Formation of Al_3Zr or Al_3(Zr_(1-x) Ti_x) precipitates with a metastable L1_2 structure was investigated in conventionally solidified Al-0.1 Zr, Al-0.2 Zr, Al-0.1 Zr-0.1 Ti, and Al-0.2 Zr-0.2 Ti (at. percent) alloys aged isothermally at 375, 400, or 425 deg C. Pronounced hardening results from nanometer-scale, spheroidal Al_3Zr or Al_3Zr_(1-x)Ti_x (L1_2) precipitates within solute-enriched dendrites. Interdendritic regions contain a significantly lower number density of coarser cauliflower-, rod- and plate-shaped precipitates with the L1_2 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 deg C, there is no difference in the mean precipitate radii of spheroidal Al_3Zr or Al_3(Zr_(1-x)Ti_x) precipitates, confirming that Ti additions do not improve the coarsening resistance. Comparison with prior coarsening studies on Al_3Zr, and Al_3(Zr_(1-x)V_x), Al_3(Zr_(1-x)Ti_x) and Al_3(Zr_(1-x-y)V_xTi_y) (L1_2) precipitates at 425 deg C confirms the lack of an effect from ternary alloying additions.
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