The influence of Cu and Mg additions on the decomposition of Al-(40 to 50) wt% Zn alloys during isothermal and continuous heating

The influence of 1 wt% copper and 0.2wt% magnesium additions on the heat evolution and structural changes of Al-40 and Al-50wt% Zn alloys during isothermal ageing at 30° C and continuous heating up to 400° C was studied using microcalorimetry, differential scanning calorimetry and transmission electron microscopy. Isothermal studies indicate rapid formation of Guinier-Preston (GP) zones at initial stages of ageing followed by discontinuous precipitation of-phase competing with the formation of transition phases also containing third elements. DSC curves show that the amount of metastable phases formed during room temperature ageing increases with ternary additions, especially to Al-50% Zn alloys. Up to 75° C, GP zones undergo composition but no size changes: during further continuous heating,' (Mg₂Zn) appears, the dissolution of which proceeds parallel to the continuous precipitation of at medium temperatures. Addition of copper reduces the discontinuous-precipitation to a lesser extent than magnesium.     

Calorimetric studies of 8090 and 1441 Al–Li–Cu–Mg–Zr alloys of conventional and retrogressed and reaged tempers

The differential scanning calorimetry (DSC) technique has been used to examine the solid state reactions of GPB zones formation, precipitation of δ′, T₁, T₂, S′, δ phases and their dissolution occurring in the 8090 and 1441 Al–Li–Cu–Mg–Zr alloys of the water-quenched (WQ), peak aged T8 and T6, over aged T7, retrogressed (R), retrogressed and reaged (RRA) T77 tempers. All the exothermic and endothermic peaks in the DSC thermograms have been identified and discussed. The noticeable differences observed in the thermograms of the 8090 and 1441 alloys have been explained and this is attributed to the variation in the concentrations of the solute elements Li, Cu and Mg in the alloys. The peak temperatures as well as the heat evolved and absorbed during the precipitation and dissolution reactions have been determined with the help of the built-in software of the simultaneous thermal analyzer (STA) used for DSC studies. X-ray diffractograms and a few TEM micrographs have been illustrated to correlate the reactions in these alloys. Further, all the thermograms of the WQ state of the 1441 alloy, taken at four different heating rates, have exhibited overlapping peaks of GPB zones formation and the δ′ phase precipitation. The overlapping peaks cause restrictions in determining the kinetic parameters of activation energy and growth parameter, but, interestingly, the thermograms of retrogressed tempers have showed separate peaks of GPB zones formation and δ′ phase precipitation which will easily enable to find out kinetic parameters, by varying heating rate method.     

Effects of Mg,Fe, Be additions and solution heat treatment on the π-AlMgFeSi iron intermetallic phase in Al–7Si–Mg alloys

The presence of magnesium in Be-free Al–7Si–Mg alloys results in the formation of an undesirable iron-intermetallic known as the π-AlMgFeSi phase. The effect of Mg, Fe, and Be on the formation of this phase in both unmodified and Sr-modified Al–7Si–xMg–yFe alloys containing 0.4–0.8-wt% Mg and 0.1–0.8-wt% Fe has been investigated at a dendrite arm spacing of 65 μm. A qualitative microstructural examination was carried out to study the effect of solution heat treatment (540 °C/8 h) on the decomposition of the π-AlMgFeSi phase (“π-phase”) in Al–7Si–xMg–0.1Fe alloys containing 0.4–1.0-wt% Mg. The results indicate that increasing the Mg and Fe content increases the amount of the π-AlMgFeSi phase formed. Quantitative measurements revealed a reduction in the surface fraction of the π-phase after solution heat treatment. Different levels of decomposition of the π-phase into needles of β-Al₅FeSi iron intermetallic phase (“β-phase”) were observed at 0.4-, 0.6-, and 0.8-wt% Mg, after solution heat treatment.     

Effect of Mg content and solution treatment on the microstructure of Al-Si-Cu-Mg alloys

Three different quaternary alloys Al-6Si-3Cu-xMg (x = 0.59, 3.80 and 6.78 wt.%) were produced using conventional ingot casting metallurgy. The study was focused to investigate the effect of magnesium and solution heat treatment on the microstructure. Results shown variations in composition and morphology for the silicon-rich phases as well as a change of the predominant copper-rich phase Al₂Cu (θ) to Al₅Cu₂Mg₈Si₆ (Q phase) when magnesium content is increased. The amount of Mg in solid solution was constant for the three different cast-alloys, increasing considerably after solution heat treatment to 2.7 at.% for the alloy with higher Mg content This fact allowed to obtain Cu:Mg ratios (in at.%) in solid solution lower than 1.0 for the alloys with 3.80 and 6.78 Mg wt.%, impossible to reach for the alloy with low Mg content. During dissolution process, Al₂Cu phase was observed to be more suitable to dissolve than Q phase. Fragmentation, spheroidization and coarsening of Q and silicon-rich phases were observed. Solution time required for these processes occurrence was longer for Q phase. Solution heat treatments at 480°C for 12 h were found to be appropriate for the studied alloys.     

Post-β″ phases and their influence on microstructure and hardness in 6xxx Al-Mg-Si alloys

Starting from solid solution (T4) or a condition with β″ precipitates (T6), three Al-Mg-Si alloys with similar total solute content (1.3 at%), but different Si/Mg ratios (2, 1.25 and 0.8) were isothermally heat-treated at 250 or 260°C and investigated by transmission electron microscopy. The result microstructure for all alloys and conditions consisted of metastable, needle-shaped precipitates growing along 〈100〉 directions in aluminium. Each of the phases β″-Mg₅Si₆, β′-Mg_1.8Si, U1-MgAl₂Si₂ and U2-MgAlSi could be identified as main precipitate in the alloy with its solute Si/Mg ratio closest to the same ratio in the composition of that particular phase: The highest Si content alloy produced coarse needles of the trigonal U1-phase coexisting with finer precipitates of hexagonal B′-phase. The most common phase in the Mg-rich alloy is coarse needles of hexagonal β′-type. The Si/Mg ratio of 1.25 in one alloy is similar to the Si/Mg ratio in β″. Here the microstructure changes from that of fine β″ needles to fine needles of the orthorhombic U2-phase. This material remains strongest during heat-treatment. Nucleation on dislocations, mainly by the B′-phase, was observed to be significant in the case of Si-rich alloys heat-treated from T4-condition.     

Effect of initial microstructures on hot forging of Ca-containing cast Mg alloys

Two cast noncombustible Mg–9Al–1Zn–1Ca alloys (composition in mass%) with coarse and fine initial microstructures were hot forged by compression at temperatures of 523–603 K and a true strain rate of 1–10⁻² s⁻¹. The compressive stress–strain curves for the two alloys were similar and typical of metals undergoing dynamic recrystallization (DRX). The alloy with the coarse initial microstructure suffered from edge crack formation during hot forging, while the alloy with the fine initial microstructure exhibited smooth peripheral surfaces after hot forging at temperatures of 573 K and above. The reduction of grain size by DRX was similar in the two hot-forged alloys, but the recrystallized volume fraction was lower in the alloy with the coarse initial microstructure. Insoluble second phases (seemingly Al₂Ca) provide additional DRX sites, and thus it is expected that the finer initial cast microstructure will improve the microstructure in the resulting hot-forged Mg parts.     

Positron lifetime studies and coincidence Doppler broadening spectroscopy of Al–6Mg–<Emphasis Type="Italic">x</Emphasis>Sc (<Emphasis Type="Italic">x</Emphasis> = 0 to 0.6 wt.%) alloy

Positron annihilation spectroscopy (PAS), comprising of both positron lifetime and coincidence Doppler broadening measurements, has been employed for studying the phase decomposition behaviour of scandium doped Al–6Mg alloys. Micro structural and age hardening studies have also been conducted to substantiate the explanation of the results of PAS. Samples with scandium concentration ranging from 0 to 0.6 wt.% have been studied. The measured positron lifetimes of undoped alloy reveal that GP zones are absent in the as-prepared Al–6Mg alloy. The observed positron lifetimes and the results of coincidence Doppler broadening measurements largely stem from the entrap of positrons at the interface between aluminium rich primary dendrites and the magnesium enriched interdendritic eutectic mixture of Mg₅Al₈ (β) and the primary solid solution of aluminium (α). The study also provides evidence of the formation of scandium vacancy complexes in Al–6Mg alloys doped with scandium upto a concentration of 0.2 wt.%. However such complex formation ceases to continue beyond 0.2 wt.% Sc; instead, the formation of fine coherent precipitates of Al₃Sc is recorded in the as prepared alloy containing 0.6 wt.% scandium. The positron annihilation studies coupled with CDBS have also corroborated with the fact that the fine coherent precipitates of Al₃Sc are formed upon annealing the Al–6Mg alloys doped with scandium of concentration 0.2 wt.% and above. Transmission electron microscopic studies have provided good evidence of precipitate formation in annealed Al–6Mg–Sc alloys. Elevated temperature annealing leads to dissociation of the scandium-vacancy complexes, thereby leading to the enhancement of the mobility of magnesium atoms. This has facilitated fresh nucleation and growth of Mg₅Al₈ precipitates in the above alloys at 673 K.     

A study of precipitation hardening of commercial Al-9 wt% Si alloy

Structural and morphological investigations of the processes occurring in Al-9 wt% Si silumin during isothermal ageing at high temperatures were performed using hardness measurements and electron microscopic observations. It was established that the alloy achieved maximum hardness after being aged for 2.7×10³ sec at a temperature of about 463 K. During this time, formation of clusters of supersaturating atoms, probably of spherical symmetry, was observed, and which, as the ageing time lengthened, assumed the form of needles (GP zones). It is suggested that GP zones contain foreign atoms (impurities, e.g. Fe, Mn, Na) which may tend to increase the stability of these zones. During continued ageing, in the period between 1.1×10⁴ and 2.2×10⁴ sec, excess silicon precipitates appeared. These precipitates increased in size progressively as the ageing time was increased, whereas the GP zones, after reaching a certain magnitude, remained unchanged. The change GP zones -phase rod-like precipitates occurs after ageing for approximately 8.6×10⁴ sec. Differences between the course of ageing in Al-9 wt% Si silumin and in (after supersaturation) single-phase alloys from a pseudobinary Al-Mg₂Si system, are discussed.     

Effects of rapid heating on aging characteristics of T6 tempered 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. Heating rate in fluidized bed (FB) is an order of magnitude greater than in conventional air convective furnaces (CF). On aging using FB, it was observed that the nucleation rate of Mg₂Si particles was greater than in CF. Thermal analyses show an endothermic reaction during aging in CF. No such transformation was observed during aging in FB. The endothermic transformation could be due to the dissolution of GP zones or metastable phase(s). The total heat treatment time for T6 temper was reduced to less than 2 h using FB.     

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.     

Calorimetric evaluation of the effects of SiC concentration on precipitation processes in SiC particulate-reinforced 7091 aluminium

A comprehensive differential scanning calorimetric (DSC) investigation has been conducted on the precipitation and dissolution behaviour of SiC particulate-reinforced 7091 aluminium. DSC is shown to be a particularly attractive experimental technique for developing new thermal and thermomechanical processes for aluminium-based metal matrix composites. These new processes are necessitated due to the deleterious effects that the SiC reinforcement causes on the aluminium matrix precipitation behaviour and the resultant ductility and fracture toughness properties. In this study the effects of SiC concentration and ageing temperatures, times, and sequences were evaluated. In addition to the unreinforced 7091 alloy, composites containing 10, 20 and 30 vol% particulate SiC were characterized. Identifications of specific phases involved in reactions detected with DSC were achieved by making direct correlations to previously published transmission electron microscopy studies. It was found that the presence of SiC in the aluminium significantly affects the solid state transformation kinetics of the 7091 aluminium matrix. Specifically, increasing the SiC concentration was found to decrease the temperatures at which GPI and GPII zones precipitate at their maximum rates and to increase the temperature at which GPI zones revert at maximum rate. The transition phase,, and equilibrium phase,, formation kinetics were observed to be insensitive to SiC concentration. Also, increasing the SiC concentration was seen to decrease the temperature at which the equilibrium phase dissolution occurred at its maximum rate. Transformation mechanics have been proposed which are consistent with these observations.     

Quasicrystal-reinforced Mg alloys

The formation of the icosahedral phase (I-phase) as a secondary solidification phase in Mg–Zn–Y and Mg–Zn–Al base systems provides useful advantages in designing high performance wrought magnesium alloys. The strengthening in two-phase composites (I-phase + α-Mg) can be explained by dispersion hardening due to the presence of I-phase particles and by the strong bonding property at the I-phase/matrix interface. The presence of an additional secondary solidification phase can further enhance formability and mechanical properties. In Mg–Zn–Y alloys, the co-presence of I and Ca₂Mg₆Zn₃ phases by addition of Ca can significantly enhance formability, while in Mg–Zn–Al alloys, the co-presence of the I-phase and Mg₂Sn phase leads to the enhancement of mechanical properties. Dynamic and static recrystallization are significantly accelerated by addition of Ca in Mg–Zn–Y alloy, resulting in much smaller grain size and more random texture. The high strength of Mg–Zn–Al–Sn alloys is attributed to the presence of finely distributed Mg₂Sn and I-phase particles embedded in the α-Mg matrix.     

Preprecipitation in Al/Zn/Mg alloys studied by hardness and small-angle X-ray scattering measurements

Small-angle X-ray scattering techniques and hardness measurements have been used to study preprecipitation in two Al/Zn/Mg alloys containing the same Zn content (2.46 at. %) but different Mg contents. Changes in Guinier-Preston (G-P) zone size are observed to follow the same pattern as changes in hardness. It is also observed that a critical zone size is required for the nucleation of the intermediate precipitate,'; the size being dependent on the Mg content. The role of Mg as a reversible-vacancy trap is also established.     

The solidification process of Al–Mg–Si alloys

The microstructure and solidification process of three Al–Mg–Si alloys with different magnesium contents have been studied using optical microscopy and the electron probe X-ray microanalysis. The results showed that Al–Mg–Si alloys possessed fairly complicated solidification path: L→α-Al+L1→α-Al+Al15Si2(FeMn)3+L2→α-Al+Al15Si2(FeMn)3+ (α-Al+Mg2Si)+L3→α-Al+Al15Si2(FeMn)3+(α-Al+Mg2Si)+(α-Al+Mg2Si+Al15Si2(FeMn)3), and wide solidification temperature of 75 °C. The magnesium content in the alloys greatly influenced the as-cast microstructure. The higher the magnesium content, the more Mg2Si structure was present. Iron and manganese segregated to the finally solidified zone, which resulted in the formation of ternary eutectic structure. Although their content in the alloys was very low, their effect on solidification behaviour cannot be ignored. This revised version was published online in November 2006 with corrections to the Cover Date.     

Clustering and formation of nano-precipitates in dilute aluminium and magnesium alloys

A nuclear magnetic resonance (NMR) method is described for monitoring the evolution of nano-scale precipitate structures in dilute aluminium and magnesium-based alloys. Three examples are given of recent work in this field: (i) ⁶³Cu detection of Guinier–Preston zones (GPZs) and θ-phases in Al(Cu), (ii) ²⁷Al detection of γ-phase (Mg₁₇Al₁₂) formation in Mg(Al) alloys and (iii) ⁴⁵Sc detection of Al₃Sc formation in Al(Sc) alloys.     

Heterogeneous crystallization and composition dependence of optical parameters in Sn–Sb–Bi–Se chalcogenides

Bulk samples of Sn₁₀Sb_20−x Bi _x Se₇₀ (0 ≤ x ≤ 8) chalcogenide alloys were prepared by the conventional melt quenching technique. Thin films were prepared on well-cleaned glass substrates by thermal evaporation technique. X-ray diffraction studies revealed that the alloys with x = 0 and 2 at.% of Bi were amorphous, whereas the alloys with x = 4, 6, 8 at.% were crystalline. The crystalline phases are identified as due to the formation of Bi₂Se₃ and BiSe₂ phases. The microstructural and differential scanning calorimetric studies show the presence of these phases. A simple, straight forward procedure suggested by Swanepoel has been used to calculate the optical parameters, refractive index, and extinction coefficient. The optical gap for all the samples has been obtained from the Tauc plots. The variation in optical parameters for different Bi concentration has been explained on the basis of presence of defect states and the change in stoichiometry with the change in Bi concentration.     

Prediction of hot deformation behavior of Al–5.9%Cu–0.5%Mg alloys with trace additions of Sn

High temperature deformation behavior of Al–5.9wt%Cu–0.5wt%Mg alloys containing trace amounts (from 0 to 0.1 wt%) of Sn was studied by hot compression tests conducted at various temperatures and strain rates. The peak flow stress of the alloys increased with increase in strain rate and decrease in deformation temperature. The peak stress could be correlated with temperature and strain rate by a suitable hyperbolic-sine constitutive equation. The activation energy for hot deformation of the alloy without Sn content was observed to be 183.4 kJ mol⁻¹ which increased to 225.5 kJ mol⁻¹ due to 0.08 wt% of Sn addition. The Zener-Hollomon parameter (Z) was determined at various deforming conditions. The tendency of dynamic recrystallization increased with low Z values, corresponding to low strain rate and high temperature. The peak flow stresses at various processing conditions have been predicted by the constitutive modeling and correlated with the experimental results with fairly good accuracy. It was possible to predict 80, 75, 100, 100, 90, and 85% of the peak stress values within an error less than ±13%, for the investigated alloys. With addition of Sn content >0.04 wt%, peak flow stress increased significantly for all strain rate and temperature combinations. Scanning electron microscope revealed two types of second phases at the grain boundary of the undeformed alloy matrix, one being an Al–Cu–Si–Fe–Mn phase while the other identified as CuAl₂. The high strength and flow stress value of the alloy with 0.06 wt% of Sn content, may be attributed to the variation in amount, composition, and morphology of the Al–Cu–Si–Fe–Mn phase, as well as to the lower value of activation energy for precipitation reaction, as revealed from differential scanning calorimetric studies.     

Effects of Sn,Ca additions on thermal conductivity of Mg matrix alloys

In the present work, the effects of Sn, Ca additions on thermal conductivity were investigated in as cast Mg–Sn–Ca alloys. The measured values of thermal conductivity of Mg–3Sn–xCa alloys obviously increased from 85.6 to 126.3?W?m^??1?K^??1 with the increasing Ca from 0 to 1.5?wt-%, and then decreased to 98.3?W?m^??1?K^??1 with the 2.5?wt-% Ca. In addition, the thermal conductivity of the Mg–Sn–Ca (Sn/Ca atomic ratio of 1) alloys decreased slightly from 154.2 to 132.1?W?m^??1?K^??1 with the increasing Sn, Ca. Meanwhile, the microstructures of the selected alloys were discussed in detail, suggesting that the solute atoms that caused lattice distortion had greater effect on thermal conductivity compared with the second phases formed in as cast Mg–Sn–Ca alloys.     

Structural transformations produced during tempering of Fe-Ni-Co-Mo alloys

In an attempt to resolve the difficulties in interpreting the behaviour of the industrial maraging steel of VASCOMAX 300 type, we have studied the influence of separate or simultaneous additions of Co and Mo on the structural transformations taking place during the heating cycles at moderate rates (300° Ch^–1) for the Fe-Ni base alloys. An interpretation of the mechanisms of the M transformation, implying preponderantly diffusional mechanisms, has been proposed for the alloys Fe-Ni, Fe-Ni-Co, Fe-Ni-Mo and Fe-Ni-Co-Mo for which the Ni/Fe, Mo/Fe ratios are equal to those of the reference maraging steel. The ageing phenomena in the martensite of the alloys containing Mo have been studied and show the formation of G.P. zones having a diameter of a few tens of Å and spaced at an average distance of 120 Å, in the case of the quaternary alloy, at temperatures of 400 to 450° C.