Materials Science Forum Vols. 783-786

Abstract: It is well established that the precipitate volume fraction and size are the dominating parameters determining the hardening in Al-Mg-Si alloys. Several empirical models of the relationship between precipitate morphology and hardening have been published, but the coherency strain field surrounding the particles has so far not been taken into account. The authors have earlier published data with characterisation of microstructure of aged Al-Mg-Si alloys, including estimation of the coherency strain field surrounding these (semi-) coherent precipitates. In this paper the effect of including the coherency strain field as part of the effective hardening diameter of the precipitates in hardening models is evaluated. The correlation between the tensile test results and mechanical properties calculated with an appropriate microstructural model increased from 0.78 to 0.87, by implementation of coherency strain field as part of the effective particle radius in the model.

Abstract: An AA2139 alloy with a chemical composition of Al–4.35Cu-0.46%Mg–0.63Ag-0.36Mn–0.12Ti (in wt.%) and an initial grain size of about 155 μm was subjected to annealing at 430°C for 3 h followed by furnace cooling. This treatment resulted in the formation of a dispersion of coarse particles having essentially plate-like shape. The over-aged alloy exhibits lower flow stress and high ductility in comparison with initial material in the temperature interval 20-450°C. Examination of microstructural evolution during high-temperature deformation showed localization of plastic flow in vicinity of coarse particles. Over-aging leads to transition from ductile-brittle fracture to ductile and very homogeneous ductile fracture at room temperature.

Abstract: Hydrogen desorption kinetics for rapidly solidified high purity Al and Al-Cr alloy foils containing 1.0, 1.5 and 3.0 at % Cr were investigated by means of thermal desorption analysis (TDA) at a heating rate of 3.3°C/min. For the first time, it was found that oxide inclusions of Al₂O₃ are dominant high-temperature hydrogen traps compared with pores and secondary phase precipitates resulted in rapid solidification of Al and its alloys. The correspondent high-temperature evolution rate peak was identified to be positioned at 600°C for high purity Al and shifted to 630°C for Al-Cr alloys. Amount of hydrogen trapped by dislocations increases in the alloys depending on Cr content. Microstructural hydrogen trapping behaviour in low-and intermediate temperature regions observed here was in coincidence with previous data obtained for RS materials using thermal desorption spectroscopy (TDS). The present results on hydrogen thermal desorption evolution indicate that the effect of oxide surface layers becomes remarkable in TDA measurements and show advantages in combinations of both desorption analysis methods to investigate hydrogen desorption kinetics in materials.

Abstract: It is reported that alloying with Cu causes the transformation from the D0₂₂ type structure of Al₃Ti into L1₂ cubic structure. Since the lattice constant of Al_2.5Cu_0.5Ti with L1₂ structure is a = 0.3927 nm, smaller disregistry value between Al and Al_2.5Cu_0.5Ti can be achieved. In this study, novel refiner for Al cast containing Al_2.5Cu_0.5Ti intermetallic compound particles with L1₂ structure is fabricated by cold pressing. It is found that the grains of Al cast are partially refined by using the novel refiner, although microstructure of Al grain is not homogeneous. In this way, the Al_2.5Cu_0.5Ti intermetallic compound particles with L1₂ structure can become favorable heterogeneous nucleation sites for Al cast.

Abstract: The microstructural and textural evolutions during annealing of a highly strained (ε=4.0) commercial purity aluminium (AA1200) were followed by electron backscatter diffraction (EBSD). Boundary spacings were analyzed for crystallites of different crystallographic orientations. It was found that initially during the annealing the microstructural evolution is dominated by recovery of the highly strained microstructure, while later the microstructure consists of identifiable grains nucleating and growing at the expense of deformed and recovered matrix. No much texture change occursduring early stages of annealing, whereas a cube texture evolves and dominates after complete recrystallization.

Abstract: The hypereutectic Al-17Si-2Cu-1Ni-0.8Mn alloys with 2% or 3%Fe were fabricated out assisted with ultrasonic vibration (USV) treatment. The coarse plate-like δ-Al4(Fe,Mn)Si2 phase was transformed into fine particles with average diameter of about 17μm~20μm after USV treatment, and the volume fraction of particle-like Fe-bearing compounds is about 3.6%~5%. The polygonal primary Si crystals were also refined into particles with average diameter of about 15μm~23μm, and the volume fraction of primary Si declined to about 5.4%~6.5% after USV treatment. The matrixes of the castings were reinforced with fine Fe-bearing compounds and Si particles. The ultimate tensile strength of the alloys, which were produced by gravity casting process assisted with USV, increases with the increase of Fe content at 350 °C. It is considered the increase of the elevated temperature strength of the samples are mainly attributed to the refinement of δ-Al4(Fe,Mn)Si2 phase by USV and the increase of the volume fraction of the Fe-bearing compounds. The die casting process assisted with USV can further improve the mechanical properties of Al-17Si-2Fe-2Cu-1Ni-0.8Mn alloy.

Abstract: Equal-channel angular pressing (ECAP) has been established as an effective method to achieve ultrafine-grained materials and ECAP combined with the Conform process (ECAP-Conform) is considered a promising candidate to realize continuous processing by ECAP. This paper describes experiments conducted on samples of an Al-6061 alloy in order to evaluate the homogeneity development of the alloy processed by ECAP and ECAP-Conform. The samples of the Al-6061 alloy were processed by ECAP and ECAP-Conform at room temperature up to 4 passes via route B_C. The microhardness measurements were recorded both in the cross-sectional planes and along the longitudinal directions of the samples. The results show the alloy obtained significant strengthening using both processes. There was reasonable plastic isotropy after 4 passes of ECAP-Conform in both the cross-sectional plane and the longitudinal direction and the alloy showed a homogeneous microhardness distribution in the cross-sectional plane after 4 passes of conventional ECAP.

Abstract: Contraction behavior of 7xxx aluminum alloys was systematically investigated using a new device for measuring linear contraction during solidification. The effects of contraction behaviors and microstructure during solidification of three types of 7xxx series aluminum alloys on the hot tearing susceptibility were studied. It was found that the contraction behaviors of the three alloys showed extremely different shrinkage characters during solidification. The linear expansion coefficient (LEC) of 7050 and 7075 alloys shown an enormous increase from rigidity point (solid fraction is 0.69 and 0.73 respectively) to the peak value and then dropped to a constant value with the decreasing temperature. The LEC of 7022 alloy is different from the 7050 and 7075 alloys that it increases from rigidity point (solid fraction around 0.8) with a small fluctuation at the temperature little lower to the rigidity point and then they increase monotonously to the room temperature. The microstructure showed that the onset of the contraction process was the point which dendrites started coalescence. The profiles of thermal expansion coefficient of 7050 alloy were greater than the 7022’s during the solidification process, so the contraction behavior of the alloy during solidification could be used as a kind of criterion of hot tearing susceptibility.

Abstract: The influence of Mg content on Al-10 mass % Si-0.05, 0.30 and 0.60 mass % Mg alloys castings were produced by high-pressure die-casting process, used by microstructure observation and evaluation of mechanical properties. With increasing Mg content, the main constituent phases (primary α-Al phase and eutectic phase) were not changed, but a small amount of phases crystallized at last stage of solidification (β-Al₅FeSi, π-Al₈Si₆Mg₃Fe and Mg₂Si) were changed in the kind and the volume. In the mechanical property, 0.2%PS and UTS were increased, elongation and absorbed energy were decreased with increasing Mg content or difference of heat treatment. Because the fracture mode in primary α-Al phase was changed from ductile to brittle by precipitation strengthening.

Abstract: The cold rolling and annealing texture formation has been investigated in electro deposited pure iron which has an extremely sharp and isotropic <111>//ND fiber. Regardless of cold rolling reduction, {111}<112> intensified texture is formed after cold rolling. Similar texture remains after recrystallization in 65% cold rolled material while {111}<110> type texture forms in 80% and 90% cold rolled ones. The recrystallized grains at the stage of 5% recrystallization have {111}<112> orientation in 65% cold rolled sheet, whereas {111}<110> is observed in 80% cold rolled one. From this aspect, it is considered that the nucleation orientation plays an important role in the recrystallization texture formation. In the meanwhile, the growth of the recrystallized nuclei is also supposed to affect the recrystallization texture formation. The nuclei with {111}<112> orientation in lightly cold rolled sheet are easier to consume the deformed matrix than they do in heavily cold rolled sheets because their frequency to encounter a deformed grain with nearly the same orientation is much smaller in lightly cold rolled specimen, which can result in a large mobility for growth. Cross cold rolling makes cold rolling texture rather homogeneous <111>//ND fiber, which gives rise to an almost homogeneous <111>//ND fiber after annealing.