Phase separation in TiAl(L10)-(Al, Cr)3Ti(L12) system

Hardness and microstructural variations during aging of Ti-Al-Cr ternary L1(0)- and L1(0)-phase alloys have been investigated in terms of hardness measurements and transmission electron microscopy. Both the L1(0) and L1(2)-phase alloys harden by aging at 973 K after solution annealing at higher temperatures. The amount of age hardening of the L1(2) phase alloy is larger than that of the L1(0) phase ahoy. The phase separation between L1(0) and L1(2) phase have not been observed by aging at 973 K. But Al2Ti was formed in each matrix alloy during aging. The crystal structure of the Al2Ti phase is a Ga2Zr type in the L1(0) and a Ga2Hf type in the L1(2) phase, respectively. At the beginning of aging the fine coherent cuboidal Al2Ti-phase are formed in the L1(0) phase. By further aging, two variants of Al2Ti precipitates grow along the two {110} habit planes. On the other hand, in the L1(2) phase, the Al2Ti phase forms on the {100} planes of the L1(2) matrix lattice. After prolonged aging the precipitates are rearranged along a preferential direction of the matrix lattice and form a domain consisting of only one variant. It is suggested that the precipitation of Al2Ti in each matrix alloy occurs to form a morphology which efficiently relaxes the elastic strain between precipitate and matrix lattices.