539 mAh g⁻¹, 1324 mAh g⁻¹ and reversible capacities above 472 mAh g⁻¹, 508 mAh g⁻¹ at the 20th cycle, respectively. The specific capacities reduced as the increase of the Cu content. AlLi intermetallic compound was detected in the lithiated alloys. It is concluded that the lithiation mechanism of the Al–Si-based alloys can be affected by the third component. The structural evolution and volume variation can be mitigated due to the formation of non-equilibrium state and the co-existence of nano-scaled α-Al, α-Si, and Al₂Cu for the present alloys.     

Microstructure and microhardness in horizontally solidified Al–7Si–0.15Fe–(3Cu; 0.3Mg) alloys

Transient horizontal directional solidification (THDS) experiments have been carried out with Al–7wt.%Si–0.15Fe, Al–7wt.%Si–3wt.%Cu–0.15wt.%Fe and Al–7wt.%Si–0.3wt.%Mg–0.15wt.%Fe alloys, to identify experimental relationships between growth rates (G_R), cooling rates (C_R), tertiary dendrite arm spacings (λ₃) and microhardness (HV). Optical microscopy and scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) were used to perform a comprehensive microstructural characterisation of the β-Al₅FeSi, ω-Al₇Cu₂Fe, θ-Al₂Cu, π-Al₈Mg₃FeSi₆ and α-Mg₂Si intermetallic phases. The addition of Cu and Mg to the Al–7wt.%Si–0.15wt.%Fe alloy led to the precipitation of ω and π phases from the β phase. It has been found for all analysed alloys that power experimental functions given by λ₃?=?constant.(G_R)^-1.1 and λ₃?=?constant.(C_R)^-0.55 best describe the variation of λ₃ with corresponding thermal and microstructural parameters.     

Mechanical properties and corrosion resistance of Ti–6Al–7Nb alloy dental castings

With the aim of applying a novel titanium alloy, Ti–6Al–7Nb, to a dental casting material, a comprehensive research work was carried out on its characteristics, such as castability, mechanical properties and corrosion resistance in the present study. As a result, Ti–6Al–7Nb alloy exhibited sufficient castability by a dental casting method for titanium alloys and enough mechanical properties for dental application. It is also showed excellent corrosion resistance through an immersion test in 1.0% lactic acid and an anodic polarization test in 0.9% NaCl solution. From these results, it is concluded that this Ti–6Al–7Nb alloy is applicable as a dental material in place of Ti–6Al–4V alloy, which includes cytotoxic vanadium.