Precipitation evolution in Al–Zr and Al–Zr–Ti alloys during isothermal aging at 375–425 °C

Formation of Al3Zr or Al3(Zr1−x Tix) precipitates with a metastable L12 structure was investigated in conventionally solidified A1–0.1 Zr, Al–0.2 Zr, Al–0.1 Zr–0.1 Ti, and Al–0.2 Zr–0.2 Ti (at.%) alloys aged isothermally at 375, 400, or 425 °C. Pronounced hardening results from nanometer-scale, spheroidal Al3Zr or Al3(Zr1−xTix) (L12) precipitates within solute-enriched dendrites. Interdendritic regions contain a significantly lower number density of coarser cauliflower-, rod- and plate-shaped precipitates with the L12 structure. Neither the magnitude of the peak-aged hardness, nor the subsequent loss in hardness due to overaging, is affected by ternary additions of Ti. After extended aging times (1600 h) at 425 °C, there is no difference in the mean precipitate radii of spheroidal Al3Zr or Al3(Zr1−xTix) precipitates, confirming that Ti additions do not improve the coarsening resistance. Comparison with prior coarsening studies on Al3Zr, and Al3(Zr1−xVx), Al3(Zr1−xTix) and Al3(Zr1−x−yVxTiy) (L12) precipitates at 425 °C confirms the lack of an effect from ternary alloying additions.