Displacement reactions in the solid state
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I N m e t a l - m a t r i x composites and other high-temperature materials, the occurrence of displacement r e a c tions between metal matrices and compound fibers, particles, or precipitates is an important consideration. The purpose of the present investigation is the prediction (or rationalization) of morphologies and kinetics for simple displacement reactions from a knowledge of the thermodynamic and kinetic properties of the phases involved. THEORETICAL ANALYSIS Consider the following simple displacement reaction involving the metals M and Me, and their lowest oxides MxO and MeuO
uMe + MxO ~ MeuO + x M
[I]
Negligible mutual solubility is assumed for the phases M and MevO , Me and MxO , MxO and MevO , and Me and M. It is assumed that no other binary compounds and no t e r n a r y compounds are formed by the reaction. Finally, the phases involved are assumed to exhibit predominant electronic conduction, and except when otherwise mentioned, the product oxide is assumed to exhibit predominant cation diffusion. Although only oxides are considered herein, the analysis should also be applicable (where the assumptions are valid) to nitrides, carbides, stlicides, borides, sulfides, and other compounds. The Gibbs free energy change per mole of oxygen for reaction [I] is =
-
aGbo
A spontaneous reaction should result from the contact at high temperatures of phase combinations such as Fe/NiO, Fe/Cu20 , Co/Cu20 , Ni/Cu20 , and so forth. While the magnitudes of the Gibbs free energies of reaction for the indicated pairs differ considerably, large differences also exist in the cation diffusivities in the product oxides and in the oxygen permeabilities in the product metals. Wagner* has considered the morphological and kinetic aspects of displacement reactions in the solid state. He pointed out that, in principle, the reaction product phases might form either a) adjacent to one another (to yield an aggregate arrangement as shown schematically in Fig. l(a)) or (b) behind one another (to yield a layered arrangement as shown schematically in Fig. l(b)). Wagner explained that significant plastic deformation must occur in the product phases for the aggregate arrangement of Fig. l(a), because of the large volume changes involved in the reaction. Furthermore, the actual m i c r o s t r u c t u r e for the r e a c tion zone in the aggregate arrangement should be a disordered conglomerate of the product phases in cont r a s t to the schematic illustration of Fig. l(a). A criterion to predict the type of product morphology for a given reaction couple has not been considered previously. The actual product morphologies resulting from displacement reactions and their relation to the limiting aggregate and layered arrangements receive further experimental investigation and rationalization in this study. First, however, a criterion to predict the arrangement of the product phases will be presented.
[1]
if metal saturated with r e s p e c t to its lowest oxide and oxide saturated with r e s p e c t to the metal are chosen as the stan
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