The formation of defects in Fe-Al alloys: Electrical resistivity and specific heat measurements

The electrical resistivity (ρ) and specific heat (C_p) were measured for Fe-Al alloys containing 30, 38, 43 and 48 at.% Al having the DO₃ (Fe₃Al) and the B2 (FeAl) structures using a knife-edge method at 25 °C and by a pulse-heating method from 25 to 1100 °C. The ρ values were identical for a step cooled (over several days) and a quenched (from 1000 °C) condition, except for a slight increase in ρ for the 48 at.% Al alloy when cooled at 1.5 °C/s or quenched. This is believed to be due to the high thermal vacancy concentration for this alloy. Within the DO₃ structure composition range, ρ at 25 °C increased with Al content due to filling of holes in the narrow d-band by electrons from the Al atoms. However, in the B2 structure range, ρ decreased with Al content due to less scattering of electrons because of decreasing Fe-Fe nearest neighbor clusters. The ρ and specific heat (C_p) were measured simultaneously using the pulse-heating calorimeter. The ρ-T behavior for these alloys up to 1100 °C was found to resemble the resistivity saturation phenomenon observed for disordered metal alloys, suggesting that the antisite defect influences ρ more than do vacancies. A particularly interesting feature of the ρ-T curves is the decrease in the thermal coefficient of resistance with decreasing Al content, becoming negative for 30 at.% Al. C_p for all alloys showed a marked (e.g., 25%) increase at elevated temperatures, taken to be due to defect formation. The temperature at which the deviation occurred was heating rate dependent. The C_p-T data were used to determine the energy of formation of the defects, which varies linearly from about 130 J/mol at 30 at.% Al to 95 J/mol at 48 at.% Al. Assuming triple defect formation, the vacancy concentration was determined, and found to be in excellent agreement with values in the literature. Published by Elsevier Science Ltd.