The structure of a〈100〉 and a〈110〉 dislocation cores in NiAl

Sessile dislocations by reactions in NiAl severely deformed at room temperature

B2 ordered NiAl is known for its poor room temperature (RT) ductility; failure occurs in a brittle like manner even in ductile single crystals deforming by single slip. In the present study NiAl was severely deformed at RT using the method of high pressure torsion (HPT) enabling the hitherto impossible investigation of multiple slip deformation. Methods of transmission electron microscopy were used to analyze the dislocations formed by the plastic deformation showing that as expected dislocations with Burgers vector [Formula: see text] carry the plasticity during HPT deformation at RT. In addition, we observe that they often form [Formula: see text] dislocations by dislocation reactions; the [Formula: see text] dislocations are considered to be sessile based on calculations found in the literature. It is therefore concluded that the frequently encountered 3D dislocation networks containing sessile [Formula: see text] dislocations are pinned and lead to deformation-induced embrittlement. In spite of the severe deformation, the chemical order remains unchanged.

A Dislocation Model for Flow at Intermediate Temperatures in Hard-Oriented NiAl

Recent studies of single crystals and bicrystals indicate that dislocations of the type a<011> are important, and may actually control, deformation at intermediate temperatures (above the brittle-to-ductile transition temperature) in hard-oriented NiAl. In the present work, the fine structure of a<011> dislocations has been examined using both high resolution and diffraction-contrast transmission electron microscopy. Evidence has been found for the decomposition of a<011> dislocations into two a<001> dislocations. The initial driving force for the decomposition is due to core effects, as revealed by molecular statics and dynamics Embedded Atom Method calculations. Additional decomposition occurs by a combination of climb and glide. A continuum-based dislocation model is introduced which incorporates these relevant microstructural features.

Microstructural Characterization of Creep Tested Cryomilled NiAl-13vol. % AlN

Cryomilling of prealloyed NiAl powders, followed by extrusion, has been used to produce a particulate strengthened NiAl-13vol.% AlN material. At 1300 K, the compressive strain rate-flow stress diagram has two distinct deformation regimes, with the transition occurring near 150 MPa. The low and the high stress regimes have power law creep exponents of ∼ 6.1 and 14.2, respectively. Microstructural characterization of the as-extruded and tested samples has been performed to develop an understanding of the superior mechanical properties of the material. The microstructure of the as-extruded material was inhomogeneous and consisted of mantle regions containing a mixture of small NiAl grains (diameter ∼ 50–150 nm) and fine AlN particles (size ∼ 5–50 nm) that surround larger NiAl grains (diameter ∼ 0.3–8.0 μm) which were mostly particle free. In the low-stress regime, samples tested to steady state exhibited a structure composed of subgrain boundaries in the particle-free NiAl grains. In addition, some of the subgrains had developed a well defined dislocation network. AlN patricles occasionally found within large NiAl grains acted as pinning centers for dislocations. Small NiAl grains and the AlN particles constituting the mantle coarsened during these tests. In the high-stress regime, samples tested to steady state exhibited a high density of dislocations in most of the particle-free NiAl grains. Subgrain boundaries were found occasionally but dislocation networks were rare. The AlN particles had not significantly coarsened due to the shorter times at temperature.

Evolution of a reactive surface via subsurface defect dynamics

We find that the topography and composition of a reactive surface can evolve during epitaxy via motion of point and line defects within the material. We observe the response of a NiAl surface to an Al atom flux with low-energy electron microscopy. Initially, new NiAl layers grow as Al atoms exchange with bulk Ni atoms. When the surface is critically enriched in Al, condensation occurs at dislocations. They dissociate, move linearly, and leave tracks of altered composition and new atomic steps. We show how these dynamics depend on the identity and quantity of point defects near the surface.

HRTEM of dislocation cores and thin-foil effects in metals and intermetallic compounds

Examples of the observation and analysis of dislocation cores and dislocation fine structure in metals and intermetallics using high resolution transmission electron microscopy are discussed. Specific examples include the 60 degrees dislocations in aluminum, a011 edge dislocations in NiAl, and screw dislocations in Ni3Al. The effect of the thin TEM foils on the structure and imaging of these dislocations is discussed in light of embedded atom method calculations for several configurations and coupled with image simulations. Some generalizations based on these calculations are discussed. These analyses enables determination of the spreading or decomposition of the edge component of the cores, both in and out of the glide plane, which can have significant implications for the modeling of macroscopic behavior.