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Precipitation evolution and strengthening effect of Al-15wt%Mg2Si-5wt%Cu alloy were studied by characterizing the change of morphology and tensile properties after ageing at 200 °C for various times. At the under-ageing condition (ageing for 0.5 h), metastable θ″, θ′, fine dots and σ phases are formed. After ageing for 1 h (peak-ageing condition), θ″ and θ′ are dominant phases, and the high number density of precipitates produces a significant improvement of the hardness (173 Hv) and ultimate tensile strength (356 MPa). As ageing time is prolonged (ageing for 24 h), plenty of σ phases are formed at the expense of θ"/θ′ phases and act as the main strengthening precipitates. The over-aged microstructure consequently causes a decrease in the hardness (134 Hv) and ultimate tensile strength (311 MPa) of the alloy. [ABSTRACT FROM AUTHOR]

In the present study, to determine the mechanism of the breakaway phenomenon, high-temperature steam oxidation of zirconium alloys was carried out at 1000 ℃. Additionally, the oxide microstructures of samples were characterized using multiple techniques. The results show that the microchannels caused an undulated structure of the oxide/metal (O-M) interface, which is a function of time. Meanwhile, after breakaway oxidation, a formation mechanism of cracks, based on vacancies, was proposed according to the crack morphologies. Finally, taking the microchannels as the center, a mechanism is established to explain the high-temperature oxidation behavior of the Zr alloy. • The undulation structure of oxide/metal (O-M) interface is caused by the formation of microchannels at different stages. • Raman spectroscopy shows that zirconia phase distributions correspond to the local curvature of the O-M interface. • Two different mechanisms on cracks formation before and after breakaway oxidation are established. • A breakaway oxidation mechanism focusing on formation of microchannels vertical to the O-M interface is proposed. [ABSTRACT FROM AUTHOR]