Influence of Phosphorus on the Nucleation of Eutectic Silicon in Al-Si Alloys

The role of phosphorus (P) in the heterogeneous nucleation of eutectic silicon (Si) and the evolution of eutectic grains in hypoeutectic aluminum-silicon alloys were investigated. Systematic additions of P in the range of 0.5 to 20 ppm to Al-7 wt pct Si alloys of different purities have shown that the morphology of the eutectic Si changes from a fine plate- to a coarse flake-like structure. The growth of eutectic grains was investigated by interrupting the eutectic reaction by quenching experiments. Moreover, the macroscopic growth mode of the eutectic grains was characterized by electron backscatter diffraction. An increase in P concentration from 2 to 3 ppm resulted in a transition of the macroscopic growth mode of the Al-Si eutectic in high purity alloys from growth with a planar front with a strong dependence of the thermal gradient, to nucleation in the vicinity of the primary Al dendrites and subsequent growth of distinct eutectic grains. It is suggested that AlP particles are the key impurities acting as potential nucleation sites for eutectic Si. This is further substantiated as with increasing P concentration nucleation and growth of the Al-Si occurred at higher temperatures close the equilibrium Al-Si eutectic solidification temperature at 850 K (577 °C). In addition, the recalescence undercooling ΔT _R,eu was reduced from 4.5 K (0.5 ppm P) to 1.5 K (20 ppm P) in high purity alloys. This was accompanied by a drastic increase of the nucleation rate of the eutectic grains.     

Effects of rapid heating on solutionizing characteristics of Al-Si-Mg alloys using a fluidized bed

Effects of rapid heat transfer using a fluidized bed on the heat-treating response of Al-Si-Mg alloys (both unmodified and Sr modified) were investigated. The heating rate in the fluidized bed is greater than in conventional air convective furnaces. Particle size analyses of eutectic Si showed that the high heating rate during fluidized bed solution heat treatment causes faster fragmentation and spherodization of Si particles compared to conventional air convective furnaces. The mechanism of Si fragmentation through fluidized bed processing is through both brittle fracture and neck formation and its propagation. In contrast to this, the mechanism of Si fragmentation using a conventional air convective furnace is through neck formation and propagation. The Sr-modified D357 alloy showed a faster spherodizing rate than the unmodified alloy. Thermal analyses showed an exothermic reaction during solution heat treatment using a fluidized bed due to recrystallization, and coarsening of eutectic Al grains. Whereas the alloy solutionized using a conventional air convective furnace showed two exothermic reactions, one due to annihilation of point defects and the other due to recrystallization, and coarsening of the eutectic grains in the aluminum matrix. The recrystallization temperature of the alloy solutionized in the fluidized bed is lower than those in the conventional air convective furnace. Both tensile strength and elongation of fluidized bed solutionized alloys are greater than those solutionized using the air convective furnace. The optimum heat-treatment time for T4 temper using a fluidized bed for unmodified and Sr-modified alloy was reduced to 60 and 30 minutes, respectively.     

The effect of Mg on the microstructure and mechanical behavior of Al-Si-Mg casting alloys

The microstructure and tensile behavior of two Al-7 pct Si-Mg casting alloys, with magnesium contents of 0.4 and 0.7 pct, have been studied. Different microstructures were produced by varying the solidification rate and by modification with strontium. An extraction technique was used to determine the maximum size of the eutectic silicon flakes and particles. The eutectic Si particles in the unmodified alloys and, to a lesser extent, in the Sr-modified alloys are larger in the alloys with higher Mg content. Large Fe-rich π-phase (Al₉FeMg₃Si₅) particles are formed in the 0.7 pct Mg alloys together with some smaller β-phase (Al₅FeSi) plates; in contrast, only β-phase plates are observed in the 0.4 pct Mg alloys. The yield stress increases with the Mg content, although, at 0.7 pct Mg, it is less than expected, possibly because some of the Mg is lost to π-phase intermetallics. The tensile ductility is less in the higher Mg alloys, especially in the Sr-modified alloys, compared with the lower Mg alloys. The loss of ductility of the unmodified alloy seems to be caused by the larger Si particles, while the presence of large π-phase intermetallic particles accounts for the loss in ductility of the Sr-modified alloy.     

Microstructural effects on the tensile and fracture behavior of aluminum casting alloys A356/357

The tensile properties and fracture behavior of cast aluminum alloys A356 and A357 strongly depend on secondary dendrite arm spacing (SDAS), Mg content, and, in particular, the size and shape of eutectic silicon particles and Fe-rich intermetallics. In the unmodified alloys, increasing the cooling rate during solidification refines both the dendrites and eutectic particles and increases ductility. Strontium modification reduces the size and aspect ratio of the eutectic silicon particles, leading to a fairly constant particle size and aspect ratio over the range of SDAS studied. In comparison with the unmodified alloys, the Sr-modified alloys show higher ductility, particularly the A356 alloy, but slightly lower yield strength. In the microstructures with large SDAS (>50 μm), the ductility of the Sr-modified alloys does not continuously decrease with SDAS as it does in the unmodified alloy. Increasing Mg content increases both the matrix strength and eutectic particle size. This decreases ductility in both the Sr-modified and unmodified alloys. The A356/357 alloys with large and elongated particles show higher strain hardening and, thus, have a higher damage accumulation rate by particle cracking. Compared to A356, the increased volume fraction and size of the Fe-rich intermetallics (π phase) in the A357 alloy are responsible for the lower ductility, especially in the Sr-modified alloy. In alloys with large SDAS (>50 μm), final fracture occurs along the cell boundaries, and the fracture mode is transgranular. In the small SDAS (<30 μm) alloys, final fracture tends to concentrate along grain boundaries. The transition from transgranular to intergranular fracture mode is accompanied by an increase in the ductility of the alloys.     

Solidification Analysis of an Al-19 Pct Si Alloy Using <Emphasis Type="Italic">In-Situ</Emphasis> Neutron Diffraction

In-situ neutron diffraction and thermal analysis techniques were used simultaneously to evaluate the kinetics of the nonequilibrium solidification process of an Al-19 pct Si binary alloy. Feasibility studies concerning the application of neutron diffraction for advanced solidification analysis were undertaken to explore its potential for high resolution phase analysis coupled with fraction solid/liquid analysis of phase constituents. Neutron diffraction patterns were collected in a stepwise mode during solidification between 983 K and 793 K (710 °C and 520 °C). The variation of intensity of the diffraction peaks was analyzed and compared to the results of conventional cooling curve analysis. Neutron diffraction was capable of detecting nucleation of the Si phase (primary and eutectic), as well as the Al phase during Al-Si eutectic nucleation. Moreover, neutron diffraction indicated the possibility of detecting the presence of Si peaks at near liquidus temperature and premature nucleation of α-Al prior to Al-Si eutectic temperature. The solid and liquid volume fractions were determined based on the change of intensity of neutron diffraction peaks over the solidification interval. Overall, the volume fraction determined was in good agreement with the results of the cooling curve thermal analysis, as well as calculations using the FactSage software. The potential of neutron diffraction for high resolution melt analysis required for advanced studies of grain refining, eutectic modification, etc. was illustrated. This study will help us better understand the solidification mechanism of Al-Si alloys used for various casting component applications.     

Effect of Compositional Variation on the Microstructural Evolution and the Castability of Al–Mg–Si Ternary Alloys

This study examined the microstructural evolution and castability of Al–Mg–Si ternary alloys with varying Si contents. Al–6Mg–xSi alloys (where x = 0, 1, 3, 5, and 7; all compositions in mass pct) were examined, with Al–6 mass pct Mg as a base alloy. The results showed that in the ternary alloys with Si ≤ 3 pct, the solidification process ended with the formation of eutectic α-Al–Mg₂Si phases generated by a univariant reaction. However, in the case of ternary alloys with Si > 3 pct, solidification was completed with the formation of α-Al–Mg₂Si–Si ternary eutectic phases generated by a three-phase invariant reaction. In addition to the eutectic Mg₂Si phases, the primary Mg₂Si phases formed in each of the ternary alloys, and the size of both sets of phases increased with increasing Si content. The two-phase eutectic α-Al–Mg₂Si nucleated from the primary Mg₂Si phases. The inoculated Al–6Mg–1Si alloy had the smallest grain size. Moreover, the grain-refining efficacy of the Al–5Ti–B master alloy in the ternary alloys decreased with increasing Si content in the alloys. Despite the poisoning effect of Si on the potency of TiB₂ compounds in the inoculated Al–6Mg–1Si alloy, the grain size of the alloy was slightly smaller than that of the Al–6Mg binary alloy. This resulted from the increasing growth restriction factor (induced by Si addition) of the Al–6Mg–1Si alloy. In terms of the castability, the examined alloys showed different levels of susceptibility to hot tearing. Among the alloys, the ternary Al–6Mg–5Si alloy exhibited the highest susceptibility to hot tearing, whereas the Al–6Mg–7Si exhibited the lowest. The severity of hot tearing initiated by the unraveling of the bifilm was determined by the freezing range, grain size, and the amount of eutectic phases at the end of the solidification process.

     

Heterogeneous nucleation of solidification of Si in Al-Si and Al-Si-P alloys

Heterogeneous nucleation of solidification in melt spun Al-Si and Al-Si-P has been studied using differential scanning calorimetry, and transmission, scanning transmission and high resolution electron microscopies. The microstructures of the heat treated melt spun alloys all consist of an Al matrix, Al-Si eutectic distributed along the Al grain boundaries, and Si embedded in the Al matrix. The Si microstructure depends on the level of P: coarse faceted Si particles are nucleated by AlP particles in Al-Si containing 2 ppm P and Al-Si-P containing 35 ppm P whereas eutectic droplets of fine Si particles are nucleated by the surrounding Al matrix at a high undercooling in Al-Si containing 0.25 ppm P. The Si nucleation onset temperature remains approximately constant while the peak and end temperatures both decrease with increasing cooling rate, in agreement with classical nucleation theory. Kinetic analysis, using the spherical cap model gives contact angles of 10°, 43° and 10° for Si nucleation in low and high purity Al-Si and Al-Si-P respectively.     

Investigation of the microstructure of the Al–Si eutectic in binary aluminium–7 wt% silicon alloys by electron backscatter diffraction (EBSD)

Binary aluminium–7 wt% silicon alloys with and without strontium modification have been cast with a cooling rate of 0.2–0.6°C/s. The level of impurities has been kept at a minimum. The crystallographic orientation of the dendritic and eutectic aluminium as well as the eutectic silicon has been studied using electron backscatter diffraction (EBSD). The crystallographic orientation of the aluminium within the eutectic is found to be strongly influenced by the orientation of neighbouring dendrites in unmodified and Sr-modified alloys. The crystallographic orientation of the silicon phase in the eutectic shows that silicon flakes/fibers within one eutectic colony can often be related to each other by the misorientation characteristic of twinning. Within one Sr-modified eutectic colony, silicon fibers are often found to have a common 〈1 1 0〉 direction. Aluminium has been found to have a 〈1 0 0〉 or a 〈1 1 0〉 direction parallel to the 〈1 1 0〉 direction of the Si fibers.     

Aging Effects on Heat Treatment Response and Mechanical Properties of Al-(1 to 13 pct)Si-Mg Cast Alloys

The effects of solution treatment time and Si content and morphology on microstructures and mechanical properties of heat-treated Al-Si-Mg cast alloys were investigated systematically. Five alloys, with Si levels ranging from 1 to 13 pct, were tested in as-cast, T4, and T61 conditions. The eutectic Si was both unmodified and Sr-modified. Results show that the microstructures are affected significantly by alloy composition, eutectic Si morphology, and solution treatment time. Si content has significant effects on ultimate tensile strength (UTS), yield strength (YS), and elongation as well as a strong influence on solution treatment response. In T61 treatment with different solutionizing times, UTS and YS reach their maximum values in ~1 hour of solutionizing followed by a decrease, then a slight increase, and finally, a plateau close to the maximum level. Elongation of alloys with a high Si content, 7 pct and 13 pct, increases rapidly at solutionizing times of 1 to 2 hours then varies in a wide range, showing improvements in the 4 to 10 hours range. The data indicate that a solution treatment time of ~1 hour is sufficient to achieve maximum strength. The changes in mechanical properties were correlated to changes in microstructure evolution—Mg-Si precipitation, Si particle fragmentation, and microstructure homogenization. Empirical models uniquely relating Si content to UTS and YS are given for T61 heat-treated alloys.     

Discussion of “Effect of Strontium and Phosphorus on Eutectic Al-Si Nucleation and Formation of <Emphasis Type="Italic">β</Emphasis>-Al<Subscript>5</Subscript>FeSi in Hypoeutectic Al-Si Foundry Alloys”*

The nucleation of Si during the solidification of Al-Si hypoeutectic alloys appears to result from a hierarchy of nucleating substrates operating at progressively lower temperatures: (1) AlP, whose generally particulate morphology can initiate the formation of compact Si particles as seen in hypereutectic alloys; (2) oxide bifilms alone, whose planar form creates platelike Si morphologies; and (3) a currently unknown nucleant that initiates the coral eutectic growth morphology. The consequential growth forms are particulate when initiated on particles and coarse (unmodified) plates when initiated on oxide films. However, when oxide films are deactivated by Sr, eutectic is forced to grow at a lower formation temperature with a consequently fine coral morphology known as “modified” Si. Increasing Sr additions progressively eliminate each substrate in turn to effect the change from the “unmodified” to the modified structure. *Y.H. CHO, H.-C. LEE, K.H. OH, and A.K. DAHLE: Metall. Mater. Trans. A, 2008, vol. 39A, pp. 2435–48. Discussion submitted October 6, 2008.     

The influence of strontium on porosity formation in Al-Si alloys

Strontium modification is known to alter the amount, characteristics, and distribution of porosity in Al-Si castings. Although many theories have been proposed to account for these effects, most can be considered inadequate because of their failure to resolve contradictions and discrepancies in the literature. In an attempt to critically appraise some of these theories, the amount, distribution, and morphology of porosity were examined in sand-cast plates of Sr-free and Sr-containing pure Al, Al-1 wt pct Si, and Al-9 wt pct Si alloys. Statistical significance testing was used to verify apparent trends in the porosity data. No apparent differences in the amount, distribution, and morphology of porosity were observed between Sr-free and Sr-containing alloys with no or very small eutectic volume fractions. However, Sr modification significantly changed the amount, distribution, and morphology of porosity in alloys with a significant volume fraction of eutectic. The addition of Sr reduced porosity in the hot spot region of the casting, and the pores became well dispersed and rounded. This result can be explained by considering the combined effect of the casting design and the differences in the pattern of eutectic solidification between unmodified and Sr-modified alloys.     

Review of Microstructure Evolution in Hypereutectic Al–Si Alloys and its Effect on Wear Properties

Al–Si alloys with silicon content more than 13 % are termed as hypereutectic alloys. In recent years, these alloys have drawn the attention of researchers due to their ability to replace cast iron parts in the transportation industry. The properties of the hypereutectic alloy are greatly dependent on the morphology, size and distribution of primary silicon crystals in the alloy. Mechanical properties of the hypereutectic Al–Si alloy can be improved by the simultaneous refinement and modification of the primary and eutectic silicon and by controlling the solidification parameters. In this paper, the effect of solidification rate and melt treatment on the evolution of microstructure in hypereutectic Al–Si alloys are reviewed. Different types of primary silicon morphology and the conditions for its nucleation and growth are explained. The paper discusses the effect of refinement/modification treatments on the microstructure and properties of the hypereutectic Al-Si alloy. The importance and effect of processing variables and phosphorus refinement on the silicon morphology and wear properties of the alloy is highlighted.     

Heterogeneous nucleation of solidification of Si by solid AI in hypoeutectic Al-Si alloy

Melt-spun Al-3 wt pct Si with and without ternary additions of Na and Sr has been heat-treated above the Al-Si eutectic temperature in a differential scanning calorimeter to form a microstructure of Al-Si eutectic liquid droplets embedded in the α-Al matrix. During subsequent cooling in the calorimeter, the heterogeneous nucleation temperature for solidification of Si in contact with the surrounding Al matrix depends sensitively on the alloy purity, with a nucleation undercooling which increases with increasing alloy purity from 9 to 63 K below the Al-Si eutectic temperature. These results are consistent with Southin’s hypothesis that low levels of trace P impurities are effective in catalyzing Si nucleation in contact with the surrounding Al matrix. With a low Al purity alloy, 0.1 wt pct Na addition increases the Si nucleation undercooling from 9 to 50 K, 0.15 wt pct Sr addition does not affect the Si nucleation temperature, and 0.3 wt pct Sr addition decreases the Si nucleation undercooling from 9 to 3 to 4 K. The solidified microstructure of the liquid Al-Si eutectic droplets embedded in the Al matrix depends on the Si nucleation undercooling. With low Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form one faceted Si particle; however, with high Si nucleation undercooling, each Al-Si eutectic droplet solidifies to form a large number of nonfaceted Si particles embedded in Al. Formerly with the Oxford Centre for Advanced Materials and Composites, Department of Materials, Oxford University     

Internal oxidation of NiAl and NiAlSi alloys at the dissociation pressure of NiO

NiAl and NiAlSi alloys were internally oxidized at temperatures of 1073–1273 K by the Rhines Pack method. For the NiAl alloy, the oxidation process follows parabolic law and the oxidation front was flat with severe integranular oxidation occurring at 1073 K and extensive grain boundary sliding at 1273 K. As for NiAlSi alloys, the oxidation rate increased with increase of Si content at 1073 K but the rate decreased at higher temperatures due to total or partial continuous oxide layer formation at the internal oxidation front. The depth of intergranular oxidation was also greatly reduced. For all samples, nickel was found to be transported out to the surface with the amount proportional to the Si content. Lattice diffusion (Nabarro-Herring creep) was believed to be the main cause for nickel transport in the NiAl alloy while dislocation pipe diffusion is the mechanism for NiAlSi alloys.     

Solidification of Aluminium Alloys Under Ultrasonication: An Overview

An overview of our investigations on solidification microstructure formation under ultrasonication in various Al alloys and comparison against unrefined or chemically modified microstructures under identical cooling conditions is presented. Primary α-Al grains show significant refinement under ultrasonication, even better than established chemical inoculation, in the small ingots investigated. Increased solute content appears to promote grain refining efficiency under ultrasonication. Regular lamellar eutectic in Al–33 wt%Cu was observed to degenerate into rounded particle morphology and the irregular eutectic of long Si plates in Al–11 wt%Si were spheroidised into compact form near the ultrasound radiator. Grain refinement under ultrasonication appears to originate from enhanced heterogeneous nucleation under cavitation showing distinct reduction in nucleation undercooling. Eutectic modification, on the other hand, appears to originate from coarsening as the strong fluid flow created under cavitation disturbs the thin diffusion boundary layer ahead of the eutectic growth front.     

Growth of interdendritic eutectic in directionally solidified Al-Si alloys

Interdendritic eutectic microstructures in Al-Si (6 to 12.6 wt pct Si) alloys have been investigated as a function of growth velocity and temperature gradient. The interface morphology, as well as the behavior of the eutectic spacing and undercooling, suggest that the resultant microstructure is governed by two different growth processes. That is, at low growth rates, steady-state columnar eutectic growth is found and obeys the relationship, λ²V = constant, where λ is the eutectic spacing andV is the growth rate. At higher growth rates, the nucleation of equiaxed eutectic grains occurs in the interdendritic liquid. The experimental findings are interpreted in the light of recently developed models for the columnar to equiaxed transition and for irregular eutectic growth.     

Modeling of equiaxed microstructure formation in casting

A general micro/macroscopic model of solidification for 2-D or 3-D castings, valid for both dendritic and eutectic equiaxed alloys, is presented. At the macroscopic level, the heat diffusion equation is solved with an enthalpy formulation using a standard FEM implicit scheme. However, instead of using a unique relationship between temperature and enthalpy (i.e., a unique solidification path), the specific heat and latent heat contributions, whose sum equals the variation of enthalpy at a given node, are calculated using a microscopic model of solidification. This model takes into account nucleation of new grains within the undercooled melt, the kinetics of the dendrite tips or of the eutectic front, and a solute balance at the scale of the grain in the case of dendritic alloys. The coupling between macroscopic and microscopic aspects is carried out using two time-steps, one at the macroscopic level for the implicit calculation of heat flow, and the other, much finer, for the microscopic calculations of nucleation and growth. This micro/macroscopic approach has been applied to one-dimensional and axisymmetric castings of Al-7 pct Si alloys. The calculated recalescences and grain sizes are compared with values measured for one-dimensional ingots cast under well-controlled conditions. Furthermore, the influence of casting conditions on temperature field, undercooling, grain size, and microstructural spacings is shown to be predicted correctly from axisymmetric calculations with regard to the expected experimental behavior.     

Experimental Investigation on the Aging Response,Hardness and Total Impact Energy Absorption of Sr-Modified Heat-Treatable Cast Automotive Aluminum Alloys

In this study, a computer-aided instrumented Charpy impact test was used to produce the load–time information in addition to data concerning the energy absorbed of unmodified and Sr-modified cast automotive Al alloys. The total energy was used to discuss the impact properties of the alloy with respect to the different alloying elements and melt processing parameters. Effects of alloying elements, Sr-modification, and cooling rate were specifically evaluated. Aging response of the Al alloys at 185 °C indicated a rise in hardness and decline in impact toughness during the first 4 h of aging. The cooling rate is found to have a direct effect on the size and distribution of microstructural phases in a casting, and in turn, on the impact toughness as well.     

Solution Treatment Effects on Microstructure and Mechanical Properties of Al-(1 to 13 pct)Si-Mg Cast Alloys

The effects of solution treatment time and Si content and morphology on microstructures and mechanical properties of heat-treated Al-Si-Mg cast alloys were investigated systematically. Five alloys, with Si levels ranging from 1 to 13 pct, were tested in as-cast, T4, and T61 conditions. The eutectic Si was both unmodified and Sr-modified. Results show that the microstructures are affected significantly by alloy composition, eutectic Si morphology, and solution treatment time. Si content has significant effects on ultimate tensile strength (UTS), yield strength (YS), and elongation as well as a strong influence on solution treatment response. In T61 treatment with different solutionizing times, UTS and YS reach their maximum values in ~1 hour of solutionizing followed by a decrease, then a slight increase, and finally, a plateau close to the maximum level. Elongation of alloys with a high Si content, 7 pct and 13 pct, increases rapidly at solutionizing times of 1 to 2 hours then varies in a wide range, showing improvements in the 4 to 10 hours range. The data indicate that a solution treatment time of ~1 hour is sufficient to achieve maximum strength. The changes in mechanical properties were correlated to changes in microstructure evolution—Mg-Si precipitation, Si particle fragmentation, and microstructure homogenization. Empirical models uniquely relating Si content to UTS and YS are given for T61 heat-treated alloys.