The formation of defects in Fe–Al alloys: electrical resistivity and specific heat measurements

Abstract

The electrical resistivity (ρ) and specific heat (Cp) 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 (Cp) 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. Cp 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 Cp–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.