EFFECT OF PRE-AGING ON THE EVOLUTION OF GE PRECIPITATES IN AN AL-1.8 AT-PERCENT GE ALLOY

The shape distribution of Ge precipitates in a quenched and ag ed Al-1 .8 at.% Ce alloy was characterized by transmission electron microscopy . In agreement with previous investigations, at least six different pr ecipitate morphologies were distinguished: tetrahedra, octahedra and t riangular plates all with the cube-cube orientation relationship; as w ell as laths along [100] Al directions, laths along [110] Al direction s and plates on {100} Al planes, with three different orientation rela tionships, respectively. The distribution of precipitate shapes was fo und to depend strongly on the heat treatment schedule. After quenching to 0 degrees C, samples aged at 200 degrees C exhibited predominantly triangular plates and tetrahedra and were insensitive to pre-aging at room temperature. By comparison, samples aged at 250 and especially a t 300 degrees C contained mainly lath-shaped precipitates after short pre-aging times, but a sharply increasing fraction of tetrahedra, octa hedra and triangular plates for longer pre-aging times. When the tempe rature of the quench bath was raised from 0 to 40 degrees C, samples a ged at 200 degrees C remained unchanged whereas samples aged at 250 an d 300 degrees C retained only lath shaped precipitates. Quenching dire ctly to the three aging temperatures changed the precipitate distribut ion to a coarse array of large {100} plates.