Micro-alloying effects of Mn and Zr on the evolution of ageing precipitates and high temperature strength of Al-11.5Si-4Cu alloy after a long-time heat exposure

Micro-alloying effects of Mn and/or Zr on the microstructure evolution of the ageing precipitates of Al-11.5Si-4Cu alloy (in wt.%) were investigated by TEM observation. The relationship of the microstructure evolution with the variation of mechanical properties is discussed. Mn micro-alloying in Al-11.5Si-4Cu alloy de-creases the yield strength at room temperature, but significantly increases the yield strength at 300 degrees C in the state after a long-time heat exposure at 300 degrees C for 100h. Zr micro-alloying remarkably improves the yield strength at room temperature and 300 degrees C, but severely degrades the yield strength at 300 degrees C in the state after the same heat exposure. Zr micro-alloying significantly rises the density of the precipitates of theta' phase formed during the ageing treatment and simultaneously decreases their size, and hence significantly increases the yield strength at room temperature. For Al-11.5Si-4Cu alloy, it is found that the nano precipitates of theta' which form during the ageing treatment are almost completely transformed into the coarse equilibrium theta phase during the long-time heat exposure. However, Mn micro-alloying greatly suppresses the formation of theta phase or the phase transformation of theta' -> theta during the long-time heat exposure. Thus, the long-time heat exposure only leads to a small decrease of 6Mpa in the yield strength at 300 degrees C of the alloy with Mn micro-alloying. Zr micro-alloying doesn't improve the thermal stability of theta' phase during the long-time heat exposure, and conversely promotes the phase trans-formation of theta'-> theta. Thus, after the heat exposure at 300 degrees C for 100h, the yield strength at 300 degrees C of the alloys with Zr micro-alloying is greatly decreased, even lower than that of the base alloy.

Automated summary

Micro-alloying effects of Mn and/or Zr on the microstructure evolution of ageing precipitates in Al-11.5Si-4Cu alloy were investigated. Mn decreases yield strength at room temperature but increases it at 300°C after long-time heat exposure, while Zr significantly improves yield strength at room temperature and 300°C, but degrades it after the same heat exposure. Zr increases precipitate density of theta' phase and yield strength at room temperature, while Mn suppresses theta phase formation during heat exposure, leading to minimal strength decrease at 300°C. Zr does not improve thermal stability of theta' phase and reduces yield strength at 300°C after prolonged heat exposure.