Dispersion of Oxide Films Hard Particles and Wear Debris

The strengthening effect produced by ultrafine oxide particles through solid-state or mechanical alloying is an important factor affecting final equilibrium particle size and the time required to reach equilibrium particle size. Oxide particles, derived from oxide films originally present on the particle surface and/or from oxygen in the chamber environment, become entrapped between cold welded surfaces by kneading. This process is illustrated in Fig. 17 using a fluid medium. The strengthening effect of the dispersoid, cold working, and the composite structure greatly increase toughness, strength, and fracture resistance of individual particles. Under these conditions, particles assume a more equiaxed shape. More importantly, the degree of hardening may limit the amount of structural refinement if milling energy is insufficient.

Fig. 17 Nonreactive milling process. (a) Initial milling: nickel flattened into flakes; dispersoid embedded in nickel particles. (b) Intermediate milling: nickel flake formation continues; flakes weld; embedding of dispersoid particles in nickel continues. (c) Final milling: multilayered nickel flakes fold and/or break and reweld; all dispersoid kneaded into milled powder. Source: Ref 8

Fig. 17 Nonreactive milling process. (a) Initial milling: nickel flattened into flakes; dispersoid embedded in nickel particles. (b) Intermediate milling: nickel flake formation continues; flakes weld; embedding of dispersoid particles in nickel continues. (c) Final milling: multilayered nickel flakes fold and/or break and reweld; all dispersoid kneaded into milled powder. Source: Ref 8

Because the mechanical alloying effect depends on achieving the ultimate degree of refinement, sufficient energy is required for milling. More information on mechanical alloying of oxide dispersion-strengthened superalloys is in the article "Production of Nickel-Base Powders" in this Volume.

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