A possible mechanism of phase transformation of Al3Hf from Ll2 to DO22 during aging in a rapidly solidified Al-3Hf-0.3Si alloy

Abstract The aging process of the rapidly solidified solid solution of Al-3 wt% Hf-0.3 wt% Si has been investigated by electron microscopy. The following precipitation stages of an Al 3 Hf phase have been obtained: metastable Ll 2 transforms to a new H-phase and finally to a DO 22 type superstructure. The H-phase was found to have the space group P mmm [Fig. 4(a)]. The transformation from one phase to the other can be explained by a periodic shear. Thus, a shear of 1 2 [110](110) Ll 2 on every second plane leads to the H phase which transforms to DO 22 by a shear of 1 2 [010](101) H on every second plane.