Phase transformation and dispersoid evolution for Al-Zn-Mg-Cu alloy containing Sn during homogenisation

Optical microscope (OM), field emission scanning electron microscope (FESEM), energy dispersive X-ray Spectroscopy (EDS), transmission electron microscope (TEM) differential scanning calorimetry (DSC) and X-ray diffraction (XRD) were employed to investigate the evolution of intermetallic phases during homogenisation at 465 degrees C for 0-48 h for Al-Zn-Mg-Cu-Sn alloy. Evidentially, casting is accompanied by severe dendritic segregation. The primary phases appeared during casting consisted of alpha (Al), eutectic mixture of eta (Mg (Zn, Cu, Al)(2)), theta (Al2Cu) and coarse Al7Cu2Fe particles. After 6 h of homogenisation, two eutectic phases namely S (Al2CuMg) and Cu3Sn were formed. The appearance of dendrites and their consequent dissolution with the progress of homogenisation time was modelled using a kinetic equation taking interdendritic spacing and temperature as two variants. TEM micrographs revealed the presence of high density of fine dispersoids after 6 h of homogenisation following almost complete dissolution of the dendrites into the matrix after 48 h of homogenisation. Coarsening of the said dispersoids was observed with further homogenisation treatment following Ostwald ripening mechanism and consequently lowering of Zener pinning pressure (ZPP). Crystallographic texture analysis revealed the formation of strong gamma fibre in both cast and homogenised samples. However, strong ca fibre along with Goss, Brass, P, and Copper components was also noticed in the cast sample. (C) 2019 The Authors. Published by Elsevier B.V.