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Mechanisms of liquid-phase sintering in Fe-Cu mixtures Magee, Brian Eric

Abstract

Continuous dilatometry and metallographic techniques have been used to study the..dimensional changes which occur when iron-copper compacts are sintered above the melting point of copper. Among the variables investigated were: a) the effect of solid-state presintering the compacts, b) copper content, c) particle size, d) initial compact density and e) the heating rate through the melting temperature. Five different processes causing dimensional change were identified. Three contraction processes operated consecutively: 1) rearrangement, 2) solution-precipitation and 3) coalescence. Two expansion processes operated simultaneously with processes (1) and (2), they were: 4) expansion due to diffusion of copper into iron and, 5) expansion resulting from the penetration of γ-iron grain boundaries by copper liquid. In the early stages of sintering the iron was in a 'dispersed' state, while in the last stage it existed as a solid network. That change is attributed to an increase in the values of the dihedral and wetting angles (⌽ and θ) of the liquid-solid system during sintering. It is suggested that initially high concentrations of oxygen at the solid-liquid interfaces causes a low value of γ [sub SL] and thus, the values of ⌽ and θ are initially low. As the oxygen concentration at those interfaces decreases with time, ⌽ and θ become positive, permitting coalescence to occur. Differences among the results of previous investigations in the Fe-Cu system have been explained by attributing them to differences in the thermal histories of compacts. It is suggested that some additions to the Fe-Cu system, which reduce expansion during liquid-phase sintering, do so by acting as internal de-oxidants and preventing a zero dihedral angle from being attained.

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