G. Sha

Atom probe tomography today

This review aims to describe and illustrate the advances in the application of atom probe tomography that have been made possible by recent developments, particularly in specimen preparation techniques (using dual-beam focused-ion beam instruments) but also of the more routine use of laser pulsing. The combination of these two developments now permits atomic-scale investigation of site-specific regions within engineering alloys (e.g. at grain boundaries and in the vicinity of cracks) and also the atomic-level characterization of interfaces in multilayers, oxide films, and semiconductor materials and devices.

Determining the composition of small features in atom probe: bcc Cu-rich precipitates in an Fe-rich matrix.

Aberrations in the ion trajectories near the specimen surface are an important factor in the spatial resolution of the atom probe technique. Near the boundary between two phases with dissimilar evaporation fields, ion trajectory overlaps may occur, leading to a biased measurement of composition in the vicinity of this interface. In the case of very small second-phase precipitates, the region affected by trajectory overlaps may extend to the centre of the precipitate prohibiting a direct measurement of composition. A method of quantifying the aberrant matrix contribution and thus estimating the underlying composition is presented. This method is applied to the Fe-Cu-alloy system, where the precipitation of low-nanometre size Cu-rich precipitates is of considerable technical importance in a number of materials applications. It is shown definitively that there is a non-zero underlying level of Fe within precipitates formed upon thermal ageing, which is augmented and masked by trajectory overlaps. The concentration of Fe in the precipitate phase is shown to be a function of ageing temperature. An estimate of the underlying Fe level is made, which is at lower levels than commonly reported by atom probe investigations.

Segregation of solute elements at grain boundaries in an ultrafine grained Al–Zn–Mg–Cu alloy

The solute segregation at grain boundaries (GBs) of an ultrafine grained (UFG) Al-Zn-Mg-Cu alloy processed by equal-channel angular pressing (ECAP) at 200 °C was characterised using three-dimensional atom probe. Mg and Cu segregate strongly to the grain boundaries. In contrast, Zn does not always show clear segregation and may even show depletion near the grain boundaries. Trace element Si selectively segregates at some GBs. An increase in the number of ECAP passes leads to a decrease in the grain size but an increase in solute segregation at the boundaries. The significant segregation of alloying elements at the boundaries of ultrafine-grained alloys implies that less solutes will be available in the matrix for precipitation with a decrease in the average grain size.

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.

Field evaporation behavior during irradiation with picosecond laser pulses

A field-ion specimen made of a low thermal diffusivity material, field evaporated using picosecond laser pulses incident from one side, is shown to develop different curvatures between the incident side and the “shadow” side of the specimen apex. Differences of approximately 2.6 and 1.5V∕nm in evaporation field were observed between the two regions of a type 304 stainless steel tip evaporated at 50K with pulsed laser intensities of 0.04 and 0.02nJ∕μm2ps, respectively. This indicates that diffraction of the laser beam cannot ensure uniform illumination and heating over the tip apex.

Overview: Recent progress in three-dimensional atom probe instruments and applications

Over the last few years there have been significant developments in the field of three-dimensional atom probe (3DAP) analysis. This article reviews some of the technical compromises that have led to different instrument designs and the recent improvements in performance. An instrument has now been developed, based around a novel reflectron configuration combining both energy compensation and focusing elements, that yields a large field of view and very high mass resolution. The use of laser pulsing in the 3DAP, together with developments in specimen preparation methods using a focused ion-beam instrument, have led to a significant widening in the range of materials science problems that can be addressed with the 3DAP. Recent studies of semiconductor materials and devices are described.

Effect of laser pulsing on the composition measurement of an Al–Mg–Si–Cu alloy using three-dimensional atom probe

The effect of laser pulse energy on the composition measurement of an Al-Mg-Si-Cu alloy (AA6111) specimen has been investigated over a base temperature range of 20-80K and a voltage range of 2.5-5kV. Laser pulse energy must be sufficiently higher to achieve pulse-controlled field evaporation, which is at least 0.9nJ with a beam spot size of about 5microm, providing an equivalent voltage pulse fraction, approximately 14% at 80K for the alloy specimen. In contrast to the cluster composition, the measured specimen composition is sensitive to base temperature and laser energy changes. The exchange charge state under the influence of laser pulsing makes the detection of Si better at low base temperature, but detection of Cr and Mn is better at a higher temperature and using higher laser energy. No such effect occurs for detection of Mg and Cu under laser pulsing, although Mg concentration is sensitive to the analysis temperature under voltage pulsing. Mass resolution at full-width half-maximum is sensitive to local taper angle near the apex, but has little effect on composition measurement.

An atom probe characterisation of grain boundaries in an aluminium alloy processed by equal-channel angular pressing

Abstract The segregation of solute elements at the grain boundaries of an Al–Zn–Mg–Cu alloy processed by equal-channel angular pressing was characterised using three-dimensional atom probe tomography. The results show that Mg and Cu segregate strongly to the grain boundaries but Zn shows no clear segregation and even becomes depleted near the boundaries. Trace elements such as Zr, Cr, Si and Mn show no clear segregation at the grain boundaries. An increase in the number of passes leads to a decrease in the grain size but there is no clear effect on the levels of solute segregation at the boundaries. The significant segregation of certain major alloying element at the boundaries of ultrafine-grained alloys implies that the less super-saturation solutes in the matrix will be available for precipitation with a decrease in the average grain size.

A comparison of the structure of solute clusters formed during thermal ageing and irradiation

Nanometre scale clusters form in Cu-containing reactor pressure vessel (RPV) steels during neutron irradiation. These clusters have a deleterious effect on mechanical properties, which can result in embrittlement and limit the reactor operating life. Thermal ageing of RPV steels can also induce the formation of solute clusters but it is not clear how similar these are to those formed during irradiation. In this work atom probe tomography, combined with detailed structural assessments of the structure of solute clusters, is used to address this issue. A series of thermal ageing heat treatments has been performed on several high- and low-Ni RPV welds to produce 1-4 nm diameter solute clusters. The same materials have also been neutron irradiated. The results show that CuMnNiSi enriched clusters formed during thermal ageing have, on average, higher Cu contents and lower Mn, Ni and Si contents than those found in irradiation-induced clusters. The effect of increasing bulk Ni is to encourage the formation of clusters with significantly higher Ni content, slightly higher Mn and Si contents and significantly lower Cu contents. At very high doses and dose rates MnNiSi enriched clusters can form even in high-Cu welds. Despite differences in the compositions of individual clusters formed during irradiation and during thermal ageing, clusters in both exhibit similar structure. In particular, well developed clusters in both materials have Cu-enriched cores whose peripheries are enriched in Ni, Mn and, in most cases, Si.

Confined Au-Pd Ensembles in Mesoporous TiO2 Spheres for the Photocatalytic Oxidation of Acetaldehyde

Titanium dioxide (TiO2) is the most widely used photocatalyst for environmental remediation and solar-fuel production, owing to its low cost, nontoxicity, and abundance. In TiO2based photocatalysis, the photogenerated electrons and holes separate and migrate to the surface to be involved in the surface redox reactions. However, a high recombination rate of the photogenerated charge carriers leads to low photocatalytic efficiency. Various noble metals have been introduced to trap photogenerated electrons across the interface and, thus, improve the charge separation within the TiO2 semiconductor. [2]