Kinga A. Unocic

Crystallographic texture engineering through novel melt strategies via electron beam melting: Inconel 718

Abstract Preliminary research has demonstrated the ability to utilise novel scan strategies in the electron beam melting (EBM) process to establish control of crystallographic texture within Inconel 718 deposits. Conventional EBM scan strategies and process parameters yield coarse columnar grains aligned parallel to the build direction. Through varying process parameters such as beam power, beam velocity, beam focus and scan strategy, the behaviour of the electron beam can be manipulated from a line source to a point source. The net effect of these variations is that the resulting crystallographic texture is controlled in a manner to produce either epitaxial deposits or fully equiaxed deposits. This research demonstrates the ability to change the crystallographic texture on the macroscale indicating that EBM technology can be used to create complex geometric components with both site-specific microstructures and material properties.

Effect of steam on high temperature oxidation behaviour of alumina-forming alloys

Abstract Alternative light water reactor fuel cladding materials are being investigated to replace Zircaloy for enhanced accident tolerance, which involves oxidation resistance to steam environments at ≧1200°C for short times. As chromia-forming alloys and Ni-containing alloys are both undesirable for this application, the focus has been on FeCrAl, although NiAl was used to evaluate the effect of steam oxidation at 1600°C for this study. For commercial and model FeCrAlY alloys, a critical Cr–Al composition was identified for 1 bar isothermal steam (100% H2O) oxidation resistance at 1200°C, which differed for exposures in Ar–50%H2O at the same temperature. Alloys with lower Cr and Al contents were not able to form a protective alumina scale under these conditions. To simulate the accident scenario, exposures were also conducted in steam with the temperature rising 5°C min−1 to 1500°C for the most oxidation resistant alloys. Using thermogravimetry, the maximum use temperature for candidate alloys was determined for different Cr and Al contents. Minor additions such as Y and Ti appeared to be beneficial for oxidation resistance. Similar to prior studies, alumina scales formed in air and in steam appeared to have only subtle differences in microstructure.

Effect of gallium focused ion beam milling on preparation of aluminium thin foils

Focussed ion beam milling has greatly extended the utility of the atom probe and transmission electron microscope because it enables sample preparation with a level of dimensional control never before possible. Using focussed ion beam it is possible to extract the samples from desired and very specific locations. The artefacts associated with this sample preparation method must also be fully understood. In this work, issues specifically relevant to the focussed ion beam milling of aluminium alloys are presented. After using the focussed ion beam as a sample preparation technique it is evident that gallium will concentrate in three areas of the sample: on the surface, on grain boundaries and at interphase boundaries. This work also shows that low‐energy Ar ion nanomilling is potentially quite effective for removing gallium implantation layers and gallium from the internal surfaces of aluminium thin foils.

Effect of environment on the scale formed on oxide dispersion strengthened FeCrAl at 1050°C and 1100°C

Abstract The surface scale formed on specimens of a commercial oxide dispersion strengthened (ODS) FeCrAl alloy (PM2000) exposed for 1 and 500 h at 1050°C in dry O2, Air+10%H2O and Ar+10%H2O consisted of a two-layer α-Al2O3 structure with a columnar grain inner layer and a finer grain outer layer. The alumina scales formed in Air+10%H2O and Ar+10%H2O were slightly more than half of the thickness of the scale formed in dry O2. The same two-layer structure was also observed after exposure for 500 h at 1100°C in dry O2 and 50%CO2+50%H2O. The alumina scales formed in both atmospheres were similar in thickness. Oxides rich in Y and Ti at the gas – scale interface grew in size and number with time in each case. Using analytical transmission electron microscopy, alumina grain boundary segregation of both Y and Ti was evident near the gas interface but only Y segregation was detected near the metal interface. This difference was attributed to Ti depletion in the adjacent metal and the rapid outward flux of the smaller Ti ion through the scale.

Microstructure and environmental resistance of low Cr ODS FeCrAl

Abstract Three oxide dispersion strengthened (ODS) Fe–12 wt-%Cr–5 wt-%Al (12 at-%Cr–9·7 at-%Al) alloys were mechanically alloyed with different oxide additions: (1) Y2O3, (2) Y2O3+HfO2 and (3) Y2O3+ZrO2. The as extruded microstructure was characterised including the oxide particle composition and size distribution. The 700°C Pb–Li compatibility was evaluated for a fusion energy application and the steam oxidation resistance at 1200°C and higher temperatures was evaluated for a nuclear accident tolerant fuel cladding application. The alloy prepared with only a Y2O3 addition contained some low Al regions and did not perform well, but provided a baseline for comparison. The other alloys contained sufficient Cr and Al to form a protective LiAlO2 surface oxide, which inhibits dissolution in isothermal Pb–Li. Also, a protective α-Al2O3 scale formed in steam oxidation at 1200°C, similar to the scale formed in dry air at 1200°C. For the alloy with HfO2, the scale remained protective to 1475°C in steam.

Flux threshold measurements of He-ion beam induced nanofuzz formation on hot tungsten surfaces

We report measurements of the energy dependence of flux thresholds and incubation fluences for He-ion induced nano-fuzz formation on hot tungsten surfaces at UHV conditions over a wide energy range using real-time sample imaging of tungsten target emissivity change to monitor the spatial extent of nano-fuzz growth, corroborated by ex situ SEM and FIB/SEM analysis, in conjunction with accurate ion-flux profile measurements. The measurements were carried out at the multicharged ion research facility (MIRF) at energies from 218 eV to 8.5 keV, using a high-flux deceleration module and beam flux monitor for optimizing the decel optics on the low energy MIRF beamline. The measurements suggest that nano-fuzz formation proceeds only if a critical rate of change of trapped He density in the W target is exceeded. To understand the energy dependence of the observed flux thresholds, the energy dependence of three contributing factors: ion reflection, ion range and target damage creation, were determined using the SRIM simulation code. The observed energy dependence can be well reproduced by the combined energy dependences of these three factors. The incubation fluences deduced from first visual appearance of surface emissivity change were (2–4) × 1023 m−2 at 218 eV, and roughly a factor of 10 less at the higher energies, which were all at or above the displacement energy threshold. The role of trapping at C impurity sites is discussed.

Material compatibility with isothermal Pb–Li

Abstract Eutectic Pb –Li is a leading candidate for current fusion blanket concepts as a coolant. However, there is very little data about the compatibility of most materials with Pb–Li above 500°C where the dissolution rate of many conventional alloys increases rapidly. Current work is beginning to assess Pb–Li compatibility from 500 to 800°C using isothermal capsule experiments. Aluminide coatings hold some promise in protecting conventional Fe-base alloys at 600 – 700°C. However, there is a significant initial Al loss that has not been clearly explained. Furthermore, the reaction product with coated materials is LiAlO2 rather than Al2O3 at 600° and 700°C. Even when pre-oxidized to form α-Al2O3, an alumina layer on FeCrAl transformed to LiAlO2 at 700° and 800°C. At 500°C, the preformed oxide partially transformed from alumina and some Li was detected in the oxide layer.

Vapor synthesis and thermal modification of supportless platinum-ruthenium nanotubes and application as methanol electrooxidation catalysts.

Metallic, mixed-phase, and alloyed bimetallic Pt-Ru nanotubes were synthesized by a novel route based on the sublimation of metal acetylacetonate precursors and their subsequent vapor deposition within anodic alumina templates. Nanotube architectures were tuned by thermal annealing treatments. As-synthesized nanotubes are composed of nanoparticulate, metallic platinum and hydrous ruthenium oxide whose respective thicknesses depend on the sample chemical composition. The Pt-decorated, hydrous Ru oxide nanotubes may be thermally annealed to promote a series of chemical and physical changes to the nanotube structures, including alloy formation, crystallite growth, and morphological evolution. Annealed Pt-Ru alloy nanotubes and their as-synthesized analogs demonstrate relatively high specific activities for the oxidation of methanol. As-synthesized, mixed-phase Pt-Ru nanotubes (0.39 mA/cm(2)) and metallic alloyed Pt64Ru36NTs (0.33 mA/cm(2)) have considerably higher area-normalized activities than PtRu black (0.22 mA/cm(2)) at 0.65 V vs RHE.

Ionic segregation on grain boundaries in thermally grown alumina scales

Abstract This study first examined segregation behaviour in the alumina scale formed after 100 h at 1100°C on bare and MCrAlYHfSi-coated single-crystal superalloys with ~10 ppma La and Y. For the bare superalloy, Hf and Ti were detected on the grain boundaries of the inner columnar alumina layer. Increasing the oxidation temperature to 1200°C for 2 h did not change the segregation behaviour. With the bond coating, both Y and Hf were segregated to the grain boundaries as expected. However, there was evidence of Ti-rich oxide particles near the gas interface suggesting that Ti diffused from the superalloy through the coating. To further understand these segregation observations with multiple dopants, other alumina-forming systems were examined. Alumina scale grain boundary co-segregation of Ti with Y is common for FeCrAl alloys. Co-segregation of Hf and Ti was observed in the scale formed on co-doped NiAl. No La segregation was detected in the scale formed on NiCrAl with only a 19 ppma La addition, however, the scale was adherent.

ToF-SIMS Imaging Study of the Early Stages of Corrosion in Al-Cu Thin Films

The pitting corrosion of Al-Cu thin film alloys was investigated using samples that were heat treated in air to form through-thickness Al 2 Cu particles within an Al-0.5% Cu matrix. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) analysis revealed Cu-rich regions 250―800 nm in lateral extent near the metal/oxide interface. Following exposure that generated pitting corrosion, secondary electron, secondary ion, and AFM images showed pits with size and density similar to those of the Cu-rich regions. The role of the Cu-rich regions is addressed.