An Energetic Study on the Preference of the Habit Plane in Fcc/bcc System

The preference of the habit planes (HPs) developed from precipitation in the fcc/bcc system has been investigated. The interfacial energy of different interface orientations has been examined with variation of the orientation relationships (OR) and lattice parameters by a classical molecular dynamics method. The results show that interface has the lowest interfacial energy when it contains parallel Burgers vectors and a set of dislocations. The local minimum of interfacial energy may not associated with a maximum of dislocation spacing. It is also found that the near Kurdjumov-Sachs OR is more preferable than the near Nishiyama-Wasserman OR. Contrary to the previous interfacial energy calculations, which usually limit to rational ORs, the present work allows ORs to be irrational, which agrees with the observations.

Effect of Dislocation Core Spreading at Interfaces on Strength of Thin-films

Critical strain arguments are often used to model the thickness dependence of the strength of thin films on substrates. In these arguments, plastic deformation occurs when the stress in a film is high enough that the strain energy relieved by the introduction of a misfit dislocation is sufficient to generate the line energy of that misfit. Such models typically assume compact dislocation cores. However, experimental evidence suggests that, under certain circumstances, dislocation cores may spread out into the interface between the film and the substrate. If this happens, the energy of the misfit dislocation, and the critical stress needed for its propagation, will be lowered. In this paper, the effect of dislocation core spreading on the critical stress has been modeled. The effects of interface strength, film thickness, and misfit dislocation spacing are considered.

Microstructure of GaN Grown on (1120) Sapphire

ABSTRACTMost of the work done on GaN has taken into account layers grown on the (0001) sapphire. However one would expect the growth on (1120) to lead to different structural defects. As has been shown, in one direction, the mismatch is rather small. In this work, we have carried out structural analysis of layers and interfacial relationship. Inside the layers, the density of defects is comparable to that found conventionally in layers grown on top of (0001) sapphire. The growth mode is also mosaic with a grain size of a few microns. One interesting result is the interface structure, which differs from conventional growth where a flat or stepped interface is formed with a large distance between steps. In this case, the interface is found to be rough at the atomic scale so that this roughness has a random distribution. Moreover, the misfit dislocation spacing is 1nm which is only half the dislocation spacing found in GaN growth on (0001) sapphire.

Dislocation Distribution and Subgrain Structure of GaN Films Deposited on Sapphire by HVPE and MOVPE

Transmission electron microscopy (TEM) was used to characterize the microstructure in GaN films deposited by two different methods. An 11 μm thick film was deposited directly on a sapphire substrate by HVPE; an 8 μm thick film was deposited on a 15 nm buffer layer of AIN on sapphire by MOVPE. The dislocation densities in the top layer of the HVPE and MOVPE ilms were ∼109 cm-2 and ∼5 x 109 cm-2 respectively. In the HVPE film this was almost exclusively threading dislocations (TDs), ∼70% of which had edge character. In addition to the TDs, the MOVPE sample also contained an appreciable number of dislocations lying in the basal plane. The microstructure of each film was dominated by a subgrain structure of slightly misoriented cells. In the MOVPE specimen, approximately 90% of the TDs were associated with subgrain walls, whereas only approximately 75% of the dislocations in the HVPE specimen were associated with walls. Both the HVPE and MOVPE samples experienced 40% coarsening of the cells through the thickness of the film. The subgrains of the MOVPE sample were 75% smaller than those in the HVPE sample (350 and 1300 nm, respectively). The average dislocation spacing in the walls was 50% smaller in the MOVPE sample than in the HVPE sample (82 and 180 nm, respectively).

Nanoindentation Studies of Yield Point Phenomena on Gold Single Crystals

Nanoindentation studies often show an instantaneous displacement-excursion in the loaddisplacement curve. This anomaly is generally associated with a surface contamination effect, dislocation emission, a phase transition, or an oxide break-through event. The determination of which effect is operative is often difficult when investigating oxide covered surfaces. We have performed a detailed nanoindentation study on clean, flame annealed single-crystal Au thus eliminating the effects of a surface oxide or contamination layer. Multiple displacement excursions were observed exhibiting a new phenomenon of “staircase” yielding. Owing to the fact that our radius of contact is more than an order of magnitude smaller than the average dislocation spacing expected for well annealed Au, the excursions are explained in terms of multiple dislocation nucleation events on parallel slip bands. Indentation data were taken on Au (111), (110), and (100) single crystal surfaces.

Localized Influence of Solute on the Stacking Fault Energy of Dilute Al-Based Solid Solutions

The stacking fault energy (SFE) is widely used to classify the mechanical behavior of pure metals. In alloys, however, the experimentally observed SFE is strongly influenced by localized solute effects. To further understand these effects on dislocation structure and on the observed SFE, solute segregation to an extended edge dislocation dipole, delineating two stacking faults, was studied in dilute Al:Cu, Al:Ag, and Al:Cu, Ag solid solutions. Cu and Ag were chosen to isolate solute size and modulus effects, Cu being smaller than Al, while Ag and Al are essentially the same size. Atomistic Monte Carlo results showed little change in the partial dislocation spacing in the binary systems as compared to the spacing in pure Al, even though Cu was observed to segregate to the compressive regions of the dislocation dipoles, forming widespread atmospheres, while Ag formed randomly distributed Ag-rich zones. However, in ternary Al:Cu,Ag simulations, the Ag apparently inhibited the Cu from distributing across the width of the extended dislocations, both Ag and Cu forming small clusters near or on the partial dislocations which increased the partial dislocation spacing. Results will be discussed in light of interpretations of experimental SFE determinations, emphasizing the importance of the localized solute distribution on the SFE.

Grain Boundary Structure and Structure-Behavior Relationships in HCP Metals

We present calculations of dislocation spacing for grain boundaries in zinc, and show that the spacings may be large enough to be resolved in conventional transmission electron microscopy even at extreme deviations from ideal coincidence misorientations. This effect is a result of the need for the dislocation arrays to accommodate slight differences between the ideal and actual axial ratios in addition to the difference between the ideal and actual misorientations. Recent observations of sliding behavior as a function of misorientation may also be rationalized within the framework of our structure calculations.