Quench rate effects in Al-Zn-Mg-Cu alloys

The effect of quench rate on alloys with a bcomposition of 6.75 pct Zn, 2.5 pct Mg, 1.2 pct Cu and various levels and combinations of the minor addition elements (MAE’s) Cr, Mn, and Zr is reported. The level of chromium and to a lesser extent the level of zirconium determine quench sensitivity. Slow quenching of chromium- or zirconium-bearing alloys led to the nucleation of (MgZn) plates or laths at MAE-bearing particles which had certain orientation relationships with the matrix. The density of the hardening precipitation (100 to 200Å in diam) was found to be high in slowly quenched, sensitive alloys. Even when the yield strength had fallen to 50 pct of that of the fast-quenched material, a large fraction of the fine precipitate was still formed. A fracture mechanism is proposed to account for the magnitude of quench sensitivity in such materials. An embrittlement, at low quench rates, was found in alloys containing manganese or very low levels of MAE. This embrittlement was thought to be due to precipitation in the grain boundaries, and not to the width of the precipitate-free zone.     

P/M Al-Zn-Mg-Cu系铝合金中的微量元素

总结P／M Al-Zn-Mg-Cu系铝合金中常用的微量元素和最新进展情况，以及各微量元素在合金中的主要作用，并提出了一些观点和建议。     

The micromechanisms of fatigue crack growth in a commercial Al-Zn-Mg-Cu alloy

A detailed fractographic and microstructural study, using combined scanning and transmission electron microscopy has been made of the fracture surfaces produced by fatigue of a commercial aluminium alloy 7010:T76 in moist air, dry argon and dry oxygen. In the dry environments fracture is entirely ductile: the fracture surface is non-crystallographic and essentially striation free. In moist air fracture occurs by cleavage on {110} coupled with plastic deformation during the blunting and closing of the crack: this results in the formation of well defined surface striations. Striation topography is controlled by the local orientation of the cleavage plane to the direction of maximum tensile stress and to the slip systems of highest local shear stress. In consequence a range of striation profiles are developed. It is suggested that the cleavage component of growth is the result of hydrogen embrittlement of the matrix ahead of the crack and that dislocation transport is necessary to introduce this hydrogen.     

Nucleation of Fe-intermetallic phases in the Al-Si-Fe alloys

Nucleation of Fe-intermetallic phases (i.e. binary Al-Fe, α-AlFeSi, β-AlFeSi, δ-AlFeSi, and q₁-AlFeSi phases) on the surface of different inclusions in six experimental Al-Si-Fe alloys was studied through a quantitative evaluation of the number of inclusion particles that have a direct physical contact with the nucleated phase as seen through the optical microscope. It was found that nucleation of each of the Fe-intermetallic phases was promoted on the surface of several inclusions under the same conditions of alloy composition and cooling rates. Some inclusions exhibited high potency for the nucleation of particular Fe-intermetallic phases under certain conditions and poor potency under other conditions. The potent nucleants for the primary α-Al phase such as γ-Al₂O₃ exhibited poor potency for the nucleation of the Fe-intermetallic particles that lie within the primary phase (intragranular particles). Reactive inclusions such as CaO and SiC are very potent nucleants for the intragranular Fe-intermetallic phase particles. The nucleation of the Fe-intermetallic phases in Al-Si-Fe alloys obeys the general features of nucleation, in particular, the effect of cooling rate and solute concentration on the potency of the nucleant particles: (1) it was observed that increasing the cooling rate enhances the heterogeneous nucleation of the Fe-intermetallic phases on the surface of different inclusions, and (2) the nucleation potency of inclusion particles in both α-Al and interdendritic regions improves with increasing solute concentration up to a certain level. Above this level, the solute concentration poisons the nucleation sites. Nucleation of the Fe-intermetallics in the alloys studied does not seem to be largely affected by the type of the nucleating surface.     

A model for the electrical conductivity of peak-aged and overaged Al-Zn-Mg-Cu alloys

A physically based model for the electrical conductivity of peak-aged and overaged Al-Zn-Mg-Cu (7xxx series) alloys is presented. The model includes calculations of the η- and the S-phase solvus (using a regular-solution model), taking account of the capillary effect and η coarsening. It takes account of the conductivity of grains (incorporating dissolved alloying elements, undissolved particles, and precipitates) and solute-depleted areas at the grain boundaries. Data from optical microscopy, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) with energy-dispersive X-ray spectrometry (EDS), and transmission electron microscopy (TEM) are consistent with the model and its predictions. The model has been successfully used to fit and predict the conductivity data of a set of 7xxx alloys including both Zr-containing alloys and Cr-containing alloys under various aging conditions, achieving an accuracy of about 1 pct in predicting unseen conductivity data from this set of alloys.     

Stress-corrosion resistance of high-strength Al-Zn-Mg-Cu alloys with and without silver additions

The separate and combined effects of adding silver, substituting other elements for chromium, increasing zinc, or varying the copper content on the tensile properties and resistance to stress-corrosion cracking of 7075-type alloys were investigated. Adding silver produced no increase in strength and marginal increases in the resistance to stress-corrosion cracking. Substituting either zirconium or manganese for chromium increased strength and decreased the resistance to stress-corrosion cracking after a given precipitation heat treatment; at equal strength levels, the resistances to stress-corrosion cracking of the chromium-free alloys below 3 in. (76 mm) thickness were comparable to those of 7075. Increasing zinc increased the resistance to stress-corrosion cracking but increased quench sensitivity. Decreasing copper decreased the resistance to stress-corrosion cracking in the atmosphere. Increasing copper in chromium-free alloys provided the best combination of strength and resistance to stress-corrosion cracking. Longer atmospheric exposures are needed to firmly establish the stress-corrosion resistance. Formerly Assistant Director, Alcoa Research Laboratories, is now retired.     

MAX phases in titanium and aluminum alloys

The crystalline structures in the Ti-Al-C and Ti-Si-C systems are analyzed, and experiments are conducted with VT6 titanium alloy and eutectoidal Silumin (Al-12% Si) subjected to electroexplosive alloying and electron-beam treatment. Diffraction analysis reveals the formation of MAX phases (Ti₃SiC₂ and Ti₃AlC) in the modified layer of these alloys.     

Comparison of the influence of chromium and zirconium on the quench sensitivity of Al-Zn-Mg-Cu alloys

The role played by small additions of chromium and zirconium on the “quench-sensitivity” of Al-Zn-Mg-Cu high purity alloys has been studied. The effects of the quench rate reduction have been evaluated by mechanical testing; the associated microstructural features have been investigated by electron microscopy. The obtained data adequately explain the mechanisms of the “quench-sensitivity”.     

The deformation of commercial aluminum-magnesium alloys

The deformation and fracture of Al-Mg alloys have been investigated over a range of strain rates and temperatures. The rate and temperature-dependence of the deformation can be understood on the basis of the mobility of Mg atoms. When the Mg atoms can diffuse at a sufficient rate they interact with dislocations to produce serrated yielding and a zero or negative strain rate sensitivity. This greatly restricts the tensile ductility. Outside of this serrated flow regime enhanced ductilities are achieved due to the positive strain rate sensitivity stabilizing any inhomogeneities which develop. However, the final fracture is also influenced by the constituent particle concentration. Voiding occurs at particles and if the concentration of particles is sufficiently large the voids link up by local shear. Decreasing the constituent particle concentration, together with a reduction in the amplitude of serrations, results in larger tensile ductilities. At high temperatures, quite large strain rate sensitivities can be achieved but the ductility is finally limited by the propensity for voiding in these alloys.     

Yield behavior of commercial Al-Si alloys in the semisolid state

Systematic experimental work and modeling efforts have been conducted to characterize the yield behavior of commercial aluminum alloys in the semisolid state. In this study, extensive compression experiments were performed to measure the yield stress of semisolid aluminum slurries at high solid fractions (0.5 to 1.0), and a cone penetration method was employed to measure yield stress at low solid fractions (<0.5). A functional relationship between yield stress and temperature/solid fraction has been established for these alloys. The effect of the processing route on the resultant yield stress of the material in the semisolid state was studied by evaluating commercial A356 billets manufactured via magnetohydrodynamic stirring, grain refining, and UBE’s new rheocasting (NRC) processes, respectively. Detailed microstructure observations and image analyses reveal that the difference in yield-stress values among the alloys evaluated is intricately related to the semisolid structure. At a given solid fraction, the yield stress of semisolid slurries depends on microstructural indices (i.e., entrapped-liquid content, shape factor of the alpha phase, and the alpha particle size). In addition, numerical simulation results indicate that the finite yield stress of semisolid metals plays a significant role in determining the flow pattern during die filling. Depending on processing conditions, five distinct filling patterns (shell, disk, mound, bubble, and transition) have been identified and confirmed through experimental observations. Recent simulations demonstrate that the finite yield stress is also responsible for flow instabilities encountered in commercial forming operations, such as “toothpaste behavior.” Specifically, most flow instabilities can be avoided by properly controlling processing parameters and the initial semisolid microstructure. A stability map that provides a control guide for semisolid processing has been developed and is presented.     

Improved fatigue resistance of Al-Zn-Mg-Cu (7075) alloys through thermomechanical processing

To decrease the accumulation of damage during long-life low-stress cyclic loading, microstructures must accommodate inelastic deformation by homogeneous or “dispersed” slip rather than by localized slip concentrations. In age-hardening aluminum alloys this requirement can be met by introducing a dense and uniform dislocation forest through suitable thermo-mechanical treatments. Such a treatment was developed for Al-Zn-Mg-Cu (7075) alloys, involving a process cycle of solution annealing, partial aging, mechanical working and final aging. The fatigue properties (S-N curves) of commercial and high-purity 7075TMT are compared with conventional 7075-T651 properties; with zero mean stress the alternating stress to cause failure in 10⁷ cycles is more than 25 pct higher for commercial-purity 7075TMT and almost 50 pct higher for high-purity 7075TMT. The results emphasize the importance of microstructural control when high fatigue resistance is required. F. OSTERMANN, formerly with Air Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio.     

Metastable phases in the early stage of precipitation in Al-Mg alloys

The growth kinetics and thermal stability of metastable phases in Al-Mg alloys have been investigated by means of electrical resistivity measurements and calorimetry. From the observation of two distinct endothermal peaks in DSC thermograms, the existence of two types of metastable phases named as G.P. zones and β" phase was recognized. G.P. zones were defined as a metastable phase precipitating directly from the supersaturated solid solution. By analogy with other systems, the β" phase could correspond to an ordered phase with an Ll₂ structure. Aging behavior was divided principally into two stages. In the first stage, mainly G.P. zones precipitated, although both metastable phases coexisted over all the aging process. In the later stage, the β" phase dominated after aging beyond 100 ks at room temperature. The phase diagram for these two metastable phases was proposed, based on both the reversion experiments and the thermal analysis. Formerly Graduate Student with Kyoto University.