http://nucapt.northwestern.edu/refbase/files/Knipling-2009_10705.pdf

Precipitation evolution in Al–0.1Sc, Al–0.1Zr and Al–0.1Sc–0.1Zr (at.%) alloys during isochronal aging

The Al-Zr (aluminum-zirconium) system

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 °C or 425 °C. This is due to the absence of Al3Ti 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 Al3Zr (L12) 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 α-Al is much greater than that of Zr in α-Al; and (2) the relatively small liquid solubility of Ti in α-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 Al3Ti (L12) with α-Al is also larger than that of Al3Zr (L12), further hindering nucleation of Al3Ti. Classical nucleation theory indicates that the minimum solute supersaturation required to overcome the elastic strain energy of Al3Ti nuclei cannot be obtained during conventional solidification of Al-Ti alloys (unlike for Al-Zr alloys), thus explaining the absence of Al3Ti precipitation and the presence of Al3Zr precipitation.

/pdf/nucleation-and-precipitation-strengthening-in-dilute-al-ti-pm20bn2142.pdf

Nucleation and Precipitation Strengthening in Dilute Al-Ti and Al-Zr Alloys

Al–5 Ti–1 Band Al–1 Ti–0·4B alloys have been held at 750° and 1000°C for different periods of time before being cooled at a rate of ∼ 1K S−1. The solidification process was followed by cooling-curve analysis which revealed a metastable phase as well as a ‘pre-peritectic’ reaction. The metastable phase was shown to grow preferentially on existing Al3Ti crystals during cooling of the melt from a high temperature. Holding the melt for 3–4h at 750°C transformed the metastable system into the stable one. The pre-peritectic reaction was found to occur at ∼672°C.

W Dahl

Papers

Solidification and precipitation in aluminium-zirconium alloys. Pt. 1