TEM investigation of dislocations, short-range ordering and incoherent interfaces in zirconium-nitrogen alloys

This research was performed to understand defect and interfacial structures associated with the formation of the ZrN phase in Zr-N alloys. Various transmission electron microscopy (TEM) techniques and near coincident-site (NCS) modeling were used to investigate three main topics: (1) the dissociation of perfect dislocations in ZrN, (2) the presence of diffuse intensity maxima in electron diffraction patterns (EDP's) from the ZrN phase, and (3) the atomic structure of incoherent interfaces formed between α-Zr and ZrN. The results from these studies show that 1/2 <110> dislocations dissociate into two 1/6 <112> Shockley partial dislocations bounding a stacking fault (SF) on the {lcub}111{rcub} planes, due to low value of the SF energy (SFE) in ZrN. The low SFE occurs due to short-range ordering of N vacancies, which leads to the appearance of {lcub}1,1/2,0{rcub} diffuse intensity maxima in EDP's from the ZrN phase. Both a Kohn construction and microdomain crystal models are presented to explain the appearance of the diffuse intensity maxima. The α-Zr and ZrN phases often have a high-index orientation relationship with {lcub}111{rcub} interfaces. Conventional and high-resolution TEM and NCS modeling show that these interfaces are atomically incoherent. Faceting of the ZrN phase on the close-packed {lcub}111{rcub} plane is the main factor that determines the interface plane, rather than the degree of atomic matching. These results are discussed in terms of similar interfaces formed during massive transformations.