Materials Science Forum Vols. 519-521

Abstract: Phase transformations in 6XXX alloys with Mn, Cu and Cr additions have been studied in the process of homogenization annealing at different temperatures. The continuous cooling transformation diagrams of decomposition of solid solution during the cooling of ingots from the homogenization temperature have been plotted. The effect of the cooling rate after homogenization on the properties of ingots during extrusion has been studied.

Abstract: In creep-ageforming a material experiences continuously variable bending stresses through its thickness, from tensile to compressive, which are maximum at the surfaces. This can potentially result in through thickness microstructural gradients, due to interactions between the bending stresses, creep, and precipitation occurring during ageing, that can alter a component’s performance. The aim of the work reported here was to develop an understanding of these effects in an industrial creep ageforming process. For this purpose two aerospace alloys, 7475 and 2022, were ageformed by Airbus UK using industrial scale ageforming tools. Their microstructures were analysed in detail by TEM, and SAXS, which revealed significant through thickness microstructural changes after forming.

Abstract: The formation of eutectics in Al–Zn–Mg–Ni and Al–Zn–Mg–Si systems is studied by means of metallography, DSC, EPMA, X-ray spectroscopy and thermodynamical calculations. Polythermal sections of the corresponding phase diagrams are constructed. The concentrations and temperatures of binary eutectic reactions L → (Al) + Al3Ni and L → (Al) + Mg2Si in quaternary alloys are determined. Nonequilibrium solidification in Al–7% Zn–3% Mg-based alloys ceases at approximately 480 °C. The alloys close by composition to binary eutectics have considerably improved casting properties as compared to the base Al–7% Zn–3% Mg composition. In particular, hot tearing susceptibility is much less in alloys with Al3Ni or Mg2Si. These results are corroborated by measurements of thermal contraction during solidification. The alloys containing binary eutectics exhibit much lower temperatures of contraction onset and less thermal strain is accumulated in the solidification range. Fine eutectic morphology enables fragmentation and spheroidization of intermetallic particles during annealing. The presence of Al3Ni and Mg2Si particles does not decrease the precipitation hardening effect associated with precipitation of the T′ (AlMgZn) phase. Improved casting properties and good mechanical properties of castings allow the application of Al–Zn–Mg alloys with binary eutectics formed by Al3Ni or Mg2Si as foundry alloys.

Abstract: In order to improve the high-temperature strength of an Al-Cu-Mg alloy, Mn was added at supersaturation to form a high-density dispersion of an intermetallic phase. In the P/M Al-3.6Mn- 6.4Cu-3.6Zn-1.7Mg alloy (mass%), rod-like Al-Mn-Cu-Zn quaternary intermetallic phases (Q phase) several hundred nanometers in length were dispersed in the matrix. The chemical composition of the Q phase was determined by TEM/EDX to be 78.8Al-12Mn-8Cu-1.2Zn (at%). The crystal system, space group, and lattice parameters of the unit cell were identified to be orthorhombic, Cmcm and a = 0.76, b = 2.11, c = 1.25 nm, respectively, by Rietveld analysis. Since the matrix of the alloy obtained was of the Al-Cu-Mg-(Zn) system, age-hardening occurred by formation of a GPB zone at room temperature and 448 K. At the peak level of age-hardening at room temperature, the tensile strength at room temperature was 704 MPa, and the elongations were 8.0%. The high temperature strengths at 523 and 573 K were 319 and 141 MPa, respectively, and the elongations were 17 and 34%, respectively.

Abstract: In this study, we investigated the effects of a small addition of Cu (0.1%) or Ge (0.1%) on the microstructure and mechanical property of Al-Si alloys. The results are as follows. The size of precipitates in the Cu-added or Ge-added alloys is smaller than that in the binary alloy. The size of the precipitates at the grain boundary in the Cu-added or Ge-added alloys is larger than the size of the precipitates in the binary alloy. However, the number of the precipitates at the grain boundary in the Cu-added and Ge-added alloys is smaller than that in the binary alloy. It was found that a small addition of Cu or Ge has the effect of raising the fatigue strength under repeated tensile loading as well as the tensile strength.

Abstract: The quantitative correlation between strength, ductility and precipitate microstructures in the vicinity of grain boundaries with precipitate free zones (PFZ) was evaluated for Al-Zn-Mg(-Ag, Cu) alloys using transmission electron microscopy (TEM), three-dimensional atom probe (3DAP) and tensile test. In the Al-Zn-Mg ternary and Cu-added alloys aged at 433K, larger widths of PFZ were observed by TEM and resulted in lower elongations to fracture, independent of the size of grain boundary precipitates. On the other hand, the elongation of the Ag-added alloy was higher, if compared at the same levels of proof stress, due to the much smaller width of PFZ. This strongly suggests that PFZ is harmful to fracture of the investigated alloys. From a 3DAP analysis, furthermore, it was revealed that Ag and Cu atoms are incorporated in the nanoclusters from the initial stage of aging. In this work, the elongation was well correlated to the width of PFZ, size of grain boundary precipitates and the level of proof stress, enabling to predict ductility of the alloys from known microstructural factors.

Abstract: The formation of nanoclusters in the early aging stage is not fully clarified due to their extremely small sizes. To clarify the atomic-scale clustering of solute atoms, a three-dimensional atom probe (3DAP) analysis and a Monte Carlo computer simulation have been conducted together for Al-Zn alloys. The nanoclusters in the alloy aged at room temperature were successfully detected in the obtained 3DAP maps. From these maps the growth behavior of nanoclusters during aging was experimentally examined. In the Monte Carlo simulation, on the other hand, many-body nearest neighbor interactions between atoms and between atoms and a vacancy were taken into account. The Zn concentration of the clusters calculated in the 4-body interaction model best agreed with that obtained experimentally, suggesting the importance of many-body interactions. Therefore, it was confirmed that the combined analysis of 3DAP and the well-constructed Monte Carlo simulation is useful to make clear the nanocluster formation in alloys.

Abstract: A diffusion controlled precipitation model based on classical nucleation and growth theory has been implemented to simulate the precipitation kinetics in a hot rolled supersaturated Al- Mn alloy (AA3103). The modelling approach explicitly includes the effect of concurrent recrystallisation on precipitation and considers the simultaneous evolution and interaction of two precipitate populations that vary significantly in size, i.e. constituent particles and dispersoids. Comparison with experimental results shows that this classical modelling approach predicts incorrect nucleus density and too high precipitation rates, which cannot be simply corrected by parameter fitting. Reasons for this discrepancy are discussed in terms of selection of nucleation sites, the effect of diffusion in a multi-component system, various diffusion paths and the possible influence of precipitate shape and size distribution. The model is subsequently altered by introducing two additional parameters that control the Mn solute concentration at the particle-matrix interface. This more phenomenological model is successful in reproducing the experimental precipitation kinetics, both in deformed and undeformed aluminium matrix, and the effect of concurrent recrystallisation for a wide temperature range.

Abstract: The new structural applications of aluminum materials are determined by intelligent lightweight design: the demand of safety, ecology, recycling and by economics. Al-Si alloys allow complex shapes to be cast. Metallurgical a high volume fraction of fine dispersoid which are less about 0.1 microns in size are useful for retaining a fine grain. In the 6061 series of alloys, iron combines with aluminium and silicon to form two types of commune inter-metallic, (beta AlFeSi and alpha AlFeSi).The type of inter-metallic that is present these alloys will have an important bearing on the homogenization time, workability and quality of the surface finish. The microstructure of the as-cast samples was evaluated by electronic microscopy and the morphology of inter-metallic compounds related to the efficiently modification. Mn-Ti-Sr modification of Al-alloys is normally accompanied by an increase in porosity in the casting.

Abstract: The microstructural evolution has been investigated in three alloys of the 7000 series possessing increasing zinc contents by combining small-angle X-ray scattering, differential scanning calorimetry and transmission electron microscopy, in order to gain understanding on the evolution of the compromise between yield strength and corrosion resistance. We show that the three materials show qualitatively identical precipitation sequences; however the precipitated volume fraction is shown to increase in parallel to the Zn content. Moreover, the precipitate size evolution is faster in the high Zn alloy. The precipitate composition is inferred to vary in the three materials, and this difference is shown to explain the differences in calorimetric behavior, precipitation kinetics and corrosion resistance.