The effect of Cu and Zn on the phase stability of L12 Al3Hf intermetallic compound synthesized by mechanical alloying

Elemental powders of Al and Hf were mechanically alloyed to produce L12 Al3Hf powder with a nanocrystalline structure. The effect of the addition of ternary elements on the mechanical alloying behavior and the thermal stability of L12 phase was investigated. The start and finish temperatures of the phase transformation from L12 to D023 varied as a function of time and temperature in binary and ternary alloys. L12 Al3Hf alloy was readily prepared by mechanical alloying for 6 h. It is thought that the homogeneous distribution of Al and Hf and following formation of the L12 phase are related with the attainment of the ultra-fine grain size of the powder. However, L12 phase was formed in the (Al+12.5 at.% M)3Hf (M=Cu, Zn) alloys after 10 hours milling. The delayed formation of L12 phase in the ternary alloys was related to the retardation of the microstructural evolution or grain size refinement. The start temperature of the L12 to D023 phase transformation in the binary Al3Hf alloy was only about 650 K, but it was increased to about 970 and 1170 K with the addition of 10 at.% Cu and 12.5 at.% Zn, respectively. Their grain size was less than 20 nm after 20 min heat treatment at each temperature. The start temperature of the phase transformation was not significantly affected by annealing time but the finish temperature of the transformation was decreased significantly with increasing annealing time. It is worth noting that the results in this study will be presented as useful data for the various consolidation processes of L12 nanocrystalline Al3Hf powder.