The evolution of microstructure, precipitate size, volume fraction and integrated intensity
of Al-8.0Zn-2.05Mg-1.76Cu (7055) and 7055-1.0Li alloys during isothermal ageing has been studied
by transmission electron microscopy (TEM) and synchrotron-radiation small angle X-ray scattering
(SAXS). According to the TEM results, referring to the thermodynamic phase diagram, it was found
that the addition of Li changed the types of the precipitates. T1 phase was observed in the 7055-1.0Li
alloy besides the GP zones, η' and η with variant orientations with matrix, η1、η2 and η4. The
super-lattice spots of L12 (Cu3Au) structure were probably due to the existence of Al3Li (δ') or
Al3(Zr,Li). Furthermore, the precipitation sequence has been modified in Li-containing
Al-Zn-Mg-Cu alloys. The precipitate volume fraction derived from the integrated intensity for 7055
alloy reached an plateau except ageing at 120°C and the maximum was about 0.052-0.054 in the
temperature range 140-160°C.
Cu99.8Y0.2, Cu99.2Y0.8 and Cu98Y2 alloy ribbons were prepared by single roller melt spinning. The microstructure was studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), high voltage electron microscope (HVEM) and high resolution electron microscopy (HREM). The results showed that α-Cu was the dominative phase in the rapid solidification ribbons of three alloys. A secondary phase Cu4Y was detected in the Cu98Y2 ribbon by XRD. The grain size was in a range of 50-200 nm in the Cu99.2Y0.8 and Cu98Y2 ribbons. Many nano-scale twins and some dislocations existed inside the larger grains. However, the grains in Cu99.8Y0.2 ribbon were in the size of microns and the sub-grains with small misorentations were in 100-200 nm. To understand the deformation mechanism, in situ tensile test were carried out at a High Voltage Electron Microscope (HVEM). The results showed that the deformation is predominated by the dislocation slip in larger grains. To accommodate the deformation, elastic deformation occured in the small grains in the initial stage of the deformation. Meanwhile, some small grains maybe deform by grain rotations. With strain increasing, some fractures generated and propagated along the grain boundaries or across the grains.
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