Electrosintering of iron powder compacts
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IN prior work by Conrad and co-workers, it was found that the application of an external electric field during the superplastic deformation of a 7475 Al alloy produced a substantial decrease in the cavitation that normally occurred. It was proposed[2,3] that this reduction in cavitation resulted from a decrease in the chemical potential of vacancies at the surface of the specimen due to the electric charge existing there. This, in turn, led to a migration to the surface of excess vacancies that might otherwise form cavities at the interior grain boundaries. The reduction in cavitation during superplastic deformation suggested that an external electric field might also reduce the porosity normally obtained in the sintering of metal powder compacts. Exploratory studies[4,5] provided support for this idea. However, more work was needed to confirm the results and to define in more detail the behavior and mechanism. These, then, were the objectives of the present investigation. [1–3]
important role in the sintering of metal powders. This leaves diffusional flow as the principal mechanism. Assuming that diffusional flow occurs by a vacancy mechanism, the Gibbs– Thompson equation gives the difference in the vacancy concentration (c) below a curved surface and that below a flat surface (c0):
⫺ 0 ⫽
␥⍀ ⫽ RT(ln c ⫺ ln c0) r
where is the chemical potential over the convex surface with a curvature of r, 0 is the chemical potential over an adjacent flat surface, ⍀ is the atomic volume, and ␥ is the surface energy. For ␥⍀/RTr Ⰶ 1, we obtain
⫺ 0 ⫽ RT
A. Sintering without Field The principal driving force for the sintering of two particles is the reduction in surface energy as the particles grow together. The major mechanisms for material transport during the sintering of two spheres are shown in Figure 1 and reviewed in References 6 and 7. These are broadly classified into (1) evaporation and condensation and (2) diffusional flow. Because of the low vapor pressure of most metals, evaporation and condensation do not generally play an
YUSEF FAHMY, Lecturer, and HANS CONRAD, Professor Emeritus, are with the Materials Science and Engineering Department, North Carolina State University, Raleigh, NC 27695-7907. Manuscript submitted May 19, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
冢
c ⫺ c0 c0
冣
[2]
and c ⫺ c0 ⫽
II. THEORETICAL CONSIDERATIONS
[1]
c0 ␥ ⍀ 1 RT r
[3]
The difference in vacancy concentration leads to a flux of vacancies away from the curved surface of the neck toward the flat contact surface between the two particles, which, in turn, gives a flow of atoms in the opposite direction, thereby increasing the neck diameter. Employing Fick’s law for diffusion, we obtain for the vacancy (or atom) flux the transport equation Jv ⫽ ⫺
D ⵜ(⌬) ⍀RT
[4]
where D is the appropriate vacancy diffusion coefficient (lattice (D1), grain-boundary (Db), or surface (Ds) diffusion. Assuming that the material transport occurs only by lattice diffusion and making certain approximations regarding the geometry of th
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