Age hardening response of AA7108A investigated by means of Synchrotron-based X-ray Absorption Spectroscopy (XAS) measurements

摘要

Enhanced strengthening achieved during age hardening of 7xxx alloys, necessitated the need to study the age hardening response of Cu-free AA7108A wire, as a potential material for automobile fastener production. Age hardening response was investigated by means of Synchrotron-based X-ray Absorption Spectroscopy (XAS) measurements. Linear combination fitting (LCF), using 'CANES spectra of Zn K-edge showed the gradual removal of Zn atoms from the aluminum matrix of the T4 condition (where Zn is in solution and in GP zones), when subjected to one-step artificial aging treatment at 130 degrees C and 200 degrees C, respectively. The reduction of matrix Zn during aging at 130 degrees C is mainly due to the precipitation of the semi-coherent eta' phase, whose number density increased with increasing aging time. eta' phase is the major phase responsible for hardness and electrical conductivity increase during aging at 130 degrees C. Reduction of matrix Zn concentration during aging at 200 degrees C is due to the formation and coarsening of the incoherent eta-MgZn2 phase. This resulted in a dramatic decrease in hardness and an increasing trend in the electrical conductivity during aging at 200 degrees C. Calorimetric study revealed that natural aging occurred at room temperature after solution treatment, and GP zone solvus is found to be below 130 degrees C. In addition, precipitation of the eta' phase at 130 degrees C is retarded, due to reduced concentration of vacancies, attributed to the presence of Zirconium (Zr) in the alloy.