Phase transformation from η phase to S phase at grain boundary during annealing in rapidly-solidified Al-Zn-Mg-Cu alloy

Phase transformation during annealing in rapidly-solidified Al-Zn-Mg-Cu alloy was investigated by differential scanning calorimetry, X-ray diffraction, transmission electron microscopy and transmission Kikuchi diffraction technologies. The results show that primary eta-Mg(Zn,Al,Cu)2 phase is formed at grain boundaries and inside grains during spray forming. During annealing above 300 degrees C, the primary eta phase particles at GBs are inclined to be decomposed as newly-oriented nano eta phase particles via lattice rotation at eta/Al interface and directly transformed to S-(Al2CuMg) phase while those inside grains are directly dissolved. Nanoscale crystallographic orientation relationships between newly-formed eta phase and transformed S phase at eta/Al interface are revealed. Due to the phase transformation from eta to S, the impact toughness of Al-Zn-Mg-Cu alloy is significantly dete-riorated because these S phase particles cannot be fully dissolved by solution treatment and inevitably increase brittle fracture sensibility. This finding provides new insights into designing the phase transformation in Al-Zn-Mg-Cu alloys for toughness enhancement.

Automated summary

Phase transformation during annealing in rapidly-solidified Al-Zn-Mg-Cu alloy was investigated. The primary eta-Mg(Zn,Al,Cu)2 phase was formed during spray forming, but decomposed as newly-oriented nano eta phase particles at eta/Al interface and directly transformed to S-(Al2CuMg) phase during annealing. The impact toughness of Al-Zn-Mg-Cu alloy was significantly deteriorated due to the phase transformation, which provides new insights for toughness enhancement in Al-Zn-Mg-Cu alloys.