Decomposition in Al-Zn alloys: Part I. Isothermal decomposition in an Al-22 at. pct Zn-0.1 at. pct Mg alloy

A small-angle X-ray scattering (SAS) study has been made on solution treated and isothermally annealed specimens of an Al-22 at. pct Zn-0.1 at. pct Mg alloy. The changes in peak position and integrated area of the SAS spectra with time and temperature indicate that de composition is nearly complete immediately after quenching, in agreement with the earlier interpretation that Gerold and Merz placed on the results of Rundman and Hilliard in the binary Al-22 at. pct Zn alloy. Furthermore, structural changes occurring during annealing are consistent with a coarsening or maturation of the fluctuations in the solution. The domi nant wavelength varies ast ^1/3 over a large time span and the temperature dependence of the coarsening process yields an activation energy of 94.2 kJ/mole. The effect of Mg is to re tard the formation of the equilibrium phases while having a small effect on the growth of the composition fluctuations during the coarsening process.     

Decomposition in Al-Zn alloys: Part II. Decomposition during continuous cooling

A small angle X-ray scattering study (SAS) has been made of decomposition during contin uous cooling in four binary Al-Zn alloys with compositions spanning the miscibility gap and in two ternary alloys, each containing 22 at. pct Zn plus small amounts of Sn and Mg. Plots of logλ _m (wavelength receiving maximum amplification during the quench)vs logQ (quench rate) yield slopes of approximately -1/3 for all alloys, indicating that coarsening plays an important role during the quench. In addition, measurements of integrated area under the SAS spectra indicate that decomposition is essentially complete in the quenched condition for all of the alloys studied. DENNIS T. LEWANDOWSKI, formerly a graduate student at Michigan Technological University. Now on sabbatical leave to Caterpillar Tractor Co., Mapleton Plant, Mapleton, Ill. 61554.     

Enhanced ductility in an aluminum-4 Pct magnesium alloy at elevated temperature

A considerable enhancement of the tensile ductility in a commercial Al-4 pct Mg alloy is observed during deformation at elevated temperatures (up to 250°C) and slow strain rates. Total elongations of ∼175 pct at 250°C were obtained compared to 27 pct at ambient temperature. Much of this ductility was a result of large increases with temperature in the post uniform or diffuse necking strain. Measurements of strain rate sensitivity,m, as a function of strain, strain rate, and temperature showed thatm near fracture was linearly related to total elongation. The mechanisms controllingm in this Al-4 pct Mg alloy were dynamic strain aging at the lower temperature range and dynamic recovery at the higher temperatures.m was found to be a function of strain only when the relative fraction of dynamic recovery was greater than ∼35 pct. A comparison ofm as measured in pure aluminum and in the commercial Al-4 pct Mg alloy suggests that Mg additions can significantly increasem during dynamic recovery.     

The atomic-structure changes in Al-16 pct Si alloy above the liquidus

The atomic-structure changes in an Al-16 pct Si alloy above the liquidus have been studied by a θ-θ high-temperature X-ray diffractometer, rapid solidification, a vertical centrifugal casting apparatus, and differential scanning calorimetry (DSC) measurements. It was found that the diffraction intensity and structure-factor (S(Q)) curves for an Al-16 pct Si alloy have small prepeaks at small Q values when the temperature is high enough. Rapid-solidification and centrifugal-casting experimental results show that the primary silicon phase can easily coarsen and segregate under additive force after an overheat at high temperatures. The DSC measurements show that the temperature and latent heat of primary solidification rise with the temperature of overheating. These experimental results suggest that a strong interaction occurs between Si-Si atoms in a liquid Al-16 pct Si alloy at high temperatures, resulting in the microsegregation of Si atoms in the melt.     

Effect of impurity type on boundary sliding behavior in the superplastic Zn-22 pct Al alloy

The effect of impurity type on boundary sliding behavior in the superplastic Zn-22 pct Al alloy was investigated using two grades of the alloy: Zn-22 pct Al-0.13 pct Cu (grade Cu) and Zn-22 pct Al-0.14 pct Fe (grade Fe). In the investigation, boundary sliding offset measurements in both grades were made at strain rates ranging from 5×10⁻⁷/s to 10⁻¹/s. This range of strain rate covered region I (the low strain rate region), region II (the intermediate strain rate region), and region III (the high strain rate region) of the sigmoidal plot between stress and strain rate that was previously reported for grade Fe. The experimental results show that the contributions of boundary sliding to the total strain, ξ, in the two grades of Zn-22 pct Al are about 20 and 52 pct at high (region III) and intermediate (region II) strain rates, respectively. By contrast, the experimental data reveal that ξ in grade Cu at low strain rates (52 pct) is essentially equal to that at intermediate strain rates (region II), while ξ in grade Fe at low strain rates (24 pct) is considerably lower than that at intermediate strain rates (56 pct). It is demonstrated that the difference in sliding behavior between grade Fe and grade Cu at low strain rates corresponds well with the difference in superplastic behavior between the two grades. In addition, consideration of the present and earlier data on sliding behavior in Zn-22 pct Al provides a correlation between two roles played by boundaries during superplastic deformation: the ability of boundaries to contribute to deformation through the process of boundary sliding and their ability to serve as favorable sites for the accumulation of impurities, i.e., boundary segregation.     

Elevated temperature deformation behavior of an Al-8.4 wt pct Fe-3.6 wt pct Ce alloy

The elevated temperature deformation characteristics of a rapidly solidified Al-8.4 wt pct Fe-3.6 wt pct Ce alloy have been investigated. Constant true strain rate compression tests were performed between 523 and 823 K at strain rates ranging from 10⁻⁶ to 10⁻³ s⁻¹. At temperatures below approximately 723 K, the alloy is significantly stronger than oxide dispersion strengthened (ODS) aluminum. However, at higher temperatures, the strength of the Al-Fe-Ce alloy falls rapidly with increasing temperature while ODS aluminum exhibits an apparent threshold stress. It is shown that particle coarsening cannot fully account for the reduction in strength of the Al-Fe-Ce alloy at elevated temperatures. The true activation energy for deformation of the Al-Fe-Ce alloy at temperatures between 723 and 773 K is significantly greater than that for self-diffusion in the matrix. This is unlike the behavior of ODS alloys, which contain nondeformable particles and exhibit true activation energies close to that for self-diffusion in the matrix. Since abnormally high true activation energies for deformation are also exhibited by materials containing deformable particles, such as γ^′ strengthened superalloys, it is concluded that elevated temperature deformation in ythe Al-Fe-Ce alloy involves deformation of both the matrix and the precipitates. The loss of strength of the Al-Fe-Ce alloy appears to be related to a reduction in strength of at least some of the second phase particles at temperatures above 723 K. Formerly Research Assistant, Department of Materials Science and Engineering, Stanford University.     

Combined effects of Pb and Sn additions on the corrosion of Zn-12 pct am pct Cu-0.02 pct Mg sand cast alloy in wet steam

The effects of varying the Sn content from 0 to 0.01 pct at 0.005 and 0.015 pct Pb on the corrosion of sand-cast Zn-12 pct Al-1 pct Cu-0.02 pct Mg alloy in saturated water vapour at 95°C were investigated. Within the relevant specification limits there was no interaction between Pb and Sn, the combined effect being equivalent to that of Sn alone. The tolerance of the alloy for Sn was so low that the maximum for this impurity was considered to be 0.001 pct. For Pb, 0.01 pct produced substantially the same corrosive attack as 0.001 pct and would be considered as the upper limit for this alloy.     

The microstructural development of Mg-9 pct Al-1 pct Zn alloy during injection molding

Ex-situ microstructural analysis was used in order to assess the high-temperature transformations of the Mg-9 pct Al-1 pct Zn alloy within the injection-molding system. In addition to as-cast ingot, chipped feedstock, and final products, the alloy samples at different stages of the process were investigated. It was revealed that the cold deformation during ingot chipping and recrystallization during initial stages of residency inside the barrel led to nucleation and growth of the equiaxed grain structure. Subsequent melting of the Al-rich phase followed by liquid-alloy wetting of α-Mg grain boundaries initiated the formation of the slurry with globular solid particles. The structural evolution of the semisolid alloy under the simultaneous influence of temperature gradient and injection-screw shear is considered, and the mechanisms involved are discussed. It is concluded that during processing of partially remelted alloy the temperature profile along the injection-molding barrel in combination with the feedstock structure are key factors that control both the transformation of the thixotropic slurry and the final morphology of the primary solid phase.     

The Structure and Kinetics of the Nanoscale Precipitation Processes in Al-1.0 wt pct Mg2Si-0.4 wt pct Mg-0.5 wt pct Ag Alloy

The influence of addition of 0.4 wt pct Mg on the precipitation sequence in the balanced Al-1.0 wt pct Mg₂Si bearing 0.5 wt pct Ag has been investigated during the continuous heating of the quenched alloy from the solid solution state. Differential scanning calorimetry (DSC) and high-resolution transmission electron microscopy techniques have been used. The DSC experiments showed that all processes occurred are thermally activated. The activation energies of the precipitation processes have been determined and hence the kinetics of these precipitates have been determined. The obtained results have shown that the existence of excess Mg inhibits the formation of the early stage clusters of solute-vacancy clusters. These clusters can be assisted by the binding energies between solute Si, Mg, and Ag atoms and the excess vacancies. On the other hand, excess Mg accelerates the precipitation of random, β′-phase and β-phase precipitates.     

Superplastic flow and cavitation in Zn-22 pct Al doped with Cu

The sigmoidal relationship between stress and steady-state strain rate that has been reported for micrograin superplastic alloys is characterized by the presence of three regions: region I at low stresses, region II (the superplastic region) at intermediate stresses, and region III at high stresses. Recent results on the superplastic Zn-22 pct Al eutectoid have shown that the characteristics of region I are influenced by the impurity level of the alloy, and that neither region I nor significant cavitation is observed when such a level is reduced to about 6 ppm. These observations are in agreement with the suggestion that the origin of region I is related to strong impurity segregation at boundaries. The present investigation was conducted to study the effect of Cu, as a selected impurity, on superplastic deformation and cavitation in Zn-22 pct Al. The results show that Zn-22 pct Al-0.13 pct Cu exhibits two primary characteristics: region I is absent and cavitation is not extensive. These characteristics, which are essentially similar to those reported previously for high-purity Zn-22 pct Al but are different from those documented for a grade of the alloy containing a comparable atomic concentration of Fe, suggest that Cu has little or no tendency to segregate at boundaries. Indirect evidence in support of this suggestion is inferred from studying the effect of impurities on former α boundaries that form in the microstructure of Zn-22 pct Al as a result of solution treatment above the eutectoid temperature. Although further studies are needed to provide direct evidence for the absence of Cu segregation at boundaries, the present results clearly indicate that superplastic flow and cavitation at low stresses are controlled not only by the impurity level, but also by its type.     

A structural investigation of multiple aging of Al-7 wt pct Zn-2.3 wt pct Mg

A transmission electron microscopy study of the structural changes which attend aging at 180°C with and without pre-aging at 100°C was conducted on a high purity aluminum alloy containing 6.8 pct Zn and 2.3 pct Mg. The refinement in precipitate dispersion accompanying multiple aging is caused by the operation of aging sequences which differ from those occurring in material given the single age at 180°C only. The high nucleation rate which occurs during the low temperature pre-aging treatment is responsible for the observed precipitate refinement. The results of this investigation appear to agree favorably with the Pashleyet al. model of multiple aging.     

Coarsening rate of Β precipitates in Al-11Mg alloy

The coarsening rates of the Β precipitates in an Al-11 wt pct Mg alloy at 250 ‡C, 316 ‡C, and 330 ‡C aging temperatures were investigated. As predicted by the modified Lifshitz-Slyozov-Wagner (MLSW) theory of diffusion controlled particle coarsening, a linear dependence ofr ³ ont in the long time aging region was observed. Interfacial energies of 8.1, 1.8, and 1.7 J/m² for 250 ‡C, 316 ‡C, and 330 ‡C, respectively, were computed from these data using the MLSW theory. The high values of interfacial energy indicate that the Β precipitates are incoherent with respect to the Al matrix. The especially high value of interfacial energy and low activation energy for the precipitate coarsening at 250 ‡C suggest that significant short circuit diffusion occurs at this aging temperature.     

The effect of cold work on the precipitation of Ω and θ′ in a ternary Al-Cu-Mg alloy

The effect of plastic deformation on the microstructural evolution of an Al-5.0Cu-0.5Mg (wt pct) ternary alloy was investigated. Hardness measurements and quantitative precipitate analysis were performed on specimens that were water quenched from a solution heat treatment, stretched either 0 or 6 pct and immediately aged at ambient temperature or artificially aged at 200 °C or 250 °C for times up to 3000 hours. Quantitative transmission electron microscopy (TEM) was used to characterize Ω and θ′ number density, diameter, and thickness as a function of preage mechanical stretch and artificial aging condition. Age hardening curves for naturally and artificially aged specimens revealed an increase in hardness corresponding with a preage stretch. Quantitative TEM verified an increase in the number density and a refinement of precipitates for both Ω and θ′ between the 0 and 6 pct stretch condition for those samples artificially aged. When aged at 200 °C, θ′ exhibited superior coarsening resistance relative to the Ω phase. The quantified Ω coarsening kinetics were greater than similar Ag-containing alloys. To investigate the effects of trace Si additions on subsequent microstructural evolution, a series of Al-Cu-Mg-Si quaternary alloys were produced. The addition of 0.1Si (wt pct) was found to suppress Ω precipitation in most Al-4.0Cu-xMg alloys investigated. These initial results indicate that Ω precipitation may be related to the Mg/Si ratio.     

Effects of fatigue on the GP zones in Al-Zn alloys

Small-angle X-ray scattering has been employed to study the stability of GP zones in two Al-Zn alloys during fatigue, in order to provide statistically sound information on this process. In a 5.3 at. pct Zn alloy containing l0²⁵ zones per m³ with an average diameter of 18 Å, the normally sluggish coarsening was accelerated by fatigue at room temperature by factors of l0⁶ to l0⁷. No coarsening occurred for larger zones, except near a fatigue crack. Reverted samples aged rapidly during fatigue at room temperature, but in a reverted sample of Al-3.5 at. pct Zn cycled at 77 K no appreciable zone growth occurred. Upon warming this sample to room temperature (without any load) rapid clustering took place. These results imply that a vacancy fraction of 10^-5 to l0^-6 was produced by the fatigue, and this excess vacancy concentration appears to cause the growth during cycling at room temperature.     

Degradation of mechanical properties after 100° to 250 °c storage of 25 μm Al-1 pct Si microelectronic interconnect wire

Degradation of mechanical properties of 25 μm Al-1 pct Si wire stored at 100° to 250 °C in an air atmosphere has been investigated utilizing mechanical, structural, and kinetic approaches. Forty pct of the strength of wires stored at 100 °C and 90 pct of the strength of wires stored at 250 °C disappeared within the first twenty-four (24) hours. Elongation measurements showed that the wire can be embrittled at temperatures as low as 200 °C within twenty-four (24) hours, and elongation can decrease to less than 1 pct within this time at 200 °C. Scanning electron micrographs of electro-polished wire revealed particles distributed throughout the wire which increase in size as a function of annealing time and temperature. A kinetic analysis showed that the particle coarsening was controlled by a 97 ±34 kJ/mole activation energy process. These observations are consistent with earlier findings^1,2,3 that silicon coarsens in aluminum with an activation energy of 118 ±8 kJ/mole. We therefore attribute the degradation of mechanical properties to the coarsening of silicon in aluminum wire. It has not been previously shown that this process proceeds during air storage and in the 100° to 250 °C temperature ranee.     

On the effect of plastic deformation on the coarsening of ϑ-phase precipitation in an Al-Cu alloy

In the present investigation, the focus is on dynamic coarsening of the equilibrium ϑ phase in an Al-4wt pct Cu alloy. For this purpose, specimens containing a uniform ϑ particle distribution have been produced and deformed in compression at two different temperatures (200 °C and 250 °C) and strain rates in the ranges of 200 °C to 250 °C and 10⁻⁵ to 10⁻² s⁻¹, respectively. The particle size distribution measurements performed in a scanning microscope in back-scattered mode demonstrated a double peak behavior depending on temperature: at the lowest test temperature, the dynamic coarsening is enhanced at the highest strain rate, while at 250 °C, the coarsening seems to be affected by crushing of small and medium size particles during straining.     

Differential scanning calorimetry and electron diffraction investigation on low-temperature aging in Al-Zn-Mg alloys

Differential scanning calorimetry (DSC) has been combined with transmission electron microscopy (TEM) to investigate the low-temperature decomposition processes taking place in an Al-5 wt pct Zn-1 wt pct Mg alloy. It was confirmed that two types of GP zones, i.e., GP(I) (solute-rich clusters) and GP(II) (vacancy-rich clusters), formed independently during decomposition of the supersaturated solid solution. The GP(I) zones form at a relatively low aging temperature and dissolve when the aging temperature is increased. The GP(II) zones are stable over a wider range of temperatures. To investigate the nature of the zones in the Al-Zn-Mg alloy, differential scanning calorimetry and transmission electron microscopy have also been carried out on binary Al-Zn alloys containing 5 wt pct and 10 wt pct Zn. In these Al-Zn alloys, GP zones formed rapidly during quenching, and they gave rise to characteristic electron diffraction patterns identical to those from GP(II) in the Al-Zn-Mg alloy system, implying that GP(II) zones in Al-Zn-Mg alloys are very similar to the zones formed in binary Al-Zn alloys. Thus, it is likely that GP(II) zones in Al-Zn-Mg alloys are zinc-rich clusters. In the Al-5 wt pct Zn-1 wt pct Mg alloy, both GP(I) and GP(II) were found to transform to η′ and/or η particles during heating in the differential scanning calorimeter. The η′ was also observed to form after prolonged isothermal aging of the Al-Zn-Mg alloy at 75 °C or after short aging times at 125 °C.     

Solidification of chill-cast Al-Zn-Mg alloys to be used as sacrificial anodes

In the present research, Al-Zn-Mg alloys were vacuum induction melted and gravity cast into steel molds. Ingots were microstructurally and electrochemically characterized to evaluate their performance as Al-sacrificial anodes for cathodic protection of structures exposed to marine environments. The microstructure observed in as-cast ingots consisted mainly of α-Al dendrites with 0.68 to 2.25 vol pct of τ phase in α-Al matrix and eutectic in interdendritic regions. After heat treatment, the presence of the τ phase increased up to 5 vol pct. Electrochemical efficiencies obtained in Al alloys showed maximum values of 73 and 87 pct in as-cast ingots and heat-treated ingots, respectively. In order to contribute to the development of Al-Zn-Mg anodes, the Al-5.3 at. pct Zn-6.2 at. pct Mg (Al-12 wt pct Zn-5.4 wt pct Mg) alloy was monitored to identify the temperature changes as it cools through phase transformation intervals. Growth temperatures of the phases present in this alloy were employed to predict the structure growing at fixed growth velocity. Predictions of variation of solute concentration with growth velocity in α-Al dendrites were included, too. The results of these analyses help to select alloy composition and to control microstructure in order to develop a new generation of Al-sacrificial anodes free of In and Hg.     

Deformation behavior of dilute SnBi(0.5 to 6 at. pct) solid solutions

Tensile and creep tests were conducted to characterize the deformation behavior of four dilute SnBi alloys: SnBi0.5 at. pct, SnBi1.5 at. pct, SnBi3 at. pct, and SnBi6 at. pct, the last two being supersaturated solid solutions at room temperature. The test temperatures were − 20 °C, 23 °C, 90 °C, and 150 °C, and the strain rates ranged from approximately 10⁻⁸ to 10⁻¹ 1/s. In the tensile tests, all the alloys showed strain-hardening behavior up to room temperature. At higher temperatures, only the higher-Bi-content alloys exhibited strain softening. The deformation behavior of the alloys can be divided into two stress regimes, and the change from the low-stress regime to the high-stress regime occurred at around 6 × 10⁻⁴<σ/E<7.5 × 10⁻⁴. The results suggest that, at the low-stress regime, the rate-controlling deformation mechanism changes from dislocation climb to viscous glide with the increasing Bi content of the alloy. At the high-stress regime, the activation energy of deformation is about equal in all the alloys (∼60 kJ/mol) and the stress exponents are high (10<n<12.5). Unlike in the other alloys, bismuth precipitated at room temperature from the solution-annealed and quenched SnBi6 at. pct alloy by the discontinuous mechanism. This strongly affects the mechanical properties and makes the alloy brittle at lower test temperatures. A comparison of the deformation behavior of the dilute SnBi alloys to that of the eutectic SnBi alloy suggests that the deformation of eutectic structure is controlled by the Sn-rich phase containing the equilibrium amount of dissolved Bi.     

Phenomenology of precipitation in copper-20 pct nickel-20 pct manganese

The precipitation phenomena in the alloy copper-20 pct nickel-20 pct manganese have been investigated. Utilizing transmission electron microscopy as the principal tool; the effects of aging temperature and time as well as prior cold work were studied. For all aging temperatures the reaction products are the solute depleted fcc solid solution and an ordered structure with fct symmetry. Three aging temperatures characterized by different precipitate morphologies were studied. At 350°C discontinuous precipitation is the predominant mode of decomposition. Precipitate colonies nucleate at grain and twin boundaries and eventually grow through the entire structure. Microtwinning of the colony matrix accompanies the precipitation reaction. At 450°C both grain boundary nucleated discontinuous precipitates and fine periodic homogeneous arrays are observed in the absence of cold work. The fine periodic arrays coarsen and eventually form nuclei for the ordered fct phase. The coarsening of the periodic arrays prohibits the growth of the discontinuous precipitate early in the process, so only a small volume fraction of discontinuous precipitate is formed at the grain boundaries. Aging subsequent to cold work results in ordered, fct precipitates heterogeneously nucleated on dislocations. At 500°C no precipitate is observed in the absence of cold work. When aging is preceded by cold work, the ordered fct phase appears as heterogeneously nucleated Widmanstatten laths. No grain boundary nucleated colonies are observed at this temperature.