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

Highlights: • Mn micro-alloying in Al-11.5Si–4Cu alloy significantly increases the strength at 300^oC after the long-time heat exposure. • Zr micro-alloying remarkably improves the yield strength at room temperature and 300^oC. • Zr micro-alloying severely degrades the strength at 300^oC after long-time heat exposure. • Mn micro-alloying greatly suppresses the coarsening of θ′ precipitates and phase transformation of θ′ .→ θ during the long-time heat exposure. • Zr micro-alloying promotes the phase transformation of θ′ .→ θ during the 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 decreases the yield strength at room temperature, but significantly increases the yield strength at 300^oC in the state after a long-time heat exposure at 300^oC for 100h. Zr micro-alloying remarkably improves the yield strength at room temperature and 300^oC, but severely degrades the yield strength at 300^oC in the state after the same heat exposure. Zr micro-alloying significantly rises the density of the precipitates of θ′ 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 θ′ which form during the ageing treatment are almost completely transformed into the coarse equilibrium θ phase during the long-time heat exposure. However, Mn micro-alloying greatly suppresses the formation of θ phase or the phase transformation of θ′ → θ 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^oC of the alloy with Mn micro-alloying. Zr micro-alloying doesn't improve the thermal stability of θ′ phase during the long-time heat exposure, and conversely promotes the phase transformation of θ′→θ. Thus, after the heat exposure at 300^oC for 100h, the yield strength at 300^oC of the alloys with Zr micro-alloying is greatly decreased, even lower than that of the base alloy.