METASTABLE STATE TRANSFOR MATIONS OF Al_3Ti BASE ALLOYS DURING MECHANICAL MILLING AND SUBSEQUENT ANNEALING

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METASTABLE STATE TRANSFORMATIONS OF Al 3Ti BASE ALLOYS DURING MECHANICAL MILLING AND SUBSEQUENT ANNEALING

Developing multi phase alloy is an effective approach for the strengthening and toughening of the L1 2 Al 3Ti alloy. A Nb modified Al 3Ti base alloy which has a L1 2 matrix and the precipitated second phase has been developed. Since the effect of second phase is strongly influenced by its size and distribution, high energy ball milling was employed to fabricate the alloy in comparison with the conventional casting process. It was found that structure evolution during mechanical milling is different for DO 22 , L1 2 and L1 2 DO 22 two phase alloys. The DO 22 Al 3Ti transformed only to a FCC structure even after long time milling. The ordering of L1 2 Al 67 Ti 25 Mn 8 decreased quickly and changed into a FCC structure after short time milling, subsequent milling led to the amorphous transition. The DO 22 phase in Al 3Ti Mn Nb alloys dissolved into the L1 2 matrix quickly in the early stage of milling and the matrix changed into a FCC supersaturated solid solution after 10 h milling, further milling led to the amorphous transition. Reordering of the metastable structures occurred during the annealing process. The consolidated materials consisting of fine equiaxed grains with dispersed particles exhibited remarkable improvement of strength and promising ductility.