HETEROGENEOUS PRECIPITATION OF INTERMEDIATE PHASES ON AL3ZR PARTICLES IN AL-CU-ZR AND AL-LI-CU-ZR ALLOYS

Heterogeneous precipitation of partially coherent precipitates on dispersoid particles of Al3Zr was examined in both Al-4%Cu-0.18%Zr and Al-2%Li-3%Cu-0.15%Zr alloys with special attention to the coherency state of the dispersoids which might change during solution-treatment on account of recrystallization. In the Al-Cu-Zr alloy the dispersoids were coherent in recovered (unrecrystallized) regions and they acted as heterogeneous nucleation sites for a theta' phase in such a way that the dispersoids were in contact with broad coherent planes of the theta' precipitates. On the other hand, the dispersoids became incoherent with matrix in recrystallized regions and they were in contact with narrow incoherent planes of the theta' precipitates. In the Al-Cu-Li-Zr alloy, however, the dispersoids were coherent both in recovered and recrystallized regions and they were in contact with coherent planes of T1 precipitates. The change in nucleation behavior was explained in terms of the reduction in the interfacial energies on nucleation.