Effect of rapid solidification on the microstructure and mechanical properties of hot-pressed Al–20Si–5Fe alloys

The aim of this work is to study the effect of cooling rate and subsequent hot consolidation on the microstructural features and mechanical strength of Al–20Si–5Fe–2X (X = Cu, Ni and Cr) alloys. Powder and ribbons were produced by gas atomization and melt spinning processes at two different cooling rates of 1 × 10⁵ K/s and 5 × 10⁷ K/s. The microstructure of the products was examined using optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The particles were consolidated by hot pressing at 400 °C/250 MPa/1 h under a high purity argon atmosphere and the microstructure, hardness and compressive strength of the compacts were evaluated. Results showed a profound effect of the cooling rate, consolidation stage, and transition metals on the microstructure and mechanical strength of Al–20Si–5Fe alloys. While microstructural refining was obtained at both cooling rates, the microstructure of the atomized powder exhibited the formation of fine primary silicon (~ 1 μm), eutectic Al–Si phase with eutectic spacing of ~ 300 nm, and δ-iron intermetallic. Supersaturated Al matrix containing 5–7 at.% silicon and nanometric Si precipitates (20–40 nm) were determined in the microstructure of the melt-spun ribbons. The hot consolidation resulted in coarsening of Si particles in the atomized particles, and precipitation of Si and Fe-containing intermetallics from the supersaturated Al matrix in the ribbons. The consolidated ribbons exhibited higher mechanical strength compared to the atomized powders, particularly at elevated temperatures. The positive influence of the transition metals on the thermal stability of the Al–20Si–5Fe alloy was noticed, particularly in the Ni-containing alloy.     

Effect of pores and acicular eutectic silicon particles on the performance of Al–Si–Mg (AS7G03) casting

Excellent castability, corrosion resistance and high specific strength has made cast Al–Si–Mg alloys a suitable candidate material for various aerospace application. Aluminium alloy casting AS7G03, belonging to Al–Si–Mg series of cast alloy in Y23 condition, is being used as outlet adaptor of liquid propellant tank for Indian space programme. During developmental stage, one of the castings namely oxidizer tank outlet adaptor failed and parted in to two pieces during the proof pressure test (PPT) at 22 bar.This paper brings out the details of investigation and correlates the effect of pores and acicular unmodified silicon particle on the performance of the material.     

Influence of additives on the impact toughness of Al–10.8% Si near-eutectic cast alloys

This article investigates the effects of melt treatment and addition of alloying elements on the impact toughness of as-cast and heat-treated Al–10.8% Si near-eutectic alloys. Increasingly precise impact behaviors are discussed in the context of differentiating between initiation and propagation energies, including the ductility index, which is the ratio of the propagation to initiation energies; total energy as a useful measure is also discussed. Details concerning the evaluation of tensile properties are reported in a separate article [Mohamed AMA, Samuel FH, Samuel AM, Doty HW. Influence of additives on the microstructure and tensile properties of near-eutectic Al–10.8%Si cast alloy. Mater Des, in press]. The concentration of elements in the alloys was changed to the following range: Fe 0.5–1 wt%, Mn 0.5–1 wt%, Cu 2.25–3.25 wt%, and Mg 0.3–0.5 wt%, while the impact toughness upon artificial aging in a temperature range of 155–240 °C for 5 h was also investigated. The results indicate that the morphology of fibrous Si in Sr-modified alloys enhances toughness because of its profound effect on crack initiation and crack propagation resistance. The combined addition of modifier and grain refiner leads to a 33% increase in the impact strength compared to the untreated alloy. In alloys containing high levels of iron, such as the RF2 (1% Fe, 1% Mn) and RF4 (1% Fe, 0.5% Mn) alloys, the addition of iron leads to an increased precipitation of sludge or β-Fe platelets, respectively; these particles also act as crack initiation sites and reduce the impact properties noticeably. In alloys already containing high levels of copper, such as the RC2 (3.25% Cu, 0.3% Mg) and RC5(0.3.25% Cu, 0.5% Mg) alloys, increasing the copper level lowers the impact properties significantly, in view of the fact that the fracture behavior is now predominantly influenced by the Al₂Cu phase rather than by the Si particles. The average crack propagation speed of impact-tested samples shows a good inverse relationship to impact energy. Crack propagation speed can thus provide a qualitative estimation of the impact energy expected for special alloy conditions.     

Microstructure and mechanical properties of Al–20Si–5Fe–2X (X = Cu, Ni, Cr) alloys produced by melt-spinning

Al–20Si–5Fe–2X (X = Cu, Ni and Cr) ribbons were produced by melt-spinning and consolidated by hot pressing at 400 °C for 60 min. The microstructure of the ribbons and the consolidated alloys was investigated using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffractometry (XRD) method, and transmission electron microscopy (TEM). The hardness and compressive strength of the specimens at ambient and elevated temperatures were examined. The microstructure of the ribbons exhibited featureless and dendritic zones. Results of XRD and TEM showed formation of spherically shaped Si particles with an average diameter of 20 nm. Ultrafine Si (110–150 nm) and iron-containing intermetallic particles were noticed in the microstructure of the consolidated ribbons. An improved strength was achieved by alloying of Al–20Si–5Fe with Cu, Ni, and Cr. Nickel was found to be the most effective element in increasing the maximum stress, particularly at elevated temperatures.     

Closure mechanisms in Al–Si–Mg cast alloys and long-crack to small-crack corrections

Crack growth behavior of small and long cracks has been a subject of much research; the differences observed in the growth response were largely attributed to crack closure. In this study, cast Al–Si–Mg alloys of various residual stress levels and Si content/morphology were investigated, and the dominant closure mechanisms were identified. The near-threshold fatigue crack growth behavior of the alloys is controlled by two types of closure: residual stress-induced (due to residual stresses introduced during heat treatment) and microstructure/roughness-induced (due to variations in Si content/morphology). These closure mechanisms were individually discussed and closure corrective techniques were applied to long crack growth results. The near-threshold closure corrected long crack growth data compare favorably with small crack growth data.     

Parameters controlling the microstructure of Al–11Si–2.5Cu–Mg alloys

This study investigated the effects of cooling rate during solidification, heat treatment, and the addition of Mn and Sr on the formation of intermetallic phases in Al–11Si–2.5Cu–Mg alloys. Microstructures were monitored using optical microscopy and EPMA techniques. The results reveal that the volume fractions of intermetallic phases are generally much lower in the furnace-cooled samples than in the air-cooled ones due to the dissolution of the β-AlFeSi and Al₂Cu phases during slow cooling at critical dissolution temperatures. Strontium additions increased the volume fraction of the Al₂Cu phase in the as-cast conditions at low and high cooling rates, as well as at varying ranges of Mn levels. Platelets of the β-AlFeSi phase were to be observed in the microstructure of the as-cast air-cooled samples with a DAS of 40 μm at both Mn levels, while none of these particles were to be found in the furnace-cooled samples with a DAS of 120 μm. Sludge particles were observed in almost all of the air-cooled alloys with sludge factors of between 1.4 and 1.9. These particles, however, were not observed in the furnace-cooled alloys with similar sludge factors. Solution heat treatment coarsens the Si particles in the non-modified alloys under both sets of cooling conditions studied. In the Sr-modified alloys, solution treatment has varied effects depending on the cooling rate and the level of Mn present.     

Processing and properties of Al–Li–SiCp composites

Al–Li–SiC_p composites were fabricated by a modified version of the conventional stir casting technique. Composites containing 8, 12 and 18 vol% SiC particles (40 μm) were fabricated. Hardness, tensile and compressive strengths of the unreinforced alloy and composites were determined. Ageing kinetics and effect of ageing on properties were also investigated. Additions of SiC particles increase the hardness, 0.2% proof stress, ultimate tensile strength and elastic modulus of Al–Li–8%SiC and Al–Li–12%SiC composites. In case of the composite reinforced with 18% SiC particles, although the elastic modulus increases the 0.2% proof stress and compressive strength were only marginally higher than the unreinforced alloy and lower than those of Al–Li–8%SiC and Al–Li–12%SiC composites. Clustering of SiC particles appears to be responsible for reduced the strength of Al–Li–18%SiC composite. The fracture surface of unreinforced 8090 Al-Li alloy (8090Al) shows a dimpled structure, indicating ductile mode of failure. Fracture in composites occurs by a mixed mode, giving rise to a bimodal distribution of dimples in the fracture surface. Cleavage of SiC particles was also observed in the fracture surface of composites. Composites show higher peak hardness and lower peak ageing time compared with unreinforced 8090Al alloy. Macro- and microhardness increase significantly after peak ageing. Ageing also results in considerable improvement in strength of the unreinforced 8090Al alloy and its composites. This is attributed to formation of δ^′ (Al₃Li) and S^′ (Al₂CuMg) precipitates during ageing. Per cent elongation, however, decreases due to age hardening. Al–Li–12%SiC, which shows marginally lower UTS and compressive strength than the Al–Li–8%SiC composite in extruded condition, exhibits higher strength than Al–Li–8%SiC in peak-aged condition.     

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.     

Microstructural investigations on as-cast and annealed Al–Sc and Al–Sc–Zr alloys

Al–Sc and Al–Sc–Zr alloys containing 0.05, 0.1 and 0.5 wt.% Sc and 0.15 wt.% Zr were investigated using optical microscopy, electron microscopy and X-ray diffraction. The phase composition of the alloys and the morphology of precipitates that developed during solidification in the sand casting process and subsequent thermal treatment of the samples were studied. XRD analysis shows that the weight percentage of the Al₃Sc/Al₃(Sc, Zr) precipitates was significantly below 1% in all alloys except for the virgin Al0.5Sc0.15Zr alloy. In this alloy the precipitates were observed as primary dendritic particles. In the binary Al–Sc alloys, ageing at 470 °C for 24 h produced precipitates associated with dislocation networks, whereas the precipitates in the annealed Al–Sc–Zr alloys were free of interfacial dislocations except at the lowest content of Sc. Development of large incoherent precipitates during precipitation heat treatment reduced hardness of all the alloys studied. Growth of the Al₃Sc/Al₃(Sc, Zr) precipitates after heat treatment was less at low Sc content and in the presence of Zr. Increase in hardness was observed after heat treatment at 300 °C in all alloys. There is a small difference in hardness between binary and ternary alloys slow cooled after sand casting.     

Dry sliding wear behaviour of Al–12Si–4Mg alloy with cerium addition

The purpose of this work is to understand the effect of cerium addition on wear resistance behaviour of as-cast alloys. Al–12Si–4 Mg alloys with 1–5 wt% cerium addition were prepared using the casting technique. A sliding wear test was carried out under applied loads of 10 N, 30 N and 50 N at a fixed sliding speed of 1 m/s using a pin-on-disc configuration. The wear test was conducted in dry conditions at room temperature of 25 °C. Detailed analysis of the microstructure, worn surface, collected debris and microhardness was undertaken in order to investigate the differences between the as-cast alloys with different levels of cerium addition. The addition of 1–5 wt% cerium was found to lead to the precipitation of intermetallic phases (Al–Ce), resulting a needle-like structures. Increasing cerium content up to 2 wt% improved both wear resistance and microhardness of as-cast alloys. Addition of more than 2 wt% cerium, however, led to a decrease in microhardness, resulting in lower wear resistance of the alloys. Moderate wear was observed at all loads, with specific wear rates (K′) ranging from 6.82 × 10⁻⁵ with 2 wt% Ce at applied load of 50 N to 21.48 × 10⁻⁵ mm³/N m without added Ce at an applied load of 10 N. Based on K′ ranges, the as-cast alloys exhibited moderate wear regimes, and the mechanism of wear is a combination of abrasion and adhesion. Alloy containing 2 wt% Ce, with the highest hardness and lowest K′ value, showed the greatest wear resistance.     

Influence of additives on the microstructure and tensile properties of near-eutectic Al–10.8%Si cast alloy

The continuing quest for aluminum castings with enhanced mechanical properties for applications in the automotive industries has intensified the interest in aluminum–silicon alloys. In Al–Si alloys, the properties are influenced by the shape and distribution of the eutectic silicon particles in the matrix, as also by the iron intermetallics and copper phases that occur upon solidification. The detailed microstructure and tensile properties of as-cast and heat-treated new experimental alloy belonging to cast Al–Si near-eutectic alloys have been investigated as a function of Fe, Mn, Cu, and Mg content. Microstructural examination was carried out using optical microscopy, image analysis, and electron probe microanalysis (EPMA), wavelength dispersive spectroscopic (WDS) analysis facilities. Tensile properties upon artificial aging in the temperature range of 155–240 °C for 5 h were also investigated. The results show that the volume fraction of Fe-intermetallics increases as the iron or manganese contents increase. Compact polygonal or star-like particles form when the sludge factor is greater than 2.1. The Al₂Cu phase was observed to dissolve almost completely during solution heat treatment of all the alloys studied, especially those containing high levels of Mg and Fe, while Al₅Cu₂Mg₈Si₆, sludge, and α-Fe phases were found to persist after solution heat treatment. The β-Al₅(Fe,Mn)Si phase dissolved partially in Sr-modified alloys, and its dissolution became more pronounced after solution heat treatment. At 0.5% Mn, the β-Fe phase forms when the Fe content is above 0.75%, causing the tensile properties to decrease drastically. The same results are obtained when the levels of both Fe and Mn are increased beyond 0.75%, because of sludge formation. On the other hand, the tensile properties of the Cu-containing alloys are affected slightly at high levels of Mg as a result of the formation of Al₅Cu₂Mg₈Si₆ which decreases the amount of free Mg available to form the Al₂CuMg phase. The results also show that, for the heat-treated alloys, peak aging is achieved at 180 °C, although the highest quality index corresponds to 155 °C aging temperature, for all the alloys investigated. Accordingly, 155 °C may be considered as the optimal aging treatment. It is also consistent with this observation that quality index is more sensitive to variations in tensile ductility than in tensile strength.     

Steady state creep and creep recovery behaviours of pre-aging Al–Si alloys

The creep and creep recovery of pre-aging Al–1 wt.%Si and Al–1 wt.%Si–0.1 wt.%Zr–0.1 wt.%Ti alloys have been investigated at room temperature under different constant stresses. The aging temperature dependence of steady creep rate, _st, and the recovery strain rate, π, show that under the same test conditions first alloy yields creep or creep recovery rates much higher as compared with those of second alloy. The stress exponent n was found to change from 2.5 to 7.43 and 4.57 to 11.99 for two alloys, respectively, characterizing dislocation slipping mechanism. The activation energies of steady state creep of the two alloys were found to be 78.4 kJ/mol and 32.8 kJ/mol for Al–Si and Al–Si–Zr–Ti alloys, respectively. The microstructure of the samples studied was investigated by optical and transmission electron microscopy (TEM).     

On the validity of Hall–Petch equation for single-phase β Ti–Al–Nb alloys undergoing stress-induced martensitic transformation

The yield strengths of Ti–Al–Nb alloys, which undergo stress-induced martensitic transformation, prior to the onset of plastic deformation during tensile testing, were found to obey the Hall–Petch relationship with grain size. The overall friction stress was observed to decrease with increase in Nb content while it remained more or less unchanged with increase in Al content in these alloys. On the other hand, the overall , the unpinning constant, which is an index of the efficiency of boundaries as obstacle to dislocation motion, was found to increase with increase in Nb and decrease with increase in Al content in these alloys.     

The tensile deformation behavior of Ti–3Al–4.5V–5Mo titanium alloy

The tensile deformation behavior of Ti–3Al–4.5V–5Mo titanium alloy was studied. The results show that there are obvious yield points on true stress–true strain curves of annealing structures, then a stress drop occurs. The curves show linear work-softening after yielding at annealing temperature of 720–780 °C and linear work-hardening at annealing temperature of 800–840 °C. Elastic energy stored in the α-phase is dramatically released after plastic deformation of the β-phase, which leads to the stress drop.     

Metallurgical assessment of an emerging Al–Zn–Mg–Cu P/M alloy

The objective of this work was to conduct a detailed assessment of the microstructure and mechanical properties of an emerging Al–Zn–Mg–Cu powder metallurgy (P/M) alloy known as Alumix 431D. A variety of techniques were considered including optical microscopy, X-ray diffraction, electron-probe micro-analysis, thermal dilatometry, and differential scanning calorimetry as well as apparent hardness, tensile testing, and bending fatigue. Alumix 431D exhibited many of the same attributes found in wrought counter parts such as 7075. A sintered density of approximately 99% of theoretical was achieved, indicating that the alloy was highly responsive to sintering. Once heat treated, a T6 hardness of 86 HRB and a room temperature ultimate tensile strength of 448 MPa were noted. Thermal analyses implied that the precipitation behaviour of Alumix 431D closely mimicked comparable 7XXX series wrought alloys and was largely premised on the precipitation of η-phase variants. Tensile properties of the alloy in a T1 temper were found to be relatively stable at temperatures up to 150 °C and 1000 h of exposure time. Those of T6 specimens degraded under the same exposure conditions to the point where equivalency with the T1 product was noted.     

Effect of Porosity on Thermal Conductivity of Al–Si–Fe–X Alloy Powder Compacts

The thermal conductivity and thermal diffusivity of hot- and cold-pressed Al–17Si–5Fe–3.5Cu–1.1Mg–0.6Zr (mass%) alloy powder compacts were investigated as a function of the porosity volume fraction. Samples with a very low degree of porosity were produced by hot-pressing air atomized alloy powder with a particle size of 45–100 m. The same powder was used to produce highly porous compacts by cold compaction using a manual press. The thermal diffusivity of the powder compacts was measured using a sinusoidal modulation photopyroelectric technique in a configuration that is similar to the laser flash method. The thermal diffusivity of the material decreases by a factor of about 13 with an increasing porosity of 25 vol% and a factor of about 300 at 60 vol % porosity. Since the calculated specific heat (weighted average of mass specific heat values of major alloy compounds) is much less porosity dependent, the porosity dependence of the thermal conductivity is similar to the thermal diffusivity and decreases exponentially with increasing porosity. Microstructural characterization of high porosity samples prepared by cold compaction indicated that the distribution of pores is not uniform over the cross-section. An interconnecting network of open and closed pores in the form of channels created pockets of porosity,clearpage 2.3pc which are largely responsible for a drastic reduction of thermal conductivity 4pc with increasing porosity.     

Fabrication and characterisation of Al–7Si–0.35Mg/fly ash metal matrix composites processed by different stir casting routes

In the present investigation, the effect of three different stir casting routes on the structure and properties of fine fly ash particles (13 μm average particle size) reinforced Al–7Si–0.35Mg alloy composite is evaluated. Among liquid metal stir casting, compocasting (semi solid processing), modified compocasting and modified compocasting followed by squeeze casting routes evaluated, the latter has resulted in a well-dispersed and relatively agglomerate and porosity free fly ash particle dispersed composites. Interfacial reactions between the fly ash particle and the matrix leading to the formation of MgAl₂O₄ spinel and iron intermetallics are more in liquid metal stir cast composites than in compocast composites.     

The effect of Li on the tensile properties of cast Al–Mg2Si metal matrix composite

The effects of both Li modification and cooling rate on the microstructure and tensile properties of an in situ prepared Al–15%Mg₂Si composite were investigated. It was found that the addition of 0.3%Li reduces the average size of Mg₂Si primary particles from 30 to 6 μm. The effect of cooling rate was investigated by the use of a mold with different section test bars. The results showed an increase in both UTS and elongation values with reduction in section thicknesses corresponding to increasing cooling rates. Adding Li also raised the tensile strength and elongation values and reduced the number of decohered particles observed in fracture surfaces thereby increasing the alloy's ductility. Data scatter and unexpected low tensile values of 3 mm sections were attributed to casting defects observed in fracture surfaces. Large clusters of Mg₂Si particles and eutectic cell boundaries were found to be potential crack propagation paths in this alloy.     

Effect of environment on fatigue crack growth in ultrafine grain Al–Mg

The fatigue crack growth rates, obtained in high vacuum and in ambient air, of ultrafine grain (UFG) Al–7.5Mg (grain size  250 nm) at various load ratios were compared to those of powder-metallurgy (P/M) Al–7Mg (grain size  2 μm) and ingot-metallurgy (I/M) Al–7Mg (grain size  100 μm). In both vacuum and ambient air, fatigue crack growth rates at all stress ratios decrease with increasing grain size. The fatigue crack growth threshold (ΔK_th) follows the reverse order, increasing with increasing grain size. These trends are interpreted in terms of fracture surface roughness effects that are correlated with grain size. In vacuum, the thresholds of all three materials exhibit no load ratio dependency at load ratios from 0.1 to 0.5. In air, the threshold of UFG Al–7.5Mg exhibits weak load ratio dependency, while P/M and I/M Al–7Mg exhibit modest load ratio dependency. The environmental effect on the fatigue crack growth rates is assessed by determining the difference in crack growth driving force (ΔK) between air and vacuum. It was found that the environmental contribution to the driving force of all three materials is similar, nearly independent of grain size.     

Microstructure, mechanical properties and aging behavior of Mg–5Li–3Al–2Zn–xAg

Mg–5Li–3Al–2Zn–xAg (x = 0, 0.1, 0.3, 0.6, 1.2) alloys were prepared by medium-frequency induction furnace under the ambient of pure argon. The effect of Ag addition on the microstructure, tensile properties, and aging behavior was investigated. Results show that the addition of Ag can restrain the decomposition from MgAlLi₂ to AlLi. With the addition of Ag, the over-aging point is retarded and the over-aging phenomenon is avoided in Mg–5Li–3Al–2Zn–1.2Ag. The solid solution of Ag in matrix phases and the restraining of the decomposition from MgAlLi₂ to AlLi are two aspects that strengthen the alloys.