Effect of Minor Amount of Scandium on the Microstructure of 7000 Series ‎Aluminium in as Cast, Homogenized and Artificial Age Hardened Conditions

Two alloys of Al-Zn-Mg-Cu with the same amount of zirconium and different amount ‎of scandium (0.05 and 0.1 wt.%) were melt in the resistance furnace and cast in the cast iron mold ‎and, after doing the tests of ICP, Spectroscopy and DSC, were put under homogenizing, ‎solubility and T6 treatments operations. Using microscopic observations and DSC analysis, the ‎temperature and time of homogenizing the alloys samples were 500°C‏ ‏and 12h and, 490°C and ‎‎12h for Al-Zn-Mg-Cu-0.1Sc-0.09Zr and Al-Zn-Mg-Cu-0.0.05Sc-0.09Zr, respectively. At the ‎dissolution temperature obtained from the results of hardness test and microscopic ‎observations by FESEM, the alloys were subjected to dissolution and then, were put under T6 ‎thermal operations at 120°C for 12 hours. Microscopic investigations showed that the ‎dominant phases in the microstructure of casting were along with T (Al2Mg3Zn3 and copper solid ‎solution and MgZn2 phase, and the eutectic phase formed 10% of the microstructure. Also, ‎investigation of the casting and homogenized microstructure revealed that no phase included ‎scandium and zirconium in the microstructure. After the homogenization ‎operation, the eutectic phase decreased to 3%. The dominant phase observed in the ‎homogeneous microstructure was the phases containing iron. After the dissolution, the volume ‎fraction of the eutectic phase remained constant and the MgZn2 phase. Having performed the ‎T6 thermal operation at 120°C, Al3(Sc,Zr) and MgZn2 nanoparticles were observed at the ‎grain boundary and inside the microstructure.‎