Metastable Ti-Al phases obtained by mechanical alloying

The Ti-Al phase formation occurring during mechanical alloying was investigated, special attention being paid to avoid contamination. The compositions Ti10Al90, Ti25Al75, Ti33Al67, Ti43Al57, Ti44Al56, Ti50Al50 and Ti70Al30 were investigated. In all cases the resulting phase composition of the mechanical alloys does not depend on the initial state, i.e. pure metals or compounds. The mechanical alloying of samples having compositions falling into the region of gamma-TiAl result in a metastable h.c.p. phase. The mechanical alloy Ti70Al30 has an h.c.p. structure if the treatment intensity is low, or a b.c.c. structure if the treatment intensity is rather high. The compositions Ti25Al75 and Ti33Al67 transform into the same phase, possibly having the TiAl2 structure. Annealing of Ti25Al75 reveals an intermediate phase, possibly Ti2Al5. This phase results also from the mechanical alloying of Ti10Al90. A possible mechanism of mechanical alloying is discussed. It is shown that the results are in good accordance with the model of local melting.