The kinetics of the transformation of pyrite to pyrrhotite have been investigated. The study was performed using thermogravimetric analysis over the temperature range of 620 to 973 K in atmospheres of H 2 , He, Ar, and in vacuo over a wide range of pressures: 0.20 Pa to 4.24 MPa. Based on the kinetic results, a mechanistic picture of the various steps exerting control over the transformation is proposed. The thermal decomposition proceeds via a two-step, consecutive process. The ratecontrolling step is the desorption of sulfur vapor from the surface. The presence of H 2 introduces different rate-controlling steps into the sequence, providing the H 2 exists at a pressure sufficiently high to suppress the rate of thermal decomposition. Rates at which the H 2 reduction occurs with pyrite samples from different sources depends upon the samples' impurity level and the extent to which various crystallographic faces are exposed.

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The effect of attrition milling on the solid-state oxidation of aluminum powder is important for the reaction-bonded aluminum oxide process. Attrition milling increased the surface area to 14.4 and 20.2 m 2 /g versus 1.2 m 2 /g for unmilled powder and smeared the Al particles, and the surface was hydrolyzed to form bayerite and boehmite. Upon heating the hydroxides decompose to form an 11-13 nm thick amorphous plus ␥-Al 2 O 3 layer which subsequently retards oxidation kinetics. The oxidation per unit area decreases for the higher surface area powders at temperatures below the critical temperature but the total oxidation of the milled powder is ϳ70% versus ϳ9% for the as-received powder because of the higher surface area. The critical temperature depends on Al particle surface characteristics and is defined as the transition temperature above which the initial rate of oxidation is linear, not parabolic. Above the critical temperature the oxidation per unit area decreases significantly. In addition, linear oxidation occurs faster than parabolic oxidation and thus the initial fast oxidation kinetics (i.e., linear) can cause thermal runaway during oxidation. Therefore, oxidation below the critical temperature is essential to maximize solid-state oxidation and to prevent thermal runaway. The critical temperatures for the as-received (1.24 m 2 /g), the 6 h (14.4 m 2 /g), and 8 h (20.2 m 2 /g) attrition-milled Al powders were 500°, 475°, and 500°C, respectively. A model for oxidation during the parabolic and linear oxidation stages is presented.

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and the technical assistance of the staff of the Department of Metallurgy and Materials Science. Gratitude is also expressed to Stelco Research Centre for providing the alloy analyses through the good offices of Mr. Dennis McCutcheon.

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