J.M. Titchmarsh

Quasi-peritectic solidification reactions in 6xxx series wrought Al alloys

Abstract Secondary intermetallic phase formation during directional solidification of two 6xxx series wrought Al alloys at low growth velocities of 5–30 mm/min has been investigated using differential scanning calorimetry, transmission electron microscopy and scanning transmission electron microscopy. Thermodynamic calculations predict that a quasi-peritectic reaction, L+Al 13 Fe 4 → α-Al+α-AlFeSi, should occur during equilibrium solidification of the alloys. However, no composite Al 13 Fe 4 /α-AlFeSi particles, but composite Al 13 Fe 4 /β-AlFeSi particles and triple phase junctions have been observed for the first time, indicating a divorced metastable β-AlFeSi quasi-peritectic reaction, L+Al 13 Fe 4 → α-Al+β-AlFeSi. More detailed analysis suggests that the metastable β-AlFeSi quasi-peritectic reaction is more favourable both at nucleation and during growth. No unique orientation relationship was found between primary Al 13 Fe 4 and peritectic β-AlFeSi. The nucleation and growth of peritectic phases and the morphology evolution of the two intermetallic phases, Al 13 Fe 4 and β-AlFeSi, are discussed.

TEM investigations of intergranular stress corrosion cracking in austenitic alloys in PWR environmental conditions

Abstract Analytical transmission microscopy has been used to investigate the initiation of stress corrosion cracking in Inconel 600 subjected to constant load testing under simulated pressured water reactor primary water conditions. The observations revealed that intergranular attack proceeded by the development of a zone of polycrystalline chromia along the boundary plane intersecting either the free surface or a blunted, open crack in contact with the free surface. Ni-rich metal particles were interspersed within the chromia. Conversely, open cracks were filled with nanocrystalline NiO and large compound particles of spinel and NiO, indicating a difference in potential between closed, attacked boundaries and open cracks. Open cracks appeared to have initiated by fracture of the chromia zones, such fracture being strongly dependent on boundary geometry with respect to loading direction. The observations suggest that stress corrosion crack initiation and propagation is dependent on diffusion of oxygen through the porous oxides. Dislocations and stress could enhance diffusion as chromia was observed along slip planes at the arrested tips of blunt cracks.

Carrier recombination at dislocations in epitaxial gallium phosphide layers

The nature of dark spots observed in cathodoluminescence micrographs of gallium phosphide epitaxial layers has been examined using the transmission electron microscope and etching studies. Each dark spot is shown to be located at the intersection of a dislocation with the layer surface. Moreover screw, edge and mixed dislocations all give rise to spots of similar size and intensity. It is suggested that enhanced non-radiative recombination which gives rise to the dark spots is due to the core structure, rather than a concentration of dopant atoms, method of layer growth, etc., and that different core structures are equally effective.

Evidence for deformation-induced transformations of Cu-rich precipitates in an aged FeCu alloy

We present experimental evidence that deformation can induce the transformation of small Cu-rich precipitates from the coherent bcc phase to the 9R phase in aged binary FeCu alloys. This is in broad agreement with molecular dynamics simulations of the interaction of dislocations with copper precipitates carried out by Bacon and co-workers [Acta Mater. 50 195/209 (2002); Mater. Sci. Eng. A 400/401 353 (2005); J. Nucl. Mater. 329/333 1233 (2004)].

Preparation of site specific transmission electron microscopy plan-view specimens using a focused ion beam system

A new technique for the preparation of site specific plan-view specimens using a focused ion beam system is presented. The technique consists of milling a wedge shaped piece of material which is free from the substrate, lifting this out using a micromanipulator and needle, and orientating it on the substrate with the original surface vertical. The plan-view specimen is then milled from this piece of material using an approach based on the “lift-out” technique for the preparation of a cross-section specimen. Advantages of this technique over current methods based on the “lift-out” and the “trench” techniques are that the plan-view specimens are site specific, the surrounding substrate is left intact, and numerous plan-view specimens can be prepared in close proximity to one another.

Preparation of transmission electron microscopy cross-section specimens of crack tips using focused ion beam milling

The preparation of transmission electron microscope (TEM) thin foil specimens from metal alloys containing cracks is usually thwarted by the difficulty in preventing preferential erosion of material along the flanks and at the tips of cracks. Recent developments in focused ion beam (FIB) micromachining methods have the potential to overcome this inherent problem. In this article we describe the development of new procedures, one using FIB alone and the other using a combination of FIB with more conventional ion milling to generate TEM specimens that largely retain the microstructural information at stress corrosion cracks in austentic alloys. Examples of corrosion product phase identification and interfacial segregation are included to verify that detailed information is not destroyed by ion bombardment during specimen preparation.

Aberration-corrected HREM/STEM for semiconductor research

Aberration correction leads to a substantial improvement in the resolution of transmission electron microscopes. The JEM-2200FS in Oxford (Begbroke site) is equipped with correctors for both TEM and STEM. Alignment of the TEM and STEM correctors is achieved through variations of the Zemlin tableaux. The microscope can be used to study the same or similar regions of a sample in both TEM and STEM modes.

A New Double-Corrected HREM/STEM and its Applications for Advanced Materials

Extended abstract of a paper presented at the Pre-Meeting Congress: Materials Research in an Aberration-Free Environment, at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, July 31 and August 1, 2004.