@Article{ Knipling-2007-2552,
  author = "Knipling, K.E. and Dunand, D.C. and Seidman, D.N.",
  title = "Nucleation and Precipitation Strengthening in Dilute Al-Ti and Al-Zr Alloys",
  journal = "Metallurgical and Materials Transactions A",
  year= "2007",
  volume = "38A",
  number = "10",
  pages = "2552--2563",
  abstract = "Two conventionally solidified Al-0.2Ti alloys (with 0.18 and 0.22 at. pct Ti) exhibit no hardening after aging up to 3200 hours at 375$^\circ$C or 425$^\circ$C. This is due to the absence of Al$_3$Ti precipitation, as confirmed by electron microscopy and electrical conductivity measurements. By contrast, an Al-0.2Zr alloy (with 0.19 at. pct Zr) displays strong age hardening at both temperatures due to precipitation of Al$_3$Zr (L1$_2$) within Zr-enriched dendritic regions. This discrepancy between the two alloys is explained within the context of the equilibrium phase diagrams: (1) the disparity in solid and liquid solubilities of Ti in $\alpha$-Al is much greater than that of Zr in $\alpha$-Al; and (2) the relatively small liquid solubility of Ti in $\alpha$-Al limits the amount of solute retained in solid solution during solidification, while the comparatively high solid solubility reduces the supersaturation effecting precipitation during post-solidification aging. The lattice parameter mismatch of Al$_3$Ti (L1$_2$) with $\alpha$-Al is also larger than that of Al$_3$Zr (L1$_2$), further hindering nucleation of Al$_3$Ti. Classical nucleation theory indicates that the minimum solute supersaturation required to overcome the elastic strain energy of Al$_3$Ti nuclei cannot be obtained during conventional solidification of Al-Ti alloys (unlike for Al-Zr alloys), thus explaining the absence of Al$_3$Ti precipitation and the presence of Al$_3$Zr precipitation.",
  doi = "10.1007/s11661-007-9283-6"
}