Materials Science Forum Vols. 519-521

Abstract: Although low-alloyed aluminum has been used as electric line and cable materials to date, there still is a great demand for higher strength with retaining the good electrical conductivity and ductility. In the study, iron has been chosen as an additive element and the addition effect on the strength and electrical has been investigated since iron is reported to have a marked solution-strengthening effect at a given addition amount. Aluminum with 99.99mass% purity and Al-Fe alloys with iron up to 0.9mass% were induction-melted, continuously cast into a rod with 8mm diameter, and cold-drawn into a wire with 0.3mm diameter. Tensile test and electrical resistivity measurement were carried out on the rod and wire after each pass. It was found that, in the rod, the strength increased while the ductility and electrical conductivity decreased as the addition iron amount was increased. Work hardening occurred clearly at an early stage of cold drawing, while it became sluggish as the cold reduction increased in each material. At the early stage, the strengths increased as the iron amount was increased, and at the later stage, the alloy with iron addition of 0.9mass% had higher strengths and larger elongation to failure.

Abstract: The Russian 1370 Al-Mg-Si-Cu alloy (analogue of 6056) is an advanced material for the skin and stringers of fuselage. However, these alloys are susceptible to the intercrystalline corrosion depending on the heat treatment regimes. The investigations of the effect of heating temperature during the solid solution treatment (530-555)°C and quenching rate, ageing regimes and interval between the quenching and ageing on the structure and properties of 1370 alloy sheets were performed. The strengthening β (Mg2Si) -phase and Q (Al5Cu2Mg8Si6) -phase relationship dependence of the alloy properties is established. Using the dark field electron microscopy which permits to intensify separate diffraction reflexes of strengthening phases the structure of Q-phase precipitates has been studied. Optimization of heat treatment of 1370 alloy sheets provided the combination of fracture toughness characteristic (Kco= 100 MPa√m) high level, the ultimate strength of 400-420 MPa and intercrystalline corrosion no more 100 μm in depth.

Abstract: A high recrystallisation resistance is required in aluminium alloys intended for processing or use at temperatures between 450°C-600°C. Additions of Hf, Sc and Zr significantly improve the resistance to recrystallisation through the formation of Al3X-dispersoids (X=Hf,Sc,Zr), and in this work different concentrations and combinations of these elements were added to five aluminium alloys. The alloys were extruded, subjected to various degrees of cold rolling (0%-80%) and finally annealed at high temperatures in order to study the structural stability. All variants displayed a high resistance towards recrystallisation, but the best results were obtained in the alloy containing only Sc and Zr. In this alloy no signs of recrystallisation were observed even after 1 hour annealing of extruded and 80% cold rolled profiles at 600°C.

Abstract: Twelve different aluminium alloys with constant iron (Fe) and scandium (Sc) contents of 0.5 wt.% and 0.2 wt.%, respectively, were cast and subsequently homogenised. The distribution of Sc in the microstructure was examined by means of Energy Dispersive X-ray Spectroscopy (EDX) in Scanning Electron Microscope (SEM). Emphasis was put on measuring the solubility of Sc in the Fe-bearing phases of the investigated alloys. It was found that the amount of Sc tied up in Febearing phases is so low that it can be regarded as negligible, with the possible exception of α- AlFeMnSi (Al15(Fe,Mn)3Si2) and the π-phase (Al8FeMg3Si6). A quaternary AlFeSiSc phase with a composition close to Al10Fe3Si5Sc2 may have been discovered.

Abstract: A356 is the aluminum casting grade which has compositions that combines outstanding casting characteristics with excellent properties after heat treatment. Mechanical properties of A356 can be improved by reducing of secondary dendrite arm spacing (SDAS), precipitation hardening, and the interaction effect of both. It is generally accepted that dendrite arm spacing and fine distribution microstructure are related to each other and they also affect the precipitation hardening in a way that smaller SDAS results in shorter time required to obtain a satisfactory degree of solution of the undissolved or precipitated soluble phase constituents and to achieve good homogeneity. Minor addition of Sb was successfully used in reducing the SDAS in previous work. However, the effect of Sb addition on age hardening has not been investigated, especially in a high cooling rate condition. In this research, effects of minor addition of Sb on SDAS, age hardening and mechanical properties; i.e. hardness and tensile properties, are reported. It was found that Sb addition did not clearly affect SDAS at the high cooling rate, i.e. as in permanent mold casting process. Moreover, we found that the addition of Sb into A356 also lowered mechanical properties.

Abstract: Hardness measurements and 3-dimensional atom probe analysis have been used to characterise the precipitation behaviour in two 6xxx series aluminium alloys, one Cu-free alloy (Al-0.78at%Mg- 0.68at%Si) and one Cu-containing alloy (Al-0.78at%Mg-0.68at%Si-0.30at%Cu). The heat treatments consisted of either natural ageing or pre-ageing at 353K followed by a paint-bake treatment at 453K. Natural ageing was seen to increase the hardness, and hence reduce formability compared to pre-ageing. In addition, the strengthening effect of artificial ageing was less after natural ageing than after pre-ageing. In the Cu-free alloy, needle-like β″ was observed to form only after a pre-ageing treatment during the first 60 minutes of a paint-bake treatment. In the Cucontaining alloy, needle-like β″ formed during paint bake in both the naturally-aged and pre-aged material, although it is formed more rapidly after pre-ageing. This was accompanied by an increase in strength over the Cu-free alloy and indicates that Cu reduces the deleterious effect of natural ageing.

Abstract: Recent 7xxx aluminum alloys have been designed for the finite use of thick semiproduct with contolled amount of constituent phases which mostly evolve during ingot preheat. In this study, the effects of constitutional change and preheat conditions of 7175 and 7050 type alloys on the evolution of constituent phases [M-, T-, S-phase and dispersoid] are presented. The constiuents evolve depending on the constitutional effect, primarily the change of Zn:Mg ratio, preheat condition comprising temperature and cooling rate following preheat. T- and M-phase are reprecipitated during cooling after preheat, depending on the alloy constitutions. S-phase is evolved depending on the constitution and preheat temperature, rather than preheat cooling rate. Prominent precipitation temperature interval of constituents are discussed in view of quaternary phase evolutions. In addition, evolutions of dispersoids together with M-phase are discussed. Specific alloy designs and preheat conditions could provide controlled microstructures for the thick 7xxx semiproducts.

Abstract: During age-hardening of certain Al-Zn-Mg-Cu alloys, a 90% hardness increase can occur with 75 seconds. The clustering and precipitation of solute element species during this early rapid hardening (RH) period has been investigated through atom probe tomography, transmission electron microscopy, and Vickers hardness measurements. This study has focussed on the effect of copper by analysing three alloys; Al-2.0Zn-1.8Mg-0.7Cu, Al-2.0Zn-1.7Mg-0.2Cu and Al-1.9Zn-1.7Mg (at.%). The early RH reaction in these alloys accounts for up to 70% of the total hardening (peak hardness minus as-quenched hardness) and takes place during the first 60 seconds of ageing. We report preferred solute-solute interactions in the as-quenched materials. This quenched-in nanostructure acts as a template for subsequent solute clustering, the nature of which we have correlated with ageing.

Abstract: The trace additions of Ag to binary Al-Mg alloys promote precipitation of a phase in the under-aged condition. The structure of the phase in an Al-10Mg-0.5Ag (mass%) alloy have been characterised using transmission electron microscopy and electron microdiffraction. An icosahedral quasicrystalline phase, which is identified by the electron microdiffraction patterns, has been found in the Al-10Mg-0.5Ag (mass%) alloy after solution treatment, water quenched and then aged during the time between 20 and 40 min. at 513K. The orientation relationship between the quasicrystalline phase and the a - Al matrix is as follows; i5 // <011>a and i3 // <111>a. A uniform distribution of finer-scale (typically 20 nm), faceted precipitate icosahedral quasicrystalline particles was observed within the core of the grains, but the precipitates became coarser (60-80 nm) approaching the grain boundary precipitate-free zone. The quasicrystalline particles contain elements of Al, Mg and Ag. The quasicrystalline precipitate particles, which are the metastable phase, appear to be the primary strengthening phase in the Al-10Mg-0.5Ag (mass%) alloy aged at 513K.

Abstract: The review of the works devoted to the influence of scandium on the structure, mechanical behavior and corrosion resistance of aluminum alloys is given. The wrought thermally non-hardenable alloys of Al- Mg system and thermally hardenable high-strength alloys of Al-Zn- Mg-Cu system are considered. The influence of Sc on the glass forming ability of amorphous alloys and on the superplastic behavior of aluminum alloys is discussed.