In-Situ TEM Studies of Sessile Dislocation Arrangements and N Vacancy Ordering in ZrN
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In-Situ TEM Studies of Sessile Dislocation Arrangements and N Vacancy Ordering in ZrN P. Li and J.M. Howe1 Center for Solid State Science, Arizona State University, Tempe, AZ, 85287-1704, U.S.A. 1 Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA, 22904-4745, U.S.A. ABSTRACT Dissociation of perfect 1/2 single dislocations into two 1/6 Shockley partial dislocations in ZrN was observed by transmission electron microscopy (TEM). The 1/2 single dislocations have a super-jog character and are not coplanar with the dissociated Shockley partials. This sessile arrangement of dislocations may be responsible for the brittleness of ZrN. The wide separation of the partial dislocations bounding stacking faults indicates that the stacking-faults energy (SFE) is low in ZrN. The low SFE can be explained on the basis of a high vacancy concentration, which was confirmed by the appearance of diffuse intensity maxima in electron diffraction patterns due to short-range ordering (SRO) of N vacancies. In-situ heating experiments in the TEM revealed that the diffuse intensity maxima disappear during heating and reappear on cooling. This indicates that N (or N vacancy) diffusion scrambles the SRO arrangement of N vacancies during heating. The width of the stacking faults in ZrN increases with temperature, indicating that the SFE decreases as the vacancy concentration increases.
INTRODUCTION The stacking-fault energy (SFE) is generally high in transition-metal carbides and nitrides with a NaCl-prototype structure. For example, the SFE ranges from 130 to 300 mJ/m2 in TiC, depending on the C concentration [1]. A recent study of defect structures in ZrN indicates that the SFE has an unusually low value of approximately 4 mJ/m2, leading to a wide separation between dissociated dislocations [2]. Both the dislocation dissociation reaction and anomalously low SFE can be explained by the large vacancy concentration in ZrN. This was confirmed by the appearance of diffuse intensity maxima in electron diffraction patterns due to short-range ordering (SRO) of N vacancies [2, 3]. The SFE has been found to dramatically decrease in TaC with increasing concentration of C vacancies [4], which in turn, causes an increase in the separation of dissociated dislocations. Since the concentration of N vacancies should increase with temperature in ZrN according to the Zr-N phase diagram [5], it is anticipated that the SFE of ZrN should decrease as the temperature increases, leading to an increase in the separation of dissociated dislocations. SRO of N vacancies in ZrN causes {1,1/2,0} diffuse intensity maxima in reciprocal space [3]. One expects the diffuse intensity to disappear as N diffusion scrambles the arrangement of SRO of N vacancies on heating. The present study describes dislocation structures and SRO of N vacancies in ZrN and their response to in-situ heating in the transmission electron microscope
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