Interaction Between Dislocations and NiF e2 O 4 Precipitates In A NiO Matrix
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INTERACTION BETWEEN DISLOCATIONS AND NIFe2O4 PRECIPITATES IN A NIO MATRDC SCOTT R. SUMMERFELT* AND C. BARRY CARTER** Dept. of Materials Science and Engineering, Bard Hall, Cornell University, Ithaca, NY 14853
ABSTRACT Three different types of dislocation interactions with NiFe20 4 (spinel crystal structure) precipitates in a NiO matrix have been studied. In the first, the movement of dislocations introduced by room temperature deformation is impeded by the spinel precipitates. Glide dislocations in the NiO with 1/2 Burgers vectors and {OTI }glide planes cannot pass through the spinel precipitates without forming stacking faults because the perfect NiO dislocations are partial dislocations in NiFe 20 4. Many dislocation loops but no stacking faults were observed in the deformed samples indicating that the gliding dislocations formed the loops when they moved past the precipitates. In the second type of interactions, cusps were formed in the spinel-NiO interface at close to the dislocation loops when the sample was heat treated; the cusps indicate preferential dissolution of the spinel. In the final interaction, the dislocations were shown to act as preferential nucleation sites when spinel was precipitated from the NiO matrix. At slow nucleation rates, NiFe20 4 precipitated only on the dislocations; when the nucleation rate was increased, precipitation occurred both on and away from the dislocations. Precipitates which form at a dislocation may contain a stacking fault extending from the partial dislocation to a cusp in the spinel-NiO interface. When this occurred, the stacking faults were observed to be faceted parallel to either ( 111 } or {011) planes. INTRODUCTION The spinel-wiistite system is particularly suitable for a precipitation study because both structures have face-centered-cubic oxygen sublattices. In the absence of misfit dislocations, precipitate growth can therefore occur by the movement of cations alone (if misfit dislocations are present movement on both ion sublattices must occur), In the NiFe 20 4 -NiO system, the small misfit in the oxygen sublattice (less than 0.2% at room temperature) can result in the growth of large coherent precipitates [1]. The dislocations in NiO and perfect dislocations in spinel have similar 1/2 Burgers vectors and (011) glide planes [2]. Since the lattice parameter of NiFe20 4 is almost twice that of NiO, a perfect dislocation in NiO is a partial dislocation in NiFe204 [3]. Previous studies of MgFe204 precipitates in MgO have shown that precipitation hardening and preferential nucleation on dislocations occur [4-7]. In the present study, the interaction between dislocations and NiFe 2O4 precipitates in a NiO matrix has been studied. The NiFe204-NiO system was chosen because precipitation [1,3], particle coarsening [1], and nucleation and growth in thin-film specimens [1] have been studied previously. The effect of the spinel precipitates on dislocation movement was studied by precipitating spinel in NiO and then introducing the dislocations by deformation. The effect
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