In this study, the microstructure of hot-extruded and under-extruded Zr bearing aluminum alloys Mesoalite10 ®-1.3 mass%Zr was investigated by high-resolution EBSP analysis and the effect of Zr addition on microstructure formation during hot extrusion was studied. In an extruded Zr free alloy, coarse fibrous grains elongated to the extrusion direction were predominantly evolved. And these grains were considered to be formed by elongation of the original equi-axial grains by extrusion. Whereas, in extruded Meso10-1.3 Zr alloy, many fine grains were evolved near the grain boundaries of the fibrous grains. In under-extruded Meso10-1.3 Zr alloy, inhomogeneous local strains were developed near random grain boundaries and new fine grains were formed by continuous dynamic recrystallization. It is considered that in aluminum alloys, the addition of Zr promotes continuous dynamic recrystallization during hot extrusion.

The stress corrosion cracking SCC resistance of hot-extruded P/M Al-Zn-Mg alloys, Mesoalite, was investigated as compared with that of I/M Al-Zn-Mg alloys and the correlation between stress corrosion resistance and microstracture was studied. SCC resistance about P/M alloys was high as compared with that of I/M alloys and when the solute concentration increased, reduction of SCC resistance about P/M alloys was not observed. The difference in width of PFZ, distribution of precipitates on grain boundary, the amount of metastable phase precipitated in the matrix, tilt angle of grain boundary, grain size and shape was investigated as a cause of the difference of SCC resistance. The large change in the size and shape of grain among these factors was observed. In the P/M alloys, the fiber-like grain elongated to the extruded direction was observed to the equi-axial grain having been observed in the I/M alloys. When stress was applied parallel to elongated grains, the stress perpendicular to the grain boundary was low. Since the SCC resistance is improved when the applied stress perpendicular to the grain boundary is small, it is concluded that the SCC resistance of the P/M alloys which has the fiber-like grain elongated to the ED became large.

The super high strength Al-Zn-Mg based alloy Mesoalite20 ® with a chemical composition of Al-9.5Zn-3Mg-1.5Cu-4Mn-0.5Zr-0.02Ag in mass% includes two type of precipitates of Q and Al 6 Mn phases. The Q precipitates have a quaternary composition and fine rod-like shape, but Al 6 Mn has round and irregular shape. After compaction of air-atomized powder by CIP, the Q and Al 6 Mn phases precipitated in sequence during heating. By hot extrusion at 773 K, their precipitates flowed along extrusion direction and only Q precipitates aligned well, which have been known to be very effective to fiber-reinforcement. When increasing Mn content from 4 to 7 mass% with constant of 1.5 mass% Cu, the amount of Al 6 Mn increased remarkably. Consequently the compressive strength and plastic elongation to fracture decreased with increasing Mn content. When increasing Cu content from 0.5 to 2.5 mass% with constant of 4 mass% Mn, on the other hand, the amount of Q phase increased and their mechanical property improved slightly with increasing Cu content. Therefore the present study has proposed a new alloy with higher Cu content, which gives higher strength than that of commercial Mesoalite 20.

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