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The microstructural evolution during long-term thermal exposure of Alloy 617 is investigated in detail. During processing, various precipitation phases were formed, including MC, M 23 C 6 , M 6 C and γ', in which the M 23 C 6 carbide is the primary strengthening phase and determines the hardness of Alloy 617. The M 23 C 6 carbides can either form from decomposing MC or precipitate directly from the γ matrix, but they most eventually transformed to M 6 C carbides after long-term aging. In addition to the phase transformation among various carbides, the morphology evolution of M 23 C 6 carbide is found to have a significant effect on the formation of γ' phase, which is closely related to the redistribution behavior of Al and Ti between M 23 C 6 and γ matrix. We also identified a faster evolution kinetic of the precipitates at grain boundary than those in grain interior, which is caused by the different local diffusion rates. This work elucidates the transformation sequence among different precipitates and the corresponding alloy element redistribution behavior during aging Alloy 617, which shed lights on the strengthening phase design and can in turn optimize the mechanical properties of Alloy 617. • The study reveals the variation of precipitates during long-term aging and its strong link to the element diffusion and redistribution behaviors. • The morphology evolution of M 23 C 6 carbide can promote the precipitation of γ' particles. • The evolution between different precipitated phases is much faster at grain boundary than that in grain interior. • M 23 C 6 carbide is mainly strengthening phase, which determines the hardness of Alloy 617. [ABSTRACT FROM AUTHOR]

In this work, eutectic Si modification grade formula was established and effect of Sr element on modification level and mechanical properties of Al-9Si-0.4Mg-0.1Cu alloy was studied. Thermal analysis, SEM, EPMA and TEM were applied to analyze cooling curves, microstructure and Si twinning of the alloys. The results showed that precision and accuracy of eutectic Si modification grade formula were significantly improved and quantitative evaluation modification level of eutectic Si could be achieved. Modification level of eutectic Si increased first and then decreased with Sr content increasing. Though twin plane re-entrant edge (TPRE) mechanism and impurity induced twinning (IIT) mechanism were effective under certain conditions, they cannot fully explain the modification mechanism of eutectic Si in Al-9Si-0.4Mg-0.1Cu alloy. Sr element caused constitutional undercooling at the front of solid-liquid interface, promoted the formation of fine Al 2 Si 2 Sr phases and nano-precipitates, resulting in the changes of size and morphology of eutectic Si. With the improvement of eutectic Si modification level, the UTS and El of the alloys increased. While, adding excessive Sr led to the formation of coarse Al 2 Si 2 Sr, which was detrimental to the improvement of modification level and mechanical properties of alloy. • Modification level function suitable for quantitatively evaluate modification grade of eutectic Si was established. • Fine Al 2 Si 2 Sr phases and nano-precipitates change the size and morphology of eutectic Si. • Coarse Al 2 Si 2 Sr reduces the modification level of eutectic Si and the mechanical properties of alloy. [ABSTRACT FROM AUTHOR]

Microalloying of an Al-15%Mg 2 Si alloy (wt%) with 1%Cu produced an enhanced age-hardening response, increasing peak hardness and ultimate tensile strength due to a modified precipitation process. The solid solubility of Cu in Al-15%Mg 2 Si alloy effectively promotes the formation of the high number density of Guinier-Preston-Bagaryasky (GPB) zones combined with S precipitates. It suppresses β′ and β″ formation during ageing at 200 °C. Consequently, Al-15%Mg 2 Si-1%Cu alloy with the high density of GPB zones and S phase exhibited high peak hardness (121 HVN) and ultimate tensile strength (284 MPa), 42.4% and 49.5% higher than that of the Al-15%Mg 2 Si alloy, respectively. • The addition of 1% Cu in Al-15%Mg 2 Si alloy has a significant influence on precipitation during ageing at 200 °C. • Cu addition promotes the formation of GPB zones and S phase, instead of β′ and β″ precipitates. • The high number density of GPB zones and S phase produces an enhanced age-hardening response. [ABSTRACT FROM AUTHOR]