| The mechanical behaviors of a wrought high strength aluminium alloy containing 4.5 Zn, 2.0 Mg, and 0.12 Zr in wt.% have been examined as a function of the concentration of chromium in the range of 0.02∼0.2wt.%. It has been reported that the chromium forms Cr-dispersoids called E-phase(Al_(18)Mg₃Cr₂) in AI-Zn-Mg alloys. The size of these Cr-dispersoids is known to be depended on the content of chromium. The alloy containing 0.1 wt.% Cr has the small spherical Cr-dispersoids. However, the Cr-dispersoids are long rod-Like in the alloy with 0.2wt.% Cr. Because these Cr-dispersoids inhibit the motion of dislocations within the grain, the strength is increased. However, increment in strength is higher in the alloy with 0.1 wt.% Cr than in that with 0.2 wt.% Cr. The reason of the above phenomena may be resulted from the decrease of planar slip and increase cross slip due to the spherical Cr-dispersiods in the alloy with 0.1 wt.% Cr. The fracture toughness of the alloy depends on the shape and size of the Cr-dispersoids. The small spherical dispersoids increase the fracture toughness but the long rod-like dispersoids reduce it. |
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