T.T. Cheng

The decomposition of the beta phase in Ti-44Al-8Nb and Ti-44Al-4Nb-4Zr-0.2Si alloys

Abstract The decomposition of the beta phase has been studied in Ti–44Al–8Nb and Ti–44Al–4Nb–4Zr–0.2Si alloys cooled from the ( α + β ) phase field using TEM and SEM. The morphology of the transformation products is shown to be sensitive to cooling rate and the results of the microstructural observations are used to construct a schematic continuous cooling transformation (CCT) diagram for these alloys. It is shown that under slow cooling, the beta phase is mainly consumed by the neighbouring alpha grains via α / β interfacial migration. During the subsequent α → α + γ transformation which occurs at lower temperatures and produces a lamellar microstructure, the retained beta phase can transform partially to γ by a process analogous to the discontinuous coarsening of γ lamellae. It has also been shown that, at higher cooling rates, the beta phase can decompose to alpha via secondary lath formation and/or a Widmanstatten transformation although transformation by α / β interfacial migration still occurs. The decomposition of beta directly to faceted gamma has also been observed both in slowly cooled and rapidly cooled materials. In the slowly cooled materials this is attributed to direct nucleation of gamma grains, whereas in the rapidly cooled materials they are thought to be the result of discontinuous coarsening. Furthermore, a B2 to ω transformation occurs readily in these alloys at temperatures higher than 700°C in both cases. Possible mechanisms for each of these transformations are discussed.

Interfacial dislocation mechanism for diffusional phase transformations exhibiting martensitic crystallography: formation of TiAl + Ti3Al lamellae

Abstract The displacive–diffusive transformation, which gives rise to lamellar γ-TiAl+α2-Ti3Al microstructures, has been analysed theoretically. Using the formulation due to Hirth and Pond, the shear produced and extent of diffusional flux associated with interfacial defect motion can be quantified in terms of the Burgers vectors, step heights of the defects and the chemical compositions of the adjacent crystals. Moreover, this formulation also enables the effects of chemical composition, misfit at the interface and ordering to be investigated systematically. It is shown that for the defects observed on such interfaces in recent high-resolution transmission electron microscopy studies, their motion would lead to the conservation of sites and, correspondingly, to equal and opposite fluxes of Ti and Al atoms. Consequently, although this transformation is diffusional, it exhibits crystallographic features consistent with martensitic processes.

A high-resolution electron microscopy study of steps on lamellar γ-α2 interfaces in a low-misfit TiAl-based alloy

Abstract The defect character of steps on lamellar γ-α2 interfaces in a quinternary TiAl-based alloy for which the misfit between the α2 and γ phases is less than 0.1 % has been studied using high-resolution transmission electron microscopy. The interfaces consisted of atomically flat coherent terraces separated by interfacial steps across which equal even numbers of {111}γ and (0002)α 2 planes match. It has been found that the diversity of step heights is greater than that reported previously for lamellar TiAl-based alloys. Circuit mapping was used to identify the Burgers vectors of these steps from lattice images obtained at [10l]γ and [l10]γ zone axes. It was found that the Burgers vectors exhibited by the steps varied with both the height and the sense of the step. In each case these Burgers vectors were consistent with the steps being perfect interfacial disconnections as described by Ponds topological theory of interfacial defects. The defect character of the steps and the occurrence of certain com...

Geometry and interface structure of island nuclei for GaSb buffer layers grown on (001) GaAs by metalorganic vapour phase epitaxy

Atomic force microscopy and transmission electron microscopy have been used to investigate the geometry and interface structure of island nuclei formed in the initial stages of buffer layer growth for MOVPE GaSb on (001) GaAs. There is a bimodal distribution of island sizes with a high density of small, homogeneous nuclei and a lower density of larger, secondary nuclei. The smaller islands have pronounced crystallographic facets which are consistent with those which would be expected for minimization of surface energy and lateral growth anisotropy. The secondary islands are present at junctions between primary nuclei and may have formed due to enhancement of growth rates at emergent threading segments of misfit dislocations. The lattice misfit is accommodated by a regular square arrangement of edge-type misfit dislocations but unusual strain contrast arises in HREM images due to either a “stand-off” of dislocations from the interface or a corrugated interface.

The influence of substrate surface preparation on the microstructure of CdTe grown on (001) GaAs by metalorganic chemical vapour deposition

Abstract A cross-sectional transmission electron microscopy study has been performed on the microstructure of CdTe epitaxial buffer layers, which were grown upon 2°-off (001) GaAs substrates by metalorganic chemical vapour deposition (MOCVD). The substrates were prebaked at various temperatures instead of the usual acid etching treatment. Ut was shown that single crystal films were only obtained for baking temperatures of 550 and 590°C. These single crystal deposits exhibited a distinctive anisotropic defect structure with subgrain boundaries and 90°-type misfit dislocations perpendicular to the off-cut axis and stacking faults and a mixture of 60°-type and 90°-type misfit dislocations parallel to the off-cut axis. All of the stacking faults and 60°-type dislocations lie on the plane (111) producing a 6° rotation between the substrate and deposit lattices about the offcut axis. The development of this microstructure has been ascribed to the formation of high aspect ratio island nuclei and differences in Schmid factors due to the offcut. This microstructure was compared to that produced on an etched substrate and it appears that fine scale substrate surface roughness eliminates both of these influences.

The role of the initial nucleation stage in microstructural development for CdTe grown on heat-cleaned 2°-off (001)GaAs by metalorganic chemical vapour deposition

Abstract A transmission electron microscopy (TEM) study of the initial nucleation layers for the two-step growth of CdTe on 2°-off (001)GaAs substrates by metalorganic chemical vapour deposition (MOCVD) is presented. For substrates which are degreased and etched in the usual manner, the initial deposits are oriented with their axes parallel to those of the substrates and the defects are distributed isotropically. Initial deposits grown on substrates which are prepared by thermal etching at 545–585°C, however, contain both (001)- and (111)-oriented grains. When these polycrystalline initial deposits are heated to the “bulk” growth temperature, epitaxial regrowth occurs giving a single crystal film with a very distinctive anisotropic defect microstructure with stacking faults and a mixture of 60°-type and 90°-type misfit dislocations parallel to the off-cut axis and subgrain boundaries and 90°-type misfit dislocations in the orthogonal direction. All of the 60°-type misfit dislocations and most of the stacking faults lie on the plane (111) producing a rotation of 5–8° between the substrate and deposit lattices about the off-cut axis. These effects are explained in terms of differences in resolved shear stresses as secondary defects are introduced during the regrowth process.

Precipitation and migration of point defects in MOCVD CdxHg1-xTe

Abstract Cross-sectional TEM studies of Cd x Hg 1− x Te layers grown by the interdiffused multilayer process have shown that the microstructures can be related to the diffusion mechanisms during the annealing. Layers on CdTe buffers on GaAs contain extrinsic faulted loops produced by point defect condensation whereas layers on lattice-matched Cd x Zn 1 − x Te substrates contain dislocation networks which have migrated away from the original interface.

The Mechanism of Mixed-Mode Phase Transformations

The topological theory of interfacial defects and the associated flux analysis are reviewed. It is shown that the shears and diffusive fluxes associated with the motion of disconnections can be determined directly from their crystallographic characteristics, and that the effects of changes in chemical composition, interfacial misfit and ordering can be incorporated into the analysis. The special conditions are identified for which there is conservation of atomic sites during the motion of disconnections. It is shown that, under these circumstances, disconnection motion may result in mixed-mode displacive-diffusive transformations whereby diffusion is required for the transformation to proceed but the interfaces exhibit crystallographic characteristics which one would normally associate with a martensitic transformation. It is shown that the growth of γ lamellae in TiAl-based alloys is an example of such a mixed-mode transformation.

The Character of Steps on Gamma/Alpha-2 Interfaces in a Low Misfit Lamellar TiAl-Based Alloy

The defect character of steps on lamellar {gamma}/{alpha}{sub 2} interfaces in a quinternary TiAl-based alloy has been studied using high resolution transmission electron microscopy. The interfaces consisted of atomically flat coherent terraces separated by interfacial steps across equal even numbers of {l{underscore}brace}111{r{underscore}brace}{gamma} and (0002){alpha}{sub 2} planes. Circuit mapping was used to identify the Burgers vectors of these steps from lattice images obtained at [10{bar 1}]{gamma} and [{bar 1}10]{gamma} zone axes. It was found that the Burgers vectors exhibited by the two-layer steps are different from those reported previously and are consistent with those expected for perfect disconnections as described by Ponds topological theory of interfacial defects, and not with the usual partial dislocation model.