Doctoral Dissertations

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Metallurgical Engineering

Major Professor

Charlie R. Brooks

Committee Members

R. A. Buchaner


The specific heat Cp and electrical resistivity p of four iron-aluminum alloys (30, 38, 43, and 48 at.% Al) were measured as a function of temperature and composition. The alloys were step-cooled from 1000°C to minimize vacancy concentration and induce maximum order. At room temperature the 30 at.% Al alloy was in the FesAl phase region (D0&sub3; structure) while the other three were in the FeAl phase region (B2 structure). The resistivity at 25°C increased with increasing disorder (increasing Al content for Fe&sub3;Al and increasing Fe content for FeAl), reaching a maximum value of ∼165 μΩ-cm at the Fe&sub3;Al-FeAl phase boundary.

A pulse-heating calorimeter was used to measure simultaneously C&subp; and ρ from 25 to approximately 1000°C. The samples were initially heated at 73°C/s (except for the 43 at.% Al which was heated at 90°C/s), then cooled back to 25°C at 1.5°C/s. They were then repulsed to examine the effect of the pulse treatment and of heating rate. For the 30 at.% Al alloy, ρ increased to a maximum at 485°C, at which a slight decrease occurred. This was taken as due to the D0&sub3 to B2 phase transformation. Then in the FeAl region, ρ decreased slightly to the upper temperature of measurement (∼1200°C). The slope of the ρ-T curves decreased with decreasing Al content, becoming negative for the 30 at.% Al alloy in the FeAl region. The relatively high resistivities and the tendency for the ρ-T curves to approximate a common value at high temperatures is »milar to the saturation behavior of some amorphous and disorder transition alloys discovered by Mooij.

For the 30, 38 and 43 at.% A1 alloys, the ρ-T curves were identical for the initial slowly cooled condition and after cooling from high temperature at 1.5°C/s. The curve for the 48 at.% A1 alloy was about 30% higher after cooling at 1.5°C/s from high temperature. The ρ-T curves were the same for the heating rates that were used in this study.

The specific heat was determined for the alloys, and it showed a dramatic increase at high temperatures. The C&subp;-T curves were the same for the initial slowly cooled condition and for pulse treatment. For the 43 and 48 at.% A1 alloys the C&subp;-T curves wCTe independent of heating rate for the rates tested, but for the 38 at.% A1 alloy, the temperature corresponding to the dramatic rise in C&subp; shifted to higher temperature with higher heating rate.

The C&subp;-T curves in the lower temperature linear region were extrapolated to higher temperature, and subtracted from the experimental C&subp; curve to determine the enthalpy of formation of defects responsible for the dramatic increase in C&subp;. The enthalpy of formation decreased from 135 kJ/mol at 30 at.% A1 to 95 kJ/mol at 48 at.% Al. The vacancy concentrations nv were calculated from these values by assuming triple defect formation. At 900°C n&subv; was constant (<0.2 at.%) from 30 to 38 at.% Al but increased to ∼1.2 at.% for 48 at.% Al. This is in agreement with the results from other experimental studies and the Chang-Neumann model.

For the 38 at.% Al alloy, C⊂p; was affected by the heating rate but ρ was not. Therefore, it was determined that the β sublattice dominates the resistivity since vacancies form primarily on the α sublattice through the formation of triple defects.

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