Abstract: The effects of a small amount of rare earth (RE) elements added on the hardness and hydrogen embrittlement of an aged and retrogressed-reaging Al-Zn-Mg alloy were investigated. Scanning electron microscopy and X-ray diffraction were employed to study the microstructure and the precipitation phase of the alloy. The addition of REimproves the precipitation of a streng thing phase (MgZn₂) and decreases the grain size. By the addition of RE, the strength, hardness and elongation of the alloy at peak-aging can be improved, the hardness and hydrogen embrittlement resistance of the alloy after aged-retrog ressed-reaged can be enhanced.

Key words: rare earths, Al-Zn-Mg alloy, aging-retrogression-re aging, hardness and hydrogen embrittlement