STRUCTURAL AND ELASTIC BEHAVIOR OF FE50AL50 NANOCRYSTALLINE ALLOYS

Pure iron and aluminum powders were mixed in the equiatomic ratio and mechanically alloyed in a high-energy ball mill for different times. S tructure refinement of x-ray powder diffraction data was performed to study the structural transformations induced by mechanical and subsequ ent thermal annealing treatments, The mechanical alloying (MA) process induces a progressive dissolution of aluminum phase into the bcc iron phase. After 32 h of MA a single-phase Fe(Al) bcc extended solid solu tion, with lattice parameter a(0)=2.891 Angstrom, average coherent dom ain size (D) approximate to 50 Angstrom, and lattice strain 0.5%, was observed. The annealing of the specimens after MA up to 8 h favored th e aluminum dissolution in alpha-iron and the precipitation of the Al5F e2 phase, whereas a nanostructured B2 FeAl intermetallic compound was observed in the annealed samples which were previously milled for 8, 1 6, and 32 h. In the same specimens a minority cubic phase Fe3AlCX, ant i-isomorphous with perovskite, derived from contamination of ethanol a nd introduced in the steel vial as a lubricant agent, was also observe d. Anelasticity measurements have shown the occurrence of two main tra nsient effects during the first thermal run. The first one occurring a t 500 K in all mechanically alloyed specimens was attributed to therma lly activated structural transformations, whereas the second at about 700 K was attributed to a magnetic order-disorder transition. During t he second run of anelasticity measurements a relaxation peak P-1 in th e nanostructured B2 FeAl intermetallic compound, attributed to grain-b oundary sliding mechanisms and with an activation energy of 1.8+/-0.2 eV was observed. In specimens milled for 8-32 h a second small peak P- 2 at the low-temperature tail of the P-1 peak was observed and tentati vely attributed to a Zener-type relaxation. (C) 1996 American Institut e of Physics.